Inherited Metabolic Disease: Amino Acids, Organic Acids, and Galactose1

Inherited Metabolic Disease: Amino Acids, Organic Acids, and Galactose¹

Louis J. Elsas II^†

Phyllis B. Acosta

^†Deceased.

¹Abbreviations: σ-ALA, σ-aminolevulinic acid; ARG, arginine; ASA, argininosuccinate lyase deficiency; ATP, adenosine triphosphate; BCAA, branched-chain amino acid; BCKA, branched-chain α-ketoacid; BCKAD, branched-chain α-ketoacid dehydrogenase; BH₄, tetrahydrobiopterin; CβS, cystathionine β-synthase; CH₃-B₁₂, methylcobalamin; CIT, citrulline; CNS, central nervous system; CoA, coenzyme A; DHPR, dihydropteridine reductase; DNPH, dinitrophenylhydrazine; EEG, electroencephalogram; ETF, electron transfer factor; FAA, fumarylacetoacetate; FAH, fumarylacetoacetic acid hydrolase; GA-I, glutaric acidemia type I; GAL, galactose; GAL-1-P, galactose-1-phosphate; GALK, galactokinase; GALT, galactose-1-phosphate uridyl transferase; GCD, glutaryl-coenzyme A dehydrogenase; GC/MS, gas chromatography/mass spectrometry; GLU, glucose; GLU-1-P, glucose-1-phosphate; GLY, glycine; HC, head circumference; HCO₃, bicarbonate; HMG, 3-hydroxy-3-methylglutaryl; ILE, isoleucine; IVA, isovaleric acid; IVD, isovaleryl-coenzyme A dehydrogenase; IVG, isovalerylglycine; l-DOPA, l-3,4-dihydroxyphenylalanine; LEU, leucine; LNAA, large neutral amino acid; LYS, lysine; MAT, methionine-S-adenosyltransferase; MBG, 2-methylbutyrylglycinuria; MET, methionine; MMA, methylmalonic acidemia; MPKUCS, Maternal Phenylketonuria Collaborative Study; MS/MS, tandem mass spectrometry; MSUD, maple syrup urine disease; NAD, nicotinamide adenine dinucleotide; NADP, nicotinamide adenine dinucleotide phosphate; NTBC, 2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione; OHIVA, 3-hydroxyisovaleric acid; ORN, ornithine; OTC, ornithine transcarbamylase; PAH, phenylalanine hydroxylase; PHE, phenylalanine; PKU, phenylketonuria; p-OHPPAD, propionic acidemia; RDA, recommended dietary allowance; SAM, S-adenosylmethionine; THR, threonine; TPP, thiamin pyrophosphate; TRP, tryptophan; TYR, tyrosine; UCED, urea cycle enzyme deficiency; UDP, uridine diphosphate; USDA, United States Department of Agriculture; VAL, valine.

GENETIC PERSPECTIVE

Geneticists approach the general subject of nutrition and the specific requirement for nutrients with the view that the recommended dietary allowance (RDA) (1) for an essential nutrient is not optimum for all individuals. Rather, individuals in a population have genetically determined variations in their nutrient requirements that extend over a wide range. This concept arose historically from two older scientific disciplines: human biochemical genetics and nutrition science. The former discipline originated with Sir Archibald Garrod’s Croonian lectures of 1908. Garrod defined four “inborn errors of metabolism” as inherited blocks in the normal flow of metabolic processes. Biochemical and clinical expression of these metabolic blocks demonstrated patterns of inheritance consistent with Mendel’s predictions for transmission
of single genes with a large effect on the phenotype. In alkaptonuria, Garrod observed that the amount of protein ingested was proportional to the darkness of urine and thus the amount of alkaptones excreted. Most individuals did not express this phenomenon, but asymptomatic carriers could be stressed with protein and excrete discernible amounts of alkaptones. Thus arose the “individualistic” concept that an individual’s genes controlled metabolism and that disease states were created by blocks in this metabolic flow that yielded accumulated precursors and deficient products.

Today, we recognize that “inborn errors” are discontinuous traits resulting from variations in the amount and function of enzymes and coenzymes (2, 3, 4). The amino acid sequences and the quantity of enzymes are dictated by genes and epigenetic regulation. The control of enzyme function is predicated by molecular regulation through gene transcription, posttranscriptional processing of RNA, translation, posttranslational modification, cofactor interaction, trafficking, and protein turnover. More than 20,000 monogenic human disorders are cataloged and available. Of these, approximately 400 have a defined biochemical basis (5). The extent of normal variation in genes controlling enzyme activity suggests that approximately 30% of our population is heterozygous for common alleles (6). Within this continuous diversity, mutations produce discontinuous, relatively rare traits that are expressed as disease under normal environmental conditions.

Mutant gene frequencies vary in populations; for example, mitochondrial branched-chain α-ketoacid dehydrogenase (BCKAD) deficiency (maple syrup urine disease [MSUD]) occurs in 1 of approximately every 185,000 newborns worldwide, but it occurs in 1 of 176 in an inbred Mennonite population (7). The Mennonite mutation is in the E1α gene and changes a tyrosine (TYR) at position 194 to asparagine (Y302N). In the homozygous state, it produces extreme toxicity caused by accumulated branchedchain α-ketoacids (BCKAs) if affected newborns are fed the RDA for branched-chain amino acids (BCAAs). Normal development is expected if dietary isoleucine (ILE), leucine (LEU), and valine (VAL) are restricted to 20% to 40% of the RDA during infancy, however (8). Considerable human variation occurs in the structure and activity of enzymes involved in the catabolism of essential amino acids, but only a few persons are so impaired that ingestion of the RDA creates severe disease. Populationbased newborn screening and dietary intervention are now applied through public health programs to more than 40 rare inborn errors for which newborn screening predicts genetic susceptibility given a normal diet (2, 3). By contrast to these relatively rare inborn errors, all humans lack the enzyme that converts L-gulono-α-lactone to ascorbic acid, but scurvy does not occur if sufficient vitamin C is ingested and absorbed (9). Thus, the frequency of genetic susceptibility given a “normal” diet ranges from rare to common and extends to the metabolism of amino acids, nitrogen, carbohydrates, lipids, fatty acids, organic acids, purines, pyrimidines, minerals, and vitamins.

GENETIC DISORDERS BENEFITED BY NUTRITION SUPPORT

More than 400 genetic disorders have been reported in which toxic manifestations relate to accumulation, deficiency, or overproduction of normally occurring substrates and products of metabolic flow (3). In many of these disorders, modification of the dietary supply alleviates the manifestations. In many others, however, irreversible damage has already occurred by the time symptoms appear. Optimum management of these disorders depends on identifying affected persons while they are presymptomatic or before irreversible disease has occurred. Because the disorders are heritable, genetic markers are present from the moment of conception, and thus the genetic power of prediction and prevention is applicable. In practice, certain disorders can be detected in the fetus in the tenth to sixteenth week of gestation by studies on chorionic villus or amniotic fluid cells. Prenatal diagnosis can be made in the ninth to twelfth week of gestation through the use of chorionic villus biopsy (10). Teratogenic sequelae of an inborn error in a pregnancy, such as birth defects in children of mothers with phenylketonuria (PKU), may be prevented by strict control of blood phenylalanine (PHE) before conception and throughout pregnancy. Other inherited metabolic alterations are detected postnatally in the presymptomatic infant by analysis of blood, urine, erythrocytes, leukocytes, or cultured skin fibroblasts for impaired enzymes, accumulated substrates, or products of alternate metabolic pathways.

A selective search for presymptomatic genetic disease is often undertaken when a family history of inherited disease is present. Selective screening for inherited disease is also initiated for relatively common symptoms such as failure to thrive in childhood. Early treatment has proven effective for many diseases such as PKU, galactosemia, isovaleric acidemia, homocystinuria, MSUD, argininosuccinic aciduria, and citrullinemia. Irreversible brain damage occurs if treatment is not initiated in PKU before the second week of life. In MSUD, galactosemia, isovaleric acidemia, and disorders of the urea cycle, irreversible damage to the brain may occur within the first week of life, whereas disorders of fatty acid oxidation may remain undetected for weeks until an intermittent infection produces hypoglycemia. To predict and prevent irreversible damage from inherited metabolic disorders, population-based, newborn screening of dried blood spots from a newborn’s heel stick has been performed since the 1960s. The screening technology has progressed from bioassay to tandem mass spectrometry (MS/MS), which has enabled expansion of detection of accumulated analytes from a few amino acids to include fatty acids and organic acids. These compounds are determined from their acylcarnitine profiles,
and amino acids are determined from their butylester derivatives. In 2006, the American College of Medical Genetics (11) recommended a uniform panel of 29 disorders with primary targets and 25 secondary disorders. Forty-two disorders are screened for by MS/MS of dried blood spots, and all these disorders require rapid retrieval of the screen-positive newborn and urgent confirmation or denial of the screening results with immediate dietary intervention for the confirmed diagnosis. These metabolic conditions were selected through an iterative process among clinicians, laboratorians, and nutritionists. Selection was based on sufficient knowledge and availability of screening and confirmatory technology, the possibility for preventive intervention, and the probability that a screenpositive infant for one of these disorders would have a good outcome from the early intervention. Some of the disorders screened for, their MS/MS profiles, and the acute interventions required for a screen-positive infant are outlined in Table 69.1 (11, 12, 13, 14, 15, 16, 17, 18, 19, 20). The impaired enzymes are listed in Table 69.2 (15, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36). Because the initiation of emergency nutrition in the newborn changes with diagnostic confirmation and with increasing age, some recommended nutritional changes over the age of the child are listed in Table 69.3.

Although many patients with inherited disorders benefit from nutrition support, each disorder would require a chapter for adequate discussion. Thus, this chapter emphasizes disorders for which population-based screening, retrieval, diagnosis, and nutrition support are available to prevent irreversible, severe pathologic problems.

GENERAL PRINCIPLES OF GENETIC DISEASE MANAGEMENT

Thirteen approaches to treatment of inherited metabolic disease are discussed here. Because all these disorders are inherited, all require genetic counseling of the parents regarding recurrence risks, burden of the disease on the affected child, and alternatives in reproduction. The focus here is on direct nutrition and medical interventions for the diagnosed infant. The choice of therapy depends on the pathophysiologic mechanisms producing disease and the medical approach to returning homeostasis to the whole body. Several therapeutic approaches may be tried sequentially or used simultaneously, depending on the acuteness of the disease process.

Enhancing anabolism and depressing catabolism. This combined approach involves the use of high-energy feedings and appropriate amino acid mixtures for aminoacidopathies and organic acidemias. An attempt should be made to prevent the physiologic catabolic state in the 4- to 14-day-old infant, and an anabolic state should be maintained throughout childhood. This therapeutic maneuver should be common to all inborn errors involving catabolic pathways. Caution must be exercised, however, to use high-energy feedings primarily during the acute presentation, but not on a long-term basis, to prevent overweight or obesity.
Correcting the primary imbalance in metabolic relationships. This correction involves using both dietary restriction to reduce accumulated substrates that are toxic and provision of products that may be deficient. For example, in phenylalanine hydroxylase (PAH) deficiency, PHE is restricted and TYR is supplemented.
Enhancing excretion of accumulated substances that are overproduced. The kidney can aid as a dialysis organ in removing accumulated toxic precursors. Maintaining diuresis with hydration is a critical component of therapy.
Providing alternative metabolic pathways to decrease accumulated toxic precursors in blocked reaction sequences. Many examples of this approach exist. For instance, the accumulated ammonia in enzyme defects of the urea cycle is reduced by removing nitrogen through administration of therapeutic amounts of phenylacetic acid (less noxious precursors, phenylbutyrate are used) to form phenylacetylglutamine from glutamine with the consequent loss of two nitrogen atoms in urine. Benzoic acid is also used to force glycine adducts of hippuric acid that result in the per mole urinary excretion of one nitrogen atom. Similarly, in isovaleric acidemia, innocuous isovalerylglycine (IVG) is formed from accumulating isovaleric acid (IVA) if supplemental glycine (GLY) is provided to drive the ubiquitous glycine-N-transacylase. IVG is then excreted in the urine. Betaine is used to drive methylation of homocysteine to methionine (MET) in cystathionine β-synthase (CβS) deficiency.
Using metabolic inhibitors to lower overproduced products. For example, allopurinol inhibits xanthine oxidase and decreases overproduction of uric acid in gout; lovastatin and compactin suppress hydroxymethylglutaryl -coenzyme A (CoA) reductase and reduce excess endogenous cholesterol biosynthesis in familial hypercholesterolemia; and 2-(2- nitro-4-trifluorome thylbenzoyl)-1,3-cyclohexanedione (NTBC) inhibits p-hydroxyphenylpyruvic acid dioxygenase (p-OHPPAD) and thus the toxic succinylacetone production in type I tyrosinemia.
Supplying products of blocked secondary pathways. In cystic fibrosis, the exocrine pancreas does not function in a normal manner to produce and secrete digestive enzymes. Administration of these pancreatic enzymes partially corrects this insufficiency and may prevent the sequelae of fat-soluble vitamin deficiency in the newborn and young child.
Stabilizing altered enzyme proteins. The rate of bio logic synthesis and degradation of holoenzymes depends on their structural conformation. In some holoenzymes, saturation by coenzyme increases their biologic halflife and thus overall enzyme activity at the equilibrium

produced by mutant proteins. This therapeutic mechanism is exemplified in PKU, homocystinuria, and MSUD. Tetrahydrobiopterin (BH₄ [Kuvan]) is available as a drug for therapy of patients with non-PKU hyperphenylalaninemia to enhance PAH activity (37). Pharmacologic intake of vitamin B₆ in homocystinuria or vitamin B₁ in MSUD increases intracellular pyridoxal phosphate or thiamin pyrophosphate (TPP) and increases the specific activity of CβS or BCKAD complex, respectively (38, 39, 40, 41). Another approach is to provide chemical chaperones to stabilize mutant proteins. For example, excess intravenous infusion of D-galactose has increased defective α-galactosidase in the cardiac variant of Fabry disease (42).
Replacing deficient coenzymes. Various vitamindependent disorders are caused by blocks in coenzyme production and are “cured” by lifetime pharmacologic intake of a specific vitamin precursor. This mechanism presumably involves overcoming a partially impaired enzyme reaction by mass action. If reactions that are required to produce methylcobalamin (CH₃-B₁₂) or adenosylcobalamin are impaired, homocystinuria or methylmalonic aciduria (or both) will result. Daily intake of milligram quantities of vitamin B₁₂ may cure both disorders (43). In biotinidase deficiency, the coenzyme biotin is not released from its covalently bound state. Reviews of “vitamin-dependency syndromes” have been published (39, 40, 41).
Artificially inducing enzyme production. If the structural gene or enzyme is intact, but suppressor, enhancer, or promoter elements are not functional, abnormal amounts of enzyme may be produced. It should be possible to “turn on” or “turn off” the structural gene and enable normal enzymatic production to occur. PAH activity has been induced in patients diagnosed with PKU when they received PHE loading for 3 days with intact protein (44, 45). Polyethylene glycol-coated phenylalanine ammonia lyase is undergoing clinical studies in patients with PKU to determine efficacy and allergenicity (46). In the acute porphyria of type I tyrosinemia, excessive σ-aminolevulinic acid (σ-ALA) production may be reduced by suppressing transcription of the σ-ALA synthase gene with excess glucose (GLU) and hematin (Figs. 69.1 and 69.2). In type I tyrosinemia, the overproduction of succinylacetone may be “turned off” by blocking an earlier enzyme p-OH-phenylpyruvate oxidase with the drug NBTC.
Replacing enzymes. Many attempts to replace deficient enzymes by plasma infusion and microencapsulation have been tried, with limited success. The use of polyethylene glycol coating of adenosine deaminase significantly prolonged the biologic half-life of this enzyme in treating severe combined immunodeficiency (47). The engineering of β- glucosidase with a high mannan receptor site enables intravenous use of

alglucerase (Ceredase) to treat type I Gaucher disease. Human α-galactosidase A replacement therapy in Fabry disease reverses substrate storage in the lysosome (42). Recombinant human α-glucosidase can prevent progression and improve cardiac and muscle function in Pompe disease (48). Polyethylene glycol-coated phenylalanine ammonia lyase is undergoing clinical studies in patients with PKU to determine efficacy and allergenicity (46).
Transplanting organs. Liver transplantation in a host of inherited metabolic disorders benefits systemic metabolism; the return of organ function replaces deficient enzyme activity (49, 50). Bone marrow and kidney transplantations are also of benefit.
Correcting the underlying defect in DNA so the body can manufacture its own functionally normal enzymes. The DNA for many proteins that are functionally deficient such as adenosine deaminase, hypoxanthine-guanine phosphoribosyl transferase, and ornithine transcarbamylase (OTC) and the lowdensity lipoprotein cholesterol receptor have been cloned, and retroviral constructs containing their cDNA have been transfected into dividing somatic
cells from affected individuals. Human gene therapy is currently contemplated for these inborn errors, although several technical problems in the toxicity of vectors, gene stability, and gene expression must be solved first. Other molecular approaches such as homologous recombination to correct mutant sequences or to inhibit RNA for dominant disorders producing antagonism to the normal allele are also future possibilities (51, 52).
Preventing absorption of a nutrient that is toxic in excess. Large neutral amino acids (LNAAs) free of PHE are available for preventing absorption of PHE from the intestinal tract and from passage across the blood-brain barrier (53).

TABLE 69.1 CORE AND SECONDARY TARGETS OF INBORN ERRORS OF METABOLISM WITH ONLINE MENDELIAN INHERITANCE IN MAN NUMBER RECOMMENDED FOR NEWBORN SCREENING, MARKER ANALYTES USED WITH TANDEM MASS SPECTROMETRY SCREENING, AND NUTRITION SUPPORT DURING DIAGNOSIS AND ACUTE THERAPY

INBORN ERROR (OMIM NO.)			MARKER ANALYTE	NUTRITION SUPPORT DURING DIAGNOSIS AND ACUTE ILLNESS
Core targets
	Amino acid disorder
		Argininosuccinic acidemia (No. 207900)	CIT	Rapid intervention is required Blood NH₃ >200 µmol/L Delete protein 1-2 d only; increase L-ARG and/or L-CIT if not arginase deficient If necessary, IV glucose and electrolytes at 150 mL/kg/24h to supply 10 mg/kg/min Give Pedialyte and sugar-sweetened, caffeine-free soft drinks with added Polycose or Moducal to maintain energy intake at 125%-150% of RDA Initiate oral medical food and complete diet as rapidly as tolerated Sodium benzoate, phenylbutyric acid, or phenylacetic acid is used to help decrease blood NH₃
		Citrullinemia (No. 215700)	CIT	Rapid intervention is required Blood NH₃ >200 µmol/L Delete protein 1-2 d only; increase L-ARG if not arginase deficient If necessary, give IV glucose and electrolytes at 150 mL/kg/24h to supply 10 mg/kg/min Give Pedialyte and sugar-sweetened, caffeine-free soft drinks with added Polycose or Moducal to maintain energy intake at 125%-150% of RDA Initiate oral medical food and complete diet as rapidly as tolerated Sodium benzoate, phenylbutyric acid, or phenylacetic acid is used to help decrease blood NH₃
		Homocystinuria (No. 236200)	MET	Maintain adequate hydration Administer 25-100 mg/kg pyridoxine in addition to usual infant formula for 1 mo to determine if patient is vitamin B₆ responsive If patient is not vitamin B₆ responsive, restrict MET (20 mg/kg) with medical food, folate, betaine, and complete diet at end of 1 mo
		Maple syrup urine disease (MSUD) (Nos. 248600, 248611, 248610, 238339)	LEU ± VAL	Rapid intervention is required Delete BCAAs 1-2 d Correct metabolic acidosis and electrolyte abnormalities Provide adequate energy to suppress endogenous protein catabolism (125%-150% of RDA for age) Monitor hydration status, electrolyte status, and clinical symptoms carefully to prevent neurologic crisis. Add L-ILE to therapy within 1-2 d when plasma ILE concentration reaches ≈ 105 µmol/L Plasma LEU concentration remains elevated for prolonged period if either ILE or VAL is deficient Suspect concurrent sepsis Initiate oral medical food and complete diet as soon as tolerated
		Phenylketonuria (PKU) (No. 261600)	PHE,PHE/TYR	Delete dietary PHE 1-2 d only For infant, offer Pedialyte with added Polycose to maintain electrolyte balance if needed If necessary, give IV glucose, electrolytes at 150 mL/kg/24h to supply a glucose infusion rate of 10 mg/kg/min, and amino acids free of PHE to maintain anabolism Give sugar-sweetened, caffeine-free soft drinks with added Polycose or Moducal to maintain energy intake at 100% of RDA Initiate oral medical food and complete diet as rapidly as tolerated
		Tyrosinemia type I (TyrI) (No. 276700)	TYR	Rapid intervention is required Delete dietary PHE, TYR, 1-2 d only For infant, offer Pedialyte with added Polycose to maintain electrolyte balance if needed If necessary, give IV glucose and electrolytes at 150 mL/kg/24h to supply a glucose infusion rate of 10 mg/kg/min, and amino acids free of PHE and TYR to maintain anabolism Give sugar-sweetened, caffeine-free soft drinks with added Polycose or Moducal to maintain energy intake at 120%-130% of RDA Initiate oral medical food and complete diet as rapidly as tolerated
	Fatty acid oxidation disorders
		Carnitine uptake deficiency (CUD) (No. 212140)	CO	Give uncooked cornstarch as needed to help prevent hypoglycemia if ≥6 mo of age Avoid fasting If necessary, give IV glucose at 150 mL/kg to supply a glucose infusion rate of 10 mg/kg/min At home, give frequent feedings of fluids containing 2.5 g carbohydrate per fluid ounce Initiate oral diet as rapidly as possible
		Long-chain-hydroxyacyl-CoA dehydrogenase deficiency (LCHAD) (No. 609016)	C16-OH; C18:1-OH,	Give uncooked cornstarch as needed to help prevent hypoglycemia if ≥6 mo of age Avoid fasting If necessary, give IV glucose at 150 mL/kg to supply a glucose infusion rate of 10 mg/kg/min At home, give frequent feedings of fluids containing 2.5 g carbohydrate per fluid ounce Initiate oral diet as rapidly as possible
		Medium-chain acyl-CoA dehydrogenase deficiency (MCAD) (No. 201450)	C8/C10 ± C6, C10:1, C8	Give uncooked cornstarch as needed to help prevent hypoglycemia if ≥6 mo of age Avoid fasting If necessary, give IV glucose at 150 mL/kg to supply a glucose infusion rate of 10 mg/kg/min At home, give frequent feedings of fluids containing 2.5 g carbohydrate per fluid ounce Initiate oral diet as rapidly as possible Avoid medium-chain triglycerides
		Trifunctional protein (TFP) deficiency (No. 609015)	C16-OH, C18:1-OH	Give uncooked cornstarch as needed to help prevent hypoglycemia if ≥6 mo of age Avoid fasting If necessary, give IV glucose at 150 mL/kg to supply a glucose infusion rate of 10 mg/kg/min At home, give frequent feedings of fluids containing 2.5 g carbohydrate per fluid ounce Initiate oral diet as rapidly as possible
		Very-long-chain acyl-CoA dehydrogenase deficiency (VLCAD) (No. 201475)	C14:1, C14:1/C12:1 ± C14, C16, C18:1	Give uncooked cornstarch as needed to help prevent hypoglycemia if ≥6 mo of age Avoid fasting If necessary, give IV glucose at 150 mL/kg to supply a glucose infusion rate of 10 mg/kg/min At home, give frequent feedings of fluids containing 2.5 g carbohydrate per fluid ounce Initiate oral diet as rapidly as possible
	Organic acid disorders
		β-Ketothiolase (BKT) deficiency^a (No. 248600)	C5:1, ± C5OH	Delete dietary ILE 1-2 d only Administer L-carnitine Offer Pedialyte with Polycose to maintain electrolyte balance if needed Give IV glucose and electrolytes at 150 mL/kg/24h to supply a glucose infusion of 10 mg/kg/min, if required Give sugar-sweetened, caffeine-free soft drinks with added Polycose or Moducal to maintain energy intake at 100%-125% of RDA Initiate oral medical food and complete diet as rapidly as possible
		β-Methylcrotonyl-CoA carboxylase (3MCC) deficiency^a (No. 210200)	C5-OH, ± C5:1	Delete dietary LEU 1-2 d only Administer IV L-carnitine Provide vigorous fluid replacement Correct metabolic acidosis and electrolyte abnormalities Provide adequate energy to suppress catabolism (125%-150% of RDA for age) If necessary, give IV glucose, lipid, and L-amino acids free of LEU Initiate oral medical food and complete diet as rapidly as tolerated
		Cobalamin A and B (Cbl A, B) defects^a (Nos. 251100, 251110)	C3, C3/C2	Delete ILE, MET, THR, VAL 1-2 d only Initiate oral diet as rapidly as possible and administer pharmacologic doses of folate and IM hydroxycobalamin
		Glutaric acidemia type I^a (GA-I) (No. 231670)	C5-DC	Delete LYS and TRP 1-2 d only For infant, offer Pedialyte with added Polycose to maintain electrolyte balance if needed If necessary, give IV glucose and electrolytes at 150 mL/kg/24h to supply a glucose infusion of 10 mg/kg/min, and amino acids free of LYS and TRP Give sugar-sweetened, caffeine-free soft drinks with added Polycose or Moducal to maintain energy intake at 100% Initiate oral medical food and complete diet as rapidly as tolerated
		HMG-CoA lyase deficiency^a (No. 246450)	C5-OH, ± C6-DC	Rapid intervention is required Delete LEU 1-2 d only Limit fat intake Administer L-carnitine Vigorous replacement of fluids Correct severe metabolic acidosis and electrolyte abnormalities Provide adequate energy to suppress endogenous protein catabolism (125%-150% of RDA for age) Suspect concurrent sepsis: have a low threshold to treat after obtaining appropriate cultures Initiate oral medical food and complete diet as rapidly as tolerated
		Isovaleric acidemia^a (IVA) (No. 243500)	C5	Delete dietary LEU 1-2 d only Administer GLY and L-carnitine For infant, offer Pedialyte with Polycose to maintain electrolyte balance if needed Give sugar-sweetened, caffeine-free soft drinks with added Polycose or Moducal to maintain energy intake at 100%-125% of RDA If necessary, give IV glucose and electrolytes at 150 mL/kg/24h to supply a glucose infusion rate of 10 mg/kg/min, and amino acids free of LEU. Initiate oral medical food and complete diet as rapidly as tolerated
		Methylmalonic acidemia^a (MUT) (No. 251000)	C3, C3/C2	Rapid intervention is required Delete ILE, MET, THR, VAL 1-2 d only Administer L-carnitine Provide vigorous replacement of fluids Correct severe metabolic acidosis and electrolyte abnormalities; provide adequate energy to suppress endogenous protein catabolism (125%-150% of RDA for age) Suspect concurrent sepsis: have a low threshold to treat after obtaining appropriate cultures
		Multiple carboxylase deficiency^a (MCD) (No. 253260)	C5-OH, ± C3	Biotin, 10-20 mg/d For infant, offer Pedialyte with added Polycose to maintain electrolyte balance if needed If necessary, give IV glucose and electrolytes at 150 mL/kg/24h to supply a glucose infusion rate of 10 mg/kg/min Give sugar-sweetened, caffeine-free soft drinks with added Polycose or Moducal to maintain energy at 100%-125% of RDA Initiate complete diet as rapidly as tolerated
		Propionic acidemia^a (PPA) (No. 606054)	C3, C3/C2	Rapid intervention is required Delete ILE, MET, THR, VAL 1-2 d Administer L-carnitine Provide vigorous replacement of fluids Correct severe metabolic acidosis and electrolyte abnormalities Provide adequate energy to suppress endogenous protein catabolism (125%-150% of RDA for age) Suspect concurrent sepsis: have a low threshold to treat after obtaining appropriate cultures Initiate complete diet as rapidly as tolerated
	Other disorders
		Biotinidase (BIOT deficiency (No. 253260)	±C5-OH, C5:1	Biotin, 10-20 mg/d For infant, offer Pedialyte with added Polycose to maintain electrolyte balance if needed Give sugar-sweetened, caffeine-free soft drinks with added Polycose or Moducal to maintain energy at 100%-125% of RDA If necessary, give IV electrolytes and glucose at 150 mL/kg/24h to supply a glucose infusion rate of 10 mg/kg/min Initiate infant formula as rapidly as tolerated
		Cystic fibrosis (CF) (No. 219700)		Refer to pediatric gastroenterologist
		Galactose-1-phosphate uridyltransferase (GALT) deficiency^b (No. 606999)		Avoid lactose- and galactose-containing infant formulas. Avoid drugs containing galactose or lactose
Secondary targets
	Amino acid disorders
		Argininemia (No. 107830)	ARG	Rapid intervention is required Blood NH₃ >200 7mgr;mol/L Delete protein 1-2 d only Give Pedialyte and sugar-sweetened, caffeine-free soft drinks with added Polycose or Moducal to maintain energy intake at 125%-150% of RDA If necessary, give IV glucose and electrolytes at 150 mL/kg/24h to supply glucose at 10 mg/kg/min Initiate oral medical food and complete diet as rapidly as tolerated Sodium benzoate, phenylbutyric acid, or phenylacetic acid is used to help decrease blood NH₃
		Biopterin regeneration (BIOPT REG) deficiency (No. 261630)	PHE, PHE/TYR
		Biopterin synthesis (BS) defect (No. 261630)	PHE, PHE/TYR
		Citrin deficiency (No. 603471)	CIT	High-protein, low-carbohydrate diet
		Hypermethioninemia (No. 250850)	MET	Delete MET 1-2 d only Initiate oral medical food and complete diet as rapidly as tolerated
		Hyperphenylalaninemia (No. 261630)	PHE	Delete dietary PHE 1-2 d only For infant, offer Pedialyte with added Polycose to maintain electrolyte balance if needed If necessary, give IV glucose, electrolytes at 150 mL/kg/24h to supply a glucose infusion rate of 10 mg/kg/min, and amino acids free of PHE to maintain anabolism Give sugar-sweetened, caffeine-free soft drinks with added Polycose or Moducal to maintain energy intake at 100% of RDA Initiate oral medical food and complete diet as rapidly as tolerated
		Tyrosinemia type II (TyrII) (No. 276600)	TYR	Delete dietary PHE, TYR, 1-2 d only For infant, offer Pedialyte with added Polycose to maintain electrolyte balance if needed; give sugar-sweetened, caffeine-free soft drinks with added Polycose or Moducal to maintain energy intake at 120%-130% of RDA If necessary, give IV glucose and electrolytes at 150 mL/kg/24h to supply a glucose infusion rate of 10 mg/kg/min, and amino acids free of PHE and TYR to maintain anabolism Initiate oral medical food and complete diet as rapidly as tolerated
		Tyrosinemia type III (TyrIII) (No. 276710)	TYR	Delete dietary PHE, TYR, 1-2 d only For infant, offer Pedialyte with added Polycose to maintain electrolyte balance if needed If necessary, give IV glucose and electrolytes at 150 mL/kg/24h to supply a glucose infusion rate of 10 mg/kg/min, and amino acids free of PHE and TYR to maintain anabolism Give sugar-sweetened, caffeine-free soft drinks with added Polycose or Moducal to maintain energy intake at 120%-130% of RDA Initiate oral medical food and complete diet as rapidly as tolerated
	Fatty acid oxidation disorders
		Carnitine acylcarnitine transporter (CACT) defect (No. 212138)	C16:1; C18:1	Rapid intervention is required Avoid fasting If necessary, give IV electrolyte and glucose at 150 mL/kg to supply a glucose infusion rate of 10 mg/kg/min. Give uncooked cornstarch as needed to help prevent hypoglycemia if ≥6 mo of age At home, give frequent feedings of fluids containing 2.5 g carbohydrate per fluid ounce Initiate oral diet as rapidly as possible
		Carnitine palmitoyl-transferase I (CPT IA) defect (No. 600528)	Carnitine	Give uncooked cornstarch as needed to help prevent hypoglycemia if ≥6 mo of age Avoid fasting If necessary, give IV glucose at 150 mL/kg to supply a glucose i nfusion rate of 10 mg/kg/min At home, give frequent feedings of fluids containing 2.5 g carbohydrate per fluid ounce Initiate oral diet as rapidly as possible
		Carnitine palmitoyltransferase II (CPT II) defect (No. 255110)	C16:1, C18:1	Give uncooked cornstarch as needed to help prevent hypoglycemia if ≥6 mo of age Avoid fasting If necessary, give IV glucose at 150 mL/kg to supply a glucose infusion rate of 10 mg/kg/min At home, frequent feedings of fluids containing 2.5 g carbohydrate per fluid ounce Initiate oral diet as rapidly as possible
		Dienoyl-CoA reductase deficiency (DE RED) (No. 222745)		Give uncooked cornstarch as needed to help prevent hypoglycemia if ≥6 mo of age Avoid fasting If necessary, give IV glucose at 150 mL/kg to supply a glucose infusion rate of 10 mg/kg/min At home, frequent feedings of fluids containing 2.5 g carbohydrate per fluid ounce Initiate oral diet as rapidly as possible
		Glutaric acidemia type II^a (GA-II) (Multiple acyl-CoA dehydrogenase deficiency) (No. 231680)	C4, C5, C5-DC, C6, 8, 12, 14, 16	Delete LYS and TRP 1-2 d only Restrict fat Administer L-carnitine and GLY Administer riboflavin Maintain anabolism, electrolyte balance and hydration Initiate oral medical food and complete diet as rapidly as tolerated
		Medium-chain ketoacyl-CoA thiolase deficiency (MCKAT) (No. 602199)	C8, C8/C10, ±C6, C6, C10:1	Give uncooked cornstarch as needed to help prevent hypoglycemia if ≥6 mo of age Avoid fasting If necessary, give IV glucose at 150 mL/kg to supply a glucose infusion rate of 10 mg/kg/min At home, give frequent feedings of fluids containing 2.5 g carbohydrate per fluid ounce Initiate oral diet as rapidly as possible
		Medium-/short-chain hydroxyacyl-CoA dehydrogenase deficiency (M/SCHAD) (No. 300256)	C4-OH	Give uncooked cornstarch as needed to help prevent hypoglycemia if ≥6 mo of age Avoid fasting If necessary, give IV glucose at 150 mL/kg to supply a glucose infusion rate of 10 mg/kg/min At home, give frequent feedings of fluids containing 2.5 g carbohydrate per fluid ounce Initiate oral diet as rapidly as possible
		Short-chain acyl-CoA dehydrogenase deficiency (SCAD) (No. 201470)	C4	Give uncooked cornstarch as needed to help prevent hypoglycemia if ≥6 mo of age Avoid fasting If necessary, give IV glucose at 150 mL/kg to supply a glucose infusion rate of 10 mg/kg/min At home, give frequent feedings of fluids containing 2.5 g carbohydrate per fluid ounce Initiate oral diet as rapidly as possible
Organic acid disorders
		2-Methyl-3-hydroxybutyric acidemia (2M3HBA)	C5, C5:1, C5-OH	Delete dietary ILE 1-2 d only Administer L-carnitine Offer Pedialyte with Polycose to maintain electrolyte balance if needed Give IV glucose and electrolytes at 150 mL/kg/24h to supply a glucose infusion rate of 10 mg/kg/min, if required Give sugar-sweetened, caffeine-free soft drinks with added Polycose or Moducal to maintain energy intake at 100%-125% of RDA Initiate oral medical food and complete diet as rapidly as tolerated
		2-Methylbutyryl-CoA dehydrogenase deficiency (2MBG) (No. 600006)	C5	? LEU restriction
		3-Methylglutaconyl hydratase deficiency^a (3MGA) (No. 250950)	C5-OH	Delete dietary LEU 1-2 d only Administer IV L-carnitine Provide vigorous fluid replacement Correct metabolic acidosis and electrolyte abnormalities Provide adequate energy to suppress catabolism (125%-150% of RDA for age) If necessary, give IV glucose, lipid, and L-amino acids free of LEU Return to oral medical food and complete diet as rapidly as tolerated
		Cobalamin C and D (Cbl C, D) defects^a (Nos. 277410, 277400)	C3/C2	Delete ILE, MET, THR, VAL 1-2 d only Initiate oral diet as rapidly as possible and administer pharmacologic doses of folate and IM hydroxycobalamin
		Isobutyryl-CoA dehydrogenase deficiency (IBG) (No. 611283)	C4	Delete dietary LEU 1-2 d only Increase GLY, VAL and L-carnitine For infant, offer Pedialyte with Polycose to maintain electrolyte balance if needed If necessary, give IV glucose and electrolytes at 150 mL/kg/24h to supply a glucose infusion rate of 10 mg/kg/min, and amino acids free of LEU Give sugar-sweetened, caffeine-free soft drinks with added Polycose or Moducal to maintain energy intake at 100%-125% of RDA Initiate oral medical food and complete diet as rapidly as tolerated
		Malonic (MAL) acidemia (No. 248360)	C3	Restrict fat Administer L-carnitine and medium-chain triglycerides Avoid fasting Give uncooked cornstarch as needed to help prevent hypoglycemia if ≥6 mo of age If necessary, give IV glucose at 150 mL/kg to supply a glucose infusion rate of 10 mg/kg/min At home, give frequent feedings of fluids containing 2.5 g carbohydrate per fluid ounce Initiate oral diet as rapidly as possible
Other disorders
		Galactokinase (GALK) deficiency^b (No. 230200)		Same as for normal infant Avoid formula, food, and drugs containing galactose or lactose Initiate oral diet as rapidly as tolerated
		Galactose epimerase (GALE) deficiency^b (No. 230350)		Same as for normal infant Avoid formula, food, and drugs containing galactose or lactose Initiate oral feedings as soon as tolerated
Colon (:) followed by number, double bonds; ARG, arginine; BCAA, branched-chain amino acid; C, acyl group or carbon chain; CIT, citrulline; CoA, coenzyme A; DC, dicarboxyl; GLY, glycine; HMG-CoA, 3-hydroxy-3-methylglutaryl-coenzyme A; ILE, isoleucine; IM, intramuscular; IV, intravenous; LEU, leucine; LYS, lysine; MET, methionine; NH3, ammonia; O, oxygen; OH, hydroxy; OMIM, Online Mendelian Inheritance in Man; PHE, phenylalanine; RDA, recommended dietary allowance; THR, theronine; TRP, tryptophan; TYR, tyrosine; VAL, valine.
^aOne or more amino acids involved in disorder.
^bScreened for by measuring blood galactose.
Data from references 2 and 12 to 20, with permission.

TABLE 69.2 CHROMOSOMAL LOCATION, GENE SIZE, NUMBER OF MUTATIONS, TISSUE DISTRIBUTION OF GENES, AND GENOTYPE/PHENOTYPE CORRELATIONS

ENZYME				CHROMOSOMAL LOCATION	GENE SIZE (kb)	NUMBER OF MUTATIONS	TISSUE DISTRIBUTION	GENOTYPE/ PHENOTYPE CORRELATION
Enzymes of amino acid metabolism
	Phenylalanine hydroxylase			12q22-q24.1	>90	>500	Liver, kidney	Genotype broadly predicts metabolic and clinical phenotype
	Dihydropteridine reductase			4p15.1-p16.1		21	Liver, fibroblasts, erythrocytes, leukocytes, platelets	?
	Guanosine triphosphate cyclohydrolase			14q22.1-q22.2	30	42	Liver	None
	6-Pyruvoyltetrahydropterin synthase			11q22.3-q23.3	?	>28	Liver, erythrocytes	Genotype associated with phenotype
	Pterin-4 α-carbinolamine dehydratase			10q22		7	Lymphocytes, scalp hair root cells	Mild phenotypes
	Fumarylacetoacetate hydrolyase			15q23-q25		34	Liver, renal tubules, lymphocytes, erythrocytes	Genotype not clearly associated with phenotype
	Maleylacetoacetate isomerase			14q24.3	?	3	Liver, fibroblasts, kidney	?
	Tyrosine aminotransferase			16q22.1	10.9	15	Liver, kidney	None
	4-Hydroxyphenylpyruvate dioxygenase			12q24-qter	21	?	Liver	?
	Cystathionine β-synthase			21q22.3	30	30	Liver, fibroblasts, brain, phytohemagglutininstimulated lymphocytes, amniotic fluid cells, chorionic villus cells	Genotype associated with phenotype I278T: vitamin B₆ responsive T353M: African, not vitamin B₆ responsive G307S: Celtic, not-vitamin B₆ responsive
Methionine-Sadenosyltransferase				10q22	20	17	Liver (22, 28)	Genotype not clearly associated with phenotype
Enzymes of organic acid metabolism
	Branched-Chain α-Ketoacid Dehydrogenase Complex
		E1α (decarboxylase)		19q13.3	55	12	Liver, fibroblasts, leukocytes, muscle	Y393W (Mennonite) (classic phenotype)
		E1β (stabilizes E1α)		6q1.4	100	4	Liver, fibroblasts, leukocytes, muscle	11bp del → stop
		E2 (transacylase)		1p31	68	6	Liver, fibroblasts, leukocytes, muscle	E163X R183P, common in Ashkenazi?
		E3 (lipoamide dehydrogenase)		7p22	20	10	Liver, fibroblasts, leukocytes, muscle	Affects pyruvate and α-ketoglutarate dehydrogenases as well
		E1α kinase (inactivating)		16p13.12	40	2	Liver, fibroblasts, leukocytes, muscle	Inhibited by tumor necrosis factor α and produces cancer cachexia
		E1α phosphatase (activating)		?	?	?	Liver, fibroblasts, leukocytes, muscle	Actives branchedchain- α-ketoacid dehydrogenase complex
		Isovaleryl-CoA dehydrogenase		15q14-q15	2.1-4.6	20	Liver, fibroblasts	Genotype not associated with phenotype
		3-Methylcrotonyl-CoA carboxylase		?	?	?	Fibroblasts, lymphocytes	Genotype associated with phenotype
		3-Methylglutaconyl-CoAhydratase (type 1)		?	?	?	Fibroblasts, lymphocytes	?
		3-Hydroxy-3-methylglutaryl-CoA lyase (HMG-CoA lyase)		1p35.1.36.1	?	?	Liver	Genotype associated with phenotype
		2-Methylbutyryl-CoA dehydrogenase (Acyl-CoA dehydrogenase-SBCAD)		10q26.13	20	>12	Fibroblasts (15, 21)	None reported
		Multiple carboxylase (Holocarboxylase synthetase)		?	?	>30	Liver, fibroblasts, leukocytes (32)	Genotype associated with phenotype
		Glutaryl-CoA dehydrogenase		19p13.2	7	>90	Liver, kidney, fibroblasts, leukocytes, amniotic fluid cells, chorionic villus cells.	None between genotype and clinical severity Specific mutations correlate with severity of organic aciduria
		Propionyl-CoA carboxylase					Heart, kidney, liver cells	None
			α-Subunit	13q32	100	?
			β-Subunit	3q13.3q22	?	?
		Methylmalonyl-CoA mutase		6p12-p21.2	?	22	Kidney, liver, placenta cells	Genotype associated with phenotype
		Biotinidase		3p25	?	>100	Serum leukocytes, fibroblasts (25)	None reported
		β-Ketothiolase (mitochondrial acetoacetyl-CoA thiolase)		11q22.3-q23.1	1.5	>40	Liver (26)	None reported
		2-Methyl-3-hydroxybutyryl-CoA dehydrogenase		XP11.2 H517B10 gene (35)	1.3	?	All human tissue Highest in liver and kidney	?
		Isobutyrl-CoA dehydrogenase		ACAD8gene			Fibroblasts
		Malonyl-CoA decarboxylase		MLYCD gene (30)		22	Fibroblasts (29)	None reported
		Cobalamin A		4q31.21	?	?	Liver, skeletal muscle (24)	?
		Cobalamin B		12q24 (24)	1.1	?	Liver, skeletal muscle, fibroblasts (36)	?
		Cobalamin C		1P34.1 MMACHC protein (34)	?	>42	Fibroblasts	None reported
		Cobalamin D		2q23.2 MMADHC protein (23)			Fibroblasts (27)	None reported
Enzymes of nitrogen metabolism
	Mitochondrial
		Carbamylphosphate synthetase 1		2q35	122	>32	Liver, intestine, kidney (trace)	Genotype associated with phenotype
		N-acetylglutamate synthetase		17q21.31	?	?	Liver, intestine, kidney (trace), spleen	Genotype associated with phenotype
		Ornithine transcarbamylase		Xp21.1	73	>230	Liver, intestine, kidney (trace)	Genotype associated with phenotype
		Citrin		7q21.3	?	30	Liver, kidney, heart, small intestine (31, 33)	?
	Cytosol
		Argininosuccinate synthetase		9q34.1 (many pseudogenes) 7cen-q11.2	53	14	Liver, kidney, fibroblasts, brain (trace)	Genotype associated with phenotype
		Argininosuccinate lyase		?	?	12	Liver, kidney, brain, fibroblasts	Genotype associated with phenotype
		Arginase		6q23	13	2	Liver, erythrocytes, lens, brain (trace)	Genotype associated with phenotype
Enzymes of galactose metabolism
		Galactose-4-epimerase		1p36-35	4	9	Erythrocytes, fibroblasts, liver	Unclear
		Galactokinase		17p24	7.3	13	Liver	Cataracts only
		Galactose-1-phosphate uridyltransferase		9p13	4.3	>150	Erythrocytes, leukocytes, fibroblasts, intestinal mucosa, liver	Genotype associated with phenotype Q188R (white) S135L (black), Δ5 kb (Ashkenazi)
CoA, coenzyme A.

TABLE 69.3 APPROXIMATE DAILY REQUIREMENTS FOR SELECTED NUTRIENTS^a BY INFANTS AND CHILDREN WITH SELECTED INHERITED DISORDERS OF AMINO ACID AND ORGANIC ACID METABOLISM

			AGE
NUTRIENT		UNIT	0 < 6 mo	6 < 12 mo	1 < 4 y	4 < 7 y	7 < 11 y	11 < 15 y	15 < 19 y
Energy		kcal/kg	145-95	135-80	—	—	—	—	—
		kcal/d	—	—	1,300	1,700	2,400	2,200-2,700	2,100-1,800
		(range)			900-1,800	1,300-2,300	1,650-3,300	1,500-3,700	1,200-3,900
Fluid		mL/kg^b	160-135	145-120	95	90	75	50-55	50-65
Protein^c		g/kg	3.5-3.0	3.0-2.5	—	—	—	—	—
		g/d	—	—	30-55	35-65	40-75	50-90	50-95
Fat		g/d	31	30	—	—	—	—	—
Linoleic acid		g/d	4.4	4.6	7.0	10	10-12	16	16
α-Linolenic acid		g/d	0.5	0.5	0.7	0.9	1.0-1.2	1.0-1.2	1.1-1.6
Isoleucine^d
	MSUD^d	mg/kg	90-30	90-30	85-20	80-20	30-20	30-20	30-10
	PPA/MMA^d	mg/kg	100-70	90-60	80-50	70-40	60-30	50-25	40-20
Leucine^d
	MSUD^d	mg/kg	100-60	75-40	70-40	65-35	60-30	50-30	41-15
	Isovaleric acidemia^d	mg/kg	150-70	130-70	100-60	90-50	80-40	70-30	60-25
Lysine^d
	GA-I^d	mg/kg	100-70	90-40	80-30	75-25	65-25	60-20	55-15
Methionine^d
	HCU^d	mg/kg	35-20	35-15	30-10	20-10	20-10	20-10	10-5
	PPA/MMA^d	mg/kg	50-30	45-25	40-20	35-15	30-10	25-10	20-10
Phenylalanine^d,
	PKU^d	mg/kg	70-20	50-15	40-15	35-15	30-15	30-15	30-10
Tyrosinemias^d		mg/kg	95-45	90-35	85-30	80-25	70-20	70-20	65-15
Threonine^d
	PPA/MMA^d	mg/kg	80-50	70-40	60-30	55-25	50-25	45-25	40-25
Tyrosine^d
	PKU^d	mg/kg	350-300	300-250	230	175	140	110-120	110-120
	Tyrosinemias	mg/kg	95-45	75-30	60-30	50-25	40-20	30-15	30-10
Tryptophan^d
	GA-I	mg/kg	40-10	30-10	20-10	15-8	10-6	8-5	8-4
Valine^d
	MSUD	mg/kg	95-40	60-30	85-30	50-30	30-25	30-20	30-15
	PPA/MMA	mg/kg	85-50	80-45	75-45	70-40	60-30	60-30	50-30
GA-I, glutaric acidemia type I; HCU, homocystinuria; MMA, methylmalonic acidemia; MSUD, maple syrup urine disease; PKU, phenylketonuria; PPA, propionic acidemia.
^aAll known essential amino acids, essential fatty acids, minerals, and vitamins must be provided in adequate amounts.
^bAt least 1.5 mL of fluid should be offered for each kilocalorie of energy ingested by the infant and 1 mL/kcal by children and adults.
^cMean daily protein intake by physiologically normal children by age is as follows: 2 < 4 years, 54.9 g; 4 < 9 years, 66.1 g; 9 < 4 years, 80.9 g; 14 < 19 ayears, 96.5 g.
^dAfter 1 to 2 days of deleting appropriate amino acids, introduce those amino acids at the maximum noted for age. Monitor plasma concentrations frequently, and modify the amino acid prescription appropriately.

Fig. 69.1. Metabolism of aromatic amino acids. The metabolic flow and nutrient interaction in disorders of phenylalanine and tyrosine are diagrammed. The black bars represent impaired enzymes in biopterin biosynthesis, phenylketonuria, and tyrosinemia. GTP, guanosine triphosphate; NAD, nicotinamide adenine dinucleotide (NADH is the reduced form).

Fig. 69.2. Inhibition site in heme biosynthesis of relevance to diagnosis and treatment of tyrosinemia type I. The black bar represents the partial block in acute intermittent porphyria with resultant overproduction of σ-aminolevulinic acid (σ-ALA) and porphobilinogen (PBG) with decreased heme biosynthesis. In type I tyrosinemia, succinylacetone is produced and inhibits σ-ALA dehydratase with accumulation of σ-ALA alone, which is neurotoxic. σ-ALA accumulation can be reduced by addition of excess dietary glucose (GLU) and by hematin infusions that negatively control σ-ALA synthase at levels of both enzyme and gene expression. CoA, coenzyme A.

Nutrition management remains a principal component in treating all these inherited disorders, and some practical considerations for nutrition support should be considered. Foremost is the need to maintain normal growth, which cannot be achieved without adequate intake of energy, amino acids, and nitrogen. Energy requirements are greater than normal when intact protein is restricted and free amino acids supply protein equivalent (54). Free amino acids administered in one daily dose are oxidized to a much greater extent than when the same dose is divided and administered throughout the day (55). Nitrogen balance was improved considerably when free amino acids were ingested in several doses throughout the day with intact protein rather than in one dose (56). With total protein intakes (protein equivalent [g N x 6.25 from free amino acids = g protein equivalent] plus intact protein) 25% or more greater than RDA for intact protein for age (1) and closer to actual intakes by physiologically normal children in the United States (57), patients with PKU in the United States have grown normally in height (58). If adequate energy and amino acids cannot be ingested to support normal growth through oral feedings, then nasogastric, gastrostomy, or parenteral feedings should be used. Failure to adapt nutrient intake to the individual needs of each patient can result in mental retardation, metabolic crises, neurologic crises, growth failure, and, with some inherited metabolic diseases, death. When specific amino acids or nitrogen require restriction, total deletion of the toxic nutrient for 1 to 3 days in the presence of excess energy intake is the best approach to initiating therapy. Longer deletion or overrestriction may precipitate deficiency of the amino acids or nitrogen. Because the most limiting nutrient in the diet determines growth rate, overrestriction of an amino acid, nitrogen, or energy will result in further intolerance of the toxic nutrients.

Diet restrictions to correct imbalances in metabolic relationships require the use of elemental medical foods. These medical foods must be accompanied by small amounts of intact protein that supply the restricted amino acids. Intact proteins seldom supply more than 50%, and often supply much less, of the protein requirements of patients. Nitrogen-free foods that provide energy are limited in their range of nutrients. Consequently, care must be taken to provide all nutrients required in adequate amounts (1), particularly because certain minerals are not well absorbed (59) and some vitamins may not be metabolized normally.

Elemental medical foods consist of small molecules that often provide an osmolality that exceeds the physiologic tolerance of the patient. Abdominal cramping, diarrhea, distention, nausea, and vomiting result from hyperosmolar feedings. Aside from gastrointestinal distress, more serious consequences can occur, such as hypertonic dehydration, hypovolemia, hypernatremia, and death. Osmolalities of selected medical foods intended for inherited diseases of amino acid metabolism have been published (60).

AROMATIC AMINO ACIDS

Inborn errors of the aromatic amino acids were historically the first to respond to nutrition support. PKU was discovered in 1933, and the prevention of its resultant mental retardation by dietary intervention is classic.

Biochemistry

The essential amino acid PHE is used for tissue protein synthesis and hydroxylation to form TYR. The hydroxylation reaction requires PAH, oxygen (O₂), BH₄, dihydropteridine reductase (DHPR), and nicotinamide adenine dinucleotide (NAD) plus hydrogen ion (H+) (see Fig. 69.1). The normal adult uses only 10% of the RDA for PHE for new protein synthesis, and approximately 90% is hydroxylated to form TYR. The growing child uses 60% of the required PHE for new protein synthesis, and 40% is hydroxylated to form TYR. Mass spectrometry and stable isotope studies of patients with PKU provide information on other pathways available for PHE metabolism. These alternative pathways (see Fig. 69.1) are minor in the metabolism of PHE at 50 µmol/L concentration in the plasma of physiologically normal individuals. Byproducts become apparent when PHE is not hydroxylated to TYR and accumulates to more than 500 µmol/L, however (61).

TYR is the normal immediate product of PHE and is essential to five pathways (see Fig. 69.1), including synthesis of protein, catecholamines, melanin pigment, and thyroid hormones. TYR also provides energy when it is catabolized through p-OHPPAD to fumarate and acetoacetate. Enzymes required in this latter degradative pathway include TYR aminotransferase, p-OHPPAD, homogentisic acid oxidase, and fumarylacetoacetic acid hydrolase (FAH) (see Fig. 69.1).

Phenylketonuria

PKU is a group of inherited disorders of PHE metabolism caused by impaired PAH activity. The disease is expressed at 3 to 6 months of age and is c haracterized by developmental delay, microcephaly, abnormal electroencephalogram (EEG), eczema, musty odor, and
hyperactivity. If not treated before 2 weeks of age, the metabolic imbalance produces mental retardation that is worsened with increasing time to treatment. The defect in metabolism in classic PKU is associated with less than 2% of the activity of normal PAH, and these classic mutations are now defined (62). The enzyme is expressed primarily in liver.

Molecular Biology

Five of the most frequent mutations in a US clinic include I65T, R408W, Y414C, L348V, and IVS10nt546, which account for more than 50% of mutant PAH alleles. Genotypes with R408W and IVS10nt546 result in more severe PAH impairment, whereas Y414C and I65T have relatively mild phenotypes (62). Heterozygous parents for classic PKU have 50% enzyme activity, and in the absence of known DNA changes, they can be identified by increased ratios of midday semifasting PHE squared to TYR (P²/T) in vivo (63).

The genetic bases for disorders of PAH followed localization of the PAH gene to chromosome 12q22-q24.1 and cloning of the gene, which has 90 kilobases (kb), 13 exons, and 12 introns (see Table 69.3). More than 500 different mutations have been identified that cause the “PKU phenotype,” and these involve deletions in coding frames, missense mutations, and intron splice site mutations. Ethnic variation occurs in the type and frequency of PAH mutations (64). Cloning of the PAH gene and identification of different mutations have assisted in genotyping probands, counseling families, and predicting the amount of dietary PHE that will be required (62). Immediate and lifelong avoidance of excess PHE in the diet continues to be the principal therapy of PKU.

Several investigators have reported oral 6-R-1-erythro-5,6,7,8- BH₄ responsiveness within 24 to 48 hours at doses of 5 to 10 mg/kg in patients with selected mild mutations in the PAH gene (65, 66, 67, 68, 69). An hypothesis has been proposed whereby oral administration of BH₄ makes it possible for the mutant enzymes to suppress their low binding affinity for BH₄, thus enabling patients with this subset of PAH mutations to perform the PHE hydroxylation reaction (69). Although certain genotypes, such as those containing L485, may respond with increased PAH gene expression, other missense mutations may have effects on increased catalysis or increased PAH stability. Many patients with “classic” PKU fail to respond to BH₄ (70).

Other forms of PKU may result from defects in other enzymes involved in the overall reaction. DHPR, an enzyme normally present in many tissues, reduces the quinonoid form of dihydrobiopterin to BH₄ (see Fig. 69.1). The gene for DHPR is located on chromosome 4p15.1-p16.1. Several other types of PKU result from defects in the synthesis of BH₄ (see Fig. 69.1) (see Table 69.3). In addition to functioning as a coenzyme for PAH, BH₄ is also required by TYR hydroxylase and tryptophan (TRP) hydroxylase (see Fig. 69.1). Because these enzymes produce essential neurotransmitters, defects in biopterin synthesis are associated with progressive neurologic disease unless BH₄, L-3,4-dihydroxyphenylalanine (L-DOPA), and serotonin are replaced (71).

Although the precise pathogenesis of mental retardation in classic PKU is not known, accumulation of PHE or its catabolic byproducts, deficiency of TYR or its products, or all four circumstances will produce central nervous system (CNS) damage if PHE accumulates in plasma at greater than normal concentrations during critical periods of brain development. The pathologic consequence varies with the time in brain development at which the chemical insult occurs. Deficient myelination and abnormalities in brain proteolipids or proteins occur in late gestation and during the first 6 to 9 months of life (72). During this period, oligodendroglia migration may also be impaired, resulting in irreversible brain damage later in childhood. Protein synthesis in the brain is also depressed, probably owing to competitive inhibition by high PHE concentrations on blood-brain barrier transport with consequent imbalance in intraneuronal amino acid concentrations (73). In the mature brain, neurodegeneration (74), behavioral difficulties, and prolonged performance times may result from depressed neurotransmitter synthesis. Impairment of these neuropsychologic functions in the mature brain may be reversible when PHE returns toward normal concentrations in cells and blood (75, 76).

Screening

The disorders of PHE metabolism require identification, diagnosis, and appropriate therapy before clinical expression of the disease. Nutrition and possibly other therapy should be instituted before the third week of life. Thus, a tetrapartite public health program involving screening, retrieval, diagnosis, and treatment must be coordinated and efficient to prevent mental retardation. A screening test of dried blood on filter paper that uses MS/MS (11) detects potential cases in the newborn population. The actions taken in retrieval depend on the concentration of blood PHE, days of age, and protein intake at the time of screening.

Protein ingestion may not be required for a positive PKU screen, but quantitative normal concentrations during the first 48 hours of life are needed for comparison (77). Almost all infants with PKU have blood PHE concentrations greater than normal during the first day of life, even before the first feeding if they have classic PKU mutations (71). Neonates with PAH gene mutations resulting in less severely impaired PAH may take longer to develop an elevated blood PHE concentration. Some infants with a relatively mild elevation of blood PHE have serious neuropathologic features that are progressive because of a defect in synthesis of BH₄.

Newborn screening in all 50 states, in conjunction with aggressive, rapid approaches to retrieval and diagnosis, has led to early institution of diet therapy and prevention
of mental retardation (77). With the present early infant discharge from the newborn nursery and the increase in breast-feeding, lower PHE concentrations of 121 to 242 µmol/L (2 to 4 mg/dL) are considered positive, and follow-up is initiated (78). Approximately 1 in 14,000 white newborns in the United States is affected with PKU (79), whereas 1 in 132,000 newborns in the black population is affected (80). For non-PKU hyperphenylalaninemia without TYR elevation, the estimated incidence is 1 in 48,000 for all newborns (79).

Diagnosis

If the follow-up screening test yields blood PHE levels higher than 242 µmol/L (4 mg/dL), all plasma amino acids should be quantitated by ion-exchange chromatography with the infant on known PHE intake from intact protein sources. A precise diagnosis is necessary to establish the mode of therapy.

Differential diagnosis requires several laboratory procedures. These include the following: ion-exchange chromatography or MS/MS to determine plasma PHE, TYR, and other amino acid concentrations; urinary organic analysis by gas chromatography/MS (GC/MS) genotyping of parents and proband (61, 62, 81); and assays of erythrocyte DHPR and urinary biopterin (82). For families with an affected child, prenatal diagnosis is available through direct mutational analysis of fetal cell DNA for known PAH genes or indirect restriction fragment length polymorphism analysis of PAH in parents and proband for unknown fetal PAH genes (61). Because PAH is not expressed in cultured amniotic fluid cells and because PHE concentrations do not increase in amniotic fluid until the last trimester, prenatal diagnosis was not possible before molecular techniques became available.

Treatment

Patients with plasma PHE concentrations greater than 250 µmol, plasma TYR concentrations lower than 50 µmol, and normal BH₄ and DHPR require prompt treatment with a PHE-restricted, TYR-supplemented diet. The objective of nutrition support of the child with classic PKU is to maintain blood PHE concentrations that will allow optimum growth and brain development by supplying adequate energy, protein, and other nutrients while restricting PHE and supplementing TYR intake (58).

Therapy of the child with biopterin-deficient forms of hyperphenylalaninemia requires administration of BH₄ and use of the PHE-restricted, TYR-supplemented diet in combination with L-DOPA and carbidopa (68, 71). Serotonin that is derived from TRP may improve behavior because TRP hydroxylase is also impaired by diminished BH₄ (70).

Initiation of Nutrition Support of Classic Phenylketonuria. Blood PHE concentration at the time of diagnosis may be rapidly lowered by feeding the infant a 20-kcal/oz (67 kcal/dL) PHE-free medical food mixture (16, 83). A minimum of 120 kcal/kg/day intake is necessary. Within a mean of 4 days (SD ±3), blood PHE concentration should decrease to treatment range on a PHE-free formula. Treatment should be initiated in hospitalized infants, if possible, to enable adequate parental information transfer and to monitor blood amino acid concentrations daily. Laboratory results should be available promptly to prevent precipitation of PHE deficiency and to permit rapid replacement of blood PHE and TYR to optimum concentrations.

If the infant or child is not hospitalized for initiation of nutrition support or if only weekly blood PHE concentrations are obtained, 48 hours of a PHE-free medical food mixture followed by a maintenance medical food formula containing adequate PHE should be prescribed. Blood PHE concentration will decrease to treatment range within a mean of 10 days (SD ±5) with this approach (16, 83). Blood PHE concentrations should be between 60 and 300 µmol no later than the third week of life.

Long-term Care. Long-term care of the patient with classic PKU dictates that medical food and intact protein provide all nutrients in required amounts.

Nutrient Requirements. Table 69.3 outlines the PHE, TYR, protein, energy, and fluid to offer. A prescription must be written that is individualized to the age, specific genotype (16, 81), growth rate, protein intake, and energy needs of each patient. Weekly adjustments in the diet prescription are necessary during the first 6 months of life, based on hunger, growth, development, and laboratory analyses of plasma PHE and TYR concentrations. The prescribed PHE should maintain the 3- to 4-hour postprandial plasma PHE concentration between 60 and 300 µmol (84). PHE is an essential amino acid (85) and cannot be deleted from the diet without producing death. Excess restriction produces growth failure, rashes, bone changes, and mental retardation (86).

The infant with classic PKU requires 20 to 50 mg PHE per kilogram body weight for growth, and younger infants require the larger amount (87). The PHE requirement declines rapidly between 3 and 6 months of age as the growth rate declines. Requirements for PHE in the 6- to 12-month-old patient with classic PKU may decrease to 15 mg/kg/day, but they vary considerably (see Table 9.3). Frequent monitoring of blood PHE and TYR concentrations and of intake is required to prevent excess intake when growth rate decelerates and to prevent inadequate intake when growth rate is at its peak, as in early infancy and during the prepubertal and pubertal growth spurts.

TYR is an essential amino acid for individuals with PKU. For this reason, plasma TYR concentrations must be monitored; if they are low, L-TYR supplements are given. To supply adequate TYR intake to patients with PKU, approximately 8% to 10% of protein prescribed should be as TYR. TYR supplements alone will not prevent mental retardation in classic PKU (88).

Protein requirements are increased to more than the 1980 RDA when a free amino acid mix, rather than intact
protein, is the primary protein source (16). Thus, recommendations for protein for nutrition support exceed the RDA. A mean protein intake 24% higher than the 1980 RDA for age was associated with greater PHE tolerance and growth in infants with PKU than was found when mean protein intake was near the RDA (89). Recommendations for energy and fluid intake (Table 69.4) (1, 90, 91, 92, 93) are the same as those for physiologically normal individuals (1).

TABLE 69.4 RECOMMENDED DIETARY INTAKES FOR MINERALS AND VITAMINS BY INFANTS AND CHILDREN

		RECOMMENDED INTAKE FOR AGE^a
NUTRIENT		0 < 6 mo	6 < 12 mo	1 < 4 y	4 < 7 y	7 < 11 y	11 < 15 y	15 < 19 y
Minerals
	Calcium^b (mg)	400	600	800	800	1,300	1,300	1,300
	Chloride (mEq)	5.1	16.1	42.3	53.6	53.6	64.9	64.9
	Chromium (µg)	0.2	5.5	11	15	20	25	25
	Copper (mg)	0.20	0.22	1.0-1.5	1.5-2.0	2.0-2.5	2.0-3.0	2.0-3.0
	Iodine (µg)	65	65	65	73	73	73	95
	Iron (mg)^c	10	15	15	10	10	18	18
	Magnesium (mg)	50	75	150	200	250	410	410
	Manganese (mg)	0.3	0.6	1.2	1.5	1.9	1.9	2.2
	Molybdenum (µg)	2.0	3.0	17	22	22	43	45
	Phosphorus (mg)	300	500	800	800	1,250	1,250	1,200
	Potassium (mEq)	9-24	11-33	76.7	97.1	97.1-115.0	115.0	126.7
	Selenium (µg)	15	20	20	30	40	55	55
	Sodium (mEq)	5-15	16.1	43.5	52.2	65.2	65.2	65.2
	Zinc (mg)	5	5	10	10	10	15	15
Vitamins
	A (µg RE)	400	500	400	500	700	1,000	1,000
	D (µg)^b	5	5	5	5	5	5	5
	E (mg µ-TE)	6	7	6	7	11	15	15
	K (µg)^b	5	10	30	55	55-60	75	75
	Ascorbic acid (mg)	40	50	45	45	45	75	90
	Biotin (µg)	35	50	65	85	120	120	120
	B₆ (mg)	0.3	0.6	0.9	1.3	1.6	1.8	2.0
	B₁₂ (µg)	0.5	1.5	2.0	2.5	3.0	3.0	3.0
	Choline (mg)	125	150	200	250	375	550	550
	Folate (µg)	65	80	150	200	300	400	400
	Niacin equiv (mg)^d	6	8	9	11	16	18	18
	Pantothenic acid (mg)	2.0	3.0	3.0	3.0	4.0	5.0	5.0
	Riboflavin (mg)	0.4	0.6	0.8	1.0	1.4	1.6	1.7
	Thiamin (mg)	0.3	0.5	0.7	0.9	1.2	1.4	1.4
^aData from Otten JJ, Hellwig JP, Meyers LD. Dietary Reference Intakes: The Essential Guide to Nutrient Requirements. Washington, DC: National Academies Press, 2006.
^bFor patients deficient in galactose-1-phosphate uridyl transferase, in addition to usual dietary intake of calcium, vitamin D, and vitamin K, add daily 1000 mg calcium, 10 µg vitamin D and 1 mg vitamin K (93).
^cIron needs of patients with phenylketonuria may be greater than the recommended dietary allowance. For male patients who are more than 11 years of age, 15 mg/day of iron is recommended (90, 91).
^dNiacin needs of patients with phenylketonuria are greater than recommended dietary allowances (92).

Adequate intakes of fat and essential fatty acids linoleic and linolenic acids have been suggested (1). The total fat requirement for children more than 12 months of age has not been determined, but adequate intakes of linoleate and α-linolenate are suggested. Current recommendations are for 31 and 30 g of fat for infants from birth to 6 months of age and from 7 through 12 months of age, respectively. See Table 69.3 for adequate intakes of essential fatty acids.

Minerals and some vitamins often must be considerably greater than the RDA for age (1), to prevent deficiency in patients on elemental diets (90, 92). See Table 69.4 for recommended mineral and vitamin intakes.

Phenylalanine-Free Medical Foods. Adequate amino acids and nitrogen cannot be obtained from intact protein without ingesting excess PHE (intact proteins contain 2% to 9% PHE by weight) (94). Thus, special elemental medical foods are used to provide protein, minerals, and vitamins. Sources of these products are given in Table 69.5.

Intact Proteins. Serving lists have long been available to simplify the PHE-restricted diet for families and professional persons guiding them. With the availability of the Internet, however, nutrient composition of foods is now available from the United States Department of Agriculture (USDA) (94).

A diet plan for an infant with PKU may be found in Table 69.6. By 4 years of age, patients with some genotypes may require as much as 25 mg/kg/day and others as little as 15 mg/kg. Of the more than 30 medical foods,
most are designed for patients older than 12 months of age, and some contain no minerals and vitamins and lead to deficiencies.

TABLE 69.5 SOURCE OF MEDICAL FOODS FOR INHERITED METABOLIC DISEASES

Abbott Nutrition: 3300 Stelzer Rd, Columbus, OH 43219; 800-551-5838 http://abbottnutrition.com Medical food for infants, children, and adults.

Applied Nutrition Corp: 10 Saddle Rd, Cedar Knolls, NJ 07927; 800-605-0410 http://www.medicalfood.com Medical food for children and adults.

Cambrooke Foods: 2 Central St, Framingham, MA 01701; 866-456-9776 http://www.cambrookefoods.com Medical food for children and adults.

Mead Johnson Nutritionals: 2400 West Lloyd Expressway, Evansville, IN 47721; 800-457-3550 http://www.mjn.com Medical food for infants, children, and adults.

Nutricia North America: P O Box 117, Gaithersburg, MD 20884; 800-365-7354 http://www.shsna.com Medical food for infants, children, and adults.

Vitaflo USA LLC: 211 N Union St Suite 100, Alexandria, VA 22314; 888-848-2356 http://www.vitaflousa.com Medical food for children and adults.

Management Problems. Management problems described for children with PKU occur in other children with inherited disorders of metabolism. Principles described here apply to children with other disorders as well but are not reiterated in other sections.

Maintenance of an adequate intake of protein and energy is important for the infant and child with PKU, although PHE must be restricted. Excess energy intake may lead to obesity (95) that is difficult to correct, and weight loss results in elevation of plasma PHE concentrations. Nutrition support must be aggressive, and if intake fails to meet the prescription, a nasogastric or gastrostomy tube should be placed to achieve anabolism. This is extremely important in disorders of BCAAs, nitrogen metabolism, and organic acidemias. Amino acids and nitrogen are obtained from medical foods; therefore, the amount of medical food offered must be varied to provide needs. Nonprotein sources of energy such as corn syrup, Moducal and Protein-Free Diet Powder (Mead Johnson Nutritionals, Evansville, IN), Polycose Glucose Polymers, Pro-Phree (Abbott Nutrition Products Division, Abbott Laboratories, Columbus, OH), Duocal (Nutricia North America, Gaithersburg, MD), sugar, and pure fats can be added to maintain energy intake and to satisfy the child’s hunger without affecting blood PHE concentrations.

TABLE 69.6 SAMPLE DIET: PHENYLKETONURIA (2 WEEKS OF AGE), WEIGHT 3.25 kg

PRESCRIPTION	TOTAL	PER kg
Phenylalanine (mg)	179	55
Tyrosine (mg)	906	278
Protein (g)	11.4	3.5
Energy (kcal)	390	120
Water to make (mL)	600
MEDICAL FOOD	AMOUNT	PHENYLALANINE (mg)	TYROSINE^a (mg)	PROTEIN (g)	ENERGY (kcal)
Phenex-1^b,^c	49 g	0	735	7.4	235
Similac Advance Infant Formula with Iron Powder^b	38 g	177	171	4.1	170
Water to make	600 mL
Total		177	906	11.5	405
^aAdd L-tyrosine to medical food mix only if plasma tyrosine is less than the lower limit of normal. L-Tyrosine is poorly soluble in water. Plasma reference range is 55 to 147 µmol/L.
^bAbbott Nutrition.
^cOther medical foods that may be used for infants are Phenyl-Free-1 (Mead Johnson Nutritionals) and Periflex Infant (Nutricia North America).

Various factors may influence blood PHE concentrations. Factors that may elevate blood PHE concentration include acute infections or trauma with concomitant tissue catabolism, excessive or inadequate PHE intake, and inadequate protein or energy intake. Any infection should be promptly diagnosed and appropriately treated. The best approach to nutrition support during short-term infections is to decrease the intake of PHE and increase the intake of fluids and carbohydrates through the use of Pedialyte with added Polycose powder (Abbott Nutrition Products Division, Abbott Laboratories, Columbus, OH), fruit juices, high-carbohydrate and protein-free beverages, and soft drinks that do not contain caffeine.

Excess PHE intake is the most common cause of elevated blood PHE concentration in the older child and adult with PKU. This condition may be caused by overprescription, misunderstanding of the diet by the caretaker, or dietary noncompliance. Frequent evaluations of blood PHE concentration with accompanying accurate diet diaries for calculation of intake are used to determine the dietary PHE prescription. By monitoring urinary organic acids, increases or decreases in ketone excretion aid in
differentiating between ingestion of inadequate calories and excess PHE intake, respectively.

PHE deficiency associated with inadequate PHE intake has three specific stages of development (96, 97). The first stage is characterized biochemically by decreased blood and urine PHE. Clinically, the child may appear normal, lethargic, or anorectic and may fail to gain length or weight. In the older child, increases in blood alanine and β-hydroxybutyric and acetoacetic acidemia result from muscle alanine production and β-lipolysis. In the second stage, blood PHE concentrations increase as a result of muscle protein degradation, although blood TYR concentrations may be low. BCAA concentrations may increase with decreases in other plasma amino acids. Aminoaciduria appears because of renal tubular malabsorption. In this stage, body protein stores are catabolized, energy sources are depleted, and active membrane transport functions are impaired. Eczema is common. In the third stage of PHE deficiency, blood PHE concentration is lower than normal, as are concentrations of other amino acids. Accompanying clinical manifestations include failure to grow, osteopenia, anemia, sparse hair, and, finally, death if the deficiency is not corrected by supplemental dietary PHE and TYR.

Insufficient protein intake results in an inadequate supply of essential amino acids or nitrogen for growth. When protein synthesis is decreased, PHE is no longer used for growth and accumulates in the blood. When catabolism occurs because of a prolonged lack of nitrogen or amino acid intake, the blood PHE concentration increases because tissue protein contains some 5.5% PHE. In instances of protein insufficiency, medical food intake should be increased to supply the required nitrogen or essential amino acids.

Energy, the first requirement of the body, is necessary for growth. When energy is provided as carbohydrate and fat, and if adequate nitrogen is available, nonessential amino acids may be synthesized from their ketoacid precursors. Further, carbohydrate ingestion leads to insulin secretion; and insulin promotes amino acid transport into the cell and consequent protein synthesis (97). When energy intake is inadequate, tissue catabolism occurs to meet energy needs. Such catabolism releases PHE, thus leading to an elevated blood PHE concentration. Sufficient energy must be provided through generous but not excess use of nonprotein and low-protein foods to ensure a normal growth rate.

A low blood PHE concentration (25 <mol/L) may lead to depressed appetite (98), decreased growth (99), and if prolonged, mental retardation (84). Low blood PHE concentrations are often caused by inadequate prescription of PHE. In such cases, the prescription for PHE can be increased by the addition of measured amounts of intact protein.

Assessment of Nutrition Support. Along with biweekly assessment of growth through measurement of length, weight, and head circumference (HC) and evaluation of development, the adequacy of PHE and TYR intake is determined by twice-weekly quantitation of plasma PHE and TYR concentrations during the first 6 months and weekly thereafter until the child is 1 year of age. The first year is the period of most rapid growth and of greatest vulnerability to nutrition insult. After 1 year of age, weekly blood tests suffice for monitoring the diet. If plasma PHE concentrations exceed 300 µmol/L (5 mg/dL), the prescription for PHE is decreased, and frequent blood tests are done until plasma PHE concentrations are between 60 and 300 µmol/L.

For blood tests to be of use in adjusting the prescription, laboratory analyses must be both accurate and prompt. Fluorometric, ion-exchange, high-performance liquid chromatography, and MS/MS methods are quantitative and are preferred to monitor plasma PHE and TYR concentrations. If properly instructed, parents may be given responsibility for obtaining the specimens on filter paper or in microcapillary tubes and mailing them to a central laboratory.

A record of food ingested before blood sampling for PHE and TYR measurement is essential and should be kept by the child’s caregiver. The correlation among (a) the child’s intake of PHE, TYR, protein, and energy; (b) the child’s clinical status; and (c) the plasma PHE and TYR concentrations is considered when the diet is altered.

Results of Therapy. Early diagnosis and treatment (before 2 weeks of age) of infants with PKU with a nutritionally adequate, PHE-restricted, TYR-supplemented diet promotes normal growth and prevents mental retardation. A national study showed that mean height, weight, HC, IQ scores of children who were treated early were the same as those of physiologically normal children at 4 years of age (100, 101). Trefz et al (81) reported that when blood PHE concentration was maintained at less than 360 µmol/L, no difference was observed in mean IQ by genotype of 9-year-old children.

The semisynthetic nature of the PHE-restricted diet has led to questions concerning its adequacy. Mean serum carnitine (total and free) of treated patients was in the reference range when patients were fed medical foods containing carnitine (102, 103). Medical foods containing increased amounts of L-TYR have alleviated the problem of low plasma TYR concentrations (104, 105). After an overnight fast, the concentrations of plasma GLY were elevated in patients, one group of whom received a GLYfree medical food (Periflex; Nutricia North America, Gaithersburg, MD) (106). Treated patients with PKU often have lower-than-normal concentrations of transthyretin when they are fed the RDA for protein (107). Arnold et al (108) and Acosta et al (95) reported a positive correlation between height and plasma transthyretin with concentrations less than 200 mg/L associated with poor linear growth.

Depressed plasma concentrations of total cholesterol have been reported in treated children and untreated adults with PKU (109, 110, 111). Pregnant women with PKU whose serum cholesterol concentrations failed to increase during early gestation often had a spontaneous abortion (112), possibly because of an inadequate hormonal response to pregnancy. Castillo et al (113) reported inhibition of brain and liver 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase and mevalonate-5-pyrophosphate decarboxylase in experimental hyperphenylalaninemia. According to Artuch et al (114), elevated plasma PHE concentrations resulted in decreased serum ubiquinone 10 concentrations. Hargreaves et al (115), however, found no difference in mononuclear cell coenzyme Q10 concentrations among control, treated, and untreated patients with PKU. Lower-than-normal plasma and erythrocyte docosahexaenoic acid concentrations and higher-than-normal n-6 series fatty acid concentrations have been found in patients undergoing therapy for PKU (116, 117). The significance of these differences is unclear, but they appear to be the result of the low fat content of the medical food (118, 119).

Iron deficiency has been reported in children undergoing therapy for PKU despite intakes greater than the RDA (120, 121). A poor selenium status has been found in children with PKU who were receiving medical foods without added selenium (122). Patients with PKU and low selenium concentrations had elevated concentrations of thyroxine and reverse triiodothyronine, which decreased significantly with selenium supplementation (123). Plasma retinol concentrations of children with PKU are often lower than concentrations in normal children (107), but they are within reference ranges with adequate medical food intake (91).

Bone changes were reported as early as 1956 in treated children with PKU. Preschool children with plasma PHE concentrations within treatment range had normal bone mineralization (124, 125, 126, 127). Greeves et al (128) reported that 25% of patients with PKU, most of whom were more than 8 years of age, had a history of fractures compared with 18% of physiologically normal siblings. Untreated PKU mice (PAH^enu-2) had reduced mean femur weight compared with treated and control mice and shorter mean femur length than control mice. Femur strength was greater in treated mice compared with control mice (129). Elevated serum prolactin concentrations in patients with poorly controlled PKU resulted in a high prevalence of menstrual irregularities in the girls (130). As blood PHE concentration increased in older patients under poor dietary control, values for bone mineral content and bone density were always lower than control values. Amino acid imbalances, inadequate protein intake, the need for phosphorus to buffer organic acids made from excess dietary PHE, and inadequate estrogen because of excess prolactin secretion could have contributed to bone abnormalities (131). Mussa et al (132) reported that as plasma PHE concentrations increased, osteoclastogenesis increased, leading to bone damage.

Mean plasma concentrations of immunoglobulin A and immunoglobulin M were significantly lower in patients with PKU undergoing therapy (133) than in physiologically normal children.

Diet Discontinuation. In the past, certain clinicians suggested that the diet could be discontinued at 4, 6, or 12 years of age with no adverse effects (134, 135, 136, 137). Investigators questioned this possibility because studies showed significant differences in performance and intelligence in children (101, 138) and in neurologic function in adults who discontinued the diet and in those who remained on the diet. Severe agoraphobia (139), reversible by a return to the PHE-restricted diet, was also reported in adults. Vitamin B₁₂ deficiency resulting in hematologic changes and neurologic disease occurs in off-diet patients who refuse foods of animal origin but who fail to ingest PHE-free medical foods (140, 141). Vegan-type methylmalonic aciduria from vitamin B₁₂ deficiency is the most likely pathophysiologic mechanism.

In studies using the patient as his or her own control, elevated plasma PHE concentrations prolonged the performance time on neuropsychologic tests of higher integrative function, reduced the mean power frequency of the EEG, and decreased urinary dopamine excretion and plasma L-DOPA in older treated patients with PKU (142, 143). A correlation was found among high plasma PHE concentrations, prolonged performance time on neuropsychologic tests, and decreased urinary dopamine in 10 patients. In a study of eight additional patients, statistically significant decreases were found in the mean power frequency of the EEG and in plasma L-DOPA when plasma PHE concentration increased (143). EEG slowing occurred in PKU heterozygotes at concentration changes of blood PHE induced by aspartame ingestion (150 µmol/L) (75). These effects were reversible and correlated in the reverse direction when the plasma PHE concentration was reduced. Severe neurologic deterioration occurred in several off-diet patients with PKU (74, 144). Reversal of most of the symptoms occurred in a patient who returned to a PHE-restricted diet that contained medical food (74).

Maternal Phenylketonuria

Pregnant women with PKU who are untreated at conception and during gestation have children with intrauterine growth retardation, microcephaly, and congenital anomalies, often severe and incompatible with life. Mental retardation is common in children of mothers whose plasma PHE concentration is higher than the normal range (145). The pathogenesis of the fetal damage is uncertain but is believed to be related to elevated maternal blood PHE concentration (146) because PHE is actively transported across the placenta to the fetus (147). Fetal plasma PHE concentrations are one and one half to two times those of maternal blood (148). Such elevated fetal plasma PHE concentrations are again concentrated twofold to fourfold
by the fetal blood-brain barrier (149). Intraneuronal PHE concentrations of 600 µmol interfere with brain development by one or more of the several previously described mechanisms, including abnormal oligodendroglial migration and myelin and other protein synthesis (150). Thus, it is extremely important to maintain normal plasma PHE concentrations in the reproductive-age woman with PKU before conception and throughout gestation. Surviving children of untreated women fail to grow and develop normally (151). In fact, Kirkman (152) predicted that if the fertility of these women is normal and they are not treated with dietary control of PHE intake, the incidence of PKU-related mental retardation could return to the prescreening level after only one generation.

In 1984, the Maternal Phenylketonuria Collaborative Study (MPKUCS) was initiated to answer questions related to diet and reproductive outcome in women with PKU (153). Results of the MPKUCS supported the premise that a PHE-restricted diet, plasma PHE concentrations lower than 360 µmol/L, and the gestational age at which the diet is initiated affect reproductive outcome (154).

Nutrition Support of Maternal Phenylketonuria. The PHE-restricted diet should be initiated at least 3 months before a planned pregnancy by women who have PKU, if they have previously discontinued the diet. The objectives of therapy for pregnant women with PKU are a healthy mother and a normal, healthy newborn. To obtain adequate protein and fat storage in early pregnancy to support third-trimester fetal growth, careful attention must be paid to diet and nutrition status. Although the plasma PHE concentration most likely to yield the best reproductive outcome is unknown, one group of investigators suggested that these objectives may be achieved by a PHE-restricted diet that maintains plasma PHE concentration between 60 and 180 µmol/L (155). Plasma PHE concentrations lower than 60 µmol/L may lead to maternal muscle wasting and poor fetal growth. The recommended PHE intake to prescribe for initiating therapy is given in Table 69.7 (1, 112, 116). Other indices of nutrition status should be in the normal range for pregnant women. After initiation of diet with the minimum recommended PHE prescription (see Table 69.7), the plasma PHE concentration should be monitored twice weekly to maintain the targeted plasma PHE concentration.

Even after the plasma PHE concentration is stabilized in the treatment range, frequent changes in the individualized diet prescription are required as pregnancy progresses, based on concentrations of plasma PHE, TYR, and other amino acids and on weight gain. PHE and TYR requirements of each pregnant woman depend on genotype, age, state of health, protein intake, and trimester of pregnancy (112). At approximately midpregnancy, PHE tolerance increases considerably.

TABLE 69.7 RECOMMENDED PHENYLALANINE, TYROSINE, PROTEIN, FAT, ESSENTIAL FATTY ACID, AND ENERGY INTAKES FOR PREGNANT WOMEN WITH PHENYLKETONURIA

		NUTRIENTS
TRIMESTER AND AGE (y)		PHENYLALANINE^a,^b (mg/d)	TYROSINE^b (mg/d)	PROTEIN (g/d)	FAT^c (g/d)	LINOLEIC ACID (g/d)	α-LINOLENIC ACID (g/d)	ENERGY^c (kcal/d)
TRIMESTER AND AGE (y)		PHENYLALANINE^a,^b (mg/d)	TYROSINE^b (mg/d)	PROTEIN (g/d)	FAT^c (g/d)	LINOLEIC ACID (g/d)	α-LINOLENIC ACID (g/d)	MEAN	RANGE
Trimester 1 (0 < 14 wk gestation)
	15 < 19	200 < 820	≥7,600	≥76	36-132	13	1.4	2,500	1,600-3,400
	19 < 24	180 < 800	≥7,400	≥74	47-124	13	1.4	2,500	2,100-3,200
	≥ 24	180 < 800	≥7,400	≥74	47-132	13	1.4	2,500	2,100-3,400
Trimester 2 (14 < 27 wk gestation)
	15 < 19	200 < 1000	≥7,600	≥76	36-132	13	1.4	2,500	1,600-3,400
	19 < 24	180 < 1000	≥7,400	≥74	47-124	13	1.4	2,500	2,100-3,200
	≥ 24	180 < 1000	≥7,400	≥74	47-132	13	1.4	2,500	2,100-3,400
Trimester 3 (27 < 41 wk gestation)
	15 < 19	330 < 1200	≥7,600	≥76	36-132	13	1.4	2,500	1,600-3,400
	19 < 24	310 < 1200	≥7,400	≥74	47-124	13	1.4	2,500	2,100-3,200
	≥ 24	310 < 1200	≥7,400	≥74	47-132	13	1.4	2,500	2,100-3,400
^aRecommended range of phenylalanine (PHE) intake covered approximately 80% of women studied in the Maternal Phenylketonuria Collaborative Study (MPKUCS). Initiate the diet with the lowest amount recommended for the trimester and age. Frequent monitoring of plasma PHE is essential to prevent deficiency or excess. Modify the prescription based on the following: frequent plasma PHE and tyrosine (TYR) concentrations; intakes of PHE, TYR, protein, and energy; and maternal weight gain. Recommended iron intake is from MPKUCS data in Acosta PB, Michals-Matalon K, Austin V et al. Nutrition findings and requirements in pregnant women with phenylketonuria. In: Platt LD, Koch R, de la Cruz F, eds. Genetic Disorders and Pregnancy Outcome. New York: Parthenon, 1997:21-32.
^bL-TYR is very insoluble in water. Consequently, any supplemental L-TYR should be mixed with fruit purees, mashed potatoes, or soup for ingestion. Recommended intake is from MPKUCS data in Acosta PB, Michals-Matalon K, Austin V et al. Nutrition findings and requirements in pregnant women with phenylketonuria. In: Platt LD, Koch R, de la Cruz F, eds. Genetic Disorders and Pregnancy Outcome. New York: Parthenon, 1997:21-32.
^cModified from Otten JJ, Hellwig JP, Meyers LD. Dietary Reference Intakes: The Essential Guide to Nutrient Requirements. Washington, DC: National Academies Press, 2006. For some women, energy requirements may be greater than the upper limit of the range given to obtain appropriate weight gain.

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