Involving Enzymes

Figure 12-2 The biochemical pathways affected in the hyperphenylalaninemias. BH4, tetrahydrobiopterin; 4αOHBH4, 4α-hydroxytetrahydrobiopterin; qBH2, quinonoid dihydrobiopterin, the oxidized product of the hydroxylation reactions, which is reduced to BH4 by dihydropteridine reductase (DHPR); PCD, pterin 4α-carbinolamine dehydratase; phe, phenylalanine; tyr, tyrosine; trp, tryptophan; GTP, guanosine triphosphate; DHNP, dihydroneopterin triphosphate; 6-PT, 6-pyruvoyltetrahydropterin; L-dopa, L-dihydroxyphenylalanine; NE, norepinephrine; E, epinephrine; 5-OH trp, 5-hydroxytryptophan.

TABLE 12-1

Locus Heterogeneity in the Hyperphenylalaninemias


*BH4 supplementation may increase the PAH activity of some patients in each of these three groups.

BH4, Tetrahydrobiopterin; DHPR, dihydropteridine reductase; GTP-CH, guanosine triphosphate cyclohydrolase; 5-HT, 5-hydroxytryptophan; PAH, phenylalanine hydroxylase; PCD, pterin 4α-carbinolamine dehydratase; PKU, phenylketonuria; 6-PTS, 6-pyruvoyltetrahydropterin synthase.


Variant Phenylketonuria and Nonphenylketonuria Hyperphenylalaninemia.


Mutant Enzymes and Disease

General Concepts

The following concepts are fundamental to the understanding and treatment of enzymopathies.

Inheritance patterns
Enzymopathies are almost always recessive or X-linked (see Chapter 7). Most enzymes are produced in quantities significantly in excess of minimal biochemical requirements, so that heterozygotes (typically with approximately 50% of residual activity) are clinically normal. In fact, many enzymes may maintain normal substrate and product levels with activities of less than 10%, a point relevant to the design of therapeutic strategies (e.g., homocystinuria due to cystathionine synthase deficiency—see Chapter 13). The enzymes of porphyrin synthesis are exceptions (see discussion of acute intermittent porphyria in main text, later).

Substrate accumulation or product deficiency
Because the function of an enzyme is to convert a substrate to a product, all of the pathophysiological consequences of enzymopathies can be attributed to the accumulation of the substrate (as in PKU), to the deficiency of the product (as in glucose-6-phosphate dehydrogenase deficiency (Case 19), or to some combination of the two (Fig. 12-3).


Figure 12-3 A model metabolic pathway showing that the potential effects of an enzyme deficiency include accumulation of the substrate (S) or derivatives of it (S1, S2, S3) and deficiency of the product (P) or compounds made from it (P1, P2). In some cases, the substrate derivatives are normally only minor metabolites that may be formed at increased rates when the substrate accumulates (e.g., phenylpyruvate in phenylketonuria).

Diffusible versus macromolecular substrates
An important distinction can be made between enzyme defects in which the substrate is a small molecule (such as phenylalanine, which can be readily distributed throughout body fluids by diffusion or transport) and defects in which the substrate is a macromolecule (such as a mucopolysaccharide, which remains trapped within its organelle or cell). The pathological change of the macromolecular diseases, such as Tay-Sachs disease, is confined to the tissues in which the substrate accumulates. In contrast, the site of the disease in the small molecule disorders is often unpredictable, because the unmetabolized substrate or its derivatives can move freely throughout the body, damaging cells that may normally have no relationship to the affected enzyme, as in PKU.

Loss of multiple enzyme activities
A patient with a single-gene defect may have a loss of function in more than one enzyme. There are several possible mechanisms: the enzymes may use the same cofactor (e.g., BH4 deficiency); the enzymes may share a common subunit or an activating, processing, or stabilizing protein (e.g., the GM2 gangliosidoses); the enzymes may all be processed by a common modifying enzyme, and in its absence, they may be inactive, or their uptake into an organelle may be impaired (e.g., I-cell disease, in which failure to add mannose 6-phosphate to many lysosomal enzymes abrogates the ability of cells to recognize and import the enzymes); and a group of enzymes may be absent or ineffective if the organelle in which they are normally found is not formed or is abnormal (e.g., Zellweger syndrome, a disorder of peroxisome biogenesis).

Phenotypic homology
The pathological and clinical features resulting from an enzyme defect are often shared by diseases due to deficiencies of other enzymes that function in the same area of metabolism (e.g., the mucopolysaccharidoses) as well as by the different phenotypes that can result from partial versus complete defects of one enzyme. Partial defects often present with clinical abnormalities that are a subset of those found with the complete deficiency, although the etiological relationship between the two diseases may not be immediately obvious. For example, partial deficiency of the purine enzyme hypoxanthine-guanine phosphoribosyltransferase causes only hyperuricemia, whereas a complete deficiency causes hyperuricemia as well as a profound neurological disease, Lesch-Nyhan syndrome, which resembles cerebral palsy.

Variant PKU includes patients who require only some dietary phenylalanine restriction but to a lesser degree than that required in classic PKU, because their increases in blood phenylalanine levels are more moderate and less damaging to the brain. In contrast to classic PKU, in which the plasma phenylalanine levels are greater than 1000 μmol/L when the patient is receiving a normal diet, non-PKU hyperphenylalaninemia is defined by plasma phenylalanine concentrations above the upper limit of normal (120 μmol/L), but less than the levels seen in classic PKU. If the increase in non-PKU hyperphenylalaninemia is small (<400 μmol/L), no treatment is required; these individuals come to clinical attention only because they are identified by newborn screening (see Chapter 17). Their normal phenotype has been the best indication of the “safe” level of plasma phenylalanine that must not be exceeded in treating classic PKU. The association of these three clinical phenotypes with mutations in the PAH gene is a clear example of allelic heterogeneity leading to clinical heterogeneity (see Table 12-1).

Allelic and Locus Heterogeneity in the Hyperphenylalaninemias

Allelic Heterogeneity in the PAH Gene.


Figure 12-4 The nature and identity of PAH mutations in populations of European and Asian descent (the latter from China, Korea, and Japan). The one-letter amino acid code is used (see Table 3-1). See Sources & Acknowledgments.

The allelic heterogeneity at the PAH locus has major clinical consequences. Most important is the fact that most hyperphenylalaninemic subjects are compound heterozygotes (i.e., they have two different disease-causing alleles) (see Chapter 7). This allelic heterogeneity accounts for much of the enzymatic and phenotypic heterogeneity observed in this patient population. Thus, mutations that eliminate or dramatically reduce PAH activity generally cause classic PKU, whereas greater residual enzyme activity is associated with milder phenotypes. However, homozygous patients with certain PAH mutations have been found to have phenotypes ranging all the way from classic PKU to non-PKU hyperphenylalaninemia. Accordingly, it is now clear that other unidentified biological variables—undoubtedly including modifier genes—generate variation in the phenotype seen with any specific genotype. This lack of a strict genotype-phenotype correlation, initially somewhat surprising, is now recognized to be a common feature of many single-gene diseases and highlights the fact that even monogenic traits like PKU are not genetically “simple” disorders.

Defects in Tetrahydrobiopterin Metabolism.

The locus heterogeneity of hyperphenylalaninemia is of great significance because the treatment of patients with a defect in BH4 metabolism differs markedly from subjects with mutations in PAH, in two ways. First, because the PAH enzyme of individuals with BH4 defects is itself normal, its activity can be restored by large doses of oral BH4, leading to a reduction in their plasma phenylalanine levels. This practice highlights the principle of product replacement in the treatment of some genetic disorders (see Chapter 13). Consequently, phenylalanine restriction can be significantly relaxed in the diet of patients with defects in BH4 metabolism, and some patients actually tolerate a normal (i.e., a phenylalanine-unrestricted) diet. Second, one must also try to normalize the neurotransmitters in the brains of these patients by administering the products of tyrosine hydroxylase and tryptophan hydroxylase, L-dopa and 5-hydroxytryptophan, respectively (see Fig. 12-2 and Table 12-1).

Remarkably, mutations in sepiapterin reductase, an enzyme in the BH4 synthesis pathway, do not cause hyperphenylalaninemia. In this case, only dopa-responsive dystonia is seen, due to impaired synthesis of dopamine and serotonin (see Fig. 12-2). It is thought that alternative pathways exist for the final step in BH4 synthesis, bypassing the sepiapterin reductase deficiency in peripheral tissues, an example of genetic redundancy.

For these reasons, all hyperphenylalaninemic infants must be screened to determine whether their hyperphenylalaninemia is the result of an abnormality in PAH or in BH4 metabolism. The hyperphenylalaninemias thus illustrate the critical importance of obtaining a specific molecular diagnosis in all patients with a genetic disease phenotype—the underlying genetic defect may not be what one first suspects, and the treatment can vary accordingly.

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Nov 27, 2016 | Posted by in GENERAL & FAMILY MEDICINE | Comments Off on Involving Enzymes

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