Nutrient and Genetic Regulation of Lipoprotein Metabolism1



Nutrient and Genetic Regulation of Lipoprotein Metabolism1


Edward A. Fisher

Raanan Shamir

Robert A. Hegele





The relationships among different dietary components and lipoprotein metabolism have been long recognized on both an experimental and an observational basis. For example, the INTERHEART study showed, among many other findings, that across many ethnic groups and areas of the world, cardiovascular disease (CVD) risk was inversely related to the consumption of “heart-healthy” foods: indeed, approximately 30% of the population-attributable risk of CVD was accounted for by an unhealthy dietary intake (1). In classic studies (2, 3, 4), the relationships between dietary cholesterol and specific characteristics of fats (particularly the degree of fatty acid saturation) and plasma levels of low-density lipoprotein (LDL) cholesterol (LDL-C) and high-density lipoprotein (HDL) cholesterol (HDL-C) were established by careful clinical experimentation. Over the subsequent years, additional studies were conducted in animals and humans to show that the other macronutrients, protein and carbohydrate, as well as other dietary components, also had effects on plasma lipid and lipoprotein levels (5).

As cell and molecular biologic techniques advanced in the last quarter of the twentieth century, a battery of studies was conducted to elucidate the mechanistic underpinnings of the clinical observations and intervention results. These studies were greatly expanded in scope by the revolution in molecular genetic manipulation of the mouse genome, which allowed the development of models in which candidate genes implicated in the response to nutritional factors could be inserted by transgenesis or inactivated by homologous recombination in the context of normal and abnormal backgrounds and conditions (e.g., atherosclerosis). The sequencing of the human genome, coupled with high-throughput technologies, led to
the next phase of discovery in many areas of physiology and pathophysiology. For lipoprotein metabolism, by 2010 (6), genome-wide association studies (GWASs) had established 95 genetic loci associated with the plasma concentrations of total lipids (cholesterol, triglycerides [TGs]) and the individual lipoprotein fractions. Some of these loci were known from previous evidence to be functional players in lipid and lipoprotein metabolism, and the regulation of many of these was known to be subject to a component of the diet. Other loci found by GWASs were completely novel discoveries, with their roles and regulation still to be established. This chapter summarizes the major genetic factors that are known to determine or have strong influence on human lipoprotein metabolism. For a detailed summary of the impact of specific nutrients on human lipoprotein plasma levels, the reader is referred to the chapter on nutrition in the prevention of coronary heart disease and in the management of lipoprotein disorders.


HIGH PLASMA LEVELS OF TOTAL AND LOW-DENSITY LIPOPROTEIN CHOLESTEROL

High blood cholesterol, especially LDL-C, is associated with increased risk of premature CVD. Measurement of serum total cholesterol is a reflection of the amount of cholesterol contained within circulating very-low-density lipoproteins (VLDLs), LDLs, HDLs, and chylomicrons (although chylomicron levels are essentially nil when cholesterol is measured in the fasting state). Thus, a fasting lipoprotein profile is needed when hypercholesterolemia is identified or suspected. Hypercholesterolemia with either a monogenic or multifactorial basis affects approximately 5% of the population, but increased risk of premature atherosclerosis has been mainly established for the monogenic disorders that result in elevated LDL (7). LDL is rich in cholesteryl esters (CEs), and each particle contains a single molecule of apolipoprotein-B-100 (Apo-B-100). LDL is derived from VLDL, and it serves as a carrier of cholesterol made in the liver to peripheral tissues. Cellular uptake of LDL-C depends on binding of LDL, through Apo-B, to the LDL receptor. Currently, three monogenic disorders causing autosomal dominant hypercholesterolemia (ADH) have been identified, as well as one autosomal recessive form (Table 64.1). Mutations in the LDL receptor gene (LDLR) are the most common among these, whereas mutations in other genes (e.g., in APOB, resulting in defective Apo-B and in proprotein convertase subtilisin/kexin type 9 [PCSK9] encoding PCSK9 enzyme account for a minor fraction of patients presenting with ADH.








TABLE 64.1 MONOGENIC DISORDERS CAUSING ELEVATED LOW-DENSITY LIPOPROTEIN CHOLESTEROL LEVELS























































DISORDER


ESTIMATED INCIDENCE


LDL SERUM LEVELS


CLINICAL FINDINGS


GENE DEFECT


TREATMENT


Heterozygous familial hypercholesterolemia (HeFH)


1:500a


Usually >200 mg/dL; can be lower in children


Tendon xanthomata (hallmark), xanthelasma, corneal arcus


Autosomal dominant (ADH) mutations in LDL receptor gene


Dietary treatmentb; drug treatmentc


Homozygous familial hypercholesterolemia (HoFH)


1 per million


LDL >400 mg/dL (average <600 mg/dL)


Planar, tendon, and tuberous xanthomata by age 6 y; death from coronary disease as early as 10 y age; irreversible aortic valve involvement by age 10 y if untreated


Mutations in LDL receptor gene in both alleles


Dietary treatment; drug treatment when some LDL receptor activity present; LDL apheresis; liver transplantation


PCSK9 mutations


≤3% of cases with ADHd


Similar to HeFH


Similar to HeFH


ADH; gain of function mutations


Similar to HeFH


Familial defective Apo-B


≤7% of cases with ADHd


Similar to HeFH


Similar to HeFH


ADH; Apo-B gene mutations in LDL receptor binding domain


Similar to HeFH


Autosomal recessive hypercholesterolemia (ARH)


Few cases


Similar to HoFH; on average, ˜100-150 mg/dL lower than HoFH


Similar to HoFH, with less aortic valve involvement and slower progression


Mutations in adaptor protein that is essential, in the liver, for clathrin-mediated endocytosis of LDL


Dietary treatment; response to statin therapy


ADH, autosomal dominant hypercholesterolemia; Apo-B, apolipoprotein B; LDL, low-density lipoprotein.


a Can be more frequent than 1:100 in various populations because of a founder effect.


b Restriction of dietary saturated fat and dietary cholesterol can reduce LDL serum levels but are insufficient to achieve normal values. They have an added effect with drug therapy.


c Statins are the main treatment. Combination of statins with ezetimibe further lowers LDL blood levels. Combination of statins with bile acid resins also has a synergistic effect.


d As reported in Rahalkar AR, Hegele RA. Monogenic pediatric dyslipidemias: classification, genetics and clinical spectrum. Mol Genet Metab 2008;93:282-94.




Familial Hypercholesterolemia

Familial hypercholesterolemia (FH), the most common form of ADH, is caused by mutations in the LDLR gene located on chromosome 19p13. This transmembrane receptor, present in almost all tissues, controls cholesterol homeostasis by a complex process that includes synthesis of the receptor in the endoplasmic reticulum, migration of the receptor protein to the Golgi apparatus and then to the cell surface, the binding of the LDL receptor to plasma LDL via Apo-B-100, internalization of the receptor-ligand complex, and recycling of the LDL receptor to the cell surface while the LDL is processed in the lysosome (8). More than 1000 mutations have been described, affecting each of the steps involved in LDL receptor biogenesis. When one allele of the LDL receptor is defective (heterozygous FH), a 30% increase in plasma LDL-C is observed. However, a twofold to fourfold increase or more in LDL-C level is observed in homozygous FH in which both alleles are mutated, thus resulting in a lack of function of the LDL receptor.

Most often, the diagnosis is made on the basis of clinical and family history. Definite diagnosis of heterozygous FH requires confirmation by identifying mutations in the LDLR gene or studies of LDL receptor function in fibroblasts. If FH is left untreated, myocardial infarction may occur at 30 to 40 years of age, and more than 50% of male patients and about 15% of female patients with heterozygous FH will die before 60 years of age (9).

Homozygous patients with FH develop cutaneous and tendinous xanthomata in the first decade of life, and death from cardiac ischemia and aortic valve involvement often occurs as early as the second decade of life and usually before 30 years of age (10). Dietary management is usually insufficient for treating children with heterozygous FH, and the use of statins (inhibitors of hydroxymethyl-glutarylcoenzyme A [HMG-CoA] reductase) is recommended from 8 years of age (11). Adults with heterozygous FH frequently need a combination of two or more medications in addition to dietary management to control plasma LDL-C levels.

For homozygous FH, it may be necessary to perform LDL apheresis as early as the first year of life. Liver transplantation is another option for homozygous FH, but it carries a small risk of mortality and requires chronic immunosuppression.


Mutations in PCSK9

PCSK9 gene encodes PCSK9, a serine protease that regulates the degradation of the LDL receptor and thus plays a major role in the control of cholesterol influx into cells (12). Loss of function mutations in PCSK9 result in increased LDL receptor expression and reduced LDL serum levels as well as reduced risk of CVD (13). In contrast, patients with a heterozygous gain of function mutation in PCSK9 present clinically with a condition similar to that of heterozygous FH and should be treated in a similar manner.


Familial Defective Apolipoprotein B

Mutations within the region of the APOB gene that encodes the LDL receptor binding domain reduce the binding affinity of LDL particles to the LDL receptor. LDL-C levels are about twice normal in familial defective apolipoprotein B (FDB), and this form of ADH is phenotypically similar to FH (14). A few APOB mutations causing high levels of LDL-C have been described. Of these, the point mutation resulting in the missense change Arg3500Gln is the most common. This mutation was seen in about 3% of the referrals in a pediatric French cohort for hyperlipidemia (15). Patients with FDB are treated in a manner similar to those with heterozygous FH, namely, with a statin inhibitor of HMG-CoA reductase and sometimes a second medication.


Autosomal Recessive Hypercholesterolemia

This rare form of hypercholesterolemia has been described mainly in probands from Italy (16), but it occurs in individuals from other regions as well (10). The disease is caused by mutations in LDL receptor adapter protein 1 (LDLRAP1), an essential adaptor protein within the liver (the organ that contains ˜60% of the body’s complement of LDL receptors). The LDLRAP1 gene product is essential for clathrin-mediated endocytosis of LDL (17). In other tissues such as fibroblasts, this mutation does not disrupt LDL uptake. Clinically, patients with autosomal recessive hypercholesterolemia (ARH) resemble patients with homozygous FH, although aortic valve involvement is less common in ARH, whereas patients with homozygous FH have, on average, higher LDL plasma levels and earlier onset of atherosclerotic disease.


HIGH PLASMA LEVELS OF HIGH-DENSITY LIPOPROTEIN CHOLESTEROL

Plasma levels of HDL-C, hyperalphalipoproteinemia, have heritability estimates of approximately 50% (18). In this section, the genetic causes of high HDL-C are discussed. In a later section, a similar summary is made for low HDL-C. Currently, a high HDL-C is defined as a plasma level of more than 60 mg/dL in men and more than 70 mg/dL in women, whereas a low HDL-C level is defined as less than 40 mg/dL in men and less than 50 mg/ dL in women (for conversion, mg/dL / 38.67 = mmol/L). The salient features of the high-HDL-C disorders are summarized in Table 64.2.


Cholesterol Ester Transfer Protein Deficiency

By far, the best characterized genetic cause of high HDL-C is the deficiency of cholesterol ester transfer protein (CETP). The primary function of CETP is to mediate the one-for-one exchange of a CE molecule in HDL with
a TG molecule in VLDL or LDL. In this way, some HDLCE, presumably derived from peripheral cells, including foam cells in atherosclerotic plaques, can be directed to the liver indirectly via uptake of the VLDL and LDL by the LDL receptor. The remaining HDL-CE can be directly delivered to the liver through the interaction of HDL with the scavenger receptor SR-B1. The general term for the delivery (direct and indirect) of CE from peripheral cells to the liver is reverse cholesterol transport (RCT), and this property of HDL is thought to be a major contributor to its atheroprotective role, as demonstrated in both human observational and animal intervention studies (19).








TABLE 64.2 GENETIC CAUSES OF HIGH PLASMA LEVELS OF HIGH-DENSITY LIPOPROTEIN CHOLESTEROL
































DISORDER


INCIDENCE


LIPID LEVELS


ASSOCIATED PHYSICAL FINDINGS


GENE DEFECT


DIETARY AND OTHER THERAPY


CETP deficiencya


Most common in Japan, where 2% and 7%, respectively, heterozygotic for mutations that cause complete or partial deficiency


Homozygotes: HDL-C levels usually >120 mg/dL; heterozygotes: HDL-C typically 70-100 mg/dL; in both cases, LDL-C levels may be slightly to modestly decreased


None noted


Complete deficiency most often caused by a splice-site mutation in intron 14; partial deficiency most often with a missense mutation (D442G) in exon 15


None at present


Hepatic lipase deficiencyb


Rare (≤20 known kindreds)


HDL-C can be >70 mg/dL; TG 200-450 mg/dL with TG-rich HDL and β-VLDL particles


None noted


Homozygous defect in the hepatic lipase gene (LIPC)


One report noting that fenofibrate in two patients with complete deficiency substantially improved plasma lipid profilec


CETP, cholesterol ester transfer protein; HDL, high-density lipoprotein; HDL-C, high-density lipoprotein cholesterol; LDL-C, low-density lipoprotein cholesterol; TG, triglyceride; VLDL, very-low-density lipoprotein.


a Data from Weissglas-Volkov D, Pajukanta P. Genetic causes of high and low serum HDL-cholesterol. J Lipid Res 2010;51:2032-57.


b Data from Weissglas-Volkov D, Pajukanta P. Genetic causes of high and low serum HDL-cholesterol. J Lipid Res 2010;51:2032-57; and Denke MA. Nutrient and genetic regulation of lipoprotein metabolism. In: Shils ME, Shike M, Ross AC et al, eds. Modern Nutrition in Health and Disease. 10th ed. Baltimore: Lippincott Williams & Wilkins, 2006.


c Data from Ruel IL, Lamarche B, Mauger JF et al. Effect of fenofibrate on plasma lipoprotein composition and kinetics in patients with complete hepatic lipase deficiency. Arterioscler Thromb Vasc Biol 2005;25:2600-7.


The relationship of CETP with plasma levels of HDL-C was initially defined in Japan, where CETP deficiency resulting from loss of function mutations was found to explain more than 50% of high HDL-C cases, with levels as high as 120 mg/dL in homozygotes and higher than 70 mg/dL in heterozygotes (18). In addition to the increased levels of HDL-C, the size of the HDL particles is larger in carriers of mutant CETP, presumably because of the inability to transfer accumulated CE to VLDL or LDL, and thus retention of CE in HDL.

CETP gene mutations resulting in a severe loss of function are relatively rare outside of Japan. Furthermore, subtle variations in or near the CETP gene related to plasma levels of HDL-C have been sought in large studies of people of European ancestry. In a large metaanalysis of GWAS results, 95 chromosomal loci were identified with a statistically significant association with plasma lipid levels. Of these, 31 were related to HDL-C, including CETP (6).

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Jul 27, 2016 | Posted by in PUBLIC HEALTH AND EPIDEMIOLOGY | Comments Off on Nutrient and Genetic Regulation of Lipoprotein Metabolism1

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