Robert A. Hegele

Hepatic lipase deficiency. Clinical, biochemical, and molecular genetic characteristics.

Hepatic lipase (HL) is an important enzyme in the metabolism of triglyceride-rich lipoproteins and high density lipoproteins. The clinical syndrome of HL deficiency is rare and difficult to identify. We studied carriers of mutant HL to ascertain whether there are distinctive clinical and/or biochemical characteristics of the heterozygous state. In an Ontario kindred, compound heterozygosity for two HL mutations, S267F and T383M, underlies the clinical syndrome of complete HL deficiency. We report that simple heterozygotes for either HL mutant do not have a discrete lipoprotein abnormality, except for relative triglyceride enrichment of lipoprotein fractions with d > 1.006 g/mL. Postheparin HL activity is depressed to a greater degree in carriers of S267F compared with carriers of T383M. Retinyl palmitate loading studies in a compound heterozygote revealed impaired clearance of chylomicron remnants. The dyslipoproteinemia in a compound heterozygote was ameliorated by lovastatin. There was no difference in the quantity and distribution of HL mRNA in the liver of a compound heterozygote when compared with that of a normal subject. Thus, HL deficiency associated with structural variation of the HL gene is characterized by premature atherosclerosis, triglyceride enrichment of lipoprotein fractions with d > 1.006 g/mL, the presence of circulating beta-very low density lipoproteins, and abnormal catabolism of postprandial triglyceride-rich lipoproteins.

A polymorphism of the angiotensinogen gene associated with variation in blood pressure in a genetic isolate.

BACKGROUND The Hutterite Brethren are a genetic isolate characterized by high indices of relatedness and a communal agrarian lifestyle. We hypothesized that variation of the angiotensinogen (AGT) and angiotensin-converting enzyme (ACE) genes would be associated with variation in resting blood pressure in this group. We also hypothesized that the association would depend on the sex of the subjects. METHODS AND RESULTS In 741 Hutterites, we measured blood pressure in quadruplicate and analyzed DNA for genotypes of an insertion/deletion (I/D) polymorphism of ACE and of two protein polymorphisms of AGT, namely, M235T and T174M. We tested for association between variation in systolic and diastolic blood pressures and genotypic class. We observed that genotypes of AGT codon 174 were significantly associated with variation in systolic blood pressure. We also tested for an interaction between the AGT genotype and sex. We observed that genotypes of AGT codon 174 were significantly associated with variation in systolic blood pressure only in men. The AGT codon 174 polymorphism accounted for 3.1% of the total variation in systolic blood pressure in men. CONCLUSIONS The association of AGT variation with resting blood pressure in men is consistent with the existence of important structural elements within, flanking, or proximal to the AGT gene, whose functional impact might be related to differences in sex.

Are Canadian Inuit at increased genetic risk for coronary heart disease

Abstract The Keewatin Inuit of the Northwest Territories of Canada have a very low age-adjusted mortality rate from coronary heart disease. We hypothesized that this apparent protection from disease has a genetic basis. We determined the prevalence of the disease-associated alleles of five candidate genes for atherosclerosis-related phenotypes. Surprisingly, four of the five alleles studied, namely AGT T235, FABP2 T54, PON R192 and APOE E4, were significantly more frequent in a sample of 175 Keewatin Inuit than among a representative control sample of whites living in the region. The high frequencies of these disease-associated alleles suggests either that they have no relationship with disease susceptibility in the Inuit, or that some unmeasured genetic and/or environmental factors mitigate disease susceptibility that is associated with these alleles. This highlights the difficulty in extrapolating findings from one population to another. Also, very modest genotype-phenotype associations were observed between APOE genotype (P=0.016) and plasma low-density lipoprotein cholesterol concentration and between FABP2 genotype and plasma 2-h postprandial glucose concentration (P=0.048). The relationship between APOE alleles and plasma low-density lipoprotein cholesterol was the same as has been previously reported in many study samples. However, the relationship between FABP2 alleles and plasma 2-h postprandial glucose concentrations was the opposite to that reported in other studies. This suggests that differences in environment, such as the type of fatty acid consumed, interacts with functional differences in gene products involved in candidate metabolic pathways to produce phenotypic differences.

A gene-gender interaction affecting plasma lipoproteins in a genetic isolate.

The Hutterite Brethren are a genetic isolate characterized by high indices of relatedness and a communal agrarian lifestyle. We hypothesized that variation in their apolipoprotein (apo) E and lipoprotein lipase (LPL) genes would be associated with variation in fasting plasma lipoproteins. We measured plasma lipids, lipoproteins, and apolipoproteins and analyzed DNA for genotypes of apoE and LPL in 803 Hutterites. We observed that apoE and LPL genotypes were significantly associated with variations in plasma total cholesterol, low-density lipoprotein (LDL) cholesterol, and apoB. When the data were subdivided by sex, apoE genotype was associated with plasma apoB-related traits in men, but LPL genotype was not. In contrast, LPL genotype was associated with plasma apoB-related traits in women, including triglycerides and LDL cholesterol. After accounting for age, body mass index, and colony of origin, the variation in these variables was much more significantly associated with LDL genotype than with apoE genotype in women but not in men. The association of LPL variation with plasma lipoproteins in women suggests that the functional effects of important structural elements within, flanking, or proximal to the LPL gene on chromosome 8p22 may be sex related.

Association and linkage of LDLR gene variation with variation in plasma low density lipoprotein cholesterol

AbstractThe role of common variation in the low density lipoprotein (LDL) receptor gene (LDLR) as a determinant of variation in plasma LDL cholesterol in normolipidemic populations is not well established. To address this question, we used both association and linkage analysis to evaluate the relationship between plasma LDL cholesterol and genetic variation in LDLR and in three other candidate genes for lipoprotein metabolism, namely, APOE, PON1, and LPL. We studied a sample of 719 normolipidemic Alberta Hutterites, who comprised 1217 sib pairs. Variation in each of the four candidate genes was significantly associated with variation in plasma LDL cholesterol, but the average effects of the alleles were small. In contrast, sib pair analysis showed that only the LDLR gene variation was linked with variation in plasma LDL cholesterol (P = 0.026). Thus, the common LDLR gene variation was both associated with and linked to variation in plasma LDL cholesterol, suggesting that there is a functional impact of structural variation in LDLR on plasma LDL cholesterol in this study sample. However, the absence of linkage of variation in LDL cholesterol with the other three candidate genes, in particular APOE, is consistent with a lower sensitivity of linkage analysis compared with association analysis for detecting modest effects on quantitative traits. Attributes such as the genetic structure of the study sample, the amount of variance attributable to the locus, and the information content of the marker appear to affect the ability to detect genotype-phenotype relationships using linkage analysis.

Successful Outcome in Severe Pregnancy-Associated Hyperlipemia: A Case Report and Literature Review

Severe hypertriglyceridemia causing pancreatitis is a rare complication of pregnancy, usually occurring in the second and third trimesters. Treatment includes a very low-fat diet, intravenous fluids, total parenteral nutrition, and plasma apheresis. In this article, the authors report the case of a pregnant woman who presented with a plasma triglyceride level of 65 mmol/L, abdominal pain, and a threatened abortion at 8 weeks of gestation. Treatment included restriction of dietary fat to below 10% of total calories, liquid protein supplementation, multiple hospitalizations for treatment with intravenous fluids, and total parenteral nutrition. Continuous intravenous heparin was started at 29 weeks of gestation for pulmonary embolism. This was associated with a dramatic decrease in plasma triglyceride levels. A normal female child was born at 37 weeks of gestation. The mothers weight at 2 weeks postpartum was 15 lb below her pregnant weight. It was concluded that a successful pregnancy is possible even when plasma triglyceride levels are very high early in the pregnancy.

Apolipoprotein A-I Deficiency Biochemical and Metabolic Characteristics

Familial HDL deficiencies are associated with variable susceptibility to premature coronary heart disease, but the mechanism underlying this association remains poorly understood. Three homozygotes with isolated complete apo A-I deficiency caused by an autosomal codominant apo A-I Q[-2]X mutation and one heterozygote developed coronary heart disease before age 40 years. We characterized the effects of this mutation on lipoprotein metabolism. LDL FC, phospholipid, and apo B were all significantly higher in homozygotes than in heterozygotes. The HDLs of the heterozygotes were apo A-I poor relative to apo A-II. Lecithin-cholesterol acyltransferase activity was 59% lower in homozygotes than in normal subjects or heterozygotes. Cholesteryl ester transfer activity was increased in a homozygote compared with a normolipidemic control subject. Postprandial lipid metabolism was studied in one homozygote and one heterozygote. Post-prandial TG response in the homozygote was significantly exaggerated, while residual plasma HDL level remained unaffected. The homozygote also had delayed clearance of retinyl ester, a marker of chylomicron remnant metabolism. Thus, homozygosity and heterozygosity for apo A-I Q[-2]X are associated with qualitative, as well as quantitative, disturbances in plasma HDLs, LDLs, lipid-modifying enzyme activities, and postprandial retinyl ester metabolism. The observed elevation of atherogenic lipoproteins and reduction in antiatherogenic lipoproteins in the affected members of the apo A-I Q[-2]X kindred are consistent with the primary deficiency in apo A-I having pleiotropic effects that markedly enhance susceptibility for coronary heart disease.

Interaction between variant apolipoproteins C-II and E that affects plasma lipoprotein concentrations.

The genes for apolipoprotein (apo) C-II, a cofactor for activation of lipoprotein lipase, and apo E, a ligand for receptor-mediated uptake of triglyceride-rich lipoproteins, are physically linked on chromosome 19q13.1. In a large Caribbean Caucasian family, several individuals had clinical features of the complete absence of lipoprotein lipase activity and were homozygous for a DNA frameshift mutation of apo C-II, imparting functional inactivity to the mutant protein. Plasma from heterozygous carriers of this mutation, when compared with plasma from relatives who were noncarriers, had significantly diminished capacity to activate lipoprotein lipase in vitro. We also observed in heterozygotes for this mutation a wide range of serum lipid and lipoprotein levels. When age and sex were taken into account, the presence of a single apo E allele encoding the E4 isoform occurring in individuals with a single mutant apo C-II allele was strongly associated with higher levels of cholesterol, triglycerides, very low density lipoprotein cholesterol, and non-high density lipoprotein cholesterol when compared with those of relatives who carried neither or only one variant allele. This suggests that a single genetic mutation that usually has a recessive effect on lipoprotein metabolism can have an interactive effect on lipid phenotype when it is coinherited with a single mutation at another gene whose product affects the same metabolic pathway.

−6A Promoter variant of angiotensinogen and blood pressure variation in Canadian Oji-Cree

AbstractWe previously reported significant associations between variation in the AGT gene at codon 235 and both systolic pressure and hypertension in Canadian Oji-Cree. Recently, Inoue et al suggested that the AGT T235 variant was not causative, but was rather in linkage disequilibrium with a variant in the AGT promoter, namely —6A, that was associated with increased in vitro expression of angiotensinogen and was thus a strong candidate to be the functional basis of the previously observed associations. We genotyped 518 adult Oji-Cree for the AGT promoter polymorphism and tested for its association with blood pressure and hypertension. We found that the frequency of the —6A variant was 0.85 in the Oji-Cree, which is much higher than the frequency observed in other human samples. We also found strong linkage disequilibrium between the AGT—6A and T235 variants. However, genetic variation of the AGT promoter was only marginally associated with variation in systolic pressure, with a trend to significantly higher systolic pressure seen in AGT—6A/A homozygotes than in subjects with other genotypes. In addition, genetic variation of the AGT promoter tended to be associated with a diagnosis of hypertension. Despite the very high prevalence of —6A, our native sample was essentially normotensive. Our findings are consistent with a marginally deleterious effect of the AGT—6A allele on blood pressure, but linkage disequilibrium with another causative variant cannot be ruled out in this sample of aboriginal Canadians.

Retinopathy and Neuropathy Associated With Complete Apolipoprotein A-l Deficiency

ABSTRACT: Genetic deficiencies of plasma high-density lipoprotein cholesterol are associated variably with diseases of the eyes and nervous system. We ascertained a proband with undetectable plasma HDL cholesterol due to homozygosity for a DNA mutation, APOA1 Q[-2]X, which encodes premature termination of translation of apolipoprotein A-I, the major structural protein in HDL. This person had a unique retinopathy, ataxia, and electrophysiologic abnormalities suggesting multifocal central nervous system deficits. Other gene carriers in this family had similar neurologic features, but only the proband had the retinopathy. The presence of retinopathy and neuropathy in affected family members was highly variable. This heterogeneity might result from time-dependent interactions with other genetic or environmental factors.