👤 Robert A Hegele

Hypertriglyceridemia (HTG) is a common diagnosis. Although secondary factors are important for clinical expression, susceptibility to HTG has a strong genetic component, which we review here. Severe HTG in a few families follows Mendelian - typically autosomal recessive - inheritance of rare loss-of-function mutations in genes such as LPL, APOC2, APOA5, LMF1, and GPIHBP1. In contrast, common complex HTG results from the cumulative influence of small-effect variants (single nucleotide polymorphisms) in genes such as APOA5, GCKR, LPL, and APOB. Intensive resequencing of these four genes has also shown accumulated heterozygous rare variants in HTG patients. Together, more than 20% of the susceptibility to HTG is now accounted for by common and rare variants. Further, classical Fredrickson HTG phenotypes, which were once considered to be distinct based on biochemical features, have a shared genetic architecture. Compared to other complex traits, genetic variants account for a high proportion of HTG diagnoses. By tallying the number of HTG risk alleles, it is possible to discriminate between individuals with HTG and normolipidemia, particularly in those with extreme scores. Future directions include finding the missing genetic component and determining whether genetic profiling can help with diagnosis or personalized treatment advice. Show less

Plasma triglyceride (TG) concentration is reemerging as an important cardiovascular disease risk factor. More complete understanding of the genes and variants that modulate plasma TG should enable development of markers for risk prediction, diagnosis, prognosis, and response to therapies and might help specify new directions for therapeutic interventions. Recent genome-wide association studies (GWAS) have identified both known and novel loci associated with plasma TG concentration. However, genetic variation at these loci explains only ∼10% of overall TG variation within the population. As the GWAS approach may be reaching its limit for discovering genetic determinants of TG, alternative genetic strategies, such as rare variant sequencing studies and evaluation of animal models, may provide complementary information to flesh out knowledge of clinically and biologically important pathways in TG metabolism. Herein, we review genes recently implicated in TG metabolism and describe how some of these genes likely modulate plasma TG concentration. We also discuss lessons regarding plasma TG metabolism learned from various genomic and genetic experimental approaches. Treatment of patients with moderate to severe hypertriglyceridemia with existing therapies is often challenging; thus, gene products and pathways found in recent genetic research studies provide hope for development of more effective clinical strategies. Show less

Genome-wide association studies (GWAS) have identified multiple loci associated with plasma lipid concentrations. Common variants at these loci together explain <10% of variation in each lipid trait. Rare variants with large individual effects may also contribute to the heritability of lipid traits; however, the extent to which rare variants affect lipid phenotypes remains to be determined. Here we show an accumulation of rare variants, or a mutation skew, in GWAS-identified genes in individuals with hypertriglyceridemia (HTG). Through GWAS, we identified common variants in APOA5, GCKR, LPL and APOB associated with HTG. Resequencing of these genes revealed a significant burden of 154 rare missense or nonsense variants in 438 individuals with HTG, compared to 53 variants in 327 controls (P = 6.2 x 10(-8)), corresponding to a carrier frequency of 28.1% of affected individuals and 15.3% of controls (P = 2.6 x 10(-5)). Considering rare variants in these genes incrementally increased the proportion of genetic variation contributing to HTG. Show less

Numerous single nucleotide polymorphisms (SNPs) have been found in recent genome wide association studies (GWAS) to be associated with subtle plasma triglyceride (TG) variation in normolipidemic subjects. However, since these GWAS did not specifically evaluate patients with rare disorders of lipoprotein metabolism--'hyperlipoproteinemia' (HLP)--it remains largely unresolved whether any of these SNP determinants of modest physiological changes in TG are necessarily also determinants of most HLP phenotypes. To address this question, we evaluated 28 TG-associated SNPs from GWAS in 386 unrelated adult patients with one of five Fredrickson phenotypes (HLP types 2A, 2B, 3, 4 and 5) and 242 matched normolipidemic controls. We found that several SNPs associated with TG in normolipidemic samples, including APOA5 p.S19W and -1131T>C, TRIB1 rs17321515, TBL2 rs17145738, GCKR rs780094, GALNT2 rs4846914 and ANGPTL3 rs12130333, were significantly associated with HLP types 2B, 3, 4 and 5. The findings indicate that: (i) the TG-associated Fredrickson HLP types 2B, 3, 4 and 5 are polygenic traits; (ii) these Fredrickson HLP types share numerous genetic determinants among themselves; and (iii) genetic determinants of modest TG variation in normolipidemic population samples also underlie--to an apparently even greater degree--susceptibility to these rare HLP phenotypes. Thus, the TG-associated Fredrickson HLP types 2B, 3, 4 and 5, although historically considered to be distinct are actually complex traits sharing among them several common genetic determinants seen in GWAS of normolipidemic population samples. Show less

Recent genome-wide association studies (GWAS) have reproducibly identified loci associated with plasma triglycerides (TG), HDL cholesterol, and LDL cholesterol. We sought to replicate these findings in a multiethnic population-based cohort using the curated single nucleotide polymorphism (SNP) set found on the new Illumina cardiovascular disease (CVD) beadchip, which contains approximately 50,000 SNPs densely mapping approximately 2,100 genes, selected based on their potential role in CVD. The sample consisted of individuals with European (n = 272), South Asian (n = 330), and Chinese (n = 304) ancestry. Identity by state clustering successfully classified individuals according to self-reported ethnicities. Associations between TG and APOA5, TG and LPL, HDL and CETP, and LDL and APOE were all identified (P < 2 x 10(-6)). In 13 loci, associations with the same SNP or a proxy SNP were identified in the same direction as previously reported (P < 0.05). Assessing the cumulative number of risk-associated alleles at multiple replicated SNPs increased the proportion of explained lipoprotein variance over and above traditional variables such as age, sex, body mass index, and ethnicity. The findings indicate the potential utility of the Illumina CVD beadchip, but they underscore the need to consider meta-analysis of results from commonly studied clinical or epidemiological samples. Show less

Several known candidate gene variants are useful markers for diagnosing hyperlipoproteinemia. In an attempt to identify other useful variants, we evaluated the association of two common APOA5 single-nucleotide polymorphisms across the range of classic hyperlipoproteinemia phenotypes. We assessed plasma lipoprotein profiles and APOA5 S19W and -1131T>C genotypes in 678 adults from a single tertiary referral lipid clinic and in 373 normolipidemic controls matched for age and sex, all of European ancestry. We observed significant stepwise relationships between APOA5 minor allele carrier frequencies and plasma triglyceride quartiles. The odds ratios for hyperlipoproteinemia types 2B, 3, 4 and 5 in APOA5 S19W carriers were 3.11 (95% CI 1.63-5.95), 4.76 (2.25-10.1), 2.89 (1.17-7.18) and 6.16 (3.66-10.3), respectively. For APOA5 -1131T>C carriers, the odds ratios for these hyperlipoproteinemia subtypes were 2.23 (95% CI 1.21-4.08), 3.18 (1.55-6.52), 3.95 (1.85-8.45) and 4.24 (2.64-6.81), respectively. The overall odds ratio for the presence of either allele in lipid clinic patients was 2.58 (95% CI 1.89-3.52). A high proportion of patients with four classic hyperlipoproteinemia phenotypes are carriers of either the APOA5 S19W or -1131T>C variant or both. These two variants are robust genetic biomarkers of a range of clinical hyperlipoproteinemia phenotypes linked by hypertriglyceridemia. Show less

Recent genome-wide association (GWA) studies have identified new genetic determinants of complex quantitative traits, including plasma triglyceride (TG). We hypothesized that common variants associated with mild TG variation identified in GWA studies would also be associated with severe hypertriglyceridemia (HTG). We studied 132 patients of European ancestry with severe HTG (fasting plasma TG > 10 mmol/l), who had no mutations found by resequencing of candidate genes, and 351 matched normolipidemic controls. We determined genotypes for: GALNT2 rs4846914, TBL2/MLXIPL rs17145738, TRIB1 rs17321515, ANGPTL3 rs12130333, GCKR rs780094, APOA5 rs3135506 (S19W), APOA5 rs662799 (-1131T > C), APOE (isoforms) and LPL rs328 (S447X). We found that: (i) genotypes, including those of APOA5 S19W, APOA5 -1131T > C, APOE, GCKR, TRIB1 and TBL2/MLXIPL, were significantly associated with severe HTG; (ii) odds ratios for these genetic variables were significant in both univariate and multivariate regression analyses, irrespective of the presence or absence of diabetes or obesity; (iii) a significant fraction-about one-quarter-of the explained variation in disease status was associated with these genotypes. Therefore, common SNPs (single nucleotide polymorphisms) that are associated with mild TG variation in GWA studies of normolipidemic subjects are also associated with severe HTG. Our findings are consistent with the emerging model of a complex genetic trait. At the extremes of a quantitative trait, such as severe HTG, are found the cumulative contributions of both multiple rare alleles with large genetic effects and common alleles with small effects. Show less

Metabolic syndrome (MetS) is a common phenotype, affecting about 24% of the US population. It is associated with an increased risk for type 2 diabetes and cardiovascular disease. Although there is no universally accepted definition for MetS, affected individuals commonly have a cluster of features, including abdominal obesity, hypertension, dyslipidemia, and dysglycemia. Recently, there has been extensive interest in potential genetic contributions to MetS. At present, no single gene or cluster of genes has been consistently replicated for MetS among different populations, likely due to the complex interplay between gene and environment necessary for expression of this phenotype. We review recent studies regarding the genetic contributions to MetS. Show less

We undertook studies of the association between common genomic variations in APOC3, APOA5, APOE and PON1 genes and variation in biochemical phenotypes in a sample of Greenlanders. Genetic association study of quantitative lipoprotein traits. In a sample of 1,310 adult Greenlanders, fasting plasma lipid, lipoprotein and apolipoprotein (apo) concentrations were assessed for association with known functional genomic variants of APOC3, APOA5, APOE and PON1. For significantly associated polymorphisms, between-genotype differences were examined in closer detail. We found that (1) the APOE restriction isotype was associated with variation in plasma total and LDL cholesterol and apo B (all p < .0001); (2) the APOC3 promoter genotype was associated with variation in plasma triglycerides, HDL cholesterol and apo A-I (all p < .002); (3) the APOA5 codon 19 genotype was associated with variation in plasma triglycerides (p = .027); and (4) the PON1 codon 192 genotype was associated with variation in total and LDL cholesterol and apo B (all p < .05). Taken together, our results suggest that common genetic variations in APOC3, APOA5, APOE and PON1 are associated with significant variation in intermediate traits in plasma lipoprotein metabolism in Greenlanders; the associations are similar to those observed for these variants in other populations. Show less

The genetic determinants of severe hypertriglyceridemia (HTG; MIM 144650) in adults are poorly defined. We therefore resequenced 3 candidate genes, namely LPL, APOC2, and APOA5, to search for accumulation of missense mutations in patients with severe HTG compared with normolipidemic subjects. We resequenced >2 million base pairs of genomic DNA from 110 nondiabetic patients with severe HTG and determined the prevalence of coding sequence variants compared with 472 age- and sex-matched normolipidemic controls. We found: (1) heterozygous mutations (LPL p.Q-12E >11X, p.D25H, p.W86R, p.G188E, p.I194T and p.P207L; APOC2 p.K19T and IVS2-30G>A) in 10.0% of severe HTG patients compared with 0.2% of controls (carrier odds ratio [OR] 52, 95% confidence interval [CI] 8.6 to 319); and (2) an association of the APOA5 p.S19W missense variant with severe HTG (carrier OR 5.5 95% CI 3.3 to 9.1). Furthermore, either rare mutations or the APOA5 p.S19W variant were found in 41.8% of HTG subjects compared with 8.9% of controls (carrier OR 7.4, 95% CI 4.5 to 12.0). Also, heterozygotes for rare mutations had a significantly reduced plasma triglyceride response to fibrate monotherapy. Both common and rare DNA variants in candidate genes were found in a substantial proportion of severe HTG patients. The findings underscore the value of candidate gene resequencing to understand the genetic contribution in complex lipoprotein and metabolic disorders. Show less

Common polymorphisms in the promoter of the APOC3 gene have been associated with hypertriglyceridemia and may impact on phenotypic expression of the metabolic syndrome (MetS). The rs7566605 marker, located near the INSIG2 gene, has been found to be associated with obesity, making it also a potential genetic determinant for MetS. The objective of this study is to examine the APOC3 -455T>C and the INSIG2 rs7566605 polymorphisms as potential genetic determinants for MetS in a multi-ethnic sample. Subjects were genotyped for both the APOC3 -455T>C and INSIG2 rs7566605 polymorphisms, and classified for the presence or absence of MetS (NCEP ATP III and IDF definitions). The total study population included 2675 subjects (> or =18 years of age) from six different geographical ancestries. For the overall study population, the prevalence of MetS was 22.6% (NCEP ATP III definition). Carriers of > or =1 copy of APOC3 -455C were more likely to have MetS (NCEP ATP III definition) than noncarriers (carrier odds ratio 1.73, 95% CI 1.40 to 2.14, adjusting for age and study group). The basis of the association was related not only to a higher proportion of -455C carriers meeting the triglyceride and high-density lipoprotein cholesterol criteria, but also the blood pressure criteria compared with wild-type homozygotes. Plasma apo C-III concentrations were not associated with APOC3 -455T>C genotype. The INSIG2 rs7566605 polymorphism was not associated with MetS or measures of obesity. Meta-analysis of the sample of multiple geographic ancestries indicated that the functional -455T>C promoter polymorphism in APOC3 was associated with an approximately 2-fold increased risk of MetS, whereas the INSIG2 rs7566605 polymorphism was not associated with MetS. Show less

To determine the prevalence of 'hypertriglyceridemic waist' (HTGW) in Oji-Cree, to examine its interaction with hepatic nuclear factor-1alpha (HNF1A) in association with type 2 diabetes, and to characterize its putative genetic determinants. The presence or absence of HTGW was determined in 522 Oji-Cree subjects (223 males, 299 females), >or=18 years of age, in whom physical measurements and fasting plasma analyte concentrations were gathered, and a 75-g oral glucose tolerance test was administered, as part of a cross-sectional study. Subjects were genotyped for HNF1A codon 319, angiotensinogen (AGT) codons 174 and 235, G-protein beta3-subunit (GNB3) nucleotide 825, fatty acid-binding protein (FABP2) codon 54, nucleotides -455 and -482 of the apolipoprotein (apo) C-III (APOC3) promoter, and a 5-bp insertion/deletion polymorphism within the 3'-untranslated region of protein phosphatase 1 regulatory subunit 3 (PPP1R3). The unadjusted prevalence of HTGW in Oji-Cree adults was 20.5%, with more males affected than females (27.8 vs 15.1%, P=0.0004). Logistic regression analysis, adjusted for age and gender, showed type 2 diabetes was associated with both HNF1A G319S (odds ratio (OR) 4.85, 95% CI 2.45, 9.58) and HTGW (OR 4.96, 95% CI 2.49, 9.88). When the HNF1A mutation and HTGW were present in combination, the OR for type 2 diabetes was markedly increased (OR 43.2, 95% CI 12.4, 150). In women only, both GNB3 825C>T and FABP2 A54T genotypes were significantly associated with HTGW (OR 2.02, 95% CI 1.01, 4.05 and OR 1.95, 95% CI 1.01, 3.74, respectively). HTGW is prevalent in Oji-Cree, especially in men. The ORs for type 2 diabetes were similar ( approximately 5-fold) for subjects with either the presence of HTGW or the private HNF1A G319S mutation. These two independent risk factors acted synergistically to confer an even greater increased risk of type 2 diabetes. Show less

The prevalence rates of type 2 diabetes (T2DM) and coronary heart disease (CHD) in Ontario Oji-Cree are among the world's highest. Since metabolic syndrome (MetS) increases risk of T2DM and CHD, we characterized prevalence and putative genetic determinants of MetS in Oji-Cree. In 515 adult (> or = 18 years old) and 115 adolescent (< 18 years old) Oji-Cree subjects, using the National Cholesterol Education Program Adult Treatment Panel III (NCEP ATP III) criteria, we determined that 29.9% of Oji-Cree adults, and 43.4% of adults > or = 35 years of age, had MetS. Furthermore, 33.9 and 8.7% of female Oji-Cree adults and adolescents, respectively, had MetS. Increased waist girth and depressed HDL cholesterol were the most prevalent individual MetS components, while increased blood pressure was least prevalent. AGT T174M, GNB3 825C>T, and APOC3 -455T>C genotypes were significantly associated with MetS (P = 0.018, 0.0056, and 0.029, respectively) for female adults, whereas FABP2 A54T genotype was associated with MetS (P = 0.040) for female adolescents. The high MetS prevalence in Oji-Cree adults, especially women, is consistent with their high risk of T2DM and CHD. Functional polymorphisms in three candidate genes for plasma lipoproteins and blood pressure were associated with MetS in adult Oji-Cree. Furthermore, several female adolescents met the adult MetS criteria, suggesting that the genesis of MetS begins in youth, especially among aboriginal females. Show less

Trinidadians of South Asian origin have a high prevalence of cardiovascular disease and diabetes compared to Trinidadians of African origin. The degree to which these differences are related to genetic and/or environmental factors is unclear. To determine whether there might be a genetic basis for this difference in prevalence of deleterious phenotypes we examined allele frequencies for candidate genes in atherosclerosis and diabetes. We genotyped 81 consecutive neonates of African origin and 103 consecutive neonates of South Asian origin. We evaluated common polymorphisms in 11 candidate genes for atherosclerosis and diabetes. We found differences between the two subpopulations in the allele frequencies of several candidate genes, including APOE, LIPC, APOC3, PON1, PON2, and PPP1R3. However, the differences in the allele frequencies were not all consistent with the pattern of CHD expression between these two ethnic groups in adulthood. Thus, differences in genetic architecture alone may not explain the wide disparities in disease prevalence between these two subpopulations. It is very likely that environmental factors, or unmeasured genetic factors, influence the genetic susceptibility to disease in these subpopulations. Show less

After more than a decade of study, investigators are grappling for a consensus regarding the relationship between variation in candidate genes and plasma triglyceride concentration. Certain variants of LPL--both rare variants, in the case of loss-of-function mutations in kindreds with chylomicronemia, and common variants, in the case of the D9N and N291S variants--appear to be fairly consistently associated with an elevated plasma triglyceride level. In addition, the variation of the recognition site for Sstl within the 3'-untranslated region of APOC3 has consistently shown an association with a variation in plasma triglycerides. The LPL and APOC3 variants thus have at least a chance in future clinical applications, but this will require more study. Common variants of some other promising candidate genes, such as HL, have not shown as consistent an association with the variation in plasma triglyceride level. Finally, studies of variants of newer candidates, such as the mitochondrial genome, LMNA, and IL-6, indicate that many different genes might be important determinants of plasma triglyceride concentration in the general population. As always, the associations of genes with a complex intermediate trait such as plasma triglyceride level depend upon interactions with modulatory factors such as genetic background and/or secondary genetic effects, in addition to the effects of gender, age, hormone replacement, and postprandial status. A key attribute for increasing confidence in the biologic or potential clinical validity of the associations of candidate gene variation with plasma triglyceride will be the development of assays that will provide a more direct mechanistic link between the genetic variant and the elevated plasma triglyceride. Show less

We hypothesized that common genomic variation that affected the expression and/or function of the products of the APOC3, APOE, FABP2, and PON1 genes would be associated with variation in biochemical phenotypes in a previously unstudied human sample. We determined genotypes of functional genomic variants of APOC3, APOE, FABP2, and PON1 in 509 adult aboriginal Canadians from an isolated community in Northern Ontario. We tested for genotype associations with plasma lipoprotein traits. We found that (1) common variation at nucleotide -455 of the APOC3 promoter was associated with variation in plasma triglycerides (P = .006) and (2) common variation of APOE determining plasma isoforms of apo E was associated with variation in plasma apo B (P = .009). Analysis of subjects classed by APOC3 markers showed that homozygosity for presence of a C at nucleotide -455 and a T at nucleotide -482 was associated with significantly increased plasma triglycerides in both men and women. Furthermore, this allele was approximately twice as frequent in subjects within the highest quartile of plasma triglycerides as in subjects within the lowest quartile. Since the DNA variation detected by the APOC3 markers affects in vitro expression of the gene product, it is possible that the marker itself caused the associations. However, the associations could also have resulted from linkage disequilibrium with other functional variants in APOC3 or the closely linked APOA1 and/or APOA4 genes. Show less

We hypothesized that common genomic variants would be associated with variation in lipoprotein phenotypes in young subjects. We determined genotypes of FABP2, PON, APOC3, and APOE in 188 aboriginal Canadians, aged 9 to 17 years. We found that 13 of 32 possible genotype-phenotype associations were significant: (1) the FABP2 codon 54 genotype was associated with variation in plasma triglycerides (P = .045); (2) the PON codon 192 genotype was associated with variation in plasma total and LDL cholesterol and apoB (P = .0099, P = .0088, and P = .016, respectively); (3) the APOC3 insulin-response-element genotype was associated with variation in plasma triglycerides, HDL cholesterol, apoA-I, the total cholesterol to HDL cholesterol ratio, and the apoB to apoA-I ratio (P = .0014, P = .0069, P = .045, P = .0021, and P = .0081, respectively); and (4) the APOE restriction isotype was associated with variation in plasma LDL cholesterol, apoB, the total cholesterol to HDL cholesterol ratio, and the apoB to apoA-I ratio (P = .025, P = .034, P = .045, and P = .047, respectively). The average young age and relative absence of age-dependent secondary environmental factors could have eased the identification of small genetic effects on lipoprotein phenotypes in this study sample. Show less

Most reported mutations that affect lipoprotein metabolism are found within the coding sequences of genes. Recently, a few mutations that occur within promoter sequences have been detected. These promoter sequence variants are the topic of the present review. Some of these variants are fairly common genomic variants in the promoter regions for candidate genes in lipoprotein metabolism, such as APOA1, APOC3, LPA, and LPL. It is possible that such regulatory sequence variants can result in chronic, modestly altered levels of expression of qualitatively normal gene products. This might have a cumulative effect on quantitative phenotypes, such as plasma lipoprotein concentrations, over the long term. Such an effect might not be detected by existing clinical, biochemical, and/or physiological assays. At present, the most consistent evidence from several lines of experiments indicates that genomic variation in the APOC3 promoter creates slightly elevated plasma triglyceride concentrations within the physiologic range. This altered expression appears to predispose to hypertriglyceridemia in the presence of secondary factors. Genetic variants that produce small effects on promoter function might thus be one component of the predisposition to complex diseases. The aggregate of many small effects may create or contribute to a background of susceptibility that, under appropriate conditions, leads to development of frank dyslipidemia and atherosclerosis. Show less

We hypothesized that variation of nine candidate genes in lipoprotein metabolism would be associated with variation in fasting plasma lipoprotein variables in 718 Alberta Hutterites, a genetic isolate. We measured plasma lipids, lipoproteins, and apolipoproteins and analyzed DNA for genotypes of apolipoprotein (apo) B (APOB), paraoxonase (PON), lipoprotein lipase (LPL), VLDL receptor (VLDLR), apo CIII (APOC3), LDL receptor-related protein (LRP), hepatic lipase (HL), LDL receptor (LDLR), and apo E (APOE). Using a multivariate analysis, we found that (1) genotypes of APOB, PON, LPL, LDLR, and APOE were significantly associated with variation of plasma apo B-related traits; (2) genotypes of PON, LPL, and APOC3 were significantly associated with variation in plasma triglycerides; and (3) genotypes of VLDLR, APOC3, LDLR, and APOE were significantly associated with variation in plasma apo AI and HDL cholesterol. Regression analysis showed that between 3.2% and 7.8% of the total variation in plasma lipoproteins was accounted for by variation in the candidate genes tested. The observations demonstrate a modest but significant genetic component of variation in plasma lipoprotein levels that is due to the candidate genes studied in this normolipemic human genetic isolate. Show less