Haplotype analyses of the APOA5 gene in patients with familial combined hyperlipidemia

The Genetic Basis of Hypertriglyceridemia

PURPOSE OF REVIEW

Hypertriglyceridemia is a common dyslipidemia associated with an increased risk of cardiovascular disease and pancreatitis. Severe hypertriglyceridemia may sometimes be a monogenic condition. However, in the vast majority of patients, hypertriglyceridemia is due to the cumulative effect of multiple genetic risk variants along with lifestyle factors, medications, and disease conditions that elevate triglyceride levels. In this review, we will summarize recent progress in the understanding of the genetic basis of hypertriglyceridemia.

RECENT FINDINGS

More than 300 genetic loci have been identified for association with triglyceride levels in large genome-wide association studies. Studies combining the loci into polygenic scores have demonstrated that some hypertriglyceridemia phenotypes previously attributed to monogenic inheritance have a polygenic basis. The new genetic discoveries have opened avenues for the development of more effective triglyceride-lowering treatments and raised interest towards genetic screening and tailored treatments against hypertriglyceridemia. The discovery of multiple genetic loci associated with elevated triglyceride levels has led to improved understanding of the genetic basis of hypertriglyceridemia and opened new translational opportunities.

ANGPTL3 gene variants in subjects with familial combined hyperlipidemia

Angiopoietin-like 3 (ANGPTL3) plays an important role in lipid metabolism in humans. Loss-of-function variants in ANGPTL3 cause a monogenic disease named familial combined hypolipidemia. However, the potential contribution of ANGPTL3 gene in subjects with familial combined hyperlipidemia (FCHL) has not been studied. For that reason, the aim of this work was to investigate the potential contribution of ANGPTL3 in the aetiology of FCHL by identifying gain-of-function (GOF) genetic variants in the ANGPTL3 gene in FCHL subjects. ANGPTL3 gene was sequenced in 162 unrelated subjects with severe FCHL and 165 normolipemic controls. Pathogenicity of genetic variants was predicted with PredictSNP2 and FruitFly. Frequency of identified variants in FCHL was compared with that of normolipemic controls and that described in the 1000 Genomes Project. No GOF mutations in ANGPTL3 were present in subjects with FCHL. Four variants were identified in FCHL subjects, showing a different frequency from that observed in normolipemic controls: c.607-109T>C, c.607-47_607-46delGT, c.835+41C>A and c.*52_*60del. This last variant, c.*52_*60del, is a microRNA associated sequence in the 3′UTR of ANGPTL3, and it was present 2.7 times more frequently in normolipemic controls than in FCHL subjects. Our research shows that no GOF mutations in ANGPTL3 were found in a large group of unrelated subjects with FCHL.

Haplotype analysis of the Apolipoprotein A5 gene in Moroccan patients with the metabolic syndrome

BACKGROUND

In this case-control study we investigated the relative contribution of commons APOA5 polymorphisms and haplotypes to the risk of metabolic syndrome in Moroccan patients.

METHODS

Using the International Diabetes Federation (IDF) criteria for metabolic syndrome, the study included 176 patients and 105 controls. We genotyped APOA5 polymorphisms (-1131 T > C, c.56C > G, c.553G > T and c.1259 T > C) by PCR-RFLP analysis. The effects of APOA5 polymorphisms and constructed haplotypes on metabolic syndrome were estimated using logistic regression analyses.

RESULTS

The statistical analysis showed a significant association between APOA5 -1131 T > C and APOA5 c.56C > G polymorphisms with metabolic syndrome in both Codominant and Dominant models. The APOA5 -1131 T > C polymorphism was associated with increased fasting glucose (p = 0.0295) and reduced HDL levels (p = 0.0091). Carriers of the APOA5 c.56G allele had increased triglyceride levels (p = 0.0435) and waist circumference (p = 0.0122). Similarly the APOA5 1259 T > C variant was associated with increased waist circumference (p = 0.0463). The haplotypes CCGT (OR = 3.223; p = 0.00278) and CGGT (OR = 8.234; p = 0.00534) were significantly associated with susceptibility to metabolic syndrome.

CONCLUSIONS

Our results confirms the association of APOA5 -1131 T > C and c.56C > G variants with the predisposition to metabolic syndrome complications.

The genetics of familial combined hyperlipidaemia

Almost 40 years after the first description of familial combined hyperlipidaemia (FCHL) as a discrete entity, the genetic and metabolic basis of this prevalent disease has yet to be fully unveiled. In general, two strategies have been applied to elucidate its complex genetic background, the candidate-gene and the linkage approach, which have yielded an extensive list of genes associated with FCHL or its related traits, with a variable degree of scientific evidence. Some genes influence the FCHL phenotype in many pedigrees, whereas others are responsible for the affected state in only one kindred, thereby adding to the genetic and phenotypic heterogeneity of FCHL. This Review outlines the individual genes that have been described in FCHL and how these genes can be incorporated into the current concept of metabolic pathways resulting in FCHL: adipose tissue dysfunction, hepatic fat accumulation and overproduction, disturbed metabolism and delayed clearance of apolipoprotein-B-containing particles. Genes that affect metabolism and clearance of plasma lipoprotein particles have been most thoroughly studied. The adoption of new traits, in addition to the classic plasma lipid traits, could aid in the identification of new genes implicated in other pathways in FCHL. Moreover, systems genetic analysis, which integrates genetic polymorphisms with data on gene expression levels, lipidomics or metabolomics, will attribute functions to genetic variants in addition to revealing new genes.

Haplotype analysis of the apolipoprotein A5 gene in patients with the metabolic syndrome

BACKGROUND AND AIMS

In recent studies, the T-1131C variant of apolipoprotein A5 (APOA5) gene was found to confer a risk for metabolic syndrome (MS). Here we determined four haplotype-tagging polymorphisms (T-1131C, IVS3+G476A, T1259C, and C56G), and studied the distribution of the naturally occurring major haplotype profiles in MS.

METHODS AND RESULTS

A total of 343 MS patients and 284 controls were genotyped using PCR-RFLP methods. Both in MS and control groups, we confirmed the already known association of -1131C, IVS3+473A and 1259C minor alleles with elevated triglyceride levels. The prevalence of the APOA5*2 haplotype (the combination of T-1131C, IVS3+G476A and T1259C SNPs) was 13.1% in MS patients, and 4.9% in controls (p<0.001); multiple logistic regression analysis revealed that this haplotype confers risk for the development of MS (OR=2.880; 95% CI: 1.567-5.292; p=0.001). We also observed a gender effect: in males a more prominent degree of susceptibility was found. Contrary to the APOA5*2 haplotype, the prevalence rate of APOA5*4 (determined by the T-1131C SNP alone) did not differ between MS patients and controls. We identified a novel haplotype, designated here as APOA5*5 (1259C allele alone); which appears to be protective against MS.

CONCLUSION

Our results refined the role of SNP T-1131C in the development of MS. The susceptibility nature of this SNP is limited to the APOA5*2 haplotype, while in APOA5*4 haplotype it did not confer a risk for the disease. In addition, as our current data suggest, the novel APOA5*5 haplotype can confer protection against MS.

Association between sequence variant of c.553 G > T in the apolipoprotein A5 gene and metabolic syndrome, insulin resistance, and carotid atherosclerosis

Common polymorphism of the apolipoprotein A5 gene (APOA5, c.553G>T) related to metabolic syndrome components, insulin resistance, and carotid atherosclerosis remains unclear. We investigated the associations of the APOA5 c.553G>T gene with various metabolic syndrome components and carotid artery atherosclerosis among family members. A total of 661 participants who provided complete genotyping and carotid artery measures were included in this study. Participants with APOA5 c.553T carrier (GT and TT) were more likely to have higher levels of triglycerides and apolipoprotein B, as well as lower levels of high-density lipoprotein (HDL) cholesterol, than participants with the GG genotype. Individuals who carried T alleles had an increased risk of a high level of triglycerides (multivariate odds ratio [OR], 3.86; 95% confidence interval [CI], 1.98-7.55; P<0.0001) and low levels of HDL cholesterol (OR, 2.32; 95% CI, 1.40-3.86; P=0.0012) compared with those without T alleles. The age was an effect modifier for the association between APOA5 genotype and smoking, alcohol drinking, obesity, and lipid profiles, including total, HDL, and low-density lipoprotein (LDL) cholesterol; triglycerides; and apolipoproteins. In addition, the association between APOA5 genotype and hypertriglyceridemia was significant only in adult groups (OR, 3.53; 95% CI, 1.79-6.94), and the association between APOA5 genotype and low HDL cholesterol was stable in young adolescents (OR, 2.39; 95% CI, 1.19-4.78) and adults (OR, 2.20; 95% CI, 1.17-4.15). Our findings indicated that the APOA5 c.553G>T polymorphism is associated with high triglycerides and low HDL cholesterol but not with other metabolic syndrome components or carotid atherosclerosis in this ethnic Chinese population.

Associations of polymorphisms in the apolipoprotein A1/C3/A4/A5 gene cluster with familial combined hyperlipidaemia in Hong Kong Chinese

BACKGROUND

Familial combined hyperlipidaemia (FCH) is the most common genetic dyslipidaemia associated with coronary artery disease. Single nucleotide polymorphisms (SNPs) and haplotypes in the APOA1/C3/A4/A5 gene cluster are associated with FCH in Caucasians and with elevated triglycerides (TG) in various ethnic groups. We examined these associations with FCH in Hong Kong Chinese.

METHODS

Fifty-six Chinese FCH patients and 176 unrelated controls were studied. Thirteen SNPs in the APOA1/C3/A4/A5 cluster were genotyped.

RESULTS

Four alleles in APOA5 were associated with FCH (P<0.001). The -1131T>C (rs662799) and -3A>G (rs651821) SNPs in APOA5 were in almost complete linkage disequilibrium (LD, r(2)=0.99), and their minor alleles were more frequent (P<0.001) in FCH than controls (0.60 vs. 0.24). The odds ratio (OR) for FCH was 6.2 (95% CI, 2.6-14.8) and 6.1 (2.6-14.6) per copy of -1131C and -3G, respectively, and 24.6 (8.4-72.0) and 24.4 (8.4-70.9) in -1131C and -3G homozygotes, respectively, as compared to wild-type homozygotes. The 1891T>C (rs2266788) SNP was in LD (r(2)=0.68) with -1131T>C and -3A>G, and the minor allele was more frequent in FCH than controls (0.42 vs. 0.19, P<0.001). The 553G>T (rs2075291) nonsynonymous variant was also associated with FCH (0.15 vs. 0.04, P=0.001) and, along with -3A>G (or -1131T>C) and 1891T>C, contributed to haplotypes predicting FCH. The two tightly linked SNPs, -1131T>C and -3A>G polymorphism were significantly associated with lipid traits in all subjects combined, with variant homozygous subjects having higher TG and LDL-C and lower HDL-C levels.

CONCLUSIONS

Some common polymorphisms and haplotypes in APOA5 are closely associated with FCH in Hong Kong Chinese, and these differ from those found in Caucasians.

APOA5 genetic variants are markers for classic hyperlipoproteinemia phenotypes and hypertriglyceridemia

BACKGROUND

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.

METHODS

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.

RESULTS

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).

CONCLUSIONS

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.

Clinical significance of apolipoprotein A5

PURPOSE OF REVIEW

We have examined the evidence from recent human studies examining the role of apolipoprotein A-V in triglyceride-rich lipoprotein metabolism and cardiovascular disease risk. Special emphasis was placed on the evidence emerging from the association between genetic variability at the apolipoprotein A5 locus, lipid phenotypes and disease outcomes. Moreover, we address recent reports evaluating apolipoprotein A5 gene-environment interactions in relation to cardiovascular disease and its common risk factors.

RECENT FINDINGS

Several genetic association studies have continued to strengthen the position of APOA5 as a major gene that is involved in triglyceride metabolism and modulated by dietary factors and pharmacological therapies. Moreover, genetic variants at this locus have been significantly associated with both coronary disease and stroke risks.

SUMMARY

Apolipoprotein A-V has an important role in lipid metabolism, specifically for triglyceride-rich lipoproteins. However, its mechanism of action is still poorly understood. Clinical significance at present comes largely from genetic studies showing a consistent association with plasma triglyceride concentrations. Moreover, the effects of common genetic variants on triglyceride concentrations and disease risk are further modulated by other factors such as diet, pharmacological interventions and BMI. Therefore, these genetic variants could be potentially used to predict cardiovascular disease risk and individualize therapeutic options to decrease cardiovascular disease risk.

Apoprotein A-V: an important regulator of triglyceride metabolism

Apolipoprotein A-V (apoA-V) was discovered in 2001 both by comparative sequencing and as a liver regeneration protein. The gene is a located at the APOA1/C3/A4/A5 gene cluster on chromosome 11q23, a locus well known for playing a major role in regulating plasma cholesterol and triglyceride (TG) levels. ApoA-V is produced in the liver and has very low plasma concentrations (0.1-0.4 mug/ml). Mice lacking apoA-V have 4-fold increased TG levels, whereas apoA-V overexpression leads to 40% plasma TG reduction. Based on metabolic studies in vivo, apoA-V enhances the catabolism of TG rich lipoproteins rather than affecting their intestinal or hepatic production. By activating proteoglycans-bound lipoprotein lipase (LPL), apoA-V can accelerate TG hydrolysis from VLDL and chylomicrons independent from other apoproteins. Several variants at the APOA5 gene locus have been detected in humans. Some single nucleotide polymorphisms (SNPs) are associated with significantly higher plasma TG levels in patients (e.g., -1131T > C, S19W, G185C). In addition, these SNPs may affect fibrate response and obesity. However, data for a possible association of APOA5 variants with coronary heart disease are not consistent. Severe structural mutations (Q139X, Q148X, IVS3 + 3G > C) predispose to familial hypertriglyceridaemia and late-onset chylomicronaemia. Thus, despite its low plasma concentration, apoA-V is a major regulator of plasma TG metabolism in humans. However, the precise mechanism of its function is not yet clear.

APOA1/C3/A5 haplotype and risk of hypertriglyceridemia in Taiwanese

BACKGROUND

Apolipoprotein A5 gene (APOA5) has been shown to modulate plasma triglyceride concentrations. We investigated 2 distinct APOA1/C3/A5 haplotypes roles for hypertriglyceridemia.

METHODS

We recruited 308 cases of hypertriglyceridemia and 281 normal controls from a hospital. Twelve single nucleotide polymorphisms (SNPs) across the APOA1/C3/A5 gene region were genotyped.

RESULTS

One haplotype containing the minor alleles of the APOA5 (-1131T>C, c.553G>T) and APOA1 (-3013C>T,-75G>A) was more prevalent in cases than in controls (11.3% vs. 1.1%, respectively) and was statistically significantly associated with high triglycerides (adjusted odds ratio: 12.83, 95% confidence interval [CI]: 5.1-32.4, P<0.001). Another haplotype that was associated with hypertriglyceridemia (adjusted odds ratio 2.13, 95% CI, 1.37-3.29, P=0.001). Participants carrying both minor alleles of APOA5-1131CC and c.553TT had a 116% higher triglyceride concentration compared with those carrying common allele.

CONCLUSIONS

The APOA1/C3/A5 haplotype represents an important locus for predicting risk of hypertriglyceridemia among Taiwanese.

APOA5 variants and metabolic syndrome in Caucasians

Apolipoprotein A5 (APOA5) gene variants were reported to be associated with two components of metabolic syndrome (MetS): higher TG levels and lower HDL levels. Moreover, a recent Japanese case-control study found variant -1131T>C associated with MetS itself. Thus, our study systematically analyzed the APOA5 gene for association with lipid parameters, any other features of MetS, including waist circumference, glucose-related parameters, blood pressure, uric acid, and MetS itself in Caucasians. Ten polymorphisms were analyzed in a large fasting sample of the population-based Cooperative Health Research in the Region of Augsburg (KORA) survey S4 (n = 1,354; southern Germany) and in a second fasting sample, the Salzburg Atherosclerosis Prevention Program in Subjects at High Individual Risk (SAPHIR) study (n = 1,770; Austria). Minor alleles of variants -1131T>C, -3A>G, c.56C>G, 476G>A, and 1259T>C were significantly associated with higher TG levels in single polymorphism (P < 0.001) and haplotype (P <or= 6.6 x 10(-6)) analysis. Besides associations with lower HDL levels in SAPHIR (P <or= 0.001), there were no significant findings with any other features of MetS. Variant c.56C>G was associated with higher risk for MetS [odds ratio (95% confidence interval) = 1.43 (1.04, 1.99), P = 0.03 for KORA and 1.48 (1.10, 1.99), P = 0.009 for SAPHIR). Our study confirms the association of the APOA5 locus with TG and HDL levels in humans. Furthermore, the data suggest a different mechanism of APOA5 impact on MetS in Caucasians, as variant c.56C>G (not analyzed in the Japanese study) and not -1131T>C, as in the Japanese subjects, was associated with MetS.