Apolipoprotein A5 and hypertriglyceridemia

Apolipoprotein A5 alleviates LPS/D-GalN-induced fulminant liver failure in mice by inhibiting TLR4-mediated NF-κB pathway

BACKGROUND

Fulminant liver failure (FHF) is a serious clinical problem and liver transplantation is the major intervention. But the overall survival rate of FHF is low owing to the donated organ shortage. Apolipoprotein A-V (ApoA5) is a regulator of triglyceride metabolism and has been reported to act as a predictor for remnant liver growth after preoperative portal vein embolization and liver surgery. This study aimed to investigate the therapeutic effect of ApoA5 on lipopolysaccharide/D-galactosamine (LPS/D-GalN)-induced fulminant liver failure in mice.

METHODS

FHF mouse model was established using LPS/D-GalN and ApoA5 plasmid was injected by tail vein prior to LPS/D-GalN treatment. The expressions of ApoA5, toll-like receptor 4 (TLR4), myeloid differentiation factor 88 (MyD88), and nuclear factor kappa B p65 (NF-κBp65) were assessed by real-time PCR and western blotting. Serum alanine aminotransferase (ALT) and tumor necrosis factor-α (TNF-α) levels were measured using automatic biochemical analyzer. Histological assessment and immunohistochemical (IHC) staining were conducted. Survival rate after LPS/D-GalN administration was also determined with Kaplan-Meier curve. Meanwhile, the expression of ApoA5 in injured huh7 cells was tested. Cell apoptosis analysis was performed after huh7 cells were transfected with ApoA5 plasmid and stimulated with LPS.

RESULTS

The expressions of ApoA5 decreased both in injured huh7 cells and FHF mice. ApoA5 overexpression reduced cell death rate using flow cytometry. ApoA5 not only decreased the serum ALT and TNF-α levels but also attenuated hepatic damage in hematoxylin-eosin (HE)-stained liver section. The protein expressions of TLR4, MyD88 and NF-κBp65 were inhibited when ApoA5 overexpressed. But the inhibitory effect would weaken with the increasing concentration of LPS in spite of ApoA5 overexpression. Besides, ApoA5 improved liver injury in a dose-dependent manner and the survival rate in FHF mice increased with increasing concentration of ApoA5.

CONCLUSION

ApoA5 had a protective effect against LPS/D-GalN-induced fulminant liver failure in mice within a certain range by inhibiting TLR4-mediated NF-κB pathway.

A genetic variant c.553G > T (rs2075291) in the apolipoprotein A5 gene is associated with altered triglycerides levels in coronary artery disease (CAD) patients with lipid lowering drug

Background

Elevated plasma triglycerides (TGs) are widely used as a major cardiovascular risk predictor and are thought to play an important role in the progression of coronary heart disease (CHD). It has been demonstrated that lipid lowering was associated with lower mortality in patients with CHD. The present study therefore aimed to investigate the consequences of the genetic variant c.553G > T (rs2075291) in apolipoprotein A5 gene to determination of triglycerides levels in CAD patients receiving, atorvastatin, lipid lowering drug.

Methods

We here report that a recently identified genetic variant, c.553G > T in the APOA5 gene which causes a substitution of a cysteine for a glycine residue at amino acid residue 185(G185C) is also associated with increased TG levels. To investigate theses effects, a case-control study compressing 608 subjects from the same area was performed.

Results

TG levels in T allele patients were significantly lower than the control GT allele patient (χ² = 2.382E2^a, P-value < 0.001). Overall, patients carrying T allele showed lower levels of TG than patients carrying GG allele. The homozygous patient for the T allele presented normal cholesterol levels of 134 mg/dl, and the levels in GG patients ranged from 25 to 340 mg/dl (P-value < 0.001). In summary, we demonstrated that the presence of c.553G > T variant (rs2075291); in APOA5 gene increases human plasma TG levels.

Conclusion

Nevertheless, T allele is found to reduce TG levels in CAD patients who are on the cholesterol medication, atorvastatin. Thus, c.553G > T variant can be considered as a significant predicator of hypertriglyceridemia. In addition, it could be used as a hallmark for the diagnosis and prognosis of CAD.

High-resolution genetic mapping in the diversity outbred mouse population identifies Apobec1 as a candidate gene for atherosclerosis

Inbred mice exhibit strain-specific variation in susceptibility to atherosclerosis and dyslipidemia that renders them useful in dissecting the genetic architecture of these complex diseases. Traditional quantitative trait locus (QTL) mapping studies using inbred strains often identify large genomic regions, containing many genes, due to limited recombination and/or sample size. This hampers candidate gene identification and translation of these results into possible risk factors and therapeutic targets. An alternative approach is the use of multiparental outbred lines for genetic mapping, such as the Diversity Outbred (DO) mouse panel, which can be more informative than traditional two-parent crosses and can aid in the identification of causal genes and variants associated with QTL. We fed 292 female DO mice either a high-fat, cholesterol-containing (HFCA) diet, to induce atherosclerosis, or a low-fat, high-protein diet for 18 wk and measured plasma lipid levels before and after diet treatment. We measured markers of atherosclerosis in the mice fed the HFCA diet. The mice were genotyped on a medium-density single-nucleotide polymorphism array and founder haplotypes were reconstructed using a hidden Markov model. The reconstructed haplotypes were then used to perform linkage mapping of atherosclerotic lesion size as well as plasma total cholesterol, triglycerides, insulin, and glucose. Among our highly significant QTL we detected a ~100 kb QTL interval for atherosclerosis on Chromosome 6, as well as a 1.4 Mb QTL interval on Chromosome 9 for triglyceride levels at baseline and a coincident 22.2 Mb QTL interval on Chromosome 9 for total cholesterol after dietary treatment. One candidate gene within the Chromosome 6 peak region associated with atherosclerosis is Apobec1, the apolipoprotein B (ApoB) mRNA-editing enzyme, which plays a role in the regulation of ApoB, a critical component of low-density lipoprotein, by editing ApoB mRNA. This study demonstrates the value of the DO population to improve mapping resolution and to aid in the identification of potential therapeutic targets for cardiovascular disease. Using a DO mouse population fed an HFCA diet, we were able to identify an A/J-specific isoform of Apobec1 that contributes to atherosclerosis.

Relationship between the distribution of plasma HDL subclasses and the polymorphisms of APOA5 in hypertriglyceridemia

AIMS

This study aims to examine the possible associations between high density lipoprotein (HDL) subclass distribution and APOA5-1131T>C polymorphism in hypertriglyceridemia.

METHODS

The distribution of HDL subclasses was quantified by 2-dimensional electrophoresis in conjunction with immunodetection method. The APOA5-1131T>C polymorphism was identified in 95 hypertriglyceridemic (HTG) patients and 102 healthy subjects by polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP).

RESULTS

The APOA5-1131C (C) allele frequency was higher in the HTG group than in the control group. Plasma triglycerides (TG) were significantly higher and apoA5 was significantly lower in patients with the C allele when compared to patients with the APOA5-1131T (T) allele, even more dramatically so in the APOA5-1131CC homozygote. In both the HTG group and the control group, the frequency of the C allele was positively correlated with levels of TG, total cholesterol (TC), low density lipoprotein cholesterol (LDL-C) and apolipoprotein B100 (apoB100), and negatively correlated with levels of high density lipoprotein cholesterol (HDL-C), apolipoprotein A1 (apoA1) and apolipoprotein A5 (apoA5) (P<0.001). In all subjects, the frequency of the C allele was positively correlated with the level of small-sized HDL (preβ(1)-HDL and HDL(3a)), and negatively correlated with levels of HDL(2a) and HDL(2b).

CONCLUSION

Changes in HDL subclass distributions in HTG may be related to the APOA5-1131T>C polymorphism. This polymorphism leads to a general shift towards smaller-sized HDL.

The complex genetic basis of plasma triglycerides

Demonstration of a direct relationship between plasma triglyceride (TG) concentration and atherosclerosis has proven difficult due to confounding variables that accompany elevated plasma TG, such as other dyslipidemias, obesity, and type 2 diabetes. However, human genetic studies have provided evidence suggesting a causal link between plasma TG and cardiovascular risk. Analyses in human patients with hypertriglyceridemia (HTG) also provides insight into the relationship between genetic variation, predisposition to elevated plasma TG, and risk of subsequent cardiovascular disease. Here, we review recent key studies that have contributed to our understanding of the genetic determinants of plasma TG concentration, including HTG susceptibility and phenotypic heterogeneity, and discuss our maturing model of the allelic and phenotypic spectrum of plasma TG.

APOA5 genotype modulates 2-y changes in lipid profile in response to weight-loss diet intervention: the Pounds Lost Trial

BACKGROUND

The apolipoprotein A5 gene (APOA5) is a major gene that regulates lipid metabolism and is modulated by dietary factors. A novel variant rs964184 in APOA5 was identified to be associated with lipids in genome-wide association studies.

OBJECTIVE

We examined whether this variant modified changes in lipid concentrations in response to a 2-y weight-loss diet intervention in a randomized trial.

DESIGN

The current analyses were secondary analyses of a data set from the Pounds Lost Trial. We genotyped APOA5 rs964184 in 734 overweight or obese adults who were randomly assigned to one of 4 diets that differed in percentages of energy derived from fat, protein, and carbohydrate for 2 y. We evaluated changes in fasting serum concentrations of total cholesterol (TC), LDL cholesterol, HDL cholesterol, and triglyceride from baseline to 2 y of follow-up.

RESULTS

After a 2-y dietary intervention, we showed significant interactions between the APOA5 rs964184 polymorphism and dietary fat intake (low compared with high) in the determination of changes in TC, LDL cholesterol, and HDL cholesterol (P-interaction = 0.007, 0.017, and 0.006, respectively). In the low-fat intake group (20% of energy derived from fat), carriers of the risk allele (G allele) exhibited greater reductions in TC and LDL cholesterol than did noncarriers (P = 0.036 and 0.039, respectively), whereas in the high-fat diet group (40% of energy derived from fat), participants with the G allele had a greater increase in HDL cholesterol than did participants without this allele (P = 0.038).

CONCLUSION

Our data showed better improvement in lipid profiles from long-term low-fat diet intake in the APOA5 rs964184 risk allele.

The paradox of ApoA5 modulation of triglycerides: evidence from clinical and basic research

Apolipoprotein A5 (ApoA5) is a key regulator of plasma triglycerides (TG), even though its plasma concentration is very low compared to other known apoproteins. Over the years, researchers have attempted to elucidate the molecular mechanisms by which ApoA5 regulates plasma TG in vivo. Though still under debate, two theories broadly describe how ApoA5 modulates TG levels: (i) ApoA5 enhances the catabolism of TG-rich lipoproteins and (ii) it inhibits the rate of production of very low-density lipoprotein (VLDL), the major carrier of TGs. This review will summarize the basic and clinical studies that describe the importance of ApoA5 in TG metabolism. Population studies conducted in various countries have demonstrated an association between single nucleotide polymorphisms (SNPs) in ApoA5 and the increased risk to cardiovascular disease and metabolic syndrome (including diabetes and obesity). ApoA5 is also highly expressed during liver regeneration and is an acute phase protein associated with HDL, which is independent of its effects on TG metabolism.

CONCLUSION

Despite considerable evidences available from clinical and basic research studies on the role of ApoA5 in TG metabolism and its indirect link to metabolic diseases, additional investigations are needed to understand the paradoxical role of this important apoprotein is modulated by both diet and its polymorphism variants.

Allelic and phenotypic spectrum of plasma triglycerides

The genetic underpinnings of both normal and pathological variation in plasma triglyceride (TG) concentration are relatively well understood compared to many other complex metabolic traits. For instance, genome-wide association studies (GWAS) have revealed 32 common variants that are associated with plasma TG concentrations in healthy epidemiologic populations. Furthermore, GWAS in clinically ascertained hypertriglyceridemia (HTG) patients have shown that almost all of the same TG-raising alleles from epidemiologic samples are also associated with HTG disease status, and that greater accumulation of these alleles reflects the severity of the HTG phenotype. Finally, comprehensive resequencing studies show a burden of rare variants in some of these same genes - namely in LPL, GCKR, APOB and APOA5 - in HTG patients compared to normolipidemic controls. A more complete understanding of the genes and genetic variants associated with plasma TG concentration will enrich our understanding of the molecular pathways that modulate plasma TG metabolism, which may translate into clinical benefit. This article is part of a Special Issue entitled Triglyceride Metabolism and Disease.

Genetic bases of hypertriglyceridemic phenotypes

PURPOSE OF REVIEW

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.

RECENT FINDINGS

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.

SUMMARY

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.

A single nucleotide polymorphism in APOA5 determines triglyceride levels in Hong Kong and Guangzhou Chinese

Single nucleotide polymorphisms (SNPs) in the apolipoprotein A5 (APOA5) gene have been associated with hypertriglyceridaemia. We investigated which SNPs in the APOA5 gene were associated with triglyceride levels in two independent Chinese populations. In all, 1375 subjects in the Hong Kong Cardiovascular Risk Factor Prevalence Study were genotyped for five tagging SNPs chosen from HapMap. Replication was sought in 1996 subjects from the Guangzhou Biobank Cohort Study. Among the five SNPs, rs662799 (-1131T>C) was strongly related to log-transformed triglyceride levels among Hong Kong subjects (β=0.192, P=2.6 × 10(-13)). Plasma triglyceride level was 36.1% higher in CC compared to TT genotype. This association was confirmed in Guangzhou subjects (β=0.159, P=1.3 × 10(-12)), and was significantly irrespective of sex, age group, obesity, metabolic syndrome, hypertension, diabetes, smoking and alcohol drinking. The odds ratios and 95% confidence interval for plasma triglycerides ≥1.7 mmol/l associated with TC and CC genotypes were, respectively, 1.81 (1.37-2.39) and 2.22 (1.44-3.43) in Hong Kong and 1.27 (1.05-1.54) and 1.97 (1.42-2.73) in Guangzhou. Haplotype analysis suggested the association was due to rs662799 only. The corroborative findings in two independent populations indicate that the APOA5-1131T>C polymorphism is an important and clinically relevant determinant of plasma triglyceride levels in the Chinese population.

DNA polymorphisms and haplotypes of apolipoprotein A5's attribution to the plasma triglyceride levels in Koreans

PURPOSE

Recent studies using human and mice reported that apolipoprotein A-V (APOA5) gene plays an important role in controlling triglyceride (TG) concentrations. The purpose of the present study was to investigate the correlation between single nucleotide polymorphisms (SNPs) and haplotypes in the APOA5 gene and TG in subjects and to search for possible associations of the APOA5 gene variants and common haplotypes with hypertriglyceridemia (HTG).

MATERIALS AND METHODS

We examined the case-control subjects including 100 HTG patients and 243 unrelated healthy control. The genes were screened for SNPs by direct sequencing in 48 genetically unrelated individuals. Six SNPs (-1390C>T, -1020G>A, -3A>G, V150M, G182C and 1259T>C) were genotyped in case and control populations.

RESULTS

In this study, our results indicated a strong association between APOA5 SNP -3A>G and G182C and elevated TG levels (p<0.001). Analysis of the SNPs from APOA5 gene has identified major haplotype showing very strong association with HTG, CGGGTT (p<0.001). Likelihood ratio test (LRT) of these six SNPs revealed that haplotypes were strong independent predictors of HTG (p<0.001). Haplotype-trend logistic regression (HTR) analysis revealed a significant association between the CGGGGC (haplotype 2) and CGGGTT (haplotype 4) and HTG (OR=2.48, 95% CI=1.06-5.76 and OR=8.54, 95% CI=2.66-27.42, respectively).

CONCLUSION

We confirm that the APOA5 variants are associated with triglyceride levels and the haplotype may be strong independent predictors of HTG among Koreans.

Identification of lipid binding and lipoprotein lipase activation domains of human apoAV

ApoAV, a newly discovered apolipoprotein, plays a key role in human triglyceride homeostasis; however, the structure-function correlation of apoAV is not clearly understood. To explore the relationship, wild type and six deletion mutants, that is (AV (Delta(1-51)), AV (Delta(51-128)), AV (Delta(132-188)), AV (Delta(192-238)), AV (Delta(246-299)), AV (Delta(301-343))), of human apoAV expressed in Escherichia coli were studied. All the deleted regions together encompass almost the entire 343 amino acid sequence of wild type apoAV. Circular dichroism spectroscopy showed that the alpha helical content of lipid-free wild type apoAV was 46%. In comparison with wild type apoAV, AV (Delta(192-238)) and AV (Delta(301-343)) displayed significantly decreased lipid binding activities, confirming the importance of these two regions in lipid binding function of apoAV. While, the LPL activation function of apoAV remarkably impaired after deletion of residues 192-238. These findings suggested that the domain (192-238) is absolutely necessary for apoAV in lipid binding and lipoprotein lipase activation.

Allelic drop-out may occur with a primer binding site polymorphism for the commonly used RFLP assay for the -1131T>C polymorphism of the Apolipoprotein AV gene

Apolipoprotein AV (ApoAV) gene variant, -1131T>C, is associated with increased triglyceride concentrations in all ethnic groups studied. An MseI based RFLP analysis is the most commonly used method for genotyping this SNP. We genotyped a large cohort comprising 1185 Asian Indians and 173 UK Caucasians for -1131T>C using an ARMS-PCR based tetra-primer method. For quality control, we re-genotyped approximately 10% random samples from this cohort utilizing the MseI RFLP, which showed a 2.9% (3/102) genotyping error rate between the two methods. To investigate further, we sequenced the 900 bp region around the -1131T>C polymorphism in 25 Asian Indians and 15 UK Caucasians and found a number of polymorphisms including the -987C>T polymorphism. Further analysis of the -987C>T SNP showed a higher rare allele frequency of 0.23 in Asian Indians (n = 158) compared to 0.09 in the UK Caucasians (n = 157). This SNP is located 4 bp from the 3' end of the RFLP forward primer and is in weak linkage disequilibrium with -1131T>C variant (r² = 0.084 and D' = 1). Repeated RFLP analysis of seven subjects heterozygous for -987C>T (seven times), showed discordant results with the sequence at -1131T>C SNP nearly one third (15/49) of the time. We conclude that presence of -987C>T polymorphism in the forward primer of the MseI RFLP assay may lead to allelic drop-out and generate unforeseen errors in genotyping the -1131T>C polymorphism. Our results also emphasise the need for careful quality control in all molecular genetic studies, particularly while transferring genotyping methods between various ethnic groups.