Hypertriglyceridemia associated with the c.553G>T APOA5 SNP results from aberrant hetero-disulfide bond formation

Apolipoprotein (apo) A-V was discovered by comparative genome analysis and shown to function as a modulator of plasma triacylglycerol (TG) levels.1 ApoA-V is a low abundance plasma protein (25 – 400 ng/ml) that associates with VLDL and HDL.2,3,4 It has been proposed that apoA-V modulates plasma TG-rich lipoprotein metabolism by binding to glycosylphosphatidylinositol high-density lipoprotein binding protein 1 (GPIHBP1), thereby promoting lipoprotein lipase (LPL) mediated TG hydrolysis.5 Genome wide association studies have identified SNPs in APOA5 that correlate with elevated plasma TG.6 Kao et al.7 described a c.553G>T APOA5 SNP in a Taiwanese population that gives rise to a Cys for Gly substitution at position 185 of the pre-protein, corresponding to residue 162 in mature, circulating apoA-V. This polymorphism introduces a second cysteine into apoA-V, with the other occurring at position 204. Minor allele frequency in control subjects is 0.042 compared to 0.27 (P T SNP has been identified in subjects of European ancestry9,10, indicating worldwide occurrence. Insofar as HTG is an independent risk factor for coronary artery disease11 and is a component of the metabolic syndrome12 there is great interest in understanding the molecular basis whereby APOA5 SNPs lead to elevated plasma TG.13 Indeed, given the frequency of the c.553G>T APOA5 SNP, it may be anticipated that millions of people worldwide are carriers. Dorfmeister et al.14 reported that G162C apoA-V forms multimers in vitro, although the bulk of the protein is monomeric. Binding studies with members of the low-density lipoprotein receptor family indicated G162C apoA-V is functional, leading these authors to propose defective LPL activation as the underlying mechanism. In another study, Huang et al.15 investigated the effect of different amino acid substitution mutations at position 162 in apoA-V on its LPL activation properties with the finding that Gly at this position is important in terms of LPL activation in vitro. In a recent study, Sharma et al. reported that adeno-associated virus (AAV) 2/8 mediated gene transfer of wild type (WT) apoA-V improved the HTG phenotype of apoa5 (−/−) mice.16 In the present study it is demonstrated that gene transfer of G162C apoA-V results in a protein that forms disulfide-linked heterodimers with extraneous plasma proteins resulting in an unusual plasma distribution pattern, defective lipoprotein binding and compromised TG-lowering activity.