👤 B Dorfmeister

The purpose of this study was to identify rare APOA5 variants in 130 severe hypertriglyceridemic patients by sequencing, and to test their functionality, since no patient recall was possible. We studied the impact in vitro on LPL activity and receptor binding of 3 novel heterozygous variants, apoAV-E255G, -G271C, and -H321L, together with the previously reported -G185C, -Q139X, -Q148X, and a novel construct -Delta139 to 147. Using VLDL as a TG-source, compared to wild type, apoAV-G255, -L321 and -C185 showed reduced LPL activation (-25% [P=0.005], -36% [P<0.0001], and -23% [P=0.02]), respectively). ApoAV-C271, -X139, -X148, and Delta139 to 147 had little affect on LPL activity, but apoAV-X139, -X148, and -C271 showed no binding to LDL-family receptors, LR8 or LRP1. Although the G271C proband carried no LPL and APOC2 mutations, the H321L carrier was heterozygous for LPL P207L. The E255G carrier was homozygous for LPL W86G, yet only experienced severe hypertriglyceridemia when pregnant. The in vitro determined function of these apoAV variants only partly explains the high TG levels seen in carriers. Their occurrence in the homozygous state, coinheritance of LPL variants or common APOA5 TG-raising variant in trans, appears to be essential for their phenotypic expression. Show less

Plasma triglyceride (TG) and apoAV levels are reported to be positively correlated, yet SNPs defining haplotype APOA52 have consistently shown association with elevated plasma triglyceride (TG) but not plasma apoAV levels. We previously reported that individually -1131T>C, -3A>G and +1891T>C did not influence luciferase activity or in vitro translation efficiency. To investigate the combined effect of these SNPs additional constructs were examined. Compared to the wildtype -1131T/-3A/+1891T (TAT), the triple rare allele construct -1131C/-3G/+1891C (CGC) conferred 46% lower luciferase activity (p<0.0001), showing these SNPs are acting co-operatively. Although only these two combinations occur in vivo, we experimentally altered the TAT construct one site at a time; -3G (TGT) had the largest effect (94% lower luciferase), with lesser effects from CAT (-77%) and TAC (-70.3%) (all p<0.0001). Deletion constructs excluding one site at a time showed that -3G/1891C ( -GC) in combination, compared to -AT, was having the largest effect on luciferase activity (-59%, p=0.055). Using sequence homology and EMSA analysis no transcription factor binding at -1131 or +1891 was identified, though +1891 lies within a putative mRNA stability motif. Taken together, these data identify -3A>G in the Kozak sequence as functional, affecting translation initiation and driving the haplotype effects, while showing interaction with +1891T>C and to a lesser extent -1131T>C. A paradox arises since these results predict that APOA52 will lead to reduced apoAV with concomitant reduced LPL activation or lipoprotein-receptor interaction, resulting in higher plasma TG levels. We conclude that APOA5 expression, and not circulating plasma apoAV levels, is causatively associated with plasma TG levels. Show less

Common variants in APOA5 and APOC3 have been associated with differences in plasma triglyceride (TG) levels in healthy individuals. The aim of this study was to examine the association of APOA5 (-1131T>C, S19W) and APOC3 (-482C>T, 1100C>T) polymorphisms in patients with type 2 diabetes (T2D) of European White (EW) (n=931), Indian Asian (IA) (n=610) and Afro-Caribbean (AC) (n=167) origin, with lipid and T2D parameters. Rare allele frequencies and linkage disequilibrium differed significantly amongst ethnic groups. Compared to APOA5 -1131T and 19S homozygotes, -1131C and 19W carriers had higher TGs in all groups, but this effect was only statistically significant for the -1131C in the EWs (P=0.04) and 19W in the IAs (P<0.001). APOC3 SNPs showed no significant association with lipid levels in any ethnic group. While haplotypes carrying -1131C allele showed significant TG-raising in the EWs only, the 19W defined haplotype showed significant TG-raising in both IAs and EWs. Comparing all four SNPs in EW T2D subjects with healthy EWs (n=2579), the APOC3 1100C>T frequency was significantly higher in T2D [0.26 (0.24, 0.28)] vs. healthy EWs [0.22 (0.20, 0.23)], P=0.001. While the variable size effects of the two APOA5 SNPs on TG levels may result from ethnically different gene-gene or gene-environment interactions, APOA5 and APOC3 variants did not affect parameters of T2D. However, comparison between EWs with T2D and healthy EWs suggest APOC3 1100C>T is associated with increased risk of diabetes probably through mechanisms other than direct effects on TG. Show less

Apolipoprotein AV (apoAV) is a recently discovered apolipoprotein with a triglyceride-lowering effect in genetically modified mice. Transcription of the human gene encoding apoAV (APOA5) is suppressed by insulin and stimulated by fibrates. Our goal was to study the expression of Apoa5, in comparison with Apoa4 and Apoc3, in hypertriglyceridaemic, obese and insulin-resistant Zucker rats receiving the insulin sensitiser rosiglitazone and/or a fish oil diet to lower triglycerides. Hepatic Apoa5, Apoa4 and Apo3 mRNA and liver and plasma apoAV were measured in lean and obese Zucker rats receiving rosiglitazone while on a coconut oil or fish oil diet. Basal hepatic Apoa5 expression was similar in obese and lean Zucker rats. Unexpectedly, obese Zucker rats tended to have higher plasma apoAV levels despite their hypertriglyceridaemic state. Both rosiglitazone and the fish oil diet significantly increased Apoa5 mRNA, by about 70%, but tended to lower liver and plasma apoAV. Rosiglitazone had no effect on Apoa5 mRNA in cultured rat hepatocytes. No intact PPAR (peroxisome proliferator-activated receptor) response element was identified in the rat Apoa5 promoter. Our data indicate that apoAV does not contribute to the hypertriglyceridaemia of obese Zucker rats or to the hypolipidaemic effect of rosiglitazone or a fish oil diet. The divergent changes of Apoa5 mRNA and apoAV levels suggest co- or post-translational regulation. The increase in Apoa5 mRNA induced by rosiglitazone is not directly mediated by peroxisome proliferator-activated receptor gamma. Show less