Functional and genetic analyses of cholesteryl ester transfer protein and hepatic lipase in Thai subjects with hyperalphalipoproteinemia

Abstract

Hyperalphalipoproteinemia (HALP), characterized by high plasma HDL level, is primarily caused by mutation in the cholesteryl ester transfer protein (CETP) and hepatic lipase (LIPC) genes resulting in decreased activities of CETP and hepatic lipase (HL). The cause of Thai HALP is still unknown. The objective of this research is to determine functional and genetic analyses of CETP and hepatic lipase (HL) in Thai HALP subjects and to determine protein composition on HDL of HALP subjects compare with normal controls. Thirty-eight subjects with HDL-cholesterol levels ≥ 100 mg/dL, and thirty-eight age- and sex-matched controls were recruited from an outpatient clinic. Secondary causes of HALP were excluded in all cases. The mean total and HDL cholesterol levels were significantly higher in the HALP group compared with the control group (259 ± 8 vs 234 ± 6 mg/dL, P=0.05 and 119 ± 2 vs 64 ± 3, P<0.01, respectively). The mean CETP and HL activities were significantly lower in the HALP group than in the control group (34 ± 4 vs. 44 ± 3 pmol/µL/hr, P = 0.04; and 150 ± 17 vs. 227 ± 16 nmol FFA/mL/min, P = 0.002, respectively). Seven and three subjects in the HALP group who had very low CETP and HL activities, respectively, were chosen for further analysis. A mutational analysis study revealed that a D442G missense mutation in exon 15 of the CETP gene was present in 10 subjects in the HALP group (26%), but it was not found in the control group. We also identified a novel 4 base-pair deletion mutation (734_737 del TCCC) in exon 9 of the CETP gene in one subject in the HALP group. Moreover, we discovered a novel missense mutation in the LIPC gene, G119S, in one subject in the HALP group, but not in the control group. The pathogenic role of G119S was supported by evolutionary conservation, its predicted damaging function, and in vitro expression studies. In this study; HDL was isolated by immunoaffinity column. HDL protein of HALP (n=7) and control group (n=7) were separated with two-dimensional gel electrophoresis and identified with mass spectrometry. MS analysis revealed the presence of 22 HDL-associated proteins including all known apolipoproteins and lipid transport proteins. However, there were no significant differences in the quantities of protein on HDL between case and control group, as examined by 2D analysis software. In Conclusions, In Thai subjects with HALP, both CETP and HL activities were significantly lower than those in the control subjects and several mutations in the CETP and LIPC genes were identified. Approximately one-third of Thai subjects with HALP were caused by either CETP or LIPC mutations.