Genetic and metabolic determinants of low-density lipoprotein in familial hypercholesterolemia

Abstract

The Low-Density Lipoprotein Receptor (LDLR) pathway plays a major role in controlling plasma cholesterol levels. Hepatic expression of LDLR is tightly regulated, both Proprotein Convertase Subtilisin/Kexin type 9 (PCSK9) and the recently identified Inducible Degrader of the LDLR (IDOL) regulate LDLR expression at a post-transcriptional level by enhancing the degradation of LDLR. Mutations in genes involved in the LDLR pathway are the main cause of Familial Hypercholesterolemia (FH). The canonical FH genes include the LDLR gene, apolipoprotein B-100 (APOB) gene and PCSK9 gene, and increased serum levels of PCSK9 have been reported in FH patients. The aim of this thesis was to investigate the genetic and metabolic determinants of LDL cholesterol (LDL-C) in FH patients. My hypothesis was that in addition to genetic heterogeneity, both IDOL and PCSK9 may affect plasma LDL-C in FH. The genetic spectrum in Chinese FH patients in Hong Kong would be defined and family screening performed to identify undiagnosed FH subjects and test the feasibility of genetic cascade screening. Serum levels of IDOL, PCSK9, and fibroblast growth factor 21 (FGF21) which might be involved in the regulation of IDOL, would be measured by ELISA in FH patients and control subjects. Ninety-six unrelated FH probands were recruited from 3 regional hospitals. Using direct sequencing method, definite or likely pathogenic mutations were identified in 67% of the subjects. Majority of them had defective LDLR mutations. The 3 most common variants were c. 682 G>A, c.1241 T>G, and c.1474 G>A, and together accounted for a combined frequency of 21%. Mutation in APOB gene accounted for 6.3%, with the variant c.10579 C>T being the most frequently found. No mutations in PCSK9 gene were identified. The optimal threshold of plasma LDL-C cutoff level to predict the presence of pathogenic mutation was 5.5 mmol/L (sensitivity 89%, specificity 80%). Incorporating genetic testing into cascade screening significantly improved the efficiency and yield of family cascade screening. In FH subjects on lipid lowering therapies, serum IDOL levels were reduced and PCSK9 levels were increased compared with control (p<0.001). In contrast, both circulating IDOL and PCSK9 levels were elevated in untreated FH subjects. No significant differences were seen in serum FGF21 levels. The discrepancy in serum IDOL levels in treated and untreated FH subjects was mainly due to the effect of statins. Serum IDOL levels were measured in a subgroup of FH subjects before and after statin therapy and there was a significant reduction in serum IDOL levels after treatment. On the whole, mutation-positive subjects tended to have higher serum levels of IDOL and PCSK9 than mutation-negative subjects. Both IDOL and PCSK9 were strongly correlated with plasma LDL-C in untreated FH subjects. Serum IDOL and PCSK9 were the major independent determinants of plasma LDL-C and accounted for 30% of the variability in plasma LDL-C on regression analysis. In conclusion, around two-third of Chinese FH patients had a discrete genetic basis. Both serum IDOL and PCSK9 contributed to the variability in plasma LDL-C in FH and IDOL may be a potential new therapeutic target.