Identification of loci conferring risk for premature CAD and heterozygous familial hyperlipidemia in the LDLR, APOB and PCSK9 genes

BackgroundHeterogeneous familial hypercholesterolemia (HFH) partly underlies polymorphic changes in the low-density lipoprotein receptor (LDLR), apolipoprotein B (APOB) and protein convertase subtilisin/kexin type 9 (PCSK9), exhibiting intra-ethnical variations in its clinical features.MethodsWe employed the Affymetrix whole genome scan 250 sty1 array to characterize possible genomic linkage to heterozygous familial hypercholesterolemia (HFH) and sequencing techniques to identify related mutations in the above three genes in a Saudi family of 11 individuals harbouring clinical features of FH. The propositus had early onset of coronary artery disease (CAD) and very significantly elevated cholesterol (Chol) level of 10.1 mmol/L and LDL-cholesterol (LDL-C) of 7.9 mmol/L as well as low HDL-C level of 0.51 mmol/L, while 4 siblings were affected with HFH.ResultsWhole genome scan for the autosomal dominant model showed high homology for the affected individuals in several regions including chromosomes (chr) 1 and 2 which harbour PCSK9 and APOB, respectively. Subsequent sequencing of the coding regions of these two and LDLR identified 11 single nucleotide polymorphisms (SNPs) in the LDLR, 8 in the APOB and 6 in the PCSK9 genes. The propositus uniquely carried the homozygous mutant genotypes (haplotype) for all 11 LDLR SNPs, in direct contrast to the only normolipidemic sibling and a control who carried the homozygous wild type genotypes at these loci. Another set of 7 SNPs in the APOB also isolated with FH. Interestingly, all family members were heterozygous for all except the rs2228671 C > T of this gene, for which the mother shared the C/C genotype with the propositus, two other affected off-springs and a control, all of whom exhibited low HDL-C levels. A confirmation experiment involving 70 individuals harbouring low HDL-C revealed 74.3% of them as C/C carriers.ConclusionsOur study identified a haplotype in the LDLR as a marker for early onset of CAD, and rs2228671 C > T in the LDLR in association with a reduction in HDL-C concentrations in FH. The results also substantiate the notion of genetic heterogeneity in HFH, underlining the essence of recognizing ethnic-specific gene variability as a potential basis for appropriate management of FH.