Abstract 905: Alternative Splicing of HMGCR is Associated with Plasma LDL Cholesterol Response to Simvastatin
Statins can substantially lower plasma LDL cholesterol and reduce risk for coronary heart disease, but their efficacy varies among individuals. To test whether this variation is related to single nucleotide polymorphisms (SNPs) in the gene for 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR), the key regulatory enzyme of cholesterol synthesis and the target of statin inhibition, we resequenced HMGCR exons, splice sites and 1.5kb of 5′ and 3′ flanking regions in 96 individuals. Subjects were equally derived from the upper and the lower 5% of the LDL-response distribution from the Cholesterol and Pharmacogenetics (CAP) study, a 6 week trial in which 944 subjects were treated with 40 mg/day simvastatin. Intronic SNP 20144 (rs3846662) was significantly (p<0.05) associated with high versus low LDL cholesterol response status, and was part of a previously reported haplotype associated with a 14.8% smaller reduction in LDL cholesterol in the CAP cohort. Since SNP 20144 is adjacent to exon 13, a site of HMGCR alternative splicing, we measured expression levels of the splice variant lacking exon 13 (HMGCRv 1) in 170 immortalized lymphocytes derived from the CAP subjects, and found that the SNP was associated with greater statin-induced HMGCR alternative splicing (p<0.05). Moreover, there was a significant correlation between the increased magnitude of expression of HMGCRv 1 following statin exposure in vitro and smaller reductions of total cholesterol, LDL cholesterol, apoB and triglycerides in the plasma of the corresponding subjects in vivo (p<0.0001). There was no relationship between the full-length transcript and in vivo response. Additionally, we used siRNA to specifically knock-down expression of the full-length HMGCR transcript by 68%, resulting in cells enriched in the HMGCRv 1 transcript. We then measured HMGCR enzyme activity and found that the remaining enzyme demonstrated 9 –15% greater residual activity in the presence of simvastatin concentrations ranging from 0.15nM to 6.0nM (n=10). These results suggest that HMGCR genetic variation influences the production of an HMGCR isoform with reduced statin sensitivity, and that variation in expression of this isoform is a determinant of inter-individual differences in statin response.