Molecular mechanism linking a novel PCSK9 copy number variant to severe hypercholesterolemia

A 42 year-old male with premature atherosclerosis, severe dyslipidemia and resistance to treatment with high dose statin and a recommended dose of a PCSK9 inhibitor, was found to have a duplication of the PCSK9 gene. However, the clinical phenotype, which included a more than 15-fold elevation in circulating PCSK9, was unexpected given that he had one additional gene copy.

Here we have carried out whole genome sequencing and transcriptional reporter assays to investigate the molecular mechanism leading to this unusual FH phenotype.

The PCSK9 duplication was found to contain the full coding sequence but with an 829 bp shorter 3'-untranslated region (UTR) sequence. All possible rearrangements include a head-to-head fusion between a completely duplicated PCSK9 and a chromosomal region, normally situated ~80 kb away, that includes HNF4 and USF1 binding sites that could promote transcription of the PCSK9 gene. Transcriptional reporter assays demonstrated that a construct harboring the HNF4 binding site significantly increased the promoter activity by 2.5-fold with a smaller effect noted for a USF1 construct.

Here we describe, in a patient with resistant hypercholesterolemia, a novel PCSK9 gene rearrangement that enables upregulation of PCSK9 expression by allowing proximity to an active enhancer binding to HNF4A.