Lifelong Reduction in LDL (Low-Density Lipoprotein) Cholesterol due to a Gain-of-Function Mutation in .

Background: Loss-of-function mutations in the LDL (low-density lipoprotein) receptor gene () cause elevated levels of LDL cholesterol and premature cardiovascular disease. To date, a gain-of-function mutation in with a large effect on LDL cholesterol levels has not been described. Here, we searched for sequence variants in that have a large effect on LDL cholesterol levels.

Methods: We analyzed whole-genome sequencing data from 43 202 Icelanders. Single-nucleotide polymorphisms and structural variants including deletions, insertions, and duplications were genotyped using whole-genome sequencing-based data. LDL cholesterol associations were carried out in a sample of >100 000 Icelanders with genetic information (imputed or whole-genome sequencing). Molecular analyses were performed using RNA sequencing and protein expression assays in Epstein-Barr virus-transformed lymphocytes.

Results: We discovered a 2.5-kb deletion (del2.5) overlapping the 3' untranslated region of in 7 heterozygous carriers from a single family. Mean level of LDL cholesterol was 74% lower in del2.5 carriers than in 101 851 noncarriers, a difference of 2.48 mmol/L (96 mg/dL; =8.4×10). Del2.5 results in production of an alternative mRNA isoform with a truncated 3' untranslated region. The truncation leads to a loss of target sites for microRNAs known to repress translation of . In Epstein-Barr virus-transformed lymphocytes derived from del2.5 carriers, expression of alternative mRNA isoform was 1.84-fold higher than the wild-type isoform (=0.0013), and there was 1.79-fold higher surface expression of the LDL receptor than in noncarriers (=0.0086). We did not find a highly penetrant detrimental impact of lifelong very low levels of LDL cholesterol due to del2.5 on health of the carriers.

Conclusions: Del2.5 is the first reported gain-of-function mutation in causing a large reduction in LDL cholesterol. These data point to a role for alternative polyadenylation of mRNA as a potent regulator of LDL receptor expression in humans.