A novel putative interactor for the low density lipoprotein receptor cytoplasmic domain.

Familial hypercholesterolemia (FH) is a genetic disease mainly caused by mutations in the low density lipoprotein receptor (LDL-R) gene. However, FH-like phenotypes may also arise from mutations occurring in other genes, the products of which normally interact with the LDL receptor. Although several FH-associated proteins have been discovered, many FH-like phenotypes cannot be linked to mutations in already characterized genes, suggesting the existence of other genes still to be identified, the mutations of which may be directly linked to the FH disorder. In order to identify new putative LDLr interactors possibly involved in its internalization and/or sorting, the cytoplasmic tail of the receptor was used as 'bait' in a two-hybrid assay. We identified an 85-amino acid protein able to bind the LDLr intracellular domain through the last 14 C-terminal amino acids. The novel protein is probably derived from the translation of an alternative open reading frame of the human MT2A gene.

A novel loss of function mutation of PCSK9 gene in white subjects with low-plasma low-density lipoprotein cholesterol.

OBJECTIVES: The PCSK9 gene, encoding a pro-protein convertase involved in posttranslational degradation of low-density lipoprotein receptor, has emerged as a key regulator of plasma low-density lipoprotein cholesterol. In African-Americans two nonsense mutations resulting in loss of function of PCSK9 are associated with a 30% to 40% reduction of plasma low-density lipoprotein cholesterol. The aim of this study was to assess whether loss of function mutations of PCSK9 were a cause of familial hypobetalipoproteinemia and a determinant of low-plasma low-density lipoprotein cholesterol in whites. METHODS AND RESULTS: We sequenced PCSK9 gene in 18 familial hypobetalipoproteinemia subjects and in 102 hypocholesterolemic blood donors who were negative for APOB gene mutations known to cause familial hypobetalipoproteinemia. The PCSK9 gene variants found in these 2 groups were screened in 42 subjects in the lowest (<5th) percentile, 44 in the highest (>95th) percentile, and 100 with the average plasma cholesterol derived from general population. In one familial hypobetalipoproteinemia kindred and in 2 hypocholesterolemic blood donors we found a novel PCSK9 mutation in exon 1 (c.202delG) resulting in a truncated peptide (Ala68fsLeu82X). Two familial hypobetalipoproteinemia subjects and 4 hypocholesterolemic blood donors were carriers of the R46L substitution previously reported to be associated with reduced low-density lipoprotein cholesterol as well as other rare amino acid changes (T77I, V114A, A522T and P616L) not found in the other groups examined. CONCLUSIONS: We discovered a novel inactivating mutation as well as some rare nonconservative amino acid substitutions of PCSK9 in white hypocholesterolemic individuals.