GPIHBP1 C89F Neomutation and Hydrophobic C-Terminal Domain G175R Mutation in Two Pedigrees with Severe Hyperchylomicronemia

Context: GPIHBP1 is a new endothelial binding site for lipoprotein lipase (LPL), the key enzyme for intravascular lipolysis of triglyceride-rich lipoproteins (TGRL). We have identified two new missense mutations of the GPIHBP1 gene, C89F and G175R, by systematic sequencing in a cohort of 376 hyperchylomicronemic patients without mutations on the LPL, APOC2, or APOA5 gene. Objective: Phenotypic expression and functional consequences of these two mutations were studied. Design: We performed clinical and genotypic studies of probands and their families. GPIHBP1 functional alterations were studied in CHO pgsA-745 transfected cells. Results: Probands are an adult with a homozygous G175R mutation and a child with a hemizygous C89F neomutation and a deletion of the second allele. C89F mutation was associated with a C14F signal peptide polymorphism on the same haplotype. Both patients had resistant hyperchylomicronemia, low LPL activity, and history of acute pancreatitis. In CHO pgsA-745 cells, both G175R and C14F variants reduce the expression of GPIHBP1 at the cell surface. C89F mutation is responsible for a drastic LPL-binding defect to GPIHBP1. C14F may further potentiate C89F effect. Conclusions: The emergence of hyperchylomicronemia in the generation after a neomutation further establishes a critical role for GPIHBP1 in TGRL physiopathology in humans. Our results highlight the crucial role of C65-C89 disulfide bond in LPL binding by GPIHBP1 Ly6 domain. Furthermore, we first report a mutation of the hydrophobic C-terminal domain that impairs GPIHBP1 membrane targeting.