A Novel Splicing Variant of Proprotein Convertase Subtilisin/Kexin Type 9

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is the most recently identified member of the proprotein convertase family. Genetic and cell biology studies have suggested a critical role of PCSK9 in regulating low-density lipoprotein receptor (LDLR) protein levels and thus modulating plasma LDL cholesterol. Recent data on the molecular basis for PCSK9 action support the model in which PCSK9 is self-cleaved, secreted, and tightly bound to the EGF-A repeat of LDLR extracellular domain. PCSK9 binding to LDLR is essential for the ensuing receptor-mediated endocytosis and is speculated to lock LDLR in a specific conformation that favors degradation in lysosomal compartment instead of recycling back to plasma membrane. We report here a novel human PCSK9 splicing variant, which we named PCSK9sv. PCSK9sv had an in-frame deletion of the eighth exon of 58 amino acids and was expressed in multiple tissues, including liver, small intestine, prostate, uterus, brain, and adipose tissue. Unlike wild-type PCSK9, which is secreted, PCSK9sv expressed in human embryonic kidney HEK293 cells failed to process the prosegment intracellularly and thus was not secreted into the medium. Examination of potential functions revealed that PCSK9sv did not change the LDLR protein levels. Two mutations that have been reported in humans with the associated changes in plasma LDL cholesterol were within exon 8, and thus the expression and function of the two mutants were studied. Both N425S and A443T mutants were processed normally, secreted, and reduced LDLR levels. However, the physiological function of this novel splicing variant of PCSK9 has yet to be determined.