Functional characterization of two protein C variants causing type I deficiency via cellular degradation or retention.

Hereditary protein C (PC) deficiency is a thrombotic disorder caused by variants in the PC gene (PROC). In this study, we identified a novel variant, p.Leu173Pro (L173P), and a previously reported variant, p.Val241Leu (V241L), in two unrelated Japanese families with venous thrombosis. We investigated the mechanisms by which these variants lead to type I PC deficiency. PC expression vectors were constructed and transiently expressed in human embryonic kidney 293 cells. Cell lysates and culture supernatants were subsequently analyzed by Western blotting, and intracellular trafficking was evaluated. Both PC-L173P and PC-V241L variants exhibited significantly reduced extracellular secretion compared to the wild-type PC. Furthermore, PC-L173P underwent proteasome-mediated intracellular degradation, whereas PC-V241L appeared to accumulate within the endoplasmic reticulum. This study elucidates the mechanism by which type I PC deficiency arises from distinct secretion defects: intracellular degradation and retention. Although both PROC variants result in type I PC deficiency, their differing intracellular fates are likely attributable to the mutation site and the physicochemical properties of the substituted amino acids. These findings underscore the heterogeneity of secretion defects in type I PC deficiency and provide novel insights into its molecular pathophysiology.