The Ser219-->Gly dimorphism of the endothelial protein C receptor contributes to the higher soluble protein levels observed in individuals with the A3 haplotype.

The endothelial cell protein C receptor (EPCR) plays an important role in regulating blood coagulation and in activated protein C-mediated anti-inflammatory and antiapoptotic processes. Recent studies reported that there are polymorphisms in the human EPCR gene. One of the polymorphisms (haplotype A3) results in substitution of the Ser at residue 219 with Gly in the transmembrane domain. This haplotype is associated with increased plasma levels of soluble EPCR and is a candidate risk factor for thrombosis. We established stable cell lines expressing either the EPCR A1 (Ser at residue 219) or A3 (Gly at residue 219) haplotype. Both constitutive and PMA-stimulated shedding are five- to sevenfold higher in the A3 cell line than the A1 cell line. We also isolated human umbilical vein endothelial cells (HUVEC) from A1/A1 or A1/A3 origins. PMA-stimulated shedding is fourfold higher in HUVEC derived from A1/A3 origin than from A1/A1 origin. After PMA treatment, the rate of human protein C activation decreased 36% in HUVEC derived from A1/A3 origin, while it only decreased 18% in HUVEC derived from A1/A1 origin. These results indicate that the A3 haplotype does promote cellular shedding in either 293 or endothelial cells and therefore is likely directly contributory to the higher soluble EPCR levels seen in patients carrying this haplotype.