Biochemical characterization of genetic mutations of GPR56 in patients with bilateral frontoparietal polymicrogyria (BFPP)

Bilateral frontoparietal polymicrogyria (BFPP) is a rare genetic disease characterized by cortical malformation associated with GPR56 mutations of frameshift, splicing, and point mutations (Science 303:2033). All the missense point mutations are located in the regions predicted to be exposed at the cell surface, e.g. the N-terminal extracellular domain (ECD), the proteolytic site (GPS), and the extracellular loops of transmembrane domain (TM), implying functionally important interaction among these domains. Wild type GPR56 protein is cleaved at the GPCR protein cleavage site (GPS) and gives rise to two subunits (ECD and TM), which are transported to cell surface. We have shown that GPR56 GPS mutant protein is defective in cleavage and surface localization, while non-GPS mutant proteins are cleaved normally but still defective in surface localization. Furthermore, all the mutant proteins demonstrated different glycosylation pattern from that of wild-type protein. PNGase F and Endo H sensitivity assays suggests that the mutant proteins are trapped in endoplasmic reticulum (ER), preventing them from trafficking to Golgi where further glycosylation modification usually occurs before destination to cell surface. Therefore, the loss-of-function of all these missense mutations is primarily caused by their failure to localize to cell surface.