Muscle-eye-brain disease (MEB) is an autosomal recessive disorder characterized by congenital muscular dystrophy, ocular abnormalities, and lissencephaly. Mammalian O-mannosyl glycosylation is a rare type of protein modification that is observed in a limited number of glycoproteins of brain, nerve, and skeletal muscle. Here we isolated a human cDNA for protein O-mannose beta-1,2-N-acetylglucosaminyltransferase (POMGnT1), which participates in O-mannosyl glycan synthesis. We also identified six independent mutations of the POMGnT1 gene in six patients with MEB. Expression of most frequent mutation revealed a great loss of the enzymatic activity. These findings suggest that interference in O-mannosyl glycosylation is a new pathomechanism for muscular dystrophy as well as neuronal migration disorder.

Defects in O-mannosylation of ␣-dystroglycan are thought to cause certain types of congenital muscular dystrophies with neuronal migration disorders. Among these muscular dystrophies, WalkerWarburg syndrome is caused by mutations in the gene encoding putative protein O-mannosyltransferase 1 (POMT1), which is homologous to yeast protein O-mannosyltransferases. However, there is no evidence that POMT1 has enzymatic activity. In this study, we first developed a method to detect protein O-mannosyltransferase activity in mammalian cells. Then, using this method, we showed that coexpression of both POMT1 and POMT2 (another gene homologous to yeast protein O-mannosyltransferases) was necessary for the enzyme activity, but expression of either POMT1 or POMT2 alone was insufficient. The requirement of an active enzyme complex of POMT1 and POMT2 suggests that the regulation of protein O-mannosylation is complex. Further, protein Omannosylation appears to be required for normal structure and function of ␣-dystroglycan in muscle and brain. In view of the potential importance of this form of glycosylation for a number of developmental and neurobiological processes, the ability to assay mammalian protein O-mannosyltransferase activity should greatly facilitate progress in the identification and localization of Omannosylated proteins and the elucidation of their functional roles.