The role of the GlcNAc(beta)1,2Man(alpha)- moiety in mammalian development. Null mutations of the genes encoding UDP-N-acetylglucosamine:alpha-3-D-mannoside beta-1,2-N-acetylglucosaminyltransferase I and UDP-N-acetylglucosamine:alpha-D-mannoside beta-1,2-N-acetylglucosaminyltransferase I.2 cause embryonic lethality and congenital muscular dystrophy in mice and men, respectively.

The GlcNAc(beta)1,2Man(alpha)- moiety can be synthesized by at least two mammalian glycosyltransferases, UDP-GlcNAc:alpha-3-D-mannoside beta1,2-N-acetylglucosaminyltransferase I (GnT I, EC 2.4.1.101) and UDP-GlcNAc:alpha-D-mannoside beta1,2-N-acetylglucosaminyltransferase I.2 (GnT I.2). GnT I adds a GlcNAc residue in beta1,2 glycosidic linkage to the Man(alpha)1,3 arm of the N-glycan core to initiate the biosynthesis of hybrid and complex N-glycans. GnT I.2 can add GlcNAc in beta1,2 linkage to any alpha-linked terminal Man residue but has a strong preference for the Man(alpha)1-O-Thr- moiety which occurs in alpha-dystroglycan and other O-mannosylated glycoproteins. Mouse embryos lacking a functional GnT I gene (MgatI) were unable to synthesize complex N-glycans and none survived past 10.5 days after fertilization. The embryos showed multisystemic defects in various morphogenic processes such as neural tube formation, vascularization and the determination of left-right body plan asymmetry. Six human patients with muscle-eye-brain disease (MEB) were recently shown to have point mutations in the gene encoding GnT I.2 (MGATI.2). MEB is an autosomal recessive disease characterized by congenital muscular dystrophy, ocular abnormalities, brain malformations and other multisystemic defects. Both the MGATI.2 gene and MEB disease have been mapped to chromosome 1p32-p34. At least one of the biochemical sites affected by the MGATI.2 mutations is probably the interaction between laminin in the extracellular matrix and the peripheral membrane glycoprotein alpha-dystroglycan since this interaction is believed to require the presence of the sialyl(alpha)2,3Gal(beta)1,4GlcNAc(beta)1,2Man(alpha)1-O-Ser/Thr moiety on alpha-dystroglycan. It can be concluded that the GlcNAc(beta)1,2Man(alpha)- moiety is important for mammalian development due to an essential role in two distinct biochemical pathways.