Congenital disorder of glycosylation type Ik (CDG-Ik): A defect of mannosyltransferase I

This study describes the discovery of a new inherited disorder of glycosylation named CDG-Ik. CDG-Ik ( congenital disorder of glycoslyation type Ik) is based on a defect of human mannosyltransferase I ( MT-I [MIM 605907]), an enzyme necessary for the elongation of dolichol-linked chitobiose during N-glycan biosynthesis. Mutations in semiconserved regions in the corresponding gene, HMT-1 ( yeast homologue, Alg1), in two patients caused drastically reduced enzyme activity, leading to a severe disease with death in early infancy. One patient had a homozygous point mutation (c.773C-->T, S258L), whereas the other patient was compound heterozygous for the mutations c. 773C-->T and c. 1025A-->C (E342P). Glycosylation and growth of Alg1-deficient PRY56 yeast cells, showing a temperature-sensitive phenotype, could be restored by the human wild-type allele, whereas only slight restoration was observed after transformation with the patients' alleles.