Deficiency of the endoplasmic reticulum enzyme dolichyl-phosphate mannose (Dol-P-Man):Man7GlcNAc2-PP-dolichyl mannosyltransferase leads to a new type of congenital disorder of glycosylation, designated type Ig. The patient 1 presented with a multisystemic disorder with microcephaly, developmental retardation, convulsions and dysmorphic signs. The isoelectric focusing pattern of the patient's serum transferrin showed the partial loss of complete N-glycan side chains. In skin fibroblasts from the patient, the activity of Dol-P-Man:Man7GlcNAc2-PP-Dol mannosyltransferase was severely reduced leading to the accumulation of Man7GlcNAc2-PP-Dol, which was transferred to newly synthesized glycoproteins. Sequencing of the Dol-P-Man:Man7GlcNAc2-PP-Dol mannosyltransferase cDNA revealed a compound heterozygosity for two point mutations, leading to the exchange of leucine158 for a proline residue and a premature translation stop with loss of the C-terminal 74 amino acids. The parents were heterozygous for one of the two mutations. Retroviral expression of the wild-type Dol-P-Man:Man7GlcNAc2-PP-Dol mannosyltransferase cDNA in patient's fibroblasts normalized the mannosyltransferase activity.
Abbreviations used: ALG, asparagine-linked glycosylation; CDG, congenital disorder of glycosylation; DMEM, Dulbecco's modified Eagle's medium; Dol-P-Man, dolichyl-phosphate mannose; DTT, dithiothreitol; FCS, fetal calf serum; IEF, isoelectric focusing.
These authors contributed equally to this work.