A novel disorder caused by defective biosynthesis of N-linked oligosaccharides due to glucosidase I deficiency

Glucosidase I is an important enzyme in N-linked glycoprotein processing, r emoving specifically distal alpha-1,2-linked glucose from the Glc(3)Man(9)G lcNAc(2) precursor after its en bloc transfer from dolichyl diphosphate to a nascent polypeptide chain in the endoplasmic reticulum. We have identifie d a glucosidase I defect in a neonate with severe generalized hypotonia and dysmorphic features. The clinical course was progressive and was character ized by the occurrence of hepatomegaly, hypoventilation, feeding problems, seizures, and fatal outcome at age 74 d, The accumulation of the tetrasacch aride Glc(alpha 1-2)Glc(alpha 1-3)Glc(alpha 1-3)Man in the patient's urine indicated a glycosylation disorder, Enzymological studies on liver tissue a nd cultured skin fibroblasts revealed a severe glucosidase I deficiency. Th e residual activity was <3% of that of controls, Glucosidase I activities i n cultured skin fibroblasts from both parents were found to be 50% of those of controls. Tissues from the patient subjected to SDS-PAGE followed by im munoblotting revealed strongly decreased amounts of glucosidase I protein i n the homogenate of the liver, and a less-severe decrease in cultured skin fibroblasts. Molecular studies showed that the patient was a compound heter ozygote for two missense mutations in the glucosidase I gene: (1) one allel e harbored a G-->C transition at nucleotide (nt) 1587, resulting in the sub stitution of Arg at. position 486 by Thr (R486T), and (2) on the other alle le a T-->C transition at nt 2085 resulted in the substitution of Phe at pos ition 652 by Leu (F652L), The mother was heterozygous for the G-->C transit ion, whereas the father was heterozygous for the T-->C transition, These ba se changes were not seen in 100 control DNA samples, A causal relationship between the alpha-glucosidase I deficiency and the disease is postulated.