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Human Molecular Genetics, 2002, Vol. 11, No. 19 2331-2339
© 2002 Oxford University Press

ALG12 mannosyltransferase defect in congenital disorder of glycosylation type lg

Claudia E. Grubenmann1,{dagger}, Christian G. Frank2,{dagger}, Susanne Kjaergaard3, Eric G. Berger1, Markus Aebi2 and Thierry Hennet1,*

1Institute of Physiology, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland, 2Institute of Microbiology, Swiss Federal Institute of Technology, Schmelzbergstrasse 7, 8092 Zürich, Switzerland and 3Department of Clinical Genetics, University Hospital of Copenhagen, 9 Blegdamsvej, 2100 Copenhagen, Denmark

Received May 22, 2002; Accepted July 11, 2002

In the endoplasmic reticulum (ER) of eukaryotes, N-linked glycans are first assembled on the lipid carrier dolichyl pyrophosphate. The GlcNAc2Man9Glc3 oligosaccharide is transferred to selected asparagine residues of nascent polypeptides. Defects along the biosynthetic pathway of N-glycans are associated with severe multisystemic syndromes called congenital disorders of glycosylation. Here, we describe a deficiency in the ALG12 ER {alpha}1,6-mannosyltransferase resulting in a novel type of glycosylation disorder. The severe disease was identified in a child presenting with psychomotor retardation, hypotonia, growth retardation, dysmorphic features and anorexia. In the patient's fibroblasts, the biosynthetic intermediate GlcNAc2Man7 oligosaccharide was detected both on the lipid carrier dolichyl pyrophosphate and on newly synthesized glycoproteins, thus pointing to a defect in the dolichyl pyrophosphate–GlcNAc2Man7-dependent ALG12 {alpha}1,6 mannosyltransferase. Analysis of the ALG12 cDNA in the CDG patient revealed compound heterozygosity for two point mutations that resulted in the amino acid substitutions T67M and R146Q, respectively. The impact of these mutations on ALG12 protein function was investigated in the Saccharomyces cerevisiae alg12 glycosylation mutant by showing that the yeast ALG12 gene bearing the homologous mutations T61M and R161Q and the human mutant ALG12 cDNA alleles failed to normalize the growth defect phenotype of the alg12 yeast model, whereas expression of the normal ALG12 cDNA complemented the yeast mutation. The ALG12 mannosyltransferase defect defines a new type of congenital disorder of glycosylation, designated CDG-Ig.

* To whom correspondence should be addressed. Tel: +41 16355080; Fax: +41 16356814; E-mail: thennet{at}access.unizh.ch

{dagger} The authors wish it to be known that, in their opinion, the first two authors should be regarded as joint First Authors.


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