Copyright © 2004 The American Society of Human Genetics. All rights reserved.
The American Journal of Human Genetics, Volume 75, Issue 1, 146-150, 1 July 2004
doi:10.1086/422367
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Identification and Functional Analysis of a Defect in the Human ALG9 Gene: Definition of Congenital Disorder of Glycosylation Type IL
Christian G. Frank*, Wafaa Eyaid3, Eric G. Berger2, Markus Aebi1, 1, *, Claudia E. Grubenmann2 and Thierry Hennet2, 
, 
1 Institute of Microbiology, Swiss Federal Institute of Technology, Zürich, Riyadh
2 Institute of Physiology, University of Zürich, Zürich, Riyadh
3 Department of Pediatrics, King Abdul Aziz Medical City, Riyadh
Address for correspondence and reprints: Dr. Thierry Hennet, Institute of Physiology, University of Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland* These two authors contributed equally to this work.
Abstract
Defects of lipid-linked oligosaccharide assembly lead to alterations of N-linked glycosylation known as “type I congenital disorders of glycosylation” (CDG). Dysfunctions along this stepwise assembly pathway are characterized by intracellular accumulation of intermediate lipid-linked oligosaccharides, the detection of which contributes to the identification of underlying enzymatic defects. Using this approach, we have found, in a patient with CDG, a deficiency of the ALG9 α1,2 mannosyltransferase enzyme, which causes an accumulation of lipid-linked-GlcNAc2Man6 and -GlcNAc2Man8 structures, which was paralleled by the transfer of incomplete oligosaccharides precursors to protein. A homozygous point-mutation 1567G→A (amino acid substitution E523K) was detected in the ALG9 gene. The functional homology between the human ALG9 and Saccharomyces cerevisiae ALG9, as well as the deleterious effect of the E523K mutation detected in the patient with CDG, were confirmed by a yeast complementation assay lacking the ALG9 gene. The ALG9 defect found in the patient with CDG—who presented with developmental delay, hypotonia, seizures, and hepatomegaly—shows that efficient lipid-linked oligosaccharide synthesis is required for proper human development and physiology. The ALG9 defect presented here defines a novel form of CDG named “CDG-IL.”
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