Abstract Congenital disorders of glycosylation (CDG), formerly known as carbohydrate-deficient glycoprotein syn- drome, represent a family of genetic diseases with vari- able clinical presentations. Common to all types of CDG characterized to date is a defective Asn-linked glycosyla- tion caused by enzymatic defects of N-glycan synthesis. Previously, we have identified a mutation in the ALG6 α1,3 glucosyltransferase gene as the cause of CDG-Ic in four related patients. Here, we present the identification of seven additional cases of CDG-Ic among a group of 35 untyped CDG patients. Analysis of lipid-linked oligo- saccharides in fibroblasts confirmed the accumulation of dolichyl pyrophosphate-Man 9 GlcNAc 2 in the CDG-Ic pa- tients. The genomic organization of the human ALG6 gene was determined, revealing 14 exons spread over 55 kb. By polymerase chain reaction amplification and se- quencing of ALG6 exons, three mutations, in addition to the previously described A333 V substitution, were de- tected in CDG-Ic patients. The detrimental effect of these mutations on ALG6 activity was confirmed by comple- mentation of alg6 yeast mutants. Haplotype analysis of CDG-Ic patients revealed a founder effect for the ALG6 allele bearing the A333 V mutation. Although more than 80% of CDG are type Ia, CDG-Ic may be the second most common form of the disease. Introduction Congenital disorders of glycosylation (CDG), previously known as carbohydrate-deficient glycoprotein syndrome, are a family of genetic diseases related to defective Asn- linked glycosylation (Jaeken et al. 1993; Krasnewich and Gahl 1997). To date, six defects causing CDG have been identified at the molecular level. Mutations in the cytoso- lic enzymes phosphomannomutase-2 (PMM2, CDG-Ia; Matthijs et al. 1997) and phosphomannose isomerase (PMI, CDG-Ib; Jaeken et al. 1998; Niehues et al. 1998) lead to hypoglycosylation by lowering the intracellular mannose-1-phosphate pool; this is paralleled by low lev- els of the donor molecules GDP-mannose and dolichyl phosphate-mannose. Deficiency in the endoplasmic-retic- ulum-located α1,3 glucosyltransferase (ALG6, CDG-Ic; Imbach et al. 1999; Körner et al. 1998) and α1,3 manno- syltransferase (ALG3, CDG-Id; Körner et al. 1999) impair the assembly of the lipid-linked oligosaccharide (LLO) core. Mutations in the dolichyl phosphate-mannose syn- thase-1 gene (DPM1, CDG-Ie; Imbach et al. 2000; Kim et al. 2000) lead to a shortage of the donor molecule dolichyl phosphate-mannose, which is required in several glycosy- lation pathways (Doucey et al. 1998; Sugiyama et al. 1991). Whereas the disorders affecting the assembly of the N-glycan core are referred to as CDG-I, defects along the N-glycan processing pathways are grouped as subtypes of CDG-II. Therefore, the deficiency of β1–2 N-acetylglucosaminyltransferase-II, which decreases the structural complexity of N-glycans by impairing the for- Timo Imbach · Stephanie Grünewald · Barbara Schenk · Patricie Burda · Els Schollen · Ron A. Wevers · Jaak Jaeken · Johannis B.C. de Klerk · Eric G. Berger · Gert Matthijs · Markus Aebi · Thierry Hennet Multi-allelic origin of congenital disorder of glycosylation (CDG)-Ic Hum Genet (2000) 106 : 538–545 Digital Object Identifier (DOI) 10.1007/s004390000293 Received: 11 January 2000 / Accepted: 20 March 2000 / Published online: 19 April 2000 ORIGINAL INVESTIGATION T. Imbach · Eric G. Berger · T. Hennet (✉) Institute of Physiology, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland e-mail: thennet@access.unizh.ch, Tel.: +41 1 635 5080, Fax: +41 1 635 6814 S. Grünewald · E. Schollen · G. Matthijs Center for Human Genetics, Gasthuisberg, Herestraat 49, 3000 Leuven, Belgium B. Schenk · P. Burda · M. Aebi Institute of Microbiology, Swiss Federal Institute of Technology, Schmelzbergstrasse 7, 8092 Zürich, Switzerland R. A. Wevers Laboratory of Neurology, Academic Hospital Nijmegen, Reinier Postlaan 4, 6525 GC Nijmegen, The Netherlands J. Jaeken Department of Pediatrics, Gasthuisberg, Herestraat 49, 3000 Leuven, Belgium J. B.C. de Klerk Department of Pediatrics, Sophia Childrens Hospital, Rotterdam, The Netherlands Present address: P. Burda Howard Hughes Medical Institute, Division of Cellular and Molecular Medicine, University of California San Diego, San Diego, Calif., USA © Springer-Verlag 2000