A mutation in the human ortholog of the Saccharomyces cerevisiae ALG6 gene causes carbohydrate-deficient glycoprotein syndrome type-Ic

A mutation in the human ortholog of the Saccharomyces cerevisiae ALG6 gene causes carbohydrate-deficient glycoprotein syndrome type-Ic

^*Institute of Physiology, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland; ^†Institute of Microbiology, Swiss Federal Institute of Technology, Schmelzbergstrasse 7, 8092 Zürich, Switzerland; ^‡Institute of Veterinary Bacteriology, University of Bern, Längassstrasse 122, 3012 Bern, Switzerland; and ^§Laboratory of Neurology, Academic Hospital Nijmegen, Reinier Postlaan 4, 6525 GC Nijmegen, The Netherlands

Edited by Phillips W. Robbins, Boston University, Cambridge, MA, and approved April 21, 1999 (received for review February 23, 1999)

Abstract

Carbohydrate-deficient glycoprotein syndrome (CDGS) represents a class of genetic diseases characterized by abnormal N-linked glycosylation. CDGS patients show a large number of glycoprotein abnormalities resulting in dysmorphy, encephalopathy, and other organ disorders. The majority of CDGSs described to date are related to an impaired biosynthesis of dolichyl pyrophosphate-linked Glc₃Man₉GlcNAc₂ in the endoplasmic reticulum. Recently, we identified in four related patients a novel type of CDGS characterized by an accumulation of dolichyl pyrophosphate-linked Man₉GlcNAc₂. Elaborating on the analogy of this finding with the phenotype of alg5 and alg6 Saccharomyces cerevisiae strains, we have cloned and analyzed the human orthologs to the ALG5 dolichyl phosphate glucosyltransferase and ALG6 dolichyl pyrophosphate Man₉GlcNAc₂ α1,3-glucosyltransferase in four novel CDGS patients. Although ALG5 was not altered in the patients, a C→T transition was detected in ALG6 cDNA of all four CDGS patients. The mutation cosegregated with the disease in a Mendelian recessive manner. Expression of the human ALG5 and ALG6 cDNA could partially complement the respective S. cerevisiae alg5 and alg6 deficiency. By contrast, the mutant ALG6 cDNA of CDGS patients failed to revert the hypoglycosylation observed in alg6 yeasts, thereby proving a functional relationship between the alanine to valine substitution introduced by the C→T transition and the CDGS phenotype. The mutation in the ALG6 α1,3-glucosyltransferase gene defines an additional type of CDGS, which we propose to refer to as CDGS type-Ic.

Footnotes

↵ ¶ To whom reprint requests should be addressed. e-mail: thennet{at}access.unizh.ch.
This paper was submitted directly (Track II) to the Proceedings Office.
Data deposition: The sequences reported in this paper have been deposited in the GenBank database (accession nos. AF102850 and AF102851).
ABBREVIATIONS:

CDGS,

carbohydrate-deficient glycoprotein syndrome;

Man9GlcNAc2-PP-Dol,

dolichyl pyrophosphate-linked Man9GlcNAc2;

EST,

expressed sequence tag;

CPY,

carboxypeptidase Y;

RT,

reverse transcription