ARTICLE Oligosaccharyltransferase-Subunit Mutations in Nonsyndromic Mental Retardation Florence Molinari, 1 Franc ¸ois Foulquier, 2,3 Patrick S. Tarpey, 4 Willy Morelle, 3 Sarah Boissel, 1 Jon Teague, 4 Sarah Edkins, 4 P. Andrew Futreal, 4 Michael R. Stratton, 4 Gillian Turner, 5 Gert Matthijs, 2 Jozef Gecz, 6,7 Arnold Munnich, 1 and Laurence Colleaux 1, * Mental retardation (MR) is the most frequent handicap among children and young adults. Although a large proportion of X-linked MR genes have been identified, only four genes responsible for autosomal-recessive nonsyndromic MR (AR-NSMR) have been described so far. Here, we report on two genes involved in autosomal-recessive and X-linked NSMR. First, autozygosity mapping in two sibs born to first-cousin French parents led to the identification of a region on 8p22-p23.1. This interval encompasses the gene N33/TUSC3 encoding one subunit of the oligosaccharyltransferase (OTase) complex, which catalyzes the transfer of an oligosaccharide chain on nascent pro- teins, the key step of N-glycosylation. Sequencing N33/TUSC3 identified a 1 bp insertion, c.787_788insC, resulting in a premature stop codon, p.N263fsX300, and leading to mRNA decay. Surprisingly, glycosylation analyses of patient fibroblasts showed normal N-glycan synthesis and transfer, suggesting that normal N-glycosylation observed in patient fibroblasts may be due to functional compensation. Subsequently, screening of the X-linked N33/TUSC3 paralog, the IAP gene, identified a missense mutation (c.932T/G, p.V311G) in a family with X-linked NSMR. Recent studies of fucosylation and polysialic-acid modification of neuronal cell-adhesion glycoproteins have shown the critical role of glycosylation in synaptic plasticity. However, our data provide the first demonstration that a defect in N-glycosylation can result in NSMR. Together, our results demonstrate that fine regulation of OTase activity is essential for normal cognitive-function development, providing therefore further insights to understand the pathophysiological bases of MR. Introduction Mental retardation (MR), defined as an intelligence quo- tient below 70, is the most frequent handicap in children, affecting 1% to 3% of the general population. 1 Despite recent advances, the causes of nearly 40% of MR remain unclear. Although a considerable number of X-linked MR (XLMR) genes are known, only four genes causing auto- somal-recessive nonsyndromic MR (AR-NSMR) have been identified so far: PRSS12 (MIM 606709), 2 CRBN (MIM 609262), 3 CC2D1A (MIM 610055), 4 and GRIK2 (MIM 138244). 5 These genes are involved in different pathways, namely synaptic proteolysis, regulation of mitochondrial energy metabolism, regulation of I-kB kinase/NF-kB cascade, and induction of long-term potentiation (LTP), underlining the extreme heterogeneity of the pathophysi- ological mechanism involved in these diseases. Glycosylation is an important posttranslational modifica- tion that occurs in every cell and has a significant impact on numerous biological processes. Many defects in the asparagine-linked glycosylation (N-glycosylation) have been identified in congenital disorders of glycosylation (CDG) syndromes. 6,7 Their diagnosis is based on the identi- fication of underglycosylated transferrin, either by isoelectric focusing or mass-spectrometry analyses. Clini- cally, CDG syndromes are characterized by psychomotor re- tardation, ataxia, failure to thrive, dysmorphic features, and coagulopathies. To date, 18 different CDG subtypes have been described: 12 types in group I CDG and six in group II. Group I CDG results in the disrupted synthesis of the oli- gosaccharide chain, whereas group II CDG is defined as defects in the processing of the protein-bound glycan. 6 In eukaryotic cells, the key step of the N-glycosylation is medi- ated by the oligosaccharyltransferase (OTase), an oligomeric membrane-protein complex that catalyzes the transfer of preassembled high-mannose oligosaccharide onto aspara- gine residues of nascent polypeptides entering the lumen of the endoplasmic reticulum. 8 There are at least two mam- malian OTase complexes that differ by the presence of N33/ TUSC3 (MIM 601385) or IAP. These complexes are composed of seven proteins: Ribophorin I (MIM 180470), Ribophorin II (MIM 180490), OST48 (MIM 602202), DAD1 (MIM 600243), STT3 A/B (MIM 601134, MIM 608605), OST4 (MIM 604059), and N33/TUSC3 or IAP, 8 and, to date, no mutation in any of these subunits has ever been highlighted in a human disease. In this article, we show that mutations in two OTase- subunit genes, N33/TUSC3 and IAP (also named MAGT1), the Ost3 and Ost6 Saccharomyces cerevisae human ortho- logs, respectively, result in NSMR. These data expand the spectrum of the CDG syndromes and emphasize the cru- cial role of glycosylation activity in higher brain functions and cognitive development. 1 Laboratoire de Ge ´ne ´ tique et Epige ´ne ´tique des Maladies Me ´taboliques, Neurosensorielles et du De ´veloppement (INSERM U781), Universite ´ Paris Descartes, Ho ˆpital Necker-Enfants Malades, F-75015 Paris, France; 2 Laboratory for Molecular Diagnostics, Center for Human Genetics, University of Leuven, 3000 B- Leuven, Belgium; 3 Unite ´ Mixte de Recherche CNRS/USTL 8576, Glycobiologie Structurale et Fonctionnelle, IFR 147, Universite ´ des Sciences et Technologies de Lille 1, F-59655 Villeneuve d’Ascq, France; 4 Cancer Genome Project, Wellcome Trust Sanger Institute, Wellcome Trust Genome Campus, Hinxton, CB10 1SA Cambridge, UK; 5 The Gold Service, Hunter Genetics and University of Newcastle, New South Wales, NSW 2308, Australia; 6 Department of Genetic Medicine, Women’s and Children’s Hospital, North Adelaide, SA 5005, Australia; 7 Department of Pediatrics and School of Molecular & Biomedical Science, University of Adelaide, Adelaide, SA 5005, Australia *Correspondence: colleaux@necker.fr DOI 10.1016/j.ajhg.2008.03.021. ª2008 by The American Society of Human Genetics. All rights reserved. 1150 The American Journal of Human Genetics 82, 1150–1157, May 2008