2134

EXTL1

EXTL;exostosin-like glycosyltransferase 1;EXTL1;exostosin-like glycosyltransferase 1

alpha-N-acetylglucosaminyltransferase II|exostoses (multiple)-like 1|exostosin-L|exostosin-like 1|glucuronosyl-N-acetylglucosaminyl-proteoglycan 4-alpha-N- acetylglucosaminyltransferase|glucuronosyl-N-acetylglucosaminyl-proteoglycan 4-alpha-N-acetylglucos

1p36.1

protein-coding

Recently, the EXTL1 gene, a member of the EXT tumor suppressor gene family, has been mapped to 1p36, a chromosome region which is frequently implicated in a wide variety of malignancies, including breast carcinoma, colorectal cancer and neuroblastoma. In this study, we show that the EXTL1 gene is located between the genetic markers D1S511 and D1S234 within 200 kb of the LAP18 gene on chromosome 1p36. 1, a region which has been proposed to harbor a tumor suppressor gene implicated in MYCN-amplified neuroblastomas. In addition, we determined the genomic structure of the EXTL1 gene, revealing that the EXTL1 coding sequence spans 11 exons within a 50-kb region.

The tumor suppressors EXT1 and EXT2 are associated with hereditary multiple exostoses and encode bifunctional glycosyltransferases essential for chain polymerization of heparan sulfate (HS) and its analog, heparin (Hep). Three highly homologous EXT-like genes, EXTL1-EXTL3, have been cloned, and EXTL2 is an alpha1,4-GlcNAc transferase I, the key enzyme that initiates the HS/Hep synthesis. In the present study, truncated forms of EXTL1 and EXTL3, lacking the putative NH2-terminal transmembrane and cytoplasmic domains, were transiently expressed in COS-1 cells and found to harbor alpha-GlcNAc transferase activity. EXTL3 used not only N-acetylheparosan oligosaccharides that represent growing HS chains but also GlcAbeta1-3Galbeta1-O-C2H4NH-benzyloxycarbonyl (Cbz), a synthetic substrate for alpha-GlcNAc transferase I that determines and initiates HS/Hep synthesis. In contrast, EXTL1 used only the former acceptor. Neither EXTL1 nor EXTL3 showed any glucuronyltransferase activity as examined with N-acetylheparosan oligosaccharides. Heparitinase I digestion of each transferase-reaction product showed that GlcNAc had been transferred exclusively through an alpha1,4-configuration. Hence, EXTL3 most likely is involved in both chain initiation and elongation, whereas EXTL1 possibly is involved only in the chain elongation of HS and, maybe, Hep as well. Thus, their acceptor specificities of the five family members are overlapping but distinct from each other, except for EXT1 and EXT2 with the same specificity. It now has been clarified that all of the five cloned human EXT gene family proteins harbor glycosyltransferase activities, which probably contribute to the synthesis of HS and Hep.

Although neuroblastoma is the most common extracranial solid tumour of childhood, little is known about its aetiology. Together with MYCN amplification and chromosome 17q gain, chromosome 1p deletion is one of the most frequently occurring genetic abnormalities in neuroblastoma. Based upon mapping of deletion breakpoints, putative tumour suppressor gene loci have been assigned to the distal part of the short arm of chromosome 1. Recently, the EXTL1 gene was suggested as a candidate neuroblastoma-suppressor gene and to evaluate this hypothesis, we performed 1p deletion analysis and mutation screening of the EXTL1-coding region on DNA from 22 primary neuroblastomas and 21 neuroblastoma cell lines. Deletions of the chromosome region 1p36.1, including the EXTL1 gene, were detected in several neuroblastoma cell lines and primary tumours. EXTL1 mutation screening resulted in the detection of one unclassified variant (Ser28Cys) but could not provide additional evidence of EXTL1 being involved in the aetiology of neuroblastoma.