Exostosin-1 Gene Research Articles

A Young Boy with Multiple Bony Overgrowths

Hereditary multiple exostoses is a rare autosomal dominant pediatric disorder with an incidence of about 1:50000 characterized by multiple cartilage-capped bony protuberances, called osteochondromas or exostoses, projecting from the metaphyses of long bones. It is caused by loss of function mutations in exostosin-1 and exostosin-2 genes that encode glycosyltransferase enzymes involved in the synthesis of heparan sulfate which has fundamental role in extracellular matrix formation during bone development. It commonly presents with compressive symptoms due to bony overgrowth involving all bones except calvarium and rarely transformed into malignancy. No definite treatment is available, but careful screening of these exostoses with timely referral to respective surgeon prevents long term complications and improves quality of life.
 J Enam Med Col 2019; 9(1): 60-63

Increased EXT1 gene copy number correlates with increased mRNA level predicts short disease-free survival in hepatocellular carcinoma without vascular invasion.

Exostosin-1 (EXT1) has been demonstrated to participate in the progression of many cancers. However, it has not been previously described in patients with hepatocellular carcinoma (HCC) without vascular invasion. In this study, we got the accurate data of EXT1 mRNA Z-score from the CBio data portal of The Cancer Genome Atlas (TCGA), which was used to express the level of EXT1 gene expression. We analyzed the EXT1 gene expression between HCC and normal liver tissue and compared the clinical significance of tumor tissue's EXT1 gene expression of HCC patients without vascular invasion based on data from TCGA database. The association between EXT1 gene expression and disease-free survival (DFS) was further analyzed. EXT1 gene copy number was also analyzed in this study. Univariate and multivariate analyses showed that high EXT1 gene expression group was significantly poorer than that of the low EXT1 gene expression group (P = .004). In addition, EXT1 gene expression was positively associated with α-fetoprotein (AFP), which is a well-known marker for HCC. There was a significant positive correlation between EXT1 copy number and upregulated EXT1 gene (P < .0001). In conclusion, upregulation of EXT1 could be an important indicator to the short DFS of HCC patients without vascular invasion. EXT1 gene copy number amplification is one of the mechanisms underlying the upregulation of EXT1.

A de novo mutation in the EXT2 gene associated with osteochondromatosis in a litter of American Staffordshire Terriers.

BackgroundWe aimed to identify mutations associated with osteochondromatosis in a litter of American Staffordshire Terrier puppies.HypothesisWe hypothesized that the associated mutation would be located in a gene that causes osteochondromatosis in humans.AnimalsA litter of 9 American Staffordshire puppies, their sire and dam, 3 of 4 grandparents, 26 healthy unrelated American Staffordshire Terriers, and 154 dogs of 27 different breeds.MethodsWhole genome sequencing was performed on the proband, and variants were compared against polymorphisms derived from 154 additional dogs across 27 breeds, as well as single nucleotide polymorphism database 146. One variant was selected for follow‐up sequencing. Parentage and genetic mosaicism were evaluated across the litter.ResultsWe found 56,301 genetic variants unique to the proband. Eleven variants were located in or near the gene exostosin 2 (EXT2), which is strongly associated with osteochondromatosis in humans. One heterozygous variant (c.969C > A) is predicted to result in a stop codon in exon 5 of the gene. Sanger sequencing identified the identical mutation in all affected offspring. The mutation was absent in the unaffected offspring, both parents, all available grandparents, and 26 healthy unrelated American Staffordshire Terriers.Conclusions and Clinical ImportanceThese findings represent the first reported mutation associated with osteochondromatosis in dogs. Because this mutation arose de novo, the identical mutation is unlikely to be the cause of osteochondromatosis in other dogs. However, de novo mutations in EXT2 are common in humans with osteochondromatosis, and by extension, it is possible that dogs with osteochondromatosis could be identified by sequencing the entire EXT2 gene.

Identification of mutations in EXT1 and EXT2 genes in six Chinese families with multiple osteochondromas.

The aim of the present study was to identify mutations of major causative genes in six unrelated Chinese families with multiple osteochondromas (MO). Radiographic examinations and genetic analyses were performed in 8patients exhibiting typical features of MO. Analysis was also performed on unaffected members of the six families and 250healthy volunteers. Radiographies of the patients revealed multiple exostoses in the cartilage of long bones. A total of five different mutations were identified, one in exostosin‑1 (EXT1) and four in exostosin‑2 (EXT2). Two novel mutations were detected in EXT2: A missense mutation, c.1385G>A, in exon 8, resulting in p.Trp462X; and a splice site mutation, c.725+1G>C, which consisted of a heterozygous guanine‑to‑cytosine transition at nucleotide 725+1 in intron3. Three common EXT mutations were also detected: c.1036C>T in exon5 of EXT2 resulting in p.Gln346X; c.1299C>A in exon8 of EXT2 resulting in p.Phe433Leu; and c.1038A>T in exon 2 of EXT1 resulting in p.Arg346Ser. In conclusion, the present study identified a novel missense mutation (c.1385G>A) in exon8 and a splicing mutation (c.725+1G>C) in intron3 of the EXT2 gene, which are responsible for MO in certain Chinese patients. The findings are useful for expanding the database of known EXT2 mutations and understanding the genetic basis of MO in Chinese patients, which may improve genetic counseling and the prenatal diagnosis of MO.

Large-scale mutational analysis in the EXT1 and EXT2 genes for Japanese patients with multiple osteochondromas.

BackgroundMultiple osteochondroma (MO) is an autosomal dominant skeletal disorder characterized by the formation of multiple osteochondromas, and exostosin-1 (EXT1) and exostosin-2 (EXT2) are major causative genes in MO. In this study, we evaluated the genetic backgrounds and mutational patterns in Japanese families with MO.ResultsWe evaluated 112 patients in 71 families with MO. Genomic DNA was isolated from peripheral blood leucocytes. The exons and exon/intron junctions of EXT1 and EXT2 were directly sequenced after PCR amplification. Fifty-two mutations in 47 families with MO in either EXT1 or EXT2, and 42.3 % (22/52) of mutations were novel mutations. Twenty-nine families (40.8 %) had mutations in EXT1, and 15 families (21.1 %) had mutations in EXT2. Interestingly, three families (4.2 %) had mutations in both EXT1 and EXT2. Twenty-four families (33.8 %) did not exhibit mutations in either EXT1 or EXT2. With regard to the types of mutations identified, 59.6 % of mutations were inactivating mutations, and 38.5 % of mutations were missense mutations.ConclusionsWe found that the prevalence of EXT1 mutations was greater than that of EXT2 mutations in Japanese MO families. Additionally, we identified 22 novel EXT1 and EXT2 mutations in this Japanese MO cohort. This study represents the variety of genotype in MO.Electronic supplementary materialThe online version of this article (doi:10.1186/s12863-016-0359-4) contains supplementary material, which is available to authorized users.

Deletion of exon 8 from the EXT1 gene causes multiple osteochondromas (MO) in a family with three affected members.

Multiple osteochondromas (also called hereditary multiple exostoses) is an autosomal dominant disorder characterized by multiple cartilaginous tumors, which are caused by mutations in the genes for exostosin-1 (EXT1) and exostosin-2 (EXT2). The goal of this study was to elucidate the genetic alterations in a family with three affected members. Isolation of RNA from the patients’ blood followed by reverse transcription and PCR amplification of selected fragments showed that the three patients lack a specific region of 90 bp from their EXT1 mRNA. This region corresponds to the sequence of exon 8 from the EXT1 gene. No splice site mutation was found around exon 8. However, long-range PCR amplification of the region from intron 7 to intron 8 indicated that the three patients contain a deletion of 4318 bp, which includes exon 8 and part of the flanking introns. There is evidence that the deletion was caused by non-homologous end joining because the breakpoints are not located within a repetitive element, but contain multiple copies of the deletion hotspot sequence TGRRKM. Exon 8 encodes part of the active site of the EXT1 enzyme, including the DXD signature of all UDP-sugar glycosyltransferases. It is conceivable that the mutant protein exerts a dominant negative effect on the activity of the EXT glycosyltransferase since it might interact with normal copies of the enzyme to form an inactive hetero-oligomeric complex. We suggest that sequencing of RNA might be superior to exome sequencing to detect short deletions of a single exon.Electronic supplementary materialThe online version of this article (doi:10.1186/s40064-016-1695-6) contains supplementary material, which is available to authorized users.

Genetic screening of EXT1 and EXT2 in Cypriot families with hereditary multiple osteochondromas.

The purpose of this study was to perform genetic screening of the exostosin 1 (EXT1) and exostosin 2 (EXT2) genes in Cypriot patients with a clinical diagnosis of hereditary multiple osteochondromas (HMO). Initially, mutation analysis of the EXT1 gene was performed by Sanger sequencing. When no point mutation was identified in EXT1, EXT2 analysis was performed. When no sequence variant was identified in either of the candidate genes, array-comparative genomic hybridization (CGH) was implemented to detect any large copy number changes (CNCs). In total, three point mutations and a large deletion were identified in EXT1: the c.2101C>T(p.Arg701*) mutation and the ∼0.16-Mb deletion removing exons 2–11, which were previously reported whereas the c.486_489delCAGA(p.Asp162Glyfs*12) and the c.868G>T(p.Glu290*) mutations which were novel. It is the first report on genetic screening of five HMO patients of Cypriot descent. The observations presented provide additional evidence for the variability in phenotypic expression and the mutational spectrum of this disorder. The term HMO, or hereditary multiple exostoses (HME), MIM: 133700 and MIM: 133701 respectively is used to describe a genetic disorder characterized by the development of multiple, circumscript, occasionally painful and usually symmetric bony protuberances called osteochondromas (benign cartilaginous tumours which most frequently grow outward from the juxtaphyseal region of the long bones or

Identification of a rare case of intra-articular osteochondroma manifesting as three loose bodies in a patient with hereditary multiple osteochondromas: A case report.

Hereditary multiple osteochondromas (HMO) is an autosomal dominant bone disorder characterised by the presence of multiple benign cartilage-capped tumours. Exostosin-1 (EXT1) and EXT2 are the major morbigenous genes associated with HMO, mutations in which are responsible for 90% of all HMO cases. In patients with HMO, osteochondromas arise adjacent to the metaphysis and typically remain in the metaphyseal region of the long bones. Therefore, it is rare for osteochondromas to be identified intra-articularly, although they may manifest as loose bodies. The present study describes a rare case of HMO manifesting as limited flexing range in the right knee joint of a 27-year-old male patient. Computed tomography and magnetic resonance imaging (MRI) revealed three intra-articular osteochondromas located in the intercondylar fossa of the patient's right knee. The intra-articular osteochondromas and protuberant extra-articular osteochondromas around the right knee were resected, resulting in improved right knee function and no postoperative recurrence. Pathological analysis revealed that the intra-articular osteochondromas had a thinner cartilage cap layer than the extra-articular osteochondromas. In addition, genetic analysis of the patient and the patient's mother was conducted. From this, it was determined that a nonsense mutation, c.115G>T (p.E39X) in exon 1 of the EXT1 gene, was the cause of HMO in this case. Thus, it is proposed that osteochondromas with a pedicle within the knee, may tear and become loose intra-articular bodies, resulting in limited joint function and thereby contributing to the progression of HMO.

Somatic loss of an EXT2 gene mutation during malignant progression in a patient with hereditary multiple osteochondromas

Multiple osteochondromas (MO) is an autosomal-dominant skeletal disorder caused by mutations in the exostosin-1 (EXT1) or exostosin-2 (EXT2) genes. In this study, we report the analysis of the mutational status of the EXT2 gene in tumor samples derived from a patient affected by hereditary MO, documenting the somatic loss of the germline mutation in a giant chondrosarcoma and in a rapidly growing osteochondroma. The sequencing of all exons and exon-intron junctions of the EXT1 and EXT2 genes from blood DNA of the proband did not reveal any mutation in the EXT1 gene but did demonstrate the presence of the transition point mutation c.67C > T in the EXT2 gene, determining the introduction of a stop codon in the coding sequence (p.Arg23*). A mutational analysis of other members of the family and the presence of osteochondromas in the metaphysis of long bones confirmed the diagnosis of hereditary multiple osteochondromas. Direct sequencing from DNA extracted from different sites of two tumor samples (a small rapidly growing osteochondroma and a giant peripheral secondary chondrosarcoma, each located at different chondrocostal junctions) revealed the loss of the germline EXT2 mutation. Analysis of microsatellite polymorphic markers in the 11p region harboring the EXT2 gene did not reveal any loss of heterozygosity. This observation supports a recent model of sarcomagenesis in which osteochondroma cells bear EXT homozygous inactivation, whereas chondrosarcoma-initiating cells are EXT-expressing cells.

Identification of a novel EXT1 mutation in patients with hereditary multiple exostosis by exome sequencing.

Hereditary multiple exostosis (HME) is an autosomal inherited skeletal disease whose etiology is not fully understood. To further understand the genetic spectrum of the disease, we analyzed a five-generation Chinese family with HME that have observable inheritance. Exome sequencing was performed on three HME individuals and three unaffected individuals from the family. A downstream study confirmed a new C deletion at codon 442 on exon 5 of the exostosin-1 (EXT1) gene as the only pathogenic site which generated a stop codon and caused the truncation of the protein. We rediscovered the deletion in other affected individuals but not in the unaffected individuals from the family. Upon immunohistochemistry assay, we found that the EXT1 protein level of the patients with the novel mutation in our study was less than the level in the patients without the EXT1 mutation from another unrelated family. For a deeper understanding, we analyzed the mutation spectrum of the EXT1 gene. The present study should facilitate a further understanding of HME.

Multiple heriditary exostoses in a family for three generation of Indian origin with review of literature.

Multiple hereditary exostoses (MHE) are an autosomal dominant disorder, consisting of multiple cartilage capped bone tumour arising from the metaphysis of long tubular bones. Mutations are seen in Exostosin-1 and Exostosin-2 genes. We present a family of MHE for three generations. The index case was a 10-year-old male presented with multiple exostoses in hand, forearm, leg, right knee and chest. Family history revealed similar complaints in younger brother, father, paternal uncle, paternal aunt & her daughter and grandfather.

Impact of variants of the EXT2 gene on Type 2 diabetes and its related traits in the Chinese han population

Purpose/Aim of the study: Exostosin 2 (EXT2) is involved in early pancreatic development and the regulation of insulin synthesis. In this study, we aim to evaluate the contribution of EXT2 to the genetic pathogenesis of type 2 diabetes and its related traits in the Chinese population. Materials and methods: A case–control study in a Chinese Han population was conducted that included 4766 patients with type 2 diabetes and 4596 control subjects from 14 different regions of China. Three single nucleotide polymorphism (SNP), rs3740878, rs11037909 and rs1113132, in the EXT2 gene were genotyped using the Illumina GoldenGate Genotyping assay. Results: After adjusting for sex, age and body mass index, logistic regression analysis revealed that the EXT2 gene had no association with type 2 diabetes using an additive genetic model [rs3740878 (Odds Ratio (OR) = 0.996, 95% confidence interval (CI) 0.928–1.069, p = 0.910), rs11037909 (OR = 1.003, 95%CI 0.933–1.078, p = 0.931), and rs1113132 (OR = 0.993, 95% CI 0.925–1.065, p = 0.842)]. None of these SNPs were associated with beta cell function as determined using the baseline disposition index, early phase insulin secretion and Oral Glucose Tolerance Test (OGTT) total disposition index. Conclusions: Our study suggests that the EXT2 gene might not have a major role in the development of type 2 diabetes in the Chinese population.

Lack of replication of common EXT2 gene variants with susceptibility to type 2 diabetes in Lebanese Arabs

ObjectiveRecent genome-wide association studies and replication analyses have reported the association of variants of the exostosin-2 (EXT2) gene and risk of type 2 diabetes mellitus (T2DM) in some populations, but not in others. This study investigated the associations of EXT2 variants rs1113132, rs3740878 and rs11037909 with T2DM in a Lebanese Arab population. MethodsThis case-control study involved 995 T2DM patients and 1076 control subjects. Genotyping was done by the allelic exclusion method. ResultsWhile minor allele frequencies (MAFs) of rs11037909 (P=0.028) and rs3740878 (P=0.048), but not rs1113132 (P=0.841), were higher in patients, this was lost after correcting for multiple testing. Apart from EXT2 rs1113132, which was marginally associated with T2DM in the additive model (P=0.054), but not after adjustment for covariates, none of the tested EXT2 SNPs were associated with T2DM in any of the genetic models tested. However, variable associations of EXT2 variants with T2DM were noted according to BMI status. While the three tested EXT2 variants were not associated with T2DM in obese subjects, rs1113132 and rs11037909, but not rs3740878, were associated with T2DM in non-obese subjects. Meta-analysis revealed a significant association of rs11037909 and a marginal association of rs3740878 with T2DM in the fixed model. Using a common (GTA) haplotype as reference, three-locus (rs1113132/rs11037909/rs3740878) haplotype analysis demonstrated no association between any of the EXT2 haplotypes with T2DM, not even before correcting for multiple testing. ConclusionThis study demonstrated no association of rs1113132, rs3740878 and rs11037909 EXT2 variants with T2DM.

Novel and recurrent mutations in the EXT1 and EXT2 genes in Chinese kindreds with multiple osteochondromas

Multiple osteochondromas (MO) is an autosomal dominant hereditary disorder caused by heterozygous germline mutations in the exostonsin-1 (EXT1) or exostosin-2 (EXT2) genes. In this study, we screened mutations in the EXT1/EXT2 genes in four Chinese MO kindreds by direct sequencing. Three point mutations were detected, including a nonsense mutation in the EXT2 gene (c.544C > T) and two splice site mutations in the EXT1 and EXT2 genes, respectively (EXT1: c.1883 + 1G > A and EXT2: c.1173 + 1G > T). Although splice site mutations constitute at least 10% of all mutations that cause MO, there has been limited research on their pathogenic effect on RNA processing due to poor availability of patient RNA samples. In this study, ex vivo and in vivo splicing assays were used to investigate the effect of EXT1 and EXT2 mutations on aberrant splicing at the mRNA level. Our results indicate that identified splice site mutations can cause either cryptic splice site usage or exon skipping.

A Novel Nonsense Mutation of the EXT1 Gene in an Argentinian Patient with Multiple Hereditary Exostoses

Multiple hereditary exostoses (MHE), also known as multiple osteochondromatosis, is an autosomal-dominant O-linked glycosylation disorder recently classified as EXT1/EXT2-CDG in the congenital disorder of glycosylation (CDG) nomenclature1. MHE is characterized by the presence of multiple cartilage-capped tumors, called “osteochondromas,” which usually develop in the juxta-epiphyseal regions of the long bones. The prevalence of MHE is estimated at 1:50,000 in the general population2,3. The Online Mendelian Inheritance in Man (OMIM) database classified it as either 133700 or 133701, according to whether the mutations occurred in the EXT1 or the EXT2 gene. These genes are located at 8q24 and 11p11-11p12, respectively, and they encode the co-polymerases responsible for heparan sulfate biosynthesis. EXT1 and EXT2 are tumor suppressor genes of the EXT gene family. The EXT1 gene contains eleven exons with a coding region of 2238 base pairs (bp), and the EXT2 gene contains sixteen exons with a coding region of 2154 bp4-7. These genes encode two glycosyltransferases involved in heparan sulfate biosynthesis, exostosin-1 (EXT1) (EC2.4.1.224) and exostosin-2 (EXT2) (EC2.4.1.225), whose impairment leads to the formation of exostoses5,8-10. Inactivating mutations (nonsense, frameshift, and splice site mutations) in EXT1 and EXT2 genes represent the majority of mutations that cause MHE. An overview of the reported variants is provided by the online Multiple Osteochondroma Mutation Database11. The most important complication of MHE is the malignant transformation of osteochondroma to chondrosarcoma, which is estimated to occur in 0.5% to 5% of patients7. Chondrosarcomas arise de novo (primary) or as a result of a preexisting cartilage lesion (secondary). The biological aggressiveness of chondrosarcomas can be predicted by means of a histological grading system (grade I to grade III), based on three parameters: cellularity, degree of nuclear atypia, …

Identification and functional characterization of the human EXT1 promoter region

Background Mutations in Exostosin-1 ( EXT1) or Exostosin-2 ( EXT2) cause the autosomal dominant disorder multiple osteochondromas (MO). This disease is mainly characterized by the appearance of multiple cartilage-capped protuberances arising from children's metaphyses and is known to display clinical inter- and intrafamilial variations. EXT1 and EXT2 are both tumor suppressor genes encoding proteins that function as glycosyltransferases, catalyzing the biosynthesis of heparan sulfate. At present, however, very little is known about the regulation of these genes. Two of the most intriguing questions concerning the pathogenesis of MO are how disruption of a ubiquitously expressed gene causes this cartilage-specific disease and how the clinical intrafamilial variation can be explained. Since mutations in the EXT1 gene are responsible for ~ 65% of the MO families with known causal mutation, our aim was to isolate and characterize the EXT1 promoter region to elucidate the transcriptional regulation of this tumor suppressor gene. Methods In the present study, luciferase reporter gene assays were used to experimentally confirm the in silico predicted EXT1 core promoter region. Subsequently, we evaluated the effect of single nucleotide polymorphisms (SNP's) on EXT1 promoter activity and transcription factor binding using luciferase assays, electrophoretic mobility shift assays (EMSA), and enzyme-linked immunosorbent assays (ELISA). Finally, a genotype–phenotype study was performed with the aim to identify one or more genetic modifiers influencing the clinical expression of MO. Results Transient transfection of HEK293 cells with a series of luciferase reporter constructs mapped the EXT1 core promoter at approximately − 917 bp upstream of the EXT1 start codon, within a 123 bp region. This region is conserved in mammals and located within a CpG-island containing a CAAT- and a GT-box. A polymorphic G/C-SNP at − 1158 bp (rs34016643) was demonstrated to be located in a USF1 transcription factor binding site, which is lost with the presence of the C-allele resulting in a ~ 56% increase in EXT1 promoter activity. A genotype–phenotype study was suggestive for association of the C-allele with shorter stature, but also with a smaller number of osteochondromas. Conclusions We provide for the first time insight into the molecular regulation of EXT1. Although a larger patient population will be necessary for statistical significance, our data suggest the polymorphism rs34016643, in close proximity of the EXT1 promoter, to be a potential regulatory SNP, which could be a primary modifier that might explain part of the clinical variation observed in MO patients.

한우 Exostosin-1 유전자의 SNP 탐색 및 경제형질 관련성 분석

The aim of this study was to identify the polymorphism on exostosin-1 (EXT1) gene and to associate with economic traits in Hanwoo (Korean cattle). We sequenced for detection of single nucleotide polymorphism (SNP) with 24 unrelated individuals and identified four SNPs (T272196A, C272359T, G290964A and A302092G). Relationship between the genotypes of 583 Hanwoo individuals by PCR-RFLP and economic traits were analyzed by general linear model. In EXT1 gene, there were four SNPs associated with economic traits such as eye muscle area breeding value, marbling score breeding value, backfat and thickness breeding value (p<0.05 to p<0.01). In conclusion, this study indicates an important role of EXT1 gene in determining the meat quality or economic characteristics in Hanwoo.

Differentiation-induced loss of heparan sulfate in human exostosis derived chondrocytes

An exostosis or osteochondroma is an aberrant bony growth occurring next to the growth plate either as an isolated growth abnormality or as part of the Hereditary Multiple Exostosis (HME) syndrome. Mutations in either exostosin 1 (EXT1) or exostosin 2 (EXT2) gene cause the HME syndrome and also some isolated osteochondromas. The EXT1 and EXT2 genes are glycosyltransferases that function as hetero-oligomers in the Golgi to add repeating glycosaminoglycans (GAGs) to heparan sulfate (HS) chains. Previously, we demonstrated that HS is markedly diminished in the exostosis cartilage cap and that the HS proteoglycan, perlecan, has an abnormal distribution in these caps. The present studies were undertaken to evaluate which chondrocyte-specific functions are associated with diminished HS synthesis in human chondrocytes harboring either EXT1 or EXT2 mutations. Systematic evaluation of exostosis cartilage caps and chondrocytes, both in vitro and in vivo, suggests that chondrocyte-specific cell functions account for diminished HS levels. In addition, we provide evidence that perichondrial cells give rise to chondrocytes that clonally expand and develop into an exostosis. Undifferentiated EXT chondrocytes synthesized amounts of HS similar to control chondrocytes; however, EXT chondrocytes displayed very poor survival in vitro under conditions that promote normal chondrocyte differentiation with high efficiency. Collectively, these observations suggest that loss of one copy of either the EXT1 or EXT2 gene product compromises the perichondrial chondrocytes' ability to differentiate normally and to survive in a differentiated state in vitro. In vivo, these compromised responses may lead to abnormal chondrocyte growth, perhaps from a perichondrial stem cell reserve.

Etiological Point Mutations in the Hereditary Multiple Exostoses Gene EXT1: A Functional Analysis of Heparan Sulfate Polymerase Activity

Hereditary multiple exostoses (HME), a dominantly inherited genetic disorder characterized by multiple cartilaginous tumors, is caused by mutations in members of the EXT gene family, EXT1 or EXT2. The corresponding gene products, exostosin-1 (EXT1) and exostosin-2 (EXT2), are type II transmembrane glycoproteins which form a Golgi-localized heterooligomeric complex that catalyzes the polymerization of heparan sulfate (HS). Although the majority of the etiological mutations in EXT are splice-site, frameshift, or nonsense mutations that result in premature termination, 12 missense mutations have also been identified. Furthermore, two of the reported etiological missense mutations (G339D and R340C) have been previously shown to abrogate HS biosynthesis (McCormick et al. 1998). Here, a functional assay that detects HS expression on the cell surface of an EXT1-deficient cell line was used to test the remaining missense mutant exostosin proteins for their ability to rescue HS biosynthesis in vivo. Our results show that EXT1 mutants bearing six of these missense mutations (D164H, R280G/S, and R340S/H/L) are also defective in HS expression, but surprisingly, four (Q27K, N316S, A486V, and P496L) are phenotypically indistinguishable from wild-type EXT1. Three of these four "active" mutations affect amino acids that are not conserved among vertebrates and invertebrates, whereas all of the HS-biosynthesis null mutations affect only conserved amino acids. Further, substitution or deletion of each of these four residues does not abrogate HS biosynthesis. Taken together, these results indicate that several of the reported etiological mutant EXT forms retain the ability to synthesize and express HS on the cell surface. The corresponding missense mutations may therefore represent rare genetic polymorphisms in the EXT1 gene or may interfere with as yet undefined functions of EXT1 that are involved in HME pathogenesis.