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PubMed2000

Molecular basis of multiple exostoses: mutations in the EXT1 and EXT2 genes.

Hereditary multiple exostoses (EXT) is an autosomal dominant disorder characterized by the formation of exostoses, which are cartilage-capped bony protuberances mainly located on long bones. Two genes, EXT1 and EXT2, and at least one other unidentified gene, are known to be involved in the formation of exostoses. To date, 49 different EXT1 and 25 different EXT2 mutations have been found in EXT patients, and there is evidence that mutations in these two genes are responsible for over 70% of the EXT cases. Among the 49 EXT1 mutations there are 9 nonsense, 21 frameshift, and 5 splice site mutations; 2 in-frame deletions of 1 and 5 amino acids respectively; and 12 missense mutations. For EXT2, 8 nonsense, 11 frameshift, 3 splice site and 3 missense mutations are described. The majority of these mutations are mutations causing loss of function, which is consistent with the presumed tumor suppressor function of the EXT genes.

Exostoses, Multiple Hereditary↗
PubMed1994

Assignment of a second locus for multiple exostoses to the pericentromeric region of chromosome 11.

Hereditary multiple exostoses (EXT) is an autosomal dominant disorder of enchondral bone formation characterized by multiple bony outgrowths (exostoses), with progression to osteosarcoma in a minority of cases. The exclusive involvement of skeletal abnormalities distinguishes EXT from the clinically more complex Langer-Giedion syndrome (LGS), which is associated with deletions at chromosome 8q24. Previously, linkage analysis has revealed a locus for EXT in the LGS region on chromosome 8q24. However, locus heterogeneity was apparent with 30% of the families being unlinked to 8q24. We report on two large pedigrees segregating EXT in which linkage to the LGS region was excluded. To localize the EXT gene(s) in these families we performed a genome search including 254 microsatellite markers dispersed over all autosomes and the X chromosome. In both families evidence was obtained for linkage to markers from the proximal short and long arms of chromosome 11. Two-point analysis gave the highest lod score for D11S554 (Zmax = 7.148 at theta = 0.03). Multipoint analysis indicated a map position for the EXT gene between D11S905 and D11S916, with a peak multipoint lod score of 8.10 at 6 cM from D11S935. The assignment of a second locus for EXT to the pericentromeric region of chromosome 11 implicates an area that is particularly rich in genes responsible for developmental abnormalities and neoplasia.

Bone Neoplasms↗
PubMed1996

The EXT2 multiple exostoses gene defines a family of putative tumour suppressor genes.

Hereditary multiple exostoses (EXT) is an autosomal dominant condition characterized by short stature and the development of bony protuberances at the ends of all the long bones. Three genetic locl have been identified by genetic linkage analysis at chromosomes 8q24.1, 11p11-13 and 19p. The EXT1 gene on chromosome 8 was recently identified and characterized. Here, we report the isolation and characterization of the EXT2 gene. This gene shows striking sequence similarity to the EXT1 gene, and we have identified a four base deletion segregating with the phenotype. Both EXT1 and EXT2 show significant homology with one additional expressed sequence tag, defining a new multigene family of proteins with potential tumour suppressor activity.

Amino Acid Sequence↗
PubMed2001

A novel deletion mutation of the EXT2 gene in a large Chinese pedigree with hereditary multiple exostosis.

Hereditary multiple exostoses (EXT) is an autosomal dominant disease characterized by the formation of cartilage-capped prominences (exostoses) that develop from the juxta-epiphyseal regions of the long bones. 3 genes are known to be involved in the formation of exostoses. Among them, EXT1 and EXT2, which encode enzymes that catalyse the biosynthesis of heparan sulfate, an important component of the extracellular matrix, are responsible for over 70% of the EXT cases. A large Chinese family with hereditary multiple exostoses has been analysed and the disease-causing mutation has been found. Blood samples were obtained from 69 family members, including 23 affected individuals. The EXT phenotype was shown to be linked to the EXT2 gene by using 2-point linkage analysis. After polymerase chain reaction (PCR)-single strand conformation polymorphism (SSCP) analysis and DNA sequencing, a previously unreported deletion of a G in exon 3 of EXT2 gene was observed. This deletion co-segregated with the disease phenotype, suggesting that it is the disease-causing mutation in this family. Furthermore, in at least 4 members chondrosarcoma occurred after either an operation or injury of the exostosis and 3 of them died of the malignancy in the family. Whether the operation or injury was responsible for the malignant transformation still needs further study.

Base Sequence↗
PubMed1997

Identification of novel mutations in the human EXT1 tumor suppressor gene.

Hereditary multiple exostoses (EXT) is a genetically heterogeneous bone disorder caused by genes segregating on human chromosomes 8, 11, and 19 and designated EXT1, EXT2 and EXT3, respectively. Recently, the EXT1 gene has been isolated and partially characterized and appears to encode a tumor suppressor gene. We have identified six mutations in the human EXT1 gene from six unrelated multiple exostoses families segregating for the EXT gene on chromosome 8. One of the mutations we detected is the same 1-bp deletion in exon 6 that was previously reported in two independent EXT families. The other five mutations, in exons 1, 6, 9, and the splice junction at the 3' end of exon 2, are novel. In each case, the mutation is likely to result in a truncated or nonfunctional EXT1 protein. These results corroborate and extend the previous report of mutations in this gene in two EXT families, and provide additional support for the EXT1 gene as the cause of hereditary multiple exostoses in families showing linkage to chromosome 8.

Alternative Splicing↗
PubMed1995

Refinement of the multiple exostoses locus (EXT2) to a 3-cM interval on chromosome 11.

Hereditary multiple exostoses (EXT) is an autosomal dominant skeletal disorder characterized by the formation of multiple exostoses on the long bones. EXT is genetically heterogeneous, with at least three loci involved: one (EXT1) in the Langer-Giedion region on 8q23-q24, a second (EXT2) in the pericentromeric region of chromosome 11, and a third (EXT3) on chromosome 19p. In this study, linkage analysis in seven extended EXT families, all linked to the EXT2 locus, refined the localization of the EXT2 gene to a 3-cM region flanked by D11S1355 and D11S1361/D11S554. This implies that the EXT2 gene is located at the short arm of chromosome 11, in band 11p11-p12. The refined localization of EXT2 excludes a number of putative candidate genes located in the pericentromeric region of chromosome 11 and facilitates the process of isolating the EXT2 gene.

Chromosomes, Human, Pair 11↗
PubMed2000

Location of the glucuronosyltransferase domain in the heparan sulfate copolymerase EXT1 by analysis of Chinese hamster ovary cell mutants.

Heparan sulfate formation occurs by the copolymerization of glucuronic acid (GlcA) and N-acetylglucosamine (GlcNAc) residues. Recent studies have shown that these reactions are catalyzed by a copolymerase encoded by EXT1 and EXT2, members of the exostosin family of putative tumor suppressors linked to hereditary multiple exostoses. Previously, we identified a collection of Chinese hamster ovary cell mutants (pgsD) that failed to make heparan sulfate (Lidholt, K., Weinke, J. L., Kiser, C. S., Lugemwa, F. N., Bame, K. J., Cheifetz, S., Massagué, J., Lindahl, U., and Esko, J. D. (1992) Proc. Natl. Acad. Sci. U. S. A. 89, 2267-2271). Here, we show that pgsD mutants contain mutations that either alter GlcA transferase activity selectively or that affect both GlcNAc and GlcA transferase activities. Expression of EXT1 corrects the deficiencies in the mutants, whereas EXT2 and the related EXT-like cDNAs do not. Analysis of the EXT1 mutant alleles revealed clustered missense mutations in a domain that included a (D/E)X(D/E) motif thought to bind the nucleotide sugar from studies of other transferases. These findings provide insight into the location of the GlcA transferase subdomain of the enzyme and indicate that loss of the GlcA transferase domain may be sufficient to cause hereditary multiple exostoses.

Acetylglucosamine↗
PubMed2000

Correction of ankle valgus deformity secondary to multiple hereditary osteochondral exostoses with Ilizarov.

The following case report highlights basic aspects of Multiple Hereditary Osteochondral Exostoses (MHOCE) and discusses the successful treatment of an adult with ankle pain secondary to growth arrest and foreshortening of the fibula. Two salient features include the age of the patient at presentation and the success of the procedure. Symptomatic valgus deformities of the ankle secondary to MHOCE are normally corrected during adolescence, prior to physeal closure. Reducing the ankle mortise by distally displacing the fibula and correcting rotational and angular ankle deformities with Ilizarov external fixation improved this patient's ankle function and relieved his pain.

Adult↗
PubMed2004

Ext1-dependent heparan sulfate regulates the range of Ihh signaling during endochondral ossification.

Exostosin1 (Ext1) belongs to a family of glycosyltransferases necessary for the synthesis of the heparan sulfate (HS) chains of proteoglycans, which regulate signaling of several growth factors. Loss of tout velu (ttv), the homolog of Ext1 in Drosophila, inhibits Hedgehog movement. In contrast, we show that reduced HS synthesis in mice carrying a hypomorphic mutation in Ext1 results in an elevated range of Indian hedgehog (Ihh) signaling during embryonic chondrocyte differentiation. Our data suggest a dual function for HS: First, HS is necessary to bind Hedgehog in the extracellular space. Second, HS negatively regulates the range of Hedgehog signaling in a concentration-dependent manner. Additionally, our data indicate that Ihh acts as a long-range morphogen, directly activating the expression of parathyroid hormone-like hormone. Finally, we propose that the development of exostoses in the human Hereditary Multiple Exostoses syndrome can be attributed to activation of Ihh signaling.

Animals↗
PubMed1999

The neoplastic pathogenesis of solitary and multiple osteochondromas.

Many theories of osteochondroma pathogenesis have been advanced. Genetic research into the inherited multiple form, hereditary multiple exostoses, has revealed a new family of tumour suppressor genes denoted EXT. Patterns of EXT gene mutation in hereditary multiple exostoses, in solitary and multiple osteochondromas, and in chondrosarcoma are analogous to those found in other tumour suppressor genes responsible for family cancer traits and associated malignancies. With one exception, most features of osteochondroma behaviour are comparable to those of benign neoplasms. The neoplastic pathogenesis of osteochondromas provides an alternative to the traditional 'skeletal dysplasia' theory to explain the growth disturbance associated with hereditary multiple exostoses. Recent studies on the physiological function of EXT genes are reviewed and implications for osteochondroma 'cell-of-origin' theories are discussed.

Bone Neoplasms↗
PubMed1998

[Hereditary exostoses--presentation of a family case].

In this article we present a Norwegian family with hereditary multiple exostoses and a review of relevant literature. 21 family members were examined; ten males and six females had multiple exostoses, the youngest from six months of age. The exostoses had led to compression of nerves, disturbance of bone growth with shortening and bowing of the forearm bones, and valgus deformity of the knee and ankle. Physical function and quality of life was generally well preserved. Contrary to most reports, nearly half of the patients had allegedly noticed growth or debut of exostoses in adulthood. Almost half of the patients had had the exostoses removed surgically twice, (median value). Malignant transformation of exostoses has not been observed in the family.

Adolescent↗
PubMed2006

Defective chondrocyte proliferation and differentiation in osteochondromas of MHE patients.

Multiple hereditary exostoses (MHE) is an autosomal dominant skeletal disorder caused by mutations in one of the two EXT genes and characterized by multiple osteochondromas that generally arise near the ends of growing long bones. Defective endochondral ossification is likely to be involved in the formation of osteochondromas. In order to investigate potential changes in chondrocyte proliferation and/or differentiation during this process, osteochondroma samples from MHE patients were obtained and used for genetic, morphological, immunohistological, and in situ hybridization studies. The expression patterns of IHH (Indian hedgehog) and FGFR3 (Fibroblast Growth Factor Receptor 3) were similar with transcripts expressed throughout osteochondromas. Expression of PTHR1 (Parathyroid Hormone Receptor 1) transcripts was restricted to a narrow zone of prehypertrophic chondrocytes. Numerous cells forming osteochondromas although resembling prehypertrophic chondrocytes, stained positively with an anti-proliferating cell nuclear antigen (PCNA) antibody. In addition, ectopic expression of collagen type I and abnormal presence of osteocalcin (OC), osteopontin (OP), and bone sialoprotein (BSP) were observed in the cartilaginous osteochondromas. These data indicate that most chondrocytes involved in the growth of osteochondromas can proliferate, and that some of them exhibit bone-forming cell characteristics. We conclude that in MHE, defective heparan sulfate biosynthesis caused by EXT mutations maintains the proliferative capacity of chondrocytes and promotes phenotypic modification to bone-forming cells.

Adolescent↗
PubMed2005

Tetraparesis due to exostotic osteochondroma at upper cervical cord in a patient with multiple exostoses-mental retardation syndrome (Langer-Giedion syndrome).

STUDY DESIGN: Case report of a severe upper cervical cord compression and tetraparesis by a massive cervical exostotic osteochondroma in a patient with multiple exostoses-mental retardation syndrome (Langer-Giedion syndrome; LGS). OBJECTIVE: To describe this very rare pathological condition and the results of surgical intervention. SETTING: Gifu, Japan. METHODS: A 23-year-old man was referred to our clinic because of progressing tetraparesis. He had previously been diagnosed with hereditary multiple exostoses and mental retardation. As he had not complained of any symptoms, his family only noticed the tetraparesis after advanced deterioration. His face possessed the pathognomic features of LGS. A postmyelogram CT scan demonstrated an exostotic mass arising from the left-side C2 pedicle with associated severe spinal cord compression. He was diagnosed with LGS. Hemilaminectomy on the left side and resection of the osteochondroma were performed. RESULTS: At 5 years postoperatively, a neurological examination showed the full return of all motor functions. The CT scan revealed no intracanalar recurrence of the tumor. CONCLUSION: In this case of severe tetraparesis due to cervical osteochondroma, decompression by hemilaminectomy provided excellent results. In patients with LGS and intracanalar osteochondroma, the neurological deficit may be masked by mental retardation. Hence, awareness of this pathological condition will help clinicians diagnose it at an early stage.

Adult↗
PubMed2005

Spinal cord compression due to vertebral osteochondroma: report of two cases.

Osteochondroma, or exostosis, is the most common of all benign bone tumors. Spinal osteochondromas are uncommon but may cause neurological compromise. We report two cases of spinal cord compression by osteochondromas. One patient was a 17-year-old man with hereditary multiple exostoses who was presented with spastic paraparesis, a sensory level at T3-T4, and a pyramidal syndrome. Vertebral exostosis was suspected by magnetic resonance imaging and confirmed by histological examination. Surgical decompression was followed by complete resolution of the neurological impairments. The other patient was a 19-year-old man with spastic paralysis of the right lower limb and a pyramidal syndrome. Whereas magnetic resonance imaging suggested a neurofibroma, histological features were those of osteochondroma. Nine months elapsed from symptom onset to surgery. This delay led to residual neurological impairments, which resolved almost completely after rehabilitation therapy. Vertebral osteochondromas contribute only 1.3-4.1% of all osteochondromas. The lesion may be solitary or a manifestation of hereditary multiple exostosis. Magnetic resonance imaging shows the exact location of the lesion, most notably with relation to neighboring neurological structures. Spinal cord compression is uncommon and usually has a favorable outcome provided surgical decompression is performed before major neurological damage develops.

Adolescent↗
PubMed2005

Pseudoaneurysm associated with multiple osteochondromatosis.

The authors present a case of a 12-year-old boy with a known family history of multiple hereditary exostoses who presented with knee swelling. Physical examination and subsequent workup revealed a pseudoaneurysm associated with an osteochondroma. We present this interesting case and review the literature.

Aneurysm, False↗
PubMed1997

Multiple hereditary osteochondromatosis. Two cases with ankle and knee deformities.

Two case histories of multiple hereditary osteochondromatosis are presented. Multiple hereditary exostoses are the most common inherited systemic disorder of bone. The presence of ankle deformity in 50% of the cases and the potential for malignant transformation make this an important disease for the clinician to be familiar with.

Adult↗