A direct interaction between EXT proteins and glycosyltransferases is defective in hereditary multiple exostoses.

Hereditary multiple exostoses (HME) is an autosomal dominant condition in which bony outgrowths occur from the juxtaepiphyseal regions of the long bones. In a few percent of cases these exostoses undergo malignant transformation to chondrosarcomas. HME results from mutations in one of two homologous genes, EXT1 and EXT2. These are members of a new gene family that is conserved from Caenorhabditis elegans to higher vertebrates. In humans this family comprises five genes which are most conserved at their C-termini, but they do not contain any discernible functional motifs and their function(s) is unclear. Indirect evidence suggests that EXT proteins are involved in glycosaminoglycan synthesis, act as tumor suppressors and affect hedgehog signaling. One recent study has also reported that these proteins co-purify with glycosyltransferase (GlcA and GlcNAc transferase) activity and on that basis it has been postulated that they are themselves glycosyl-transferases. We performed two-hybrid screens with a fragment of EXT2 from the region that is most highly conserved in the gene family and identified two interacting proteins: the tumor necrosis factor type 1 associated protein and a novel UDP-GalNAc:poly-peptide N -acetylgalactosaminyltransferase. Significantly, both these interactions were abrogated by a disease-causing EXT mutation, indicating that they are important in the etiology of HME. The EXT2-GalNAc-T5 interaction provides the first direct physical link between EXT proteins and known components of glycosamino-glycan synthesis.     

遗传性多发性骨软骨瘤的基因诊断及产前诊断

目的 研究家族遗传性骨软骨瘤病(hereditary multiple exostoses,HME)的致病基因及产前诊断.方法 应用连锁分析方法对一个HME家系EXT1、EXT2和EXT3基因进行分析.致病基因定位后,用PCR-测序法进行了突变分析.结果 在该家系中EXT2基因第6外显子发生1个新的无义突变(c.1006C>T),该突变导致第336位编码谷氨酰胺的密码子CAA变为终止密码子TAA(Gln336X).根据上述结果配合遗传咨询进行了产前诊断,结果显示胎儿正常.结论 在家族遗传性骨软骨瘤家系中发现一新的EXT2基因突变,并应用于产前诊断.     

遗传性多发性骨软骨瘤的基因诊断及产前诊断

目的 研究家族遗传性骨软骨瘤病(hereditary multiple exostoses,HME)的致病基因及产前诊断.方法 应用连锁分析方法对一个HME家系EXT1、EXT2和EXT3基因进行分析.致病基因定位后,用PCR-测序法进行了突变分析.结果 在该家系中EXT2基因第6外显子发生1个新的无义突变(c.1006C>T),该突变导致第336位编码谷氨酰胺的密码子CAA变为终止密码子TAA(Gln336X).根据上述结果配合遗传咨询进行了产前诊断,结果显示胎儿正常.结论 在家族遗传性骨软骨瘤家系中发现一新的EXT2基因突变,并应用于产前诊断.     

遗传性多发性骨软骨瘤的基因诊断及产前诊断

目的 研究家族遗传性骨软骨瘤病(hereditary multiple exostoses,HME)的致病基因及产前诊断.方法 应用连锁分析方法对一个HME家系EXT1、EXT2和EXT3基因进行分析.致病基因定位后,用PCR-测序法进行了突变分析.结果 在该家系中EXT2基因第6外显子发生1个新的无义突变(c.1006C>T),该突变导致第336位编码谷氨酰胺的密码子CAA变为终止密码子TAA(Gln336X).根据上述结果配合遗传咨询进行了产前诊断,结果显示胎儿正常.结论 在家族遗传性骨软骨瘤家系中发现一新的EXT2基因突变,并应用于产前诊断.     

遗传性多发性骨软骨瘤的基因诊断及产前诊断

目的 研究家族遗传性骨软骨瘤病(hereditary multiple exostoses,HME)的致病基因及产前诊断.方法 应用连锁分析方法对一个HME家系EXT1、EXT2和EXT3基因进行分析.致病基因定位后,用PCR-测序法进行了突变分析.结果 在该家系中EXT2基因第6外显子发生1个新的无义突变(c.1006C>T),该突变导致第336位编码谷氨酰胺的密码子CAA变为终止密码子TAA(Gln336X).根据上述结果配合遗传咨询进行了产前诊断,结果显示胎儿正常.结论 在家族遗传性骨软骨瘤家系中发现一新的EXT2基因突变,并应用于产前诊断.     

遗传性多发性骨软骨瘤的基因诊断及产前诊断

目的 研究家族遗传性骨软骨瘤病(hereditary multiple exostoses,HME)的致病基因及产前诊断.方法 应用连锁分析方法对一个HME家系EXT1、EXT2和EXT3基因进行分析.致病基因定位后,用PCR-测序法进行了突变分析.结果 在该家系中EXT2基因第6外显子发生1个新的无义突变(c.1006C>T),该突变导致第336位编码谷氨酰胺的密码子CAA变为终止密码子TAA(Gln336X).根据上述结果配合遗传咨询进行了产前诊断,结果显示胎儿正常.结论 在家族遗传性骨软骨瘤家系中发现一新的EXT2基因突变,并应用于产前诊断.     

遗传性多发性骨软骨瘤的基因诊断及产前诊断

目的 研究家族遗传性骨软骨瘤病(hereditary multiple exostoses,HME)的致病基因及产前诊断.方法 应用连锁分析方法对一个HME家系EXT1、EXT2和EXT3基因进行分析.致病基因定位后,用PCR-测序法进行了突变分析.结果 在该家系中EXT2基因第6外显子发生1个新的无义突变(c.1006C>T),该突变导致第336位编码谷氨酰胺的密码子CAA变为终止密码子TAA(Gln336X).根据上述结果配合遗传咨询进行了产前诊断,结果显示胎儿正常.结论 在家族遗传性骨软骨瘤家系中发现一新的EXT2基因突变,并应用于产前诊断.     

遗传性多发性骨软骨瘤的基因诊断及产前诊断

目的 研究家族遗传性骨软骨瘤病(hereditary multiple exostoses,HME)的致病基因及产前诊断.方法 应用连锁分析方法对一个HME家系EXT1、EXT2和EXT3基因进行分析.致病基因定位后,用PCR-测序法进行了突变分析.结果 在该家系中EXT2基因第6外显子发生1个新的无义突变(c.1006C>T),该突变导致第336位编码谷氨酰胺的密码子CAA变为终止密码子TAA(Gln336X).根据上述结果配合遗传咨询进行了产前诊断,结果显示胎儿正常.结论 在家族遗传性骨软骨瘤家系中发现一新的EXT2基因突变,并应用于产前诊断.     

遗传性多发性骨软骨瘤的基因诊断及产前诊断

目的 研究家族遗传性骨软骨瘤病(hereditary multiple exostoses,HME)的致病基因及产前诊断.方法 应用连锁分析方法对一个HME家系EXT1、EXT2和EXT3基因进行分析.致病基因定位后,用PCR-测序法进行了突变分析.结果 在该家系中EXT2基因第6外显子发生1个新的无义突变(c.1006C>T),该突变导致第336位编码谷氨酰胺的密码子CAA变为终止密码子TAA(Gln336X).根据上述结果配合遗传咨询进行了产前诊断,结果显示胎儿正常.结论 在家族遗传性骨软骨瘤家系中发现一新的EXT2基因突变,并应用于产前诊断.     

遗传性多发性骨软骨瘤的基因诊断及产前诊断

目的 研究家族遗传性骨软骨瘤病(hereditary multiple exostoses,HME)的致病基因及产前诊断.方法 应用连锁分析方法对一个HME家系EXT1、EXT2和EXT3基因进行分析.致病基因定位后,用PCR-测序法进行了突变分析.结果 在该家系中EXT2基因第6外显子发生1个新的无义突变(c.1006C>T),该突变导致第336位编码谷氨酰胺的密码子CAA变为终止密码子TAA(Gln336X).根据上述结果配合遗传咨询进行了产前诊断,结果显示胎儿正常.结论 在家族遗传性骨软骨瘤家系中发现一新的EXT2基因突变,并应用于产前诊断.     

Reevaluation of a genetic model for the development of exostosis in hereditary multiple exostosis

EXT1 and EXT2 are genes that have been shown to cause hereditary multiple exostosis (HME), a syndrome marked by the formation of bony growths juxtaposed to the growth plate. These genes are members of a growing family of proteins with glycosyltransferase activity required for the synthesis of heparan sulfate chains. This protein activity is predicted to play a role in the expression of proteoglycans on the cell surface and in the extracellular matrix. We and others have previously suggested that a two-hit mutational model applies to the development of an exostosis where a germline mutation coupled with a somatic mutation results in the loss of EXT1 or EXT2 function and subsequent tumor formation. We report the direct sequencing and loss of heterozygosity (LOH) analysis of 12 exostoses from 10 HME families, 4 solitary exostoses, and their corresponding constitutional DNA. Of the 16 exostoses screened, we find only one solitary case in which two somatic mutations, a deletion and an LOH, are present. This provides limited support for the two-hit hypothesis involving the EXT1 and EXT2 genes for the development of an exostosis. Alternative models are developed based on the functional significance of EXT proteins in heparan sulfate biosynthesis.     

Comparison of fluorescent single-strand conformation polymorphism analysis and denaturing high-performance liquid chromatography for detection of EXT1 and EXT2 mutations in hereditary multiple exostoses

EXT1 and EXT2 are two genes responsible for the majority of cases of hereditary multiple exostoses (HME), a dominantly inherited bone disorder. In order to develop an efficient screening strategy for mutations in these genes, we performed two independent blind screens of EXT1 and EXT2 in 34 unrelated patients with HME, using denaturing high-performance liquid chromatography (DHPLC) and fluorescent single-strand conformation polymorphism analysis (F-SSCP). The mutation likely to cause HME was found in 29 (85%) of the 34 probands: in 22 of these (76%), the mutation was in EXT1; seven patients (24%) had EXT2 mutations. Nineteen of these disease mutations have not been previously reported. Of the 42 different amplicon variants identified in total in the cohort, 40 were detected by DHPLC and 39 by F-SSCP. This corresponds to mutation detection efficiencies of 95% and 93% respectively. We have also found that we can confidently distinguish between different sequence variants in the same fragment using F-SSCP but not DHPLC. In light of this, and the similarly high sensitivities of the two techniques, we propose to continue screening with F-SSCP.     

遗传性多发性骨软骨瘤的基因诊断及产前诊断

目的 研究家族遗传性骨软骨瘤病(hereditary multiple exostoses,HME)的致病基因及产前诊断.方法 应用连锁分析方法对一个HME家系EXT1、EXT2和EXT3基因进行分析.致病基因定位后,用PCR-测序法进行了突变分析.结果 在该家系中EXT2基因第6外显子发生1个新的无义突变(c.1006C>T),该突变导致第336位编码谷氨酰胺的密码子CAA变为终止密码子TAA(Gln336X).根据上述结果配合遗传咨询进行了产前诊断,结果显示胎儿正常.结论 在家族遗传性骨软骨瘤家系中发现一新的EXT2基因突变,并应用于产前诊断.     

遗传性多发性骨软骨瘤的基因诊断及产前诊断

目的 研究家族遗传性骨软骨瘤病(hereditary multiple exostoses,HME)的致病基因及产前诊断.方法 应用连锁分析方法对一个HME家系EXT1、EXT2和EXT3基因进行分析.致病基因定位后,用PCR-测序法进行了突变分析.结果 在该家系中EXT2基因第6外显子发生1个新的无义突变(c.1006C>T),该突变导致第336位编码谷氨酰胺的密码子CAA变为终止密码子TAA(Gln336X).根据上述结果配合遗传咨询进行了产前诊断,结果显示胎儿正常.结论 在家族遗传性骨软骨瘤家系中发现一新的EXT2基因突变,并应用于产前诊断.     

Intermediate grade osteosarcoma and chondrosarcoma arising in an osteochondroma. A case report of a patient with hereditary multiple exostoses

A 40 year old man with hereditary multiple exostoses (HME), affecting predominantly his left proximal tibia, distal femur, and proximal femur, underwent resection of an osteochondroma near the trochanter major of his left proximal femur because of malignant transformation of the cartilaginous cap towards secondary peripheral chondrosarcoma. The patient had a history of a papillary thyroid carcinoma four years previously. At examination of the resected specimen, a third malignant tumour, an intermediate grade osteosarcoma (grade II/IV), was found in the osseous stalk of the osteochondroma. Although no mutations were found in the EXT1 and EXT2 genes, the genes involved in HME, or in exons 5-8 of the p53 gene, the development of three malignancies before the age of 40 suggests that this patient is genetically prone to malignant transformation.     

Genotype-phenotype correlation in hereditary multiple exostoses

Hereditary multiple exostoses (HME) is a genetically heterogeneous autosomal dominant disorder characterised by the development of bony protuberances mainly located on the long bones. Three HME loci have been mapped to chromosomes 8q24 (EXT1), 11p11-13 (EXT2), and 19p (EXT3). The EXT1 and EXT2 genes encode glycosyltransferases involved in biosynthesis of heparan sulphate proteoglycans. Here we report on a clinical survey and mutation analysis of 42 HME French families and show that EXT1 and EXT2 accounted for more than 90% of HME cases in our series. Among them, 27/42 cases were accounted for by EXT1 (64%, four nonsense, 19 frameshift, three missense, and one splice site mutations) and 9/42 cases were accounted for by EXT2 (21%, four nonsense, two frameshift, two missense, and one splice site mutation). Overall, 31/36 mutations were expected to cause loss of protein function (86%). The most severe forms of the disease and malignant transformation of exostoses to chondrosarcomas were associated with EXT1 mutations. These findings provide the first genotype-phenotype correlation in HME and will, it is hoped, facilitate the clinical management of these patients.


Keywords: hereditary multiple exostoses; EXT1; EXT2; chondrosarcoma     

一个遗传性多发性骨软骨瘤家系的基因突变检测

目的 确定一个遗传性多发性骨软骨瘤(hereditary multiple exostoses,HME)家系的致病基因.方法 应用与EXT1、EXT2紧密连锁的短串联重复序列(short tandem repeat,STR)对该家系进行连锁分析,确定候选基因,然后对候选基因的编码区及外显子与内含子交界处进行PCR-测序法突变分析.结果 该家系致病基因被定位在EXT2基因区,测序发现EXT2基因536G>A无义突变,该突变位于EXT2基因第3外显子,导致编码第180位氨基酸的密码子成为终止密码,突变与疾病共分离,其余外显子未发现突变.另发现1例外显不全.结论 EXT2基因536G>A突变是导致这个家系发生骨软骨瘤的原因.     

Novel EXT1 and EXT2 mutations identified by DHPLC in Italian patients with multiple osteochondromas

We describe the results of an optimised DHPLC-based mutation screening of the EXT1 and EXT2 genes in Italian patients affected by multiple osteochondromas [MO; also referred to as hereditary multiple exostoses (HME) in the literature], using a multistep approach. We first analysed 36 unrelated probands for EXT1 mutations by DHPLC analysis and subsequent direct sequencing of all samples with abnormal elution profile. Negative cases were then screened for EXT2 mutations using the same approach. In patients who tested normal at DHPLC screening, all EXT1 and EXT2 exons and splice-site junctions were directly sequenced. In 7 informative families, we also performed a pre-screening linkage analysis to selectively focus the DHPLC testing on the EXT1 or EXT2 gene. We detected 31 MO-related mutations, of which 23 (74%) were novel. Seven polymorphisms were also found. Twenty-four mutations (77%) were found in EXT1 and 7 (23%) in EXT2. No disease-causing mutations were detected in five of 36 patients, with a mutation frequency of 86%. According with previous studies, most mutations (90%) are loss of function. Neither false positive nor false negative results were obtained. This multistep method can be considered a fast and reliable diagnostic strategy for the detection of EXT1/2 mutations, with excellent sensitivity and specificity.     

EXT genes are differentially expressed in bone and cartilage during mouse embryogenesis.

Hereditary multiple exostoses (HME) is a genetically heterogeneous disease characterized by the development of bony protuberances at the ends of all long bones. Genetic analyses have revealed HME to be a multigenic disorder linked to three loci on chromosomes 8q24 (EXT1), 11p11-13 (EXT2), and 19p (EXT3). The EXT1 and EXT2 genes have been cloned and defined as glycosyltransferases involved in the synthesis of heparan sulfate. EST database analysis has demonstrated additional gene family members, EXT-like genes (EXTL1, EXTL2, and EXTL3), not associated with a HME locus. The mouse homologs of EXT1 and EXT2 have also been cloned and shown to be 99% and 95% identical to their human counterparts, respectively. Here, we report the identification of the mouse EXTL1 gene and show it is 74% identical to the human EXTL1 gene. Expression studies of all three mouse EXT genes throughout various stages of embryonic development were carried out and whole-mount in situ hybridization in the developing limb buds showed high levels of expression of all three EXT genes. However, in situ hybridization of sectioned embryos revealed remarkable differences in expression profiles of EXT1, EXT2, and EXTL1. The identical expression patterns found for the EXT1 and EXT2 genes support the recent observation that both proteins form a glycosyltransferase complex. We suggest a model for exostoses formation based on the involvement of EXT1 and EXT2 in the Indian hedgehog/parathyroid hormone-related peptide (PTHrP) signaling pathway, an important regulator of the chondrocyte maturation process.