Mutation screening of the EXT1 and EXT2 genes in patients with hereditary multiple exostoses.

Hereditary multiple exostoses (HME), the most frequent of all skeletal dysplasias, is an autosomal dominant disorder characterized by the presence of multiple exostoses localized mainly at the end of long bones. HME is genetically heterogeneous, with at least three loci, on 8q24.1 (EXT1), 11p11-p13 (EXT2), and 19p (EXT3). Both the EXT1 and EXT2 genes have been cloned recently and define a new family of potential tumor suppressor genes. This is the first study in which mutation screening has been performed for both the EXT1 and EXT2 genes prior to any linkage analysis. We have screened 17 probands with the HME phenotype, for alterations in all translated exons and flanking intronic sequences, in the EXT1 and EXT2 genes, by conformation-sensitive gel electrophoresis. We found the disease-causing mutation in 12 families (70%), 7 (41%) of which have EXT1 mutations and 5 (29%) EXT2 mutations. Together with the previously described 1-bp deletion in exon 6, which is present in 2 of our families, we report five new mutations in EXT1. Two are missense mutations in exon 2 (G339D and R340C), and the other three alterations (a nonsense mutation, a frameshift, and a splicing mutation) are likely to result in truncated nonfunctional proteins. Four new mutations are described in EXT2. A missense mutation (D227N) was found in 2 different families; the other three alterations (two nonsense mutations and one frameshift mutation) lead directly or indirectly to premature stop codons. The missense mutations in EXT1 and EXT2 may pinpoint crucial domains in both proteins and therefore give clues for the understanding of the pathophysiology of this skeletal disorder.     

Mutations in the EXT1 and EXT2 genes in hereditary multiple exostoses.

Hereditary multiple exostoses (EXT; MIM 133700) is an autosomal dominant bone disorder characterized by the presence of multiple benign cartilage-capped tumors (exostoses). Besides suffering complications caused by the pressure of these exostoses on the surrounding tissues, EXT patients are at an increased risk for malignant chondrosarcoma, which may develop from an exostosis. EXT is genetically heterogeneous, and three loci have been identified so far: EXT1, on chromosome 8q23-q24; EXT2, on 11p11-p12; and EXT3, on the short arm of chromosome 19. The EXT1 and EXT2 genes were cloned recently, and they were shown to be homologous. We have now analyzed the EXT1 and EXT2 genes, in 26 EXT families originating from nine countries, to identify the underlying disease-causing mutation. Of the 26 families, 10 families had an EXT1 mutation, and 10 had an EXT2 mutation. Twelve of these mutations have never been described before. In addition, we have reviewed all EXT1 and EXT2 mutations reported so far, to determine the nature, frequency, and distribution of mutations that cause EXT. From this analysis, we conclude that mutations in either the EXT1 or the EXT2 gene are responsible for the majority of EXT cases. Most of the mutations in EXT1 and EXT2 cause premature termination of the EXT proteins, whereas missense mutations are rare. The development is thus mainly due to loss of function of the EXT genes, consistent with the hypothesis that the EXT genes have a tumor- suppressor function.     

Mutation screening of the EXT genes in patients with hereditary multiple exostoses in Taiwan

Hereditary multiple exostoses (HME) is an autosomal dominant disorder characterized by growth of benign bone tumors. This genetically heterozygous disease comprises three chromosomal loci: the EXT1 gene on chromosome 8q23-q24, EXT2 on 11p11-p13, and EXT3 on 19p. Both EXT1 and EXT2 have been cloned and defined as a new family of potential tumor suppressor genes in previous work. However, no studies have been conducted in the Taiwanese population. To determine if previous results can also be applied to the Taiwanese, we analyzed 5 Taiwanese probands with clinical features of HME: 1 of them is a sporadic case, and the others are familial cases. Linkage studies were performed in the familial cases before the mutation analysis to determine to which of the three EXT chromosomes these cases could be assigned. Our results showed that one proband is linked to the EXT1 locus and three are linked to the EXT2 locus; the sporadic case was subsequently found to involve EXT1. We then identified four new mutations that have not been found in other races: two in EXT1--frameshift (K218fsX247) and nonsense (Y468X) mutations and two in EXT2-missense (R223P) and nonsense (Y394X) mutations. Our results indicate that in familial cases, linkage analysis can prove useful for preimplantation genetic diagnosis.     

Association of EXT1 and EXT2, hereditary multiple exostoses gene products, in Golgi apparatus

We prepared the specific antibodies for EXT1 and EXT2, hereditary multiple exostoses (HME) gene products, and characterized their expression, subcellular localization, and protein association among EXT members. Biochemical analyses indicate that EXT1 and EXT2 can associate and form homo/hetero-oligomers in vivo with or without HME-linked mutations, EXT1 (R340C) and EXT2 (D227N), when exogenously expressed in COS-7 cells. An immunocytochemical analysis showed that both EXT1 and EXT2 localized in Golgi apparatus, irrespective of HME mutations. An immunohistochemical analysis on developing bones further showed that both EXT1 and EXT2 were concomitantly expressed in hypertrophic chondrocytes of forelimb bones from 1-day-old neonatal mouse, but down-regulated in maturing chondrocytes of developing cartilage from 21-day-old mouse. Taken together with the recent finding that EXTs encode for the glycosyltransferase required for the synthesis of heparan sulfate [Lind, T., Tufaro, F., McCormick, C., Lindahl, U., and Lindholt, K. (1998) J. Biol. Chem. 273, 26265-26268], our results implied a molecular basis that a HME-linked mutation found in EXT genes could interfere the physiological function(s) of EXT homo/hetero-oligomers as glycosyltransferases in the developing bones of HME patients.     

Mutation analysis of EXT1 and EXT2 Genes in a Korean family with multiple exostoses

A 35-year-old female patient diagnosed clinically as multiple exostosis visited the hospital for infertility evaluation and treatment. She had an operation in pelvis, humerus, tibia and femur in 1993. An extended pedigree analysis showed three of her siblings and several cousins have suffered from the same disease with a typical autosomal dominant pattern of inheritance. So she wanted a genetic test for her disease before having a child. For mutation analysis, DNAs were extracted from the patient and her brother. All exons and exon-intron boundaries of EXT1 and EXT2 genes were amplified by polymerase chain reactions. The PCR products were directly sequenced and analyzed by ABI genetic analyzer. A single base pair deletion c.2241delC in the exon 6 of EXT1 gene was detected in both patient and her brother. Generation of a premature stop codon resulting from frameshift of codons might be a causative of the disease. According to the human genome mutation data base (HGMD), the mutation detected is not previously documented.     

A novel mutation in EXT2 gene in a Chinese family with hereditary multiple exostoses

Hereditary multiple exostoses (HME) is an autosomal-dominant disorder characterized by the presence of bony outgrowths on the long bones. In this report, we describe a Chinese family with HME. Linkage analysis and mutation detection were performed. Linkage with the EXT2 was established in this family. A novel mutation, EXT2 c239-244delG, was identified. Mutation analysis in a family with HME allows for genetic counseling and prenatal diagnosis.     

Mutation screening of EXT genes in Chinese patients with multiple osteochondromas

Since vascular risk factors commonly act for susceptibility to Alzheimer's disease (AD) and vascular dementia (VaD) by declining cognitive abilities, we conducted a genetic association study to identify their common underlying genetic factors. We selected single nucleotide polymorphisms (SNPs) which had been previously discovered for association with AD, and case and control associations of VaD were examined with the individual SNPs using 207 patients with VaD and 207 sex- and age-matched control subjects. As a result, no significant associations of susceptibility to VaD with 13 selected SNPs were observed even without employing a multiple test (P>0.05). This study suggests that genetics of VaD might be quite different from that of AD, and cautions should be taken especially when inferences about genetic factors are made with patients with mixed dementia.     

A novel mutation in the EXT2 gene identified in two unrelated Chinese families with hereditary multiple exostoses

Hereditary multiple exostoses (HME) is an autosomal dominant disorder characterized by benign bone tumors. In this report, we describe two unrelated Chinese families with HME. Linkage analysis and mutation detection was performed. Clinical analysis was also performed for some affected individual in both families. Linkage with the EXT2 was established in both families. A novel mutation, c505 G > T, was identified in both families. Further allelic heterogeneity of EXT2 was demonstrated by the intrafamilial and interfamilial variability in clinical phenotype.     

One third of Japanese patients with multiple osteochondromas may have mutations in genes other than EXT1 or EXT2

Multiple osteochondromas (MO; also referred to as hereditary multiple exostoses [HME] in the literature) is an autosomal dominant disorder characterized by benign, cartilage-capped bone tumors that grow from the metaphyses of long bones. Two genes are associated with this disease: EXT1 on 8q24.11-q24.13 and EXT2 on 11p12-p11. Mutations in EXT1 and EXT2 are found in 54-96% of patients with MO and are generally more frequent in EXT1 than in EXT2. We previously studied 43 Japanese families with MO using single-strand conformation polymorphism analysis for EXT1 and EXT2, and reported 23 families (54%) with mutations and 20 families (46%) with no mutations in these genes. Among the families with mutations, 17 families (40%) had mutations in EXT1, and 6 families (14%) had mutations in EXT2. Here we examined the same 43 Japanese families using denaturing high-performance liquid chromatography as an alternative technique. We detected five mutations, three of which are novel, in seven families in addition to the previously described mutations. In summary, we detected mutations in EXT1 or EXT2 in 30 (70%) out of 43 families. Our result suggests the presence of other gene(s) responsible for MO, at least in Japanese patients.     

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.     

Hereditary multiple exostoses (EXT): mutational studies of familial EXT1 cases and EXT-associated malignancies.

Hereditary multiple exostoses (EXT) is an autosomal dominant disorder characterized by the formation of cartilage-capped prominences that develop from the growth centers of the long bones. EXT is genetically heterogeneous, with three loci, currently identified on chromosomes 8q24.1, 11p13, and 19q. The EXT1 gene, located on chromosome 8q24.1, has been cloned and is encoded by a 3.4-kb cDNA. Five mutations in the EXT1 gene have been identified--four germ-line mutations, including two unrelated families with the same mutation, and one somatic mutation in a patient with chondrosarcoma. Four of the mutations identified resulted in frameshifts and premature termination codons, while the fifth mutation resulted in a substitution of leucine for arginine. Loss of heterozygosity (LOH) analysis of chondrosarcomas and chondroblastomas revealed multiple LOH events at loci on chromosomes 3q, 8q, 10q, and 19q. One sporadic chondrosarcoma demonstrated LOH for EXT1 and EXT3, while a second underwent LOH for EXT2 and chromosome 10. A third chondrosarcoma underwent LOH for EXT1 and chromosome 3q. These results agree with previous findings that mutations at EXT1 and multiple genetic events that include LOH at other loci may be required for the development of chondrosarcoma.     

Genetic heterogeneity in families with hereditary multiple exostoses

We have carried out a linkage analysis on 11 families segregating gene(s) for hereditary multiple exostoses (EXT). Four highly informative, short tandem-repeat (STR) markers that have been physically mapped to an interval surrounding the Langer-Giedion chromosomal region (8q24.11-q24.13) were used in a multipoint linkage analysis. Significant evidence for linkage of EXT with genetic heterogeneity was found. A model of heterogeneity with linkage of the disease gene to the STR markers in 70% of the families (with a 95% confidence interval of 26%–96%) produced a maximum LOD score of 8.11, with the most likely position of EXT between D8S85 and D8S199. Thus there are at least two genes that are capable of causing hereditary multiple exostoses, one in the Langer-Giedion region and one at another, unlinked location.     

Mutation analysis of hereditary multiple exostoses in the Chinese

Hereditary multiple exostoses (EXT; MIM 133700) is an autosomal dominant bone disorder. It is genetically heterogeneous with at least three chromosomal loci: EXT1 on 8q24.1, EXT2 on 11p11, and EXT3 on 19p. EXT1 and EXT2, the two genes responsible for EXT1 and EXT2, respectively, have been cloned. Recently, three other members of the EXT gene family, named the EXT-like genes (EXTL: EXTL1, EXTL2, and EXTL3), have been isolated. EXT1, EXT2, and the three EXTLs are homologous with one another. We have identified the intron-exon boundaries of EXTL1 and EXTL3 and analyzed EXT1, EXT2, EXTL1, and EXTL3, in 36 Chinese families with EXT, to identify underlying disease-related mutations in the Chinese population. Of the 36 families, five and 12 family groups have mutations in EXT1 and EXT2, respectively. No disease-related mutation has been found in either EXTL1 or EXTL2, although one polymorphism has been detected in EXTL1. Of the 15 different mutations (three families share a common mutation in EXT2), 12 are novel. Most of the mutations are either frameshift or nonsense mutations (12/15). These mutations lead directly or indirectly to premature stop codons, and the mutations generate truncated proteins. This finding is consistent with the hypothesis that the development of EXT is mainly attributable to loss of gene function. Missense mutations are rare in our families, but these mutations may reflect some functionally crucial regions of these proteins. EXT1 is the most frequent single cause of EXT in the Caucasian population in Europe and North America. It accounts for about 40% of cases of EXT. Our study of 36 EXT Chinese families has found that EXT1 seems much less common in the Chinese population, although the frequency of the EXT2 mutation is similar in the Caucasian and Chinese populations. Our findings suggest a possibly different genetic spectrum of this disease in different populations. Electronic Publication     

Mutational screening of SCN5A linked disorders in Polish patients and their family members

Mutations in SCN5A lead to a broad spectrum of phenotypes, including the Long QT syndrome, Brugada syndrome, Idiopathic ventricular fibrillation (IVF), Sudden infant death syndrome (SIDS) (probably regarded as a form of LQT3), Sudden unexplained nocturnal death syndrome (SUNDS) and isolated progressive cardiac conduction defect (PCCD) (Lev-Lenegre disease). Brugada Syndrome (BS) is a form of idiopathic ventricular fibrillation characterized by the right bundle-branch block pattern and ST elevation (STE) in the right precordial leads of the ECG. Mutations of the cardiac sodium channel SCN5A cause the disorder, and an implantable cardioverter-defibrillator is often recommended for affected individuals. In this study sequences of the coding region of the SCN5A gene were analysed in patients with the LQT3, Brugada Syndrome and other arrythmogenic disorders. Different mSSCP patterns are described with no disease-related SSCP conformers in any sample. Direct sequencing of the SCN5A gene confirmed the absence of mutations. This suggests that the analysed region of the SCN5A gene is not commonly involved in the pathogenesis of the Brugada Syndrome and associated disorders.     

Mutational analysis in podocin-associated hereditary nephrotic syndrome in Polish patients: founder effect in the Kashubian population

Hereditary nephrotic syndrome is caused by mutations in a number of different genes, the most common being NPHS2. The aim of the study was to identify the spectrum of NPHS2 mutations in Polish patients with the disease. A total of 141 children with steroid-resistant nephrotic syndrome (SRNS) were enrolled in the study. Mutational analysis included the entire coding sequence and intron boundaries of the NPHS2 gene. Restriction fragment length polymorphism (RFLP) and TaqMan genotyping assay were applied to detect selected NPHS2 sequence variants in 575 population-matched controls. Twenty patients (14 %) had homozygous or compound heterozygous NPHS2 mutations, the most frequent being c.1032delT found in 11 children and p.R138Q found in four patients. Carriers of the c.1032delT allele were exclusively found in the Pomeranian (Kashubian) region, suggesting a founder effect origin. The 14 % NPHS2 gene mutation detection rate is similar to that observed in other populations. The heterogeneity of mutations detected in the studied group confirms the requirement of genetic testing the entire NPHS2 coding sequence in Polish patients, with the exception of Kashubs, who should be initially screened for the c.1032delT deletion.     

Mutational spectrum of the C1INH (SERPING1) gene in patients with hereditary angioedema

Hereditary angioedema (HAE) is an autosomal dominant disease that manifests as intermittent acute swellings of the skin and mucosal surfaces, which, in the gastrointestinal tract and larynx, may even be fatal. HAE results from functional deficiency of the C1 inhibitor (C1INH) protein, which plays a key role in the classical pathway of complement activation. C1INH is the sole inhibitor of the activated proteases C1r and C1s, and is the major regulator of activated coagulation Factor XII and plasma kallikrein, which limits the generation of the vasoactive peptide bradykinin. In this paper, we report on the genetic analysis of 173 families (including 326 members) with a clinical diagnosis of HAE. Direct sequencing, Southern blotting and quantitative PCR by the MLPA method were used to screen for mutations in C1INH (SERPING1). In 142 families (82.1%), a causative C1INH gene mutation could be identified. A total of 80 novel point mutations of C1INH not published previously were detected in 96 pedigrees (including 172 members). Our results corroborate C1INH (SERPING1) deficiency as a disease of extreme allelic heterogeneity with almost each individual family carrying their own mutation. Routine molecular genetic analysis is an effective way of confirming the clinical diagnosis and identifying mutation carriers early on before any clinical manifestation becomes apparent. It is, therefore, a valuable tool in prevention and adequate treatment of acute and life-threatening oedema.