Multiple Exostoses Syndrome Research Articles

Hereditary Multiple Exostoses-A Review of the Molecular Background, Diagnostics, and Potential Therapeutic Strategies.

Hereditary multiple exostoses (HMEs) syndrome, also known as multiple osteochondromas, represents a rare and severe human skeletal disorder. The disease is characterized by multiple benign cartilage-capped bony outgrowths, termed exostoses or osteochondromas, that locate most commonly in the juxta-epiphyseal portions of long bones. Affected individuals usually complain of persistent pain caused by the pressure on neighboring tissues, disturbance of blood circulation, or rarely by spinal cord compression. However, the most severe complication of this condition is malignant transformation into chondrosarcoma, occurring in up to 3.9% of HMEs patients. The disease results mainly from heterozygous loss-of-function alterations in the EXT1 or EXT2 genes, encoding Golgi-associated glycosyltransferases, responsible for heparan sulfate biosynthesis. Some of the patients with HMEs do not carry pathogenic variants in those genes, hence the presence of somatic mutations, deep intronic variants, or another genes/loci is suggested. This review presents the systematic analysis of current cellular and molecular concepts of HMEs along with clinical characteristics, clinical and molecular diagnostic methods, differential diagnosis, and potential treatment options.

Identification and characterization of a novel member of the EXT gene family, EXTL2.

Recently, two homologous genes, EXT1 and EXT2, with a putative tumor suppressor function have been described. Mutations in both genes are responsible for multiple exostosis syndrome (EXT), an autosomal dominant condition characterized by the presence of multiple osteochondromas, bony excrescences that sometimes undergo malignant transformation to chondrosarcoma. This family of EXT genes has been extended by the identification of an EXT-like (EXTL) gene showing a high degree of homology with the EXT genes. We report here a second EXT-like gene (EXTL2) which is homologous to the EXT and EXTL genes. EXTL2 consists of 5 exons encoding an ubiquitously expressed protein of 330 amino acids. In addition, a putative pseudogene, EXTL2P was also identified. The EXTL2 gene was assigned to chromosome 1p11-p12, whereas EXTL2P was mapped on chromosome 2q24-q31.

Osteopoikilosis and multiple exostoses caused by novel mutations in LEMD3 and EXT1 genes respectively - coincidence within one family

BackgroundOsteopoikilosis is a rare autosomal dominant genetic disorder, characterised by the occurrence of the hyperostotic spots preferentially localized in the epiphyses and metaphyses of the long bones, and in the carpal and tarsal bones [1]. Heterozygous LEMD3 gene mutations were shown to be the primary cause of the disease [2]. Association of the primarily asymptomatic osteopokilosis with connective tissue nevi of the skin is categorized as Buschke-Ollendorff syndrome (BOS) [3]. Additionally, osteopoikilosis can coincide with melorheostosis (MRO), a more severe bone disease characterised by the ectopic bone formation on the periosteal and endosteal surface of the long bones [4-6]. However, not all MRO affected individuals carry germ-line LEMD3 mutations [7]. Thus, the genetic cause of MRO remains unknown. Here we describe a familial case of osteopoikilosis in which a novel heterozygous LEMD3 mutation coincides with a novel mutation in EXT1, a gene involved in aetiology of multiple exostosis syndrome. The patients affected with both LEMD3 and EXT1 gene mutations displayed typical features of the osteopoikilosis. There were no additional skeletal manifestations detected however, various non-skeletal pathologies coincided in this group.MethodsWe investigated LEMD3 and EXT1 in the three-generation family from Poland, with 5 patients affected with osteopoikilosis and one child affected with multiple exostoses.ResultsWe found a novel c.2203C > T (p.R735X) mutation in exon 9 of LEMD3, resulting in a premature stop codon at amino acid position 735. The mutation co-segregates with the osteopoikilosis phenotype and was not found in 200 ethnically matched controls. Another new substitution G > A was found in EXT1 gene at position 1732 (cDNA) in Exon 9 (p.A578T) in three out of five osteopoikilosis affected family members. Evolutionary conservation of the affected amino acid suggested possible functional relevance, however no additional skeletal manifestations were observed other then those specific for osteopoikilosis. Finally in one member of the family we found a splice site mutation in the EXT1 gene intron 5 (IVS5-2 A > G) resulting in the deletion of 9 bp of cDNA encoding three evolutionarily conserved amino acid residues. This child patient suffered from a severe form of exostoses, thus a causal relationship can be postulated.ConclusionsWe identified a new mutation in LEMD3 gene, accounting for the familial case of osteopoikilosis. In the same family we identified two novel EXT1 gene mutations. One of them A598T co-incided with the LEMD3 mutation. Co-incidence of LEMD3 and EXT1 gene mutations was not associated with a more severe skeletal phenotype in those patients.

Peroneal Nerve Palsy Resulting From Fibular Head Osteochondroma

This article describes a case of a 11-year-old boy with an osteochondroma of the peroneal head causing peroneal nerve palsy. Physical examination disclosed large exostoses palpated at the right fibular head. Neurological examination revealed paresis of the tibialis anterior, lateral peroneal, and extensor digitorum muscles with a muscle strength grade of 2. Electrophysiological studies confirmed denervation of the muscles supplied by the right peroneal nerve. Radiological examination showed an osteochondroma in the head of the right fibula. The patient underwent surgical decompression of the right peroneal nerve after resection of the bone tumor. At 36-month follow-up, there was a complete recovery of the deficits. Peroneal mononeuropathy in children is uncommon. Osteochondroma is a benign tumor consisting of projecting bone capped by cartilage. These tumors may be solitary or multiple and occur in hereditary multiple exostoses syndrome. The conjunction of this lesion with peroneal nerve palsy has been exceptionally reported for children, usually linked to hereditary multiple exostoses syndrome. Most peroneal nerve trauma occurs at the fibular head, where the common nerve has not yet divided into its deep and superficial peroneal nerve and where most peroneal nerve lesions, therefore, involve both branches, although motor deficits are more frequently involved than sensory ones. Surgical treatment should not be delayed because neurological improvement may be achieved if surgery is performed before severe neurological deficits become irreversible.

Anatomical variation of co-existence of 4th and 5thshort metacarpal bones, sesamoid ossicles and exostoses of ulna and radius in the same hand: a case report

IntroductionThe anatomical variations of bones in the hand are common. The existence of exostosis and shortening of metacarpal bones has been described in the literature as part of the hereditary multiple exostosis syndrome but no case has been reported with the co-existence of sesamoid ossicles in the same patient.Case presentationWe report a case with co-existence of distal ulnar and radial exostoses, 4th and 5th short metacarpals and sesamoid ossicles in the wrist area.ConclusionThis variation may help the interpretation of pain or sensory disorders in the hand and wrist areas.

Occlusion of the Popliteal Artery in Hereditary Multiple Exostosis Syndrome

IntroductionHereditary multiple exostosis syndrome is a rare condition which can occasionally cause arterial occlusion. Case reportA 50-year-old man was treated for acute leg ischaemia but no cause found. Two years later, acute ischaemia recurred and attempts at limb salvage failed. Hereditary multiple exostosis syndrome was then recognised as the cause due to a femoral exostosis compressing the popliteal artery. DiscussionThe case highlights a rare cause of acute limb ischaemia and emphasises the importance of arterial imaging when no embolic source is found.

Occlusion of the Popliteal Artery in Hereditary Multiple Exostosis Syndrome

IntroductionHereditary multiple exostosis syndrome is a rare condition which can occasionally cause arterial occlusion. Case reportA 50-year-old man was treated for acute leg ischaemia but no cause found. Two years later, acute ischaemia recurred and attempts at limb salvage failed. Hereditary multiple exostosis syndrome was then recognised as the cause due to a femoral exostosis compressing the popliteal artery. DiscussionThe case highlights a rare cause of acute limb ischaemia and emphasises the importance of arterial imaging when no embolic source is found.

Peroneal nerve compression resulting from fibular head osteophyte-like lesions

Background The anatomical relationship of the fibular head with the fibular nerve is a critical point in regard to injuries of peripheral nerves in the lower extremities. In this location, the peroneal nerve may be injured due to several mechanisms, and osteophyte-like lesions can be considered as a differential diagnosis. Methods The suitable literature concerning this association is reviewed and a case is presented. A 15-year-old adolescent boy presented with right peroneal nerve palsy on admission. The radiological examinations (computed tomography and magnetic resonance imaging) demonstrated an osteophytic lesion in the head of the right fibula. The patient underwent surgical decompression of the nerve and resection of the lesion. Postoperatively, there was a complete recovery of the deficits. Conclusions The association of osteophyte-like bone changes and peroneal nerve palsy is rare. The differential diagnoses of these lesions include cartilaginous exostoses and osteochondromas, which may be related to hereditary multiple exostoses syndrome. The timing of the treatment plays an important role in the neurological recovery.

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.

Syndecan-3: a cell-surface heparan sulfate proteoglycan important for chondrocyte proliferation and function during limb skeletogenesis

Syndecans are single-pass integral membrane components that serve as co-receptors for growth factors and cytokines and can elicit signal transduction via their cytoplasmic tails. We review here previous studies from our groups on syndecan-3 biology and function in the growth plates of developing long bones in chick and mouse embryos. Gain- and loss-of-function data indicate that syndecan-3 has important roles in restricting mitotic activity to the proliferative zone of growth plate and may do so in close cooperation and interaction with the signaling molecule Indian hedgehog (IHH). Biochemical and protein-modeling data suggest a dimeric/oligomeric syndecan-3 configuration on the chondrocyte's cell surface. Analyses of embryos misexpressing syndecan-3 or lacking IHH provide further clues on syndecan-3/IHH interdependence and interrelationships. The data and the conclusions reached provide insights into mechanisms fine-tuning chondrocyte proliferation, maturation, and function in the developing and growing skeleton and into how abnormalities in these fundamental mechanisms may subtend human congenital pathologies, including osteochondromas in hereditary multiple exostoses syndrome.

Epigenetic loss of the familial tumor-suppressor gene exostosin-1 (EXT1) disrupts heparan sulfate synthesis in cancer cells.

Germline mutations in the Exostoses-1 gene (EXT1) are found in hereditary multiple exostoses syndrome, which is characterized by the formation of osteochondromas and an increased risk of chondrosarcomas and osteosarcomas. However, despite its putative tumor-suppressor function, little is known of the contribution of EXT1 to human sporadic malignancies. Here, we report that EXT1 function is abrogated in human cancer cells by transcriptional silencing associated with CpG island promoter hypermethylation. We also show that, at the biochemical and cellular levels, the epigenetic inactivation of EXT1, a glycosyltransferase, leads to the loss of heparan sulfate (HS) synthesis. Reduced HS production can be reversed by the use of a DNA demethylating agent. Furthermore, the re-introduction of EXT1 into cancer cell lines displaying methylation-dependent silencing of EXT1 induces tumor-suppressor-like features, e.g. reduced colony formation density and tumor growth in nude mouse xenograft models. Screening a large collection of human cancer cell lines (n=79) and primary tumors (n=454) from different cell types, we found that EXT1 CpG island hypermethylation was common in leukemia, especially acute promyelocytic leukemia and acute lymphoblastic leukemia, and non-melanoma skin cancer. These findings highlight the importance of EXT1 epigenetic inactivation, leading to an abrogation of HS biosynthesis, in the processes of tumor onset and progression.

Congenital disorders of glycosylation (CDG): It's all in it!

Congenital disorders of glycosylation (CDGs) are due to defects in the synthesis of the glycan moiety of glycoproteins or other glycoconjugates. This review is devoted mainly to the clinical aspects of protein glycosylation defects. There are two main types of protein glycosylation: N-glycosylation and O-glycosylation. N-glycosylation generally consists of an assembly pathway (in cytosol and endoplasmic reticulum) and a processing pathway (in endoplasmic reticulum and Golgi). O-glycosylation lacks a processing pathway but is otherwise more complex. Sixteen disease-causing defects are known in protein glycosylation: 12 in N-glycosylation and four in O-glycosylation. The N-glycosylation defects comprise eight assembly defects (CDG-I) designated CDG-Ia to CDG-Ih, and four processing defects (CDG-II) designated CDG-IIa to CDG-IId. By far the most frequent is CDG-Ia (phosphomannomutase-2 deficiency). It affects the nervous system and many other organs. Its clinical expression varies from extremely severe to very mild (and thus probably underdiagnosed). The most interesting disease in this group is CDG-Ib (phosphomannose isomerase deficiency) because it is so far the only efficiently treatable CDG (mannose treatment). It has a hepatic-intestinal presentation. The O-glycosylation defects comprise two O-xylosylglycan defects (a progeroid variant of Ehlers-Danlos syndrome and the multiple exostoses syndrome) and two O-mannosylglycan defects (Walker-Warburg syndrome and muscle-eye-brain disease). All known CDGs have a recessive inheritance except for multiple exostoses syndrome, which is dominantly inherited. There is a rapidly growing group of putative CDGs with a large spectrum of clinical presentations (CDG-x). Serum transferrin iso-electrofocusing remains the cornerstone of the screening for N-glycosylation defects associated with sialic acid deficiency. Abnormal patterns can be grouped in to type 1 and type 2. However, a normal pattern does not exclude these defects. Screening for the other CDGs is much more difficult, particularly when the defect is organ- or system-restricted. The latter group promises to become an important new chapter in CDG. It is concluded that CDGs will eventually cover the whole clinical spectrum of paediatric and adult disease manifestations.

Developmental pathways in musculoskeletal neoplasia: involvement of the Indian Hedgehog-parathyroid hormone-related protein pathway.

There are many crucial genes and signaling pathways in the proper development of an organism. Pathologies may arise from a deregulation of these pathways. The Indian Hedgehog-PTH-related protein (Ihh-PTHrP) pathway is vital in the proper development of endochondral bones, such as the long bones. The Ihh-PTHrP pathway regulates the rate at which chondrocytes within the growth plate proliferate and differentiate. Thus, this pathway allows for the longitudinal growth of bones. However, a disruption in this pathway may lead to enchondromas and osteochondromas, which are both childhood cartilaginous neoplasms. Recently, our lab identified a mutant receptor for PTHrP in enchondroma samples. Mice expressing this mutant receptor and mice with increased Ihh activity develop conditions similar to human enchondromatosis. Linkage analysis shows an association between EXT genes and osteochondromas in hereditary multiple exostoses syndrome. Studies in Drosophila and mice suggest EXT gene products play a role in the diffusion of hedgehog proteins. A mutation in EXT genes may result in an abnormal Ihh diffusion pattern leading to an osteochondroma. There are agents that inhibit Hedgehog signaling. These agents may be useful in the treatment of enchondromas and osteochondromas. This review will discuss the discovery of the Ihh-PTHrP pathway and its involvement in neoplasia, and will suggest possible novel therapeutic agents in the treatment of these cartilaginous neoplasms.

EXT-Mutation Analysis and Loss of Heterozygosity in Sporadic and Hereditary Osteochondromas and Secondary Chondrosarcomas

Osteochondromas occur as sporadic solitary lesions or as multiple lesions, characterizing the hereditary multiple exostoses syndrome (EXT). Approximately 15% of all chondrosarcomas arise within the cartilaginous cap of an osteochondroma. EXT is genetically heterogeneous, and two genes, EXT1 and EXT2, located on 8q24 and 11p11-p12, respectively, have been cloned. It is still unclear whether osteochondroma is a developmental disorder or a true neoplasm. Furthermore, it is unclear whether inactivation of both alleles of an EXT gene, according to the tumor-suppressor model, is required for osteochondroma development, or whether a single EXT germline mutation acts in a dominant negative way. We therefore studied loss of heterozygosity and DNA ploidy in eight sporadic and six hereditary osteochondromas. EXT1- and EXT2-mutation analysis was performed in a total of 34 sporadic and hereditary osteochondromas and secondary peripheral chondrosarcomas. We demonstrated osteochondroma to be a true neoplasm, since aneuploidy was found in 4 of 10 osteochondromas. Furthermore, LOH was almost exclusively found at the EXT1 locus in 5 of 14 osteochondromas. Four novel constitutional cDNA alterations were detected in exon 1 of EXT1. Two patients with multiple osteochondromas demonstrated a germline mutation combined with loss of the remaining wild-type allele in three osteochondromas, indicating that, in cartilaginous cells of the growth plate, inactivation of both copies of the EXT1 gene is required for osteochondroma formation in hereditary cases. In contrast, no somatic EXT1 cDNA alterations were found in sporadic osteochondromas. No mutations were found in the EXT2 gene.

Tout-velu is a Drosophila homologue of the putative tumour suppressor EXT-1 and is needed for Hh diffusion.

Hedgehog (Hh) proteins act through both short-range and long-range signalling to pattern tissues during invertebrate and vertebrate development. The mechanisms allowing Hedgehog to diffuse over a long distance and to exert its long-range effects are not understood. Here we identify a new Drosophila gene, named tout-velu, that is required for diffusion of Hedgehog. Characterization of tout-velu shows that it encodes an integral membrane protein that belongs to the EXT gene family. Members of this family are involved in the human multiple exostoses syndrome, which affects bone morphogenesis. Our results, together with the previous characterization of the role of Indian Hedgehog in bone morphogenesis, lead us to propose that the multiple exostoses syndrome is associated with abnormal diffusion of Hedgehog proteins. These results show the existence of a new conserved mechanism required for diffusion of Hedgehog.

Multiple exostoses syndrome

Multiple exostoses syndrome (MES) is a inheritable disorder that affects the enchondral skeleton during growth. It is characterized by thickening and deformity of the growing bone with the formation of numerous cartilage-capped exostoses clustered around areas of most active growth