EXT1 Research Articles

Multiple Hereditary Exostoses: Its Burden on Childhood and Beyond

Multiple hereditary exostoses is a rare autosomal dominant condition that affects approximately one in 50,0001. The genetic mutations leading to multiple hereditary exostoses are located in the EXT1 or EXT2 genes, which are in part responsible for the heparan sulfate chain elongation or formation2. The type of genotypical abnormality may have direct influence in the phenotypical appearance of the affected children2. The literature is filled with studies on the etiology and genetics of this condition. There are also several reports on different surgical techniques for osteochondroma removal and for reconstruction of secondary deformities resulting from the osteochondromas. There is, however, a paucity of studies on the natural history of the disease and the quality of life of these patients as it relates to development and growth, physical activities, integration into society, and the presence …

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, …

Third case of 8q23.3‐q24.13 deletion in a patient with Langer–Giedion syndrome phenotype without TRPS1 gene deletion

Langer-Giedion syndrome (LGS) is a contiguous gene syndrome caused by a hemizygous deletion on chromosome 8q23.3-q24.11 involving TRPS1 and EXT1 genes. We report on a girl with LGS phenotype and a 7.5 Mb interstitial deletion at chromosome 8q23.3-q24.13. Array-comparative genomic hybridization (a-CGH) revealed a deletion encompassing only the EXT1 and not the TRPS1 gene. Even though the deletion of TRPS1 and EXT1 genes is responsible for craniofacial and skeletal features of LGS, there have been previous reports of patients with LGS phenotype and 8q24 deletions leaving the TRPS1 gene intact. To our knowledge, this is the third such case. Our patient differs from previously reported LGS patients without TRPS1 gene deletion in that she has the typical LGS facial dysmorphism and skeletal abnormalities. However, the girl is of normal height and has only a mild developmental delay. Additionally, she has dyslalia and premature adrenarche classified as Tanner stage 3 premature pubarche which have not yet been described as features of LGS. We examine the molecular breakpoints and phenotypes of our patient and previously reported cases.

Expression of Ext1, Ext2, and heparanase genes in brain of senescence-accelerated OXYS rats in early ontogenesis and during development of neurodegenerative changes

Heparan sulfate (HS) and heparan sulfate proteoglycans (HSPG) play a significant role in brain development, and their structural and quantitative changes are revealed during aging and in neurodegenerative disorders. The mechanism of these changes is not clear, but is likely to be associated with alteration in the expression and/or activity of enzymes responsible for HSPG biosynthesis and degradation. The contents of mRNAs of the genes Ext1 and Ext2 encoding polymerization enzymes and of gene Hpse of heparanase degrading HS were determined in the brain of prematurely aging OXYS rats during early postnatal development and during appearance of signs of brain accelerated aging (at age of 1, 7, 14, 30, 60, and 420 days). Wistar rats of the same age were used as controls. Expression levels of the genes Ext1, Ext2, and Hpse in the brain of rats of both strains were maximal during the two first weeks of life, and the contents of mRNAs of all genes in the brain of newborn and 7-day-old OXYS rats were significantly higher than in Wistar rats. By the 14th day of life the differences leveled, but at the age of 30 days on the background of a decrease in the contents of mRNAs of Ext1, Ext2, and Hpse in OXYS rats they became more pronounced (three-, four-, and twofold, respectively). Differences between the strains were absent at the age of 60 days and 14 months, and expression of all the genes was significantly lower than in the newborn animals. A strong positive correlation was found between contents of mRNAs of all the studied genes, and this suggested that heparanase should be involved in HSPG metabolism together with Ext1 and Ext2. Based on these and earlier findings, we conclude that development of the OXYS rat brain occurs on the background of significant alterations in HSPG metabolism that precede the development of neurodegenerative manifestations recently detected by magnetic resonance imaging.

Similar cytogenetic findings in two synchronous secondary peripheral chondrosarcomas in a patient with multiple osteochondromas

Secondary peripheral chondrosarcoma is a malignant chondroid tumor arising in a benign precursor, either an osteochondroma or an enchondroma. Multiple osteochondromas syndrome (MO) is an autosomal dominant skeletal disorder associated with bony growths in the form of osteochondromas that occasionally undergo malignant transformation to secondary peripheral chondrosarcomas. We describe the genetic examination of three secondary peripheral chondrosarcomas that had arisen synchronously from osteochondromas in a patient with MO by chromosome banding, high resolution chromosomal comparative genomic hybridization, and mutation analysis of the EXT1 and EXT2 genes. In two of the tumors (the third was not genetically informative), very similar chromosome abnormalities were found, indicating that they must somehow be part of the same neoplastic process in spite of being anatomically distinct.

Tibial hemimelia in Langer–Giedion syndrome with 8q23.1‐q24.12 interstitial deletion

Langer-Giedion syndrome (LGS) (OMIM 150230) is defined as a contiguous gene syndrome caused by loss of functional copies of the TRPS1 and EXT1 genes usually secondary to 8q microdeletion. Tibial hemimelia (TH) is the least common lower limb deficiency characterized by hypoplasia of the tibia with relatively intact fibula. We describe the third report of LGS with bilateral TH and an 8q23.1-q24.12 interstitial deletion. It is not possible to exclude that this association is fortuitous, but our report reinforces the suggestion of a putative gene involved in limb development in this chromosomal region interval.

Cell Surface Heparan Sulfate Chains Regulate Local Reception of FGF Signaling in the Mouse Embryo

Heparan sulfate (HS) proteoglycans modulate the activity of multiple growth factors on the cell surface and extracellular matrix. However, it remains unclear how the HS chains control the movement and reception of growth factors into targeted receiving cells during mammalian morphogenetic processes. Here, we found that HS-deficient Ext2 null mutant mouse embryos fail to respond to fibroblast growth factor (FGF) signaling. Marker expression analyses revealed that cell surface-tethered HS chains are crucial for local retention of FGF4 and FGF8 ligands in the extraembryonic ectoderm. Fine chimeric studies with single-cell resolution and expression studies with specific inhibitors for HS movement demonstrated that proteolytic cleavage of HS chains can spread FGF signaling to adjacent cells within a short distance. Together, the results show that spatiotemporal expression of cell surface-tethered HS chains regulate the local reception of FGF-signaling activity during mammalian embryogenesis.

Mutations in the EXT1 and EXT2 genes in patients with multiple osteochondromas from Spain

Mutations in the EXT1 and EXT2 genes in patients with multiple osteochondromas from Spain

Growth plate regulation and osteochondroma formation: insights from tracing proteoglycans in zebrafish models and human cartilage

Proteoglycans are secreted into the extracellular matrix of virtually all cell types and function in several cellular processes. They consist of a core protein onto which glycosaminoglycans (e.g., heparan or chondroitin sulphates), are attached. Proteoglycans are important modulators of gradient formation and signal transduction. Impaired biosynthesis of heparan sulphate glycosaminoglycans causes osteochondroma, the most common bone tumour to occur during adolescence. Cytochemical staining with positively charged dyes (e.g., polyethyleneimine-PEI) allows, visualisation of proteoglycans and provides a detailed description of how proteoglycans are distributed throughout the cartilage matrix. PEI staining was studied by electron and reflection contrast microscopy in human growth plates, osteochondromas and five different proteoglycan-deficient zebrafish mutants displaying one of the following skeletal phenotypes: dackel (dak/ext2), lacking heparan sulphate and identified as a model for human multiple osteochondromas; hi307 (β3gat3), deficient for most glycosaminoglycans; pinscher (pic/slc35b2), presenting with defective sulphation of glycosaminoglycans; hi954 (uxs1), lacking most glycosaminoglycans; and knypek (kny/gpc4), missing the protein core of the glypican-4 proteoglycan. The panel of genetically well-characterized proteoglycan-deficient zebrafish mutants serves as a convincing and comprehensive study model to investigate proteoglycan distribution and the relation of this distribution to the model mutation status. They also provide insight into the distributions and gradients that can be expected in the human homologue. Human growth plate, wild-type zebrafish and fish mutants with mild proteoglycan defects (hi307 and kny) displayed proteoglycans distributed in a gradient throughout the matrix. Although the mutants pic and hi954, which had severely impaired proteoglycan biosynthesis, showed no PEI staining, dak mutants demonstrated reduced PEI staining and no gradient formation. Most chondrocytes from human osteochondromas showed normal PEI staining. However, approximately 10% of tumour chondrocytes were similar to those found in the dak mutant (e.g., lack of PEI gradients). The cells in the reduced PEI-stained areas are likely associated with loss-of-function mutations in the EXT genes, and they might contribute to tumour initiation by disrupting the gradients.

Clinical and molecular studies of EXT1/EXT2 in Bulgaria

EXT1/EXT2-CDG (Multiple cartilagineous exostoses, hereditary multiple osteochondroma (MO); OMIM 133700/133701) are common defects of O-xylosylglycan glycosylation. The diagnostic criteria are at least two osteochondromas of the juxta-epiphyseal region of long bones with in the majority of cases a positive family history and/or mutation in one of the EXT genes. The authors report data on clinical symptoms and complications of 23 patients (from 16 families), discussing the family history, age of diagnosis, new clinical and molecular data. Fifteen mutations and large deletions, of which nine are new, were detected in the EXT1 and EXT2 gene by sequence analysis, FISH and MLPA analysis.

Pregnancy-specific Glycoprotein 1 Induces Endothelial Tubulogenesis through Interaction with Cell Surface Proteoglycans

Pregnancy-specific β1 glycoproteins (PSGs) are the most abundant fetal proteins in the maternal bloodstream in late pregnancy. They are secreted by the syncytiotrophoblast and are detected around day 14 postfertilization. There are 11 human PSG genes, which encode a family of proteins exhibiting significant conservation at the amino acid level. We and others have proposed that PSGs have an immune modulatory function. In addition, we recently postulated that they are proangiogenic due to their ability to induce the secretion of VEGF-A and the formation of tubes by endothelial cells. The cellular receptor(s) for human PSGs remain unknown. Therefore, we conducted these studies to identify the receptor for PSG1, the highest expressed member of the family. We show that removal of cell surface glycosaminoglycans (GAGs) by enzymatic or chemical treatment of cells or competition with heparin completely inhibited binding of PSG1. In addition, PSG1 did not bind to cells lacking heparan or chondroitin sulfate on their surface, and binding was restored upon transfection with all four syndecans and glypican-1. Importantly, the presence of GAGs on the surface of endothelial cells was required for the ability of PSG1 to induce tube formation. This finding indicates that the PSG1-GAG interaction mediates at least some of the PSG1 proposed functions.

EXT1 (exostoses (multiple) 1)

Review on EXT1 (exostoses (multiple) 1), with data on DNA, on the protein encoded, and where the gene is implicated.

Gene expression pattern in canine mammary osteosarcoma

Canine mammary sarcomas are usually very aggressive and easily metastasize. Unfortunately the biology of this type of tumor is not well known because they are a very rare type of tumors. The aim of this study was to find differences in gene expression patterns in canine mammary osteosarcomas (malignant) versus osteomas (benign) using DNA microarrays. Our microarray experiment showed that 11 genes were up-regulated in osteosarcoma in comparison to osteoma whereas 36 genes were down-regulated. Among the up-regulated genes were: PDK1, EXT1, and EIF4H which are involved in AKT/PI3K and GLI/Hedgehog pathways. These genes play an important role in cell biology (cancer cell proliferation) and may be essential in osteosarcoma formation and development. Analyzing the down-regulated genes, the most interesting seemed to be HSPB8 and SEPP1. HSPB8 is a small heat shock protein that plays an important role in cell cycle regulation, apoptosis, and breast carcinogenesis. Also SEPP1 may play a role in carcinogenesis, as its down-regulation may induce oxidative stress possibly resulting in carcinogenesis. The preliminary results of the present study indicate that the up-regulation of three genes EXT1, EIF4H, and PDK1 may play an essential role in osteosarcoma formation, development and proliferation. In our opinion the cross-talk between GLI/Hedgehog and PI3K/AKT pathways may be a key factor to increase tumor proliferation and malignancy.

Tiling resolution array-CGH shows that somatic mosaic deletion of the EXT gene is causative in EXT gene mutation negative multiple osteochondromas patients

Multiple osteochondromas (MO) is a hereditary skeletal disorder characterized by the presence of cartilage capped bony outgrowths at bone surface. Causative mutations in EXT1 or EXT2 genes have been described in 85-90 % of MO cases. However, in about 10-15 % of the MO cases, genomic alterations can not be detected, implying the potential role of other alterations. We have designed a custom-made Agilent oligonucleotide-based microarray, containing 44,000 probes, with tiling coverage of EXT1/2 genes and addition of 68 genes involved in heparan sulfate biosynthesis and other related pathways. Out of the 17 patient samples with previously undetected mutations, a low level of deletion of the EXT1 gene in about 10-15% of the blood cells was detected in two patients and mosaic deletion of the EXT2 was detected in one patient. Here we show that for the first time somatic mosaicism with large genomic deletions as the underlying mechanism in MO formation was identified. We propose that the existence of mosaic mutations and not alterations of other heparan sulfate biosynthesis related genes play a significant role in the development of MO in patients who are tested negative for mutations in Exostosins.

Podocytes require the engagement of cell surface heparan sulfate proteoglycans for adhesion to extracellular matrices

Podocytes adhere to the glomerular basement membrane by cell surface receptors. Since in other cells these adhesions are enhanced by cell surface proteoglycans, we examined the contribution of these molecules and their glycosaminoglycan side chains to podocyte adhesion by developing immortalized podocyte cell lines with (control) or without (mutant) heparan sulfate glycosaminoglycan chains. In adhesion assays control podocytes attached, spread, and migrated more efficiently compared with mutants, indicating a requirement for heparan sulfate chains in these processes. The proteoglycan syndecan-4 is known to have direct effects on cell attachment, spreading, and cytoskeletal organization. We found it localized to focal adhesions in control podocytes coincident with stress fiber formation. In mutant cells, syndecan-4 was associated with smaller focal contacts and cortical actin organization. Analysis by flow cytometry showed that mutant cells had twice the amount of surface syndecan-4 of control cells. Protein kinase Cα, a signaling molecule bound to and activated by syndecan-4, showed a fourfold increase in membrane localization-activation than that seen in control cells. In vivo, the loss of heparan sulfate glycosaminoglycans in PEXTKO mice led to a loss of glomerular syndecan-4. Overall, our study provides further evidence for a dynamic role of cell surface heparan sulfate glycosaminoglycans in podocyte activity.

MicroRNA profiling of multiple osteochondromas: identification of disease‐specific and normal cartilage signatures

Multiple osteochondroma (MO) is a rare skeletal disease characterized by the formation of multiple benign cartilage-capped bone tumors; in 1-5% of patients, a malignant transformation into peripheral chondrosarcoma may occur. This disorder is characterized by a large spectrum of germline mutations scattered along EXT1/EXT2 genes, the presence of a significant percentage of patients without alterations in EXT genes, and a large phenotypic variability. The molecular basis of MO genetic and clinical heterogeneity, including the causes underlying malignant transformation, is currently unknown. This leads to the lack of appropriate diagnostic/prognostic markers as well as of therapeutic options. Recently, specific microRNAs (miRNAs) were reported to be involved in chondrogenesis and inflammatory cartilage diseases. We therefore hypothesized a role for microRNAs in cartilaginous tumors and investigated microRNA expression in osteochondroma and normal cartilage tissues to evaluate whether they could affect osteochondromas onset and/or clinical manifestations. Our results indicate that miRNAs differentially expressed in MO samples may hamper the molecular signaling responsible for normal differentiation of chondrocytes, contributing to pathogenesis and clinical outcome. Although further studies are needed to validate our observations and to identify targets of miRNAs, this is the first study reporting on miRNA expression in growth plate and its comparison with pathological conditions.

Gene mutation screening and the genotype-phenotype correlation of hereditary multiple exostoses

Objective To establish the method of gene mutation screening for HME and investigate the relationship between genotype and clinical phenotype in HME patients. Methods Fifteen cases of HME probands were divided into the following four subgroups: mild (M) and severe ( Ⅰ S, Ⅱ S, Ⅲ S) according to the clinical diagnosis. DNA samples were obtained from the probands and family members. All of the EXT1 and EXT2 gene exons and their boundary sequences were amplified by PCR, and sequenced by directsequencing. Then the relationship between the genotypes and clinical phenotype was analyzed. Results Among the fifteen cases of HME probands, nine harbored EXT1 gene mutation, while the other 6 were positive for EXT2 gene mutation. Moreover, six novel mutations in EXT1 gene, including I8 + 2T ＞ G, c. 1182delG,c. 1108G ＞T(p. E370X) ,c. 335delA,c. 361C ＞T(p. Q121X) and c. 1879_1881delCAC were identified. In 9 patients with EXT1 gene mutation, 2 (22. 2% ) were M-type, 2 (22. 2% ) were Ⅰ S -type, 4 (44. 4% )were Ⅱ S-type,and 1 ( 11.1% ) was ⅢS-type. Whereas, 5 cases (83.3%) were M-type and only one case was Ⅱ S-type( 16. 7% ) in 6 patients with EXT2 gene mutation. Conclusions An accurate and simple gene diagnostic method for HME was established. Six novel EXT1 gene mutations, including I8 + 2T ＞ G,c. 1182delG, c. 1108G ＞T(p. E370X), c. 335delA, c. 361C ＞T(p. Q121X)and c. 1879_1881delCAC were identified as well. The clinical phenotype of the patients with EXT1 gene mutation was more severe compared to those with EXT2 gene mutations. Key words: Exostoses, multiple hereditary; N-Acetylglucosaminyltransferase; Genes, tumor suppressor; Mutation; Genotype; Phenotype

No Haploinsufficiency but Loss of Heterozygosity for EXT in Multiple Osteochondromas

Multiple osteochondromas (MO) is an autosomal dominant disorder caused by germline mutations in EXT1 and/or EXT2. In contrast, solitary osteochondroma (SO) is nonhereditary. Products of the EXT gene are involved in heparan sulfate (HS) biosynthesis. In this study, we investigated whether osteochondromas arise via either loss of heterozygosity (2 hits) or haploinsufficiency. An in vitro three-dimensional chondrogenic pellet model was used to compare heterozygous bone marrow-derived mesenchymal stem cells (MSCs EXT(wt/-)) of MO patients with normal MSCs and the corresponding tumor specimens (presumed EXT(-/-)). We demonstrated a second hit in EXT in five of eight osteochondromas. HS chain length and structure, in vitro chondrogenesis, and EXT expression levels were identical in both EXT(wt/-) and normal MSCs. Immunohistochemistry for HS, HS proteoglycans, and HS-dependent signaling pathways (eg, TGF-β/BMP, Wnt, and PTHLH) also showed no differences. The cartilaginous cap of osteochondroma contained a mixture of HS-positive and HS-negative cells. Because a heterozygous EXT mutation does not affect chondrogenesis, EXT, HS, or downstream signaling pathways in MSCs, our results refute the haploinsufficiency theory. We found a second hit in 63% of analyzed osteochondromas, supporting the hypothesis that osteochondromas arise via loss of heterozygosity. The detection of the second hit may depend on the ratio of HS-positive (normal) versus HS-negative (mutated) cells in the cartilaginous cap of the osteochondroma.

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.

Primary cilia organization reflects polarity in the growth plate and implies loss of polarity and mosaicism in osteochondroma

Primary cilia are specialized cell surface projections found on most cell types. Involved in several signaling pathways, primary cilia have been reported to modulate cell and tissue organization. Although they have been implicated in regulating cartilage and bone growth, little is known about the organization of primary cilia in the growth plate cartilage and osteochondroma. Osteochondromas are bone tumors formed along the growth plate, and they are caused by mutations in EXT1 or EXT2 genes. In this study, we show the organization of primary cilia within and between the zones of the growth plate and osteochondroma. Using confocal and electron microscopy, we found that in both tissues, primary cilia have a similar formation but a distinct organization. The shortest ciliary length is associated with the proliferative state of the cells, as confirmed by Ki-67 immunostaining. Primary cilia organization in the growth plate showed that non-polarized chondrocytes (resting zone) are becoming polarized (proliferating and hypertrophic zones), orienting the primary cilia parallel to the longitudinal axis of the bone. The alignment of primary cilia forms one virtual axis that crosses the center of the columns of chondrocytes reflecting the polarity axis of the growth plate. We also show that primary cilia in osteochondromas are found randomly located on the cell surface. Strikingly, the growth plate-like polarity was retained in sub-populations of osteochondroma cells that were organized into small columns. Based on this, we propose the existence of a mixture (‘mosaic’) of normal lining (EXT+/− or EXTwt/wt) and EXT−/− cells in the cartilaginous cap of osteochondromas.