👤 K Szuhai

Multiple osteochondromas (MO) is a syndrome in which benign cartilage-capped neoplasms develop at the surface of the long bones. Most cases are caused by exonic changes in EXT1 or EXT2, but 15% are negative for these changes. Here we report for the first time a family of MO patients with germline genomic alterations at the EXT1 locus without detectable mutations or copy number alterations of EXT exonic sequences. Array-CGH showed an 80.7 kb deletion of Intron 1 of EXT1 and a 68.9 kb duplication proximal of EXT1. We identified a breakpoint between the distal end of the duplicated region and a sequence distal of the deleted region in the first intron. This breakpoint was absent in non-affected family members. The configuration of the breakpoint indicates a direct insertion of the duplicated region into the deletion. However, no other breakpoint was found, which suggests a more complex genomic rearrangement has occurred within the duplicated region. Our results reveal intronic deletion and duplication as a new causative mechanism for MO not detected by conventional diagnostic methods. Show less

Secondary peripheral chondrosarcoma is the result of malignant transformation of a pre-existing osteochondroma, the most common benign bone tumor. Osteochondromas are caused by genetic abnormalities in EXT1 or EXT2: homozygous deletion of EXT1 characterizes sporadic osteochondromas (non-familial/solitary), and germline mutations in EXT1 or EXT2 combined with loss of heterozygosity define hereditary multiple osteochondromas. While cells with homozygous inactivation of EXT and wild-type cells shape osteochondromas, the cellular composition of secondary peripheral chondrosarcomas and the role of EXT in their formation have remained unclear. We report using a targeted-tiling-resolution oligo-array-CGH (array comparative genomic hybridization) that homozygous deletions of EXT1 or EXT2 are much less frequently detected (2/17, 12%) in sporadic secondary peripheral chondrosarcomas than expected based on the assumption that they originate in sporadic osteochondromas, in which homozygous inactivation of EXT1 is found in ~80% of our cases. FISH with an EXT1 probe confirmed that, unlike sporadic osteochondromas, cells from sporadic secondary peripheral chondrosarcomas predominantly retained one (hemizygous deleted loci) or both copies (wild-type) of the EXT1 locus. By immunohistochemistry, we confirm the presence of cells with dysfunctional EXT1 in sporadic osteochondromas and show cells with functional EXT1 in sporadic secondary peripheral chondrosarcomas. These immuno results were verified in osteochondromas and secondary peripheral chondrosarcomas in the setting of hereditary multiple osteochondromas. Our data therefore point to a model of oncogenesis in which the osteochondroma creates a niche in which wild-type cells with functional EXT are predisposed to acquire other mutations giving rise to secondary peripheral chondrosarcoma, indicating that EXT-independent mechanisms are involved in the pathogenesis of secondary peripheral chondrosarcoma. Show less

Osteochondromas (cartilage-capped bone tumors) are by far the most commonly treated of all primary benign bone tumors (50%). In 15% of cases, these tumors occur in the context of a hereditary syndrome called multiple osteochondromas (MO), an autosomal dominant skeletal disorder characterized by the formation of multiple cartilage-capped bone tumors at children's metaphyses. MO is caused by various mutations in EXT1 or EXT2, whereby large genomic deletions (single-or multi-exonic) are responsible for up to 8% of MO-cases. Here we report on the first molecular characterization of ten large EXT1- and EXT2-deletions in MO-patients. Deletions were initially identified using MLPA or FISH analysis and were subsequently characterized using an MO-specific tiling path array, allele-specific PCR-amplification and sequencing analysis. Within the set of ten large deletions, the deleted regions ranged from 2.7 to 260 kb. One EXT2 exon 8 deletion was found to be recurrent. All breakpoints were located outside the coding exons of EXT1 and EXT2. Non-allelic homologous recombination (NAHR) mediated by Alu-sequences, microhomology mediated replication dependent recombination (MMRDR) and non-homologous end-joining (NHEJ) were hypothesized as the causal mechanisms in different deletions. Molecular characterization of EXT1- and EXT2-deletion breakpoints in MO-patients indicates that NAHR between Alu-sequences as well as NHEJ are causal and that the majority of these deletions are nonrecurring. These observations emphasize once more the huge genetic variability which is characteristic for MO. To our knowledge, this is the first study characterizing large genomic deletions in EXT1 and EXT2. Show less

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. Show less

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. Show less

The tumor suppressor genes EXT1 and EXT2 are involved in the formation of multiple osteochondromas, which can progress to become secondary peripheral chondrosarcomas. The most common chondrosarcoma subtype is primary central chondrosarcoma, which occurs in the medullar cavity of bone. The EXT1/EXT2 protein complex is involved in heparan sulfate proteoglycan (HSPG) biosynthesis, which is important for signal transduction of Indian hedgehog (IHH), WNT, and transforming growth factor (TGF)-beta. The role of EXT and its downstream targets in central chondrosarcomas is currently unknown. EXT1 and EXT2 were therefore evaluated in central chondrosarcomas at both the DNA and mRNA levels. Immunohistochemistry was used to assess HSPG (CD44v3 and SDC2), WNT (beta-catenin), and TGF-beta (PAI-1 and phosphorylated Smad2) signaling, whereas IHH signaling was studied both by quantitative polymerase chain reaction and in vitro. mRNA levels of both EXT1 and EXT2 were normal in central chondrosarcomas; genomic alterations were absent in these regions and in 30 other HSPG-related genes. Although HSPGs were aberrantly located (CD44v3 in the Golgi and SDC2 in cytoplasm and nucleus), this was not caused by mutation. WNT signaling negatively correlated with increasing histological grade, whereas TGF-beta positively correlated with increasing histological grade. IHH signaling was active, and inhibition decreased cell viability in one of six cell lines. Our data suggest that, despite normal EXT in central chondrosarcomas, HSPGs and HSPG-dependent signaling are affected in both central and peripheral chondrosarcomas. Show less

Multiple osteochondromas is a hereditary syndrome that is characterized by the formation of cartilage-capped bony neoplasms (osteochondromas), for which exostosis (multiple)-1 (EXT1) has been identified as a causative gene. However, 85% of all osteochondromas present as solitary (nonhereditary) lesions in which somatic mutations in EXT1 are extremely rare, but loss of heterozygosity and clonal rearrangement of 8q24 (the chromosomal locus of EXT1) are common. We examined whether EXT1 might act as a classical tumor suppressor gene for nonhereditary osteochondromas. Eight nonhereditary osteochondromas were subjected to high-resolution array-based comparative genomic hybridization (array-CGH) analysis for chromosome 8q. The array-CGH results were validated by subjecting tumor DNA to multiple ligation-dependent probe amplification (MLPA) analysis for EXT1. EXT1 locus-specific fluorescent in situ hybridization (FISH) was performed on nuclei isolated from the three tissue components of osteochondroma (cartilage cap, perichondrium, bony stalk) to examine which parts of the tumor are of clonal origin. Array-CGH analysis of tumor DNA revealed that all eight osteochondromas had a large deletion of 8q; five tumors had an additional small deletion of the other allele of 8q that contained the EXT1 gene. MLPA analysis of tumor DNA confirmed these findings and identified two additional deletions that were smaller than the limit of resolution of array-CGH. FISH analysis of the cartilage cap, perichondrium, and bony stalk showed that these homozygous EXT1 deletions were present only in the cartilage cap of osteochondroma. EXT1 functions as a classical tumor suppressor gene in the cartilage cap of nonhereditary osteochondromas. Show less