Osteochondroma is a cartilage-capped bony projection arising from the external surface of bone, containing a marrow cavity that is continuous with that of the underlying bone (1). Osteochondroma is the most common benign bone tumour, mainly arising in the juxta-epiphyseal region of long bones (1). Approximately 15% of osteochondromas occur in the context of multiple osteochondromas (MO), previously referred to as hereditary multiple exostoses, which is an autosomal dominantly inherited disorder (2,3), while the majority of osteochondromas present as solitary (non-hereditary) lesions. Solitary and multiple osteochondromas are histologically indistinguishable (4). Osteochondromas have been associated with defects in the exostosin glycosyltransferase (EXT) 1 gene on chromosome 8q24.11-q24.13 (MIM 608177) and EXT2 on chromosome 11p12-p11 (MIM 608210) (5–7).

Osteochondroma may be an incidental finding, or diagnosed due to secondary events, such as esthetic or mechanical problems. Osteochondromas of the mandibular condyle have been associated with facial asymmetry, preauricular pain or edema, occlusal disorders, limitation of mouth opening, even condylar neck fracture. The most severe complication is malignant transformation into chondrosarcoma. According to Roychoudhury et al (8), by 2011 at least 108 cases had been reported in the English-language literature; those studies were mainly focused on clinical reports, such as clinical manifestations, radiological findings, treatment and prognosis, whereas the study on pathogenesis and molecular biology remains at the initial stage. However, it is not known whether the EXT genes are also involved in condylar osteochondromas. We performed a mutation analysis of the EXT1 and EXT2 genes in 12 cases of solitary osteochondroma of the condyle, to estimate the distribution of mutations that lead to the development of condylar osteochondromas.

Osteochondroma was previously considered as a perversion in the direction of normal bone growth, resulting from aberrant epiphyseal development (9). However, later studies demonstrated that loss or mutation of EXT1 or EXT2 are crucial in the pathogenesis of solitary as well as hereditary osteochondromas (10,11). Germline mutations of either the EXT1 or EXT2 gene may be identified in >85% of the analyzed MO cases and MO families (12–16). Approximately 77–80% of intragenic EXT1 and EXT2 mutations are inactivating mutations (44–66% associated with EXT1 and 27% with EXT2) (16,17). These alterations result in truncated (non-functional) EXT proteins (18,19). The protein products of EXT1 and EXT2 are type II transmembrane glycoproteins and comprise a Golgi-localized hetero-oligomeric complex involved in heparan sulphate proteoglycan (HSPG) biosynthesis. Hameetman et al (20) found that EXT1 or EXT2 mRNA expression in osteochondromas was decreased; however, in non-hereditary tumors, only decreased EXT1 mRNA expression was detected. Decreased EXT1 or EXT2 mRNA expression in osteochondromas was associated with intracellular accumulation of HSPGs in the Golgi apparatus. It has been shown that a lack of HSPGs on the cell surface affects growth signaling pathways in the growth plate [e.g., Indian hedgehog (IHH) signaling] and, possibly, those in osteochondromas (21–23). In the growth plate, IHH requires interaction with HSPGs to diffuse through the extracellular matrix to its receptor (21).

Somatic mutations in the EXT genes are extremely rare in non-hereditary osteochondromas and have been described in only 3 cases (24–26). However, the observation that loss of heterozygosity (LOH) and clonal rearrangement at 8q24 (EXT1 locus) are as frequent in non-hereditary osteochondromas as are EXT1 gene mutations in patients with hereditary osteochondromas, suggests that EXT1 may be involved in the development of non-hereditary osteochondromas (10,11,27). By contrast, LOH at the EXT2 locus has been reported in only 1 case of non-hereditary osteochondroma (28).

Structural changes in the EXT1 locus have been reported in 10/30 non-hereditary and in 1/13 hereditary osteochondromas (10,11). LOH detected by microsatellite analysis using DNA isolated from the cartilaginous cap was found almost exclusively at the EXT1 locus (27). Fluorescence in situ hybridization revealed loss of the 8q24.1 locus in 27/34 (79%) osteochondromas (29). Finally, in 7/8 solitary osteochondromas, homozygous deletion of EXT1 was detected (30). Of note, EXT2 was found to be affected only in MO and not in solitary lesions. In our study, we analyzed 12 patients with solitary osteochondroma for the presence of mutations in the EXT1 and EXT2 genes. A novel nonsense mutation (chr11_44117899_G/T in the intronic region in cases 7 and 12) was identified. The other 7 variations were already in the databases. Apart from known polymorphisms, no sense somatic or germline mutations were detected in tumor DNA. The results were consistent with the rarity of EXT gene mutations in non-hereditary osteochondromas. As the LOH and clonal rearrangement in EXT genes are common, we should analyse the incidence of deletions and rearrangement in condylar osteochondromas.

In conclusion, we screened EXT1 and EXT2 mutations in 12 patients with non-hereditary osteochondromas of the mandibular condyle. To the best of our knowledge, this is the first study to identify mutations of EXT1 and EXT2 in condylar osteochondromas. Our study extended the mutational spectrum in EXT1 and EXT2 and may facilitate a better understanding of the pathophysiology of condylar osteochondromas.