To determine whether the spectrum of genetic mutation in Hereditary Multiple Exostoses supports a neoplastic aetiology for this condition. Historically, experts have been cautious in attributing neoplastic qualities to the osteochondroma. Solomon states ‘[osteochondromas] are not neoplastic in the ordinary sense of the word’; Morton that it ‘is not a tumour but a growth-aberration; Peterson that the ‘osteochondroma is not a true neoplasm’; and Schmale that ‘exostoses are the result of dysplasia of the lateral apect of the growth-plate’. There are, however, several features of osteochondroma behaviour common to other neoplasms which suggest a neoplastic aetiology:
the existence of an autosomal dominant inherited multiple form, in which lesions are histologically identical to the solitary form. lesions which are distributed randomly and perhaps asymmetrically at ‘high-risk’ anatomical sites (usually adjacent to those physes with greatest growth potential). evidence of behavioural or cellular disorder. a potential for malignant transformation. Recent genetic data has supported a fresh look at the neoplastic nature of osteochondromas. EXT1 and EXT2 genes are responsible for Hereditary Multiple Exostoses (HME). EXT1 codes for a protein which alters the synthesis and display of cell-surface heparan sulphate glycosaminoglycans; molecules which affect cellular growth, differentiation, motility and adhesion. Loss-of-function of such a gene may initiate a neoplastic pathogenesis in osteochondromas. From 1996–1999, 51 families with HME were screened for EXT mutation, with mutations identified in 41 families. EXT mutation was assessed by means of fluorescent single-strand conformational polymorphism (f-SSCP) screening, followed by sequencing analysis.
All frame-shift, splice-site and nonsense mutations are loss-of-function. Missense mutations may result in partial or complete dysfunction if a crucial folding or binding site is involved. Since no missense mutations were new, this suggested their mutation sites are important, and may effectively result in loss-of-function. These data strongly support a tumour suppressor gene function for EXT genes, and a neoplastic pathogenesis for HME.
To identify if disease severity and cancer-risk might depend on genotype in Hereditary Multiple Exostoses (HME). The discovery that the EXT family of tumour suppressor genes is responsible for Hereditary Multiple Exostoses (HME) now enables correlation of clinical features with genetic defects. Genetic epidemiological studies, such as this, may provide additional data of use to the clinician. In most population-based HME cohorts, the incidence of sarcomatous degeneration has been estimated as 1–5%. This is not high, but occurs at a younger age (on average 2–3 decades younger) than chondrosarcoma in the general population. Genetic stratification might allow a very high-risk subgroup to be identified, within which surveillance for neoplastic change in osteochondromas could be concentrated. In a pilot study, 29 affected individuals from 17 families with HME were screened for EXT mutation, with mutations identified in 12 families. Pedigrees were obtained and a complete assessment of disease severity made. We have since expanded this cohort; a further 71 affected individuals from 34 families with HME have provided detailed pedigree data and undergone a simple clinical examination to assess number of palpable osteo-chondromas. EXT mutation was assessed by means of fluorescent single-strand conformational polymorphism (f-SSCP) screening, followed by sequencing analysis.
Validation of clinical examination : In those who underwent radiographic examination for clinical purposes, number of palpable osteochondromas correlated strongly with number seen on radiographs at 146 anatomical sites (r= 0.814, p<
0.001), validating the usefulness of clinical examination in a population analysis, and negating the need for a radiographic skeletal survey in individuals at risk from malignant change. EXT mutation : EXT mutation detection rates for f-SSCP were calculated to be 93%. As suggested in the pilot study, most (84%) were loss-of-function mutations. 60% had not previously been reported in the literature. There were 42 individuals with EXT1 and 29 with EXT2 mutations. Disease severity and EXT mutation: In the pilot study, median number of palpable osteochondromas were about twice as frequent in the 7 families with EXT1 mutation than in the 5 families with EXT2 mutation (p<
0.05). This was also reflected in overall disease severity scores. In the larger follow-up study, individuals with EXT1 mutation had a median number of 32 osteochondromas, compared with 16 osteochondromas in those with EXT2 mutation (Wilcoxon rank sum test p<
0.0005). Cancer risk: Six chondrosarcomas occurred, and were only found in individuals with EXT1 mutation. The observation that osteochondromas are more frequent in patients with EXT1 than EXT2 mutations is an important message in genetic counselling. If disease severity and cancer risk is greater in individuals with EXT1 mutation, screening for neoplastic change might be targeted on this group, in which lifetime risk of malignant change in osteochondromas could be increased to between 5% and 10%.
