MUTATION ANALYSIS IN HEREDITARY MULTIPLE EXOSTOSES SUPPORTS A NEOPLASTIC PATHOGENESIS IN THIS CONDITION

Abstract

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:

1. the existence of an autosomal dominant inherited multiple form, in which lesions are histologically identical to the solitary form.

2. lesions which are distributed randomly and perhaps asymmetrically at ‘high-risk’ anatomical sites (usually adjacent to those physes with greatest growth potential).

3. evidence of behavioural or cellular disorder.

4. 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.

Results : All missense mutations had previously been reported in the literature. In contrast, only 9 of 34 loss-of function mutations (frame-shift, splice-site and nonsense) had previously been reported.

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.