Mutations in the EXT1 and EXT2 genes in Spanish patients with multiple osteochondromas

Multiple osteochondromas (MO, MIM#133700, #133701), also known as multiple hereditary exostoses, is an autosomal dominant skeletal disorder characterized by the formation of multiple cartilage-capped tumours (exostoses or osteochondromas). The prevalence of MO is estimated at 1/50,000 in the Western population1. Osteochondromas are the result of excessive chondrocyte proliferation and bone growth at the juxtaepiphyseal regions of long bones and are the most common benign bone tumour. MO is characterized by an important inter- and intra-familial phenotypic variability, including variations in the number and size of osteochondromas and in the number and location of bones involved. Secondary complications are heterogeneous too, and may consist of deformities, functional limitations, compression of nerves and blood vessels, pain caused by pressure on neighbouring tissues and short stature2. The most serious secondary complication is the malignant transformation toward a secondary peripheral chondrosarcoma, occurring in 0.5–5% of patients3. MO is a genetically heterogeneous disease. In almost 90% of MO patients, germline mutations in EXT1 (MIM #608177)4 or EXT2 (MIM #608210)5,6 are found. EXT1 maps to chromosome 8q24.11–q24.13, comprises 11 exons and spans approximately 350 kb7, while EXT2, located at 11p11–p11.2, consists of 16 exons and spans almost 108 kb8. Both genes act as tumour suppressors. Involvement of other genes has been suggested, since there are families with no mutation identified in either EXT1 or EXT2. The EXT genes encode ubiquitous glycosyltransferases, catalyzing heparan sulfate (HS) polymerization at HS proteoglycans (HSPGs). These HSPGs play a major role in the diffusion of signalling molecules such as Indian Hedgehog (Ihh), which is an important regulator of chondrocyte proliferation and differentiation in the growth plate9. Approximately 10% of patients have a de novo mutation10. Loss of the wild-type allele in hereditary cases indicates that inactivation of both EXT alleles is required for osteochondroma formation11, confirming their tumour suppressor action that results in a loss of chondrocyte polarization12. However, the inactivation of both alleles probably occurs only in some of the cells in the cartilaginous cap of osteochondromas12,13. Several studies have reported on MO causing mutations in different populations. These are being gathered in the Multiple Osteochondromas Mutation Database (MOdb) (http://medgen.ua.ac.be/LOVDv.2.0/), currently listing over 400 and 200 different mutations in EXT1 and EXT2, respectively. Most of the mutations (80%) are nonsense, frame-shift and splice-site mutations, resulting in a premature termination of translation, or involve partial or total deletion of the gene. Although it has been suggested that mutations in EXT1 are associated with a more severe phenotype than mutations in EXT214,15,16, many aspects of the phenotypic variability observed in patients have yet to be understood at the genetic level. As suggested by the Human Variome Project initiative, characterization of causative mutations in familial and sporadic cases in diverse populations is needed for full understanding of Mendelian diseases17. Here we present the mutational analysis of 39 unrelated Spanish MO patients and the clinical features of most of them. This is the first report of MO mutations in the Spanish population.