Hereditary multiple exostoses and enchondromatosis.

Hereditary multiple exostoses (HME) is an autosomal-dominant disorder characterized by the development of benign tumours, multiple osteochondromas (exostoses), growing outward from the metaphyses of long bones. Birth prevalence is estimated to be one in 50,000, and the severity of the disease is variable. Osteochondromas may cause complications including pain, deformities and shortening of the long bones, restricted motion of joints, nerve or blood vessel compression, and malignant transformation (5% of cases) in adulthood. HME is a genetically heterogeneous disorder and is associated with mutations in EXT1 or EXT2 genes, which are both tumour suppressor genes. EXT genes encode glycosyltransferases, termed 'exostosins', which are involved in the biosynthesis of heparan sulphate. Enchondromatosis (or Ollier disease) is characterized by the presence of intra-osseous benign cartilaginous tumours. The estimated prevalence of the disease is one in 100,000. An asymmetrical distribution of cartilage lesions is observed in the disease. The number, size and location of the enchondromas can be extremely variable between patients. Clinical problems caused by enchondromas include skeletal deformities, limb length discrepancy, pain and the potential risk for malignant change to chondrosarcoma (20-50% of cases). The condition in which multiple enchondromas is associated with haemangiomas is known as 'Maffucci syndrome'. Ollier disease and Maffucci syndrome are not usually inherited disorders.