Chondrodysplasias are a mixed group of hereditary disorders affecting cartilage. They are manifested by short-limbed dwarfism and numerous skeletal deformities. A few are due to a variety of mutations in the COL2A1 gene, leading to abnormal forms of type II collagen. One example is the Stickler syn drome, manifested by degeneration of the joint cartilage and of the vitreous body of the eye.
The best known of the chondrodysplasias is achondroplasia, the most common cause of short-limbed dwarfism. Affected individuals have short limbs, normal trunk size, macrocephaly, and a variety of other skeletal abnormalities.
The condition is often inherited as an autosomal dominant trait, but many cases are due to new mutations. Achondroplasia is not a collagen disorder but is due to mutations in the gene encoding fibroblast growth factor receptor 3 (FGFR3). Fibroblast growth factorsare a family of more than 20 proteins that affect the growth and differentiation of cells of mesenchymal and neuroectodermal origin. Their receptors are trans membrane proteins and form a subgroup of four in the family of receptor tyrosine kinases. FGFR3 is one member of this sub group and mediates the actions of FGF3 on cartilage. In almost all cases of achondroplasia that have been investigated, the mutations were found to involve nucleotide 1138 and resulted in substitution of arginine for glycine (residue number 380) in the transmembrane domain of the protein, rendering it inactive. No such mutation was found in unaffected individuals.
Other mutations in the same gene can result in hypo chondroplasia, thanatophoric dysplasia (types I and II) (other forms of short-limbed dwarfism), and the SADDAN phenotype (severe achondroplasia with developmental delay and acanthosis nigricans [the latter is a brown to black hyper pigmentation of the skin]).
As indicated in Table 1, other skeletal dysplasias (including certain craniosynostosis syndromes) are also due to mutations in genes encoding FGF receptors. Another type of skeletal dysplasia, diastrophic dysplasia has been found to be due to mutation in a sulfate transporter.
Table1. Some Metabolic & Genetic Diseases Affecting Bone & Cartilage
