It has been known for some time that osteo-arthritis runs in families, but to what extent this is due to shared genetic factors or to family environment is still uncertain. In recent years, there has been a lot of interest in searching for the genetic basis of OA. The disease is clearly multifactorial and polygenetic; that is, it results from the interaction of a number of genes. This fact and the late onset of the disease make linkage studies and identification of susceptibility genes very difficult. Genes coding for the extracellular matrix proteins of cartilage have been obvious candidates. A defect in type II collagen has been demonstrated in a Finnish family with a very early onset of OA , but this is not a general defect in OA [10,50]. Moreover, the disease in the Finnish family was probably secondary to mild skeletal dysplasia. Indeed, OA forms part of several inherited chondrodysplasias, conditions in which mutations in collagen genes have been identified (reviewed in ). In recent years, the search for genetic susceptibility genes has turned to study genes whose expression is involved in cartilage development and homeostasis. An example is the gene for secreted frizzled-related protein 3, involved in the wnt signaling pathway. Other examples are the genes for asporin or calmod-ulin 1, molecules involved in maintaining cartilage matrix. For further discussion, see Chapter 8.
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