Genes and arthritis.
Familial clustering is a common feature of many different arthritic conditions which can be explained by shared genetic or environmental influences or a combination of these. The aetiology of some of these diseases (e.g. multiple epiphyseal dysplasia, Lesch-Nyan syndrome) is clearly monogenic but the majority of common rheumatic conditions, such as osteoarthritis (OA) and rheumatoid arthritis (RA) are complex and multi-factorial with a polygenic component. In the last 10 years there has been spectacular progress in unravelling the underlying genetic mechanisms of many monogenic disorders including many that affect the musculoskeletal system. Among these osteogenesis imperfecta (OI) and the Marfan syndrome are striking examples in which molecular defects of matrix protein components of the mesodermal tissues have been revealed. In particular, the power of modern molecular genetics both to test candidate genes (e.g. collagen in OI) and to identify genes where no prior knowledge of the protein defects exists (e.g. fibrillin in the Marfan syndrome) has proved quite remarkable. In the near future many of the techniques that have been applied successfully to these monogenic diseases can be expected to provide insights into the genetic component of common diseases, such as rheumatoid arthritis. However, it is important to realize before embarking on these studies that even monogenic diseases may provide significant problems of analysis and interpretation. Some of these problems are exemplified below (e.g. phenocopies, mosaicism, imprinting).