Genetics of the idiopathic inflammatory myopathies.

Genetic predisposition to development of the idiopathic inflammatory myopathies is probably multifactorial. Major histocompatibility complex associations with these diseases provide the strongest evidence for a genetic component. In Caucasoids, haplotypes marked by B8/DR3 are associated with each of the clinical subgroups, except mixed connective tissue disease (DR4). The strongest associations are with inclusion body myositis, polymyositis in the presence of anti-Jo-1, and with antibodies to PM-Scl in overlap syndromes. The underlying mechanisms of these associations are probably different. Unique major histocompatibility complex associations are seen with other myositis-associated autoantibodies. The association can vary between racial groups as can the type of autoantibody produced within a disease subgroup, perhaps reflecting different T cell receptor repertoires or different inducing agents. The mapping of a gene for one form of hereditary inclusion body myositis to chromosome 9p1-q1 provides a lead for the investigation of sporadic inclusion body myositis, as does the expanding knowledge of genetic factors in Alzheimer's disease. The demonstration of deletions of mitochondrial DNA in the muscle of patients with inclusion body myositis raises the question of their role in the pathogenesis of the disease.