Overlap syndromes and mixed connective tissue disease.

While the etiology of connective tissue diseases remains unknown, the classification of individual cases will continue to depend on identifying certain patterns of clinical and laboratory features. As many as 25% of connective tissue disease patients present with an overlap syndrome with features of systemic lupus erythematosus, systemic sclerosis, polymyositis, or dermatomyositis, with rheumatoid arthritis and Sjögren's syndrome evolving concurrently or consecutively during the course of the disease. The term overlap syndrome is applied to what appears to be a heterogeneous group of disorders, but in recent years there have been attempts to identify antibody markers within this population to identify subsets with particular patterns of disease expression. Thus, anti-U1-ribonucleoprotein is associated with overlap syndromes in which features of systemic lupus erythematosus are accompanied by features of systemic sclerosis or myositis; antibodies to polymyositis-scleroderma, Ku, and U2-ribonucleoprotein are associated with overlaps of systemic sclerosis and polymyositis, and anti-Jo-1 is associated with polymyositis and pulmonary fibrosis. A practical reason for subdividing cases in this way relates to prognosis and treatment, but at a more fundamental level it is hoped that the study of the origin of these antibodies and their antigen targets will provide clues to pathogenesis and even etiology.