Overlap syndromes with features of both PM and SSc are associated with PM-Scl and Ku antibodies. However, negative tests for these AABs do not exclude the possibility of PM-scleroderma overlap syndrome.
PM-Scl antibodies are directed against components of the exosome, a complex consisting of 11-16 proteins (20-110 kDa) located in the granular part of the nucleoli and in the nucleoplasm. The main targets of PM-Scl antibodies are proteins of 100 kDa (PM-Scl-100) and of 75 kDa (PM-Scl-75). PM-Scl antibodies are highly characteristic of, although not specific for, PM-scleroderma overlap syndrome . They are detectable in approximately 24% of patients with PM-scle-roderma overlap syndrome, in 8-12% with IIM, and in 1-16% with SSc. Apart from myositis, arthritis and Raynaud's phenomenon are the most common manifestations observed in PM-Scl antibody-positive patients. Cardiac and renal involvement is very rare. The prognosis for these patients is therefore relatively good. In childhood, the PM-Scl antibody-positive scleromyositis appears to be the most common scleroderma-like disease . The clinical course is relatively benign compared to that of juvenile dermatomyositis or scleroderma.
Ku antibodies are directed against DNA-binding, non-histone proteins (p70/ p80 heterodimers) and the catalytic subunit (p350) of DNA-dependent protein kinase (DNA-PK). Ku antibodies are detectable in patients with myositis-sclero-derma overlap syndrome but can also be found in patients with primary pulmonary hypertension, SLE, and other connective tissue diseases (reviewed in ).
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