Anti-RNA polymerase antibodies in systemic sclerosis (SSc): association with anti-topoisomerase I antibodies and identification of autoreactive subunits of RNA polymerase II.

The prevalence of autoantibodies to the three RNA polymerase (RNAP) enzymes in the sera of 249 SSc patients was measured using the technique of immunoprecipitation of 35S-methionine-labelled K562 cell extracts. Forty-six anti-RNAP sera were detected (18.5%) and three main groups were identified: anti-RNAP I/III sera (10; 4.0%), anti-RNAP I/II/III sera (15; 6.0%), and sera precipitating the phosphorylated (IIO) form of RNAP II (18; 7.2%). All sera in the third group also precipitated topoisomerase I (topo I), and six of them also precipitated the unphosphorylated (IIA) form of RNAP II. Although RNAP II/topo I multienzyme complexes may occur in cell extracts, autoreactive epitopes were shown to be located on both enzymes by a combination of antigen depletion studies, and in vitro assays which demonstrated functional inhibition of topo I activity. Furthermore, immunoblotting experiments using affinity-purified extracts demonstrated that all sera with anti-RNAP II antibodies recognized the largest RNAP II subunit in its phosphorylated form (IIo; 240 kD), whereas the unphosphorylated subunit (IIa; 220 kD) was only recognized by sera which also precipitated RNAP IIA. Therefore at least two different sites on the largest subunit of RNAP II are recognized by SSc sera, and one of these sites is unique to the phosphorylated (IIO) form.