Objective. To identify one nuclear autoantigenic protein within a complex of DNA binding proteins that bind to GC-rich sequences in Epstein-Barr virus and cellular DNA, and to describe the clinical characteristics of patients whose sera contained autoantibodies to this novel autoantigen.Methods. Antibodies to autoantigen Spl were initially measured by an electrophoretic mobility shift assay to detect DNA binding proteins. Nuclear extracts and purified Spl protein were used in these assays. Recognition of the autoantigen by autoimmune sera was confirmed by immunoprecipitation and immunoblotting.Results. The autoantigen was identified as Spl. Anti-Spl was detected in sera from 8 (3%) of 230 patients. These sera contained antinuclear antibodies, but lacked antibodies to double-stranded DNA or to several extractable nuclear antigens. The patients whose sera contained antibodies to Spl were white women with fatigue, arthritis, Raynaud's phenomenon, malar rash, and photosensitivity.Conclusion. Spl is the first described example of an RNA polymerase I1 transcription activator as an autoantigen. The presence of Spl autoantibodies is associated with undifferentiated connective tissue disease.The present experiments were initiated as part of a study to identify proteins that might play a role in recombination events that are common to Epstein-Barr virus (EBV) DNA and cellular DNA. Circular EBV DNA is found during latency, and linear viral DNA is present during lytic replication and is found in virions (1-5). We previously identified a group of cellular Submitted for publication October 16, 1996; accepted in revised form December 31, 1996. proteins that recognized GC-rich motifs in the EBV terminal repeats, at a locus responsible for the interconversion of linear and circular viral DNA (6). These same proteins, called terminal repeat binding proteins (TRBP), also bound to GC-rich sequences in repetitive cellular DNA, within areas such as variable number tandem repeats and Ig heavy chain class switch regions.In efforts to further characterize TRBP, we found that it was a conserved novel autoantigen that was detected by serum antibodies from a group of patients with undifferentiated connective tissue disorders. The sera from these patients demonstrated antinuclear antibody (ANA) reactivity, but were nonreactive with double-stranded DNA (dsDNA) or with several extractable nuclear antigens, including Sm, Ro, La, U1 RNP, topoisomerase I, and Ku. Since the novel autoantigen was a DNA binding protein, electrophoretic mobility shift assay (EMSA) could be used to detect serum antibodies to the autoantigen (7). In this assay, the autoantibodies supershifted a DNA-protein complex. Human B cell nuclear extract was the source of antigen and duplex DNA probes were derived from GC-rich sequences in the EBV terminal repeats. TRBP was found to consist of 3 DNA-protein complexes, A, B, and C, in order of increasing electrophoretic mobility. The autoantisera supershifted TRBP complexes A and B, but not C. Therefore, we sought to character...
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