Anti-NOR 90: A new autoantibody in scleroderma that recognizes a 90-kDa component of the nucleolus-organizing region of chromatin

We identified a patient (CAG) with scleroderma whose serum contained a high titer of IgG class antibodies that stained nucleoli in a pattern of independent tiny spots. When tested on isolated chromosomes, these antibodies selectively stained the nucleolus-organizing regions (NOR) of chromosomes 13, 14, 15, 21, and 22. These staining patterns were not altered when substrate cells and chromosomes were treated with RNase, 0.1 M HCl, or 4 M urea, but they were abolished by treatment of DNase and trypsin. Immunoblots performed with serum CAG on isolated nucleolar substrates identified a protein antigen of approximately 90 kDa. Antibodies affinity-purified from this protein selectively stained nucleoli and NOR chromosomal regions. Therefore, this protein is the antigen that accounts for the ability of serum CAG to recognize the NOR. In a search for the NOR 90-kDa specificity among 254 patients with various rheumatic diseases, we found nine additional patients whose sera stained metaphase chromosomes selectively at the NOR. Sera from five of them (three with scleroderma, two of unknown diagnosis) recognized a protein that electrophoretically co-migrated with the CAG antigen. Thus, scleroderma is present in at least four of six who appear to have this specificity. We conclude that autoantibodies to the NOR 90-kDa antigen have an association with scleroderma and may be useful diagnostically and as a probe for further studies of the biology of the cell nucleolus.