Heterogeneous nuclear ribonucleoproteins C1/C2 identified as autoantigens by biochemical and mass spectrometric methods

The determination of the molecular and submolecular specificity of ANAs is important for diagnosis and prognosis in clinical medicine and for understanding the molecular pathology of autoimmune disease. Different antigenic specificities of ANA are associated with different diseases and different patterns of disease manifestations. The various protein antigens associated with ANAs characteristically represent molecules that bind nucleic acids. Thus, many of the proteins that associate with RNA in the RNP complexes of the spliceosome have been found to react with some types of ANA [ 1 ]. Also, the hnRNP complex that associates with newly transcribed pre-mRNA contains proteins that have been shown to be targets of ANAs in some rheumatic diseases. The protein components of hnRNP are the most abundant nonhistone proteins of the cell nucleus and are classified roughly on the basis of their molecular masses into at least 21 protein groups (A through U). Autoantibodies to the A2, B1, and B2 proteins of hnRNP found in some patients may be markers of several overlap syndromes [ 2 ]. However, ANAs with specificity for these proteins as well as for the D protein and the I protein also appear to occur in many distinct connective-tissue diseases, although epitope specificities may differ [ 3 , 8 ]. Despite the discovery of the A, B, and R hnRNP ANA specificities in patients [ 2 , 9 , 10 ], antibodies against the C component of hnRNP (consisting of the C1 and C2 proteins) have to our knowledge so far been described in only one case [ 4 ]. The identification in this study was based on the colocalization of patient immunoreactivity on immunoblots of nuclear extracts with the band developed with a monoclonal anti-C1/C2 antibody. In the present study, we identify the C1/C2 specificity of circulating ANAs in three patients by high-resolution separation techniques (2D gel electrophoresis) combined with the assignment of protein identities by mass-spectrometric (MS and tandem MS) peptide mapping. This protein was also found to be the target of the ANA found in the patient described earlier [ 4 ].