Enhanced Expression and Autoimmunity of Recombination Signal Binding Protein-jκ in Human Dilated Cardiomyopathy

Dilated cardiomyopathy (DCM) is a major cause of heart failure in younger individuals. Its prognosis is poor with 40–50% of patients dying within 2 years after diagnosis. Although the etiology of DCM is poorly understood, there is increasing evidence that DCM may represent an autoimmune disease in a significant subset of patients. In order to identify candidate antigens in DCM, we applied a molecular strategy which combines recombinant expression cloning and autoimmunological screening procedures. A left ventricle from a male DCM patient was explanted at heart transplantation and a human DCM left ventricular cDNA-expression library was constructed. 2 × 106 clones were immunologically screened with serum collected from the same patient prior transplantation. Subsequent rounds of screening and purification allowed isolation of a positive clone which was sequenced and identified as Recombination Signal Binding Protein-jκ (RBP-jκ). RBP-jκ is an already identified transcription factor, e.g., involved in Epstein-Barr-virus-induced immortalization processes. Radioactively labeled RBP-jκ protein was synthesized via in vitro translation using the isolated RBP-jκ cDNA. This RBP-jκ protein was used for immunoprecipitation reactions to screen sera of healthy controls and patients suffering of DCM for the presence of RBP-jκ autoantibodies. Analysis revealed that only 31% (n = 16) of healthy but 70.6% of DCM patients (n = 17) carry an autoantibody against RBP-jκ. Patients suffering from ischemic cardiomyopathy showed a prevalence of 22% of RBP-jκ autoantibodies. Western analysis with an monoclonal antibody raised against RBP-jκ showed that RBP-jκ was overexpressed to 488 ± 140% in DCM hearts compared to non-failing controls (n = 8). Autologous immunological screening of a cDNA expression library is a powerful and novel technology to gain insights into the etiology of human idiopathic DCM. Human DCM displays an autoimmune response against RBP-jκ and an overexpression of RBP-jκ. Since RBP-jκ is involved in cellular immortalization and exerts antiapoptotic effects, the increased RBP-jκ autoantibody level during DCM may inhibit this growth-regulating feature of RBP-jκ. In this setting, enhanced myocardial RBP-jκ expression could represent a compensatory but ineffective response to counteract the increased rate of apoptosis in DCM. Furthermore, RBP-jκ may be a useful diagnostic marker for DCM.