Ro52 antibodies and susceptibility genes in congenital heart block

Sammanfattning: Congenital heart block (CHB) develops in fetuses of Ro/SSA and La/SSB positive women. During pregnancy, the autoantibodies cross the placenta and affect the fetus in which a potentially lethal atrioventricular (AV) block may develop. This thesis is aimed at identifying clinically useful maternal serologic markers predictive of risk for CHB, and to define genes linked to susceptibility in the child. Ro52-p200 antibodies, binding amino acid 200-239 of the Ro52 protein, was recently suggested by our group as a marker for high risk pregnancies. Performing a multinational study we now show that p200 antibodies are highly relevant as a second step analysis in Ro52-positive pregnancies, and increase the positive predictive value for fetal cardiac involvement. The incidence of CHB in Ro/La positive women is 1-2%. This risk is only increased to 20% in subsequent pregnancies despite persisting antibodies, indicating that there are other factors involved in disease susceptibility than antibody specificity alone. Ro/La antibody levels and Ro52 subclass profiles were investigated longitudinally through pregnancies and revealed no significant differences between affected and healthy pregnancy outcomes. There were no significant decreases or peaks in antibody levels corresponding to or preceding the time point when CHB is usually detected. We therefore investigated differences in fetal susceptibility to CHB. Fetal genetic factors in susceptibility to CHB have been suggested, but not previously investigated experimentally, To investigate MHC and non-MHC associations of the disease, an immunization model of CHB was established in rat. Analysis of MHC and non-MHC genetic influences using congenic rat strains and an F2 cross revealed significant associations with MHC encoded genes. Maternal generation of pathogenic antibody specificity was linked to a specific MHC haplotype, whereas fetal susceptibility to development of CHB was linked to a separate MHC haplotype in the fetus. Patterns of inheritance also indicated a possible epigenetic influence in susceptibility to CHB. Our data suggest complex genetic prerequisites for susceptibility, and explain why simple associations with MHC genes have not been observed in human studies of CHB. The cellular function of the Ro52 autoantigen was also investigated. We show that Ro52 is an E3 ubiquitin ligase and using a panel of Ro52 monoclonal antibodies which was generated, we show that R52 locates predominantly to the cytoplasm. Stimulation of cells with the systemic autoimmune-related cytokine IFN-alpha induced translocation of Ro52 from the cytoplasm to the nucleus, which preceded apoptosis of the cells. In summary, we identify Ro52-p200 antibodies as a clinically useful marker for risk of CHB and show that fetal susceptibility to these pathogenic autoantibodies depend on fetal MHC-encoded genes. We also demonstrate that Ro52 is an E3 ligase, and that cytokines involved in systemic autoimmunity regulate the cellular localization of the Ro52 autoantigen.