XLID Syndrome Gene Med12 Promotes Ig Isotype Switching through Chromatin Modification and Enhancer RNA regulation

The transcriptional co-activator Med12 regulates gene expression through the function of its kinase module and by interacting with the larger Mediator complex and associated RNA Polymerase II (RNAPII). Here, we show a kinase module-independent function of Med12 in antibody class switching recombination (CSR). Med12 is essential for IgH 3 regulatory region (3RR) or super-enhancer activation and functions with p300 and Jmjd6/Carm1 coactivator complexes. Med12 deficiency leads to a dramatic decrease in H3K27 acetylation and enhancer RNA (eRNA) transcription at 3RR, with concomitant impairment of AID-induced DNA double strand breaks, long-range S-S synapse formation, and 3RR-E interaction. CRISPR/dCas9-mediated enhancer activation re-establishes the epigenomic and transcriptional hallmarks of the 3RR super-enhancer, fully restoring Med12 depletion defects. Notably, we find that 3RR derived eRNAs are critical for promoting proper S region epigenetic regulation, S-S synapse formation and recruitment of Med12 and AID to the IgH locus. We find specific X-Linked intellectual disability syndrome associated Med12 mutations are defective in both 3RR eRNA transcription and CSR, suggesting B and neuronal cells may have cell-specific super-enhancer dysfunctions. We conclude Med12 is essential for IgH3RR activation and eRNA transcription and plays a central role in AID-induced antibody gene diversification and genomic instability in B cells.