Regulation of DNA Repair and Recombination by Mre11 and Cis-Acting Elements in B Lymphocytes

Abstract

Many B cell lymphomas are driven by translocations between a proto-oncogene and the B cell specific immunoglobulin heavy chain (IgH) gene. These translocations are thought to be the result of aberrant class switch recombination (CSR), a developmental process in B cells that requires programmed DNA double strand breaks (DSBs). The Mre11/Rad50/Nbs1 (MRN) complex plays multiple roles in DNA repair including sensing and binding DSBs, tethering and processing broken ends, and signaling to downstream repair pathways. To explore role for MRN in the translocation process, we engineered mice that harbor Mre11 deficiencies in B lymphocytes. These mice exhibit an 80% defect in CSR, with a significant accumulation of unrepaired DNA breaks in IgH. These unrepaired breaks are capable of participating in translocations but, surprisingly, mice do not succumb to B cell lymphoma. Thus, it is possible that MRN has crucial roles in tumor formation. Following translocation, the proto-oncogene must become deregulated in order to confer growth advantage to a cell. Regulatory elements within IgH are known to be required for this deregulation; however the regulatory role of distal cis-acting elements in deregulation has not been examined. To determine if there are elements outside of IgH that are responsible for this, we utilize a transgenic mouse model in which a transgene containing the entire IgH locus is inserted into different chromosomal locations in different mouse lines. We observe that the transgene is able to undergo oncogenic translocations at multiple chromosomal sites and in multiple orientations with respect to the endogenous locus. Sequencing reveals that these translocations are similar to translocations to endogenous IgH. These results suggest that Mre11 plays multiple roles in DNA repair during B lymphocyte development, which has implications for the formation of lymphomagenic translocations. Additionally, the larger chromosomal environment, outside of the 230kb IgH locus, plays only a minor role in translocation and subsequent deregulation of an oncogene.