AID-Initiated Off-Target DNA Breaks are Detected and Resolved During S-Phase

• Published in: The Journal of immunology (1950) 2012-07, Vol.189 (5), p.2374-2382
• Main Authors: Hasham, Muneer G., Snow, Kathy J., Donghia, Nina M., Branca, Jane A., Lessard, Mark D., Stavnezer, Janet, Shopland, Lindsay S., Mills, Kevin D.
• Format: Article
• Language: English
• ISSN: 0022-1767; 1550-6606
• DOI: 10.4049/jimmunol.1200414

Activation-induced cytidine deaminase (AID) initiates DNA double strand breaks (DSBs) in the immunoglobulin heavy chain gene ( Igh ) to stimulate isotype class switch recombination (CSR), and widespread breaks in non- Igh (off-target) loci throughout the genome. Because the DSBs that initiate class switching occur during the G1 phase of the cell cycle, and are repaired via end joining, CSR is considered a predominantly G1 reaction. By contrast, AID-induced non- Igh DSBs are repaired by homologous recombination. Although little is known about the connection between the cell cycle and either induction or resolution of AID-mediated non- Igh DSBs, their repair by homologous recombination implicates post-G1 phases. Coordination of DNA breakage and repair during the cell cycle is critical to promote normal class switching and prevent genomic instability. To understand how AID-mediated events are regulated through the cell cycle, we have investigated G1-to-S control in AID-dependent genome-wide DSBs. We find that AID-mediated off-target DSBs, like those induced in the Igh locus, are generated during G1. These data suggest that AID-mediated DSBs can evade G1/S checkpoint activation and persist beyond G1, becoming resolved during S-phase. Interestingly, DSB resolution during S-phase can promote not only non- Igh break repair, but also immunoglobulin CSR. Our results reveal novel cell cycle dynamics in response to AID-initiated DSBs, and suggest that the regulation of the repair of these DSBs through the cell cycle may ensure proper class switching while preventing AID-induced genomic instability.