Widespread genomic breaks generated by activation-induced cytidine deaminase are prevented by homologous recombination

Abstract

Activation induced cytidine deaminase (AID) is required for somatic hypermutation and immunoglobulin class switching in activated B cells. Because AID possesses no known target site specificity, there have been efforts to identify non-immunoglobulin AID targets. We show that AID acts promiscuously, generating widespread DNA double strand breaks (DSB), genomic instability and cytotoxicity in B cells with diminished homologous recombination (HR) capability. We demonstrate that the HR factor XRCC2 suppresses AID-induced off-target DSBs, promoting B cell survival. Finally, we suggest that aberrations affecting human chromosome 7q36, including XRCC2, correlate with genomic instability in B cell cancers. Our findings demonstrate that AID has promiscuous genomic DSB-inducing activity, identify HR as a safeguard against off-target AID action, and have implications for genomic instability in B cell cancers.

Cyclin-Dependent Kinases Regulate Ig Class Switching by Controlling Access of AID to the Switch Region

“…Therefore, CSR was postulated to occur in the G 1 phase. However, other studies indicate that the G 1 /S checkpoint is not fully functional in activated B cells and that AID-dependent DSBs can leak into S phase (14)(15)(16). This raises the question whether Ig class switching itself is subjected to cell cycle regulation, for example by cyclindependent kinases (CDKs).…”

“…In the absence of factors from the HR pathway, off-target activity of AID seems to be able to generate numerous non-Ig locus DNA DSBs, dramatically increasing genome instability and B cell cytotoxicity (14). Therefore, it may be greatly beneficial that class switching ends in early S phase, because the remaining off-target activity of AID can be quickly and faithfully repaired through HR, diminishing the risk for genomic instability triggered by mechanisms such as A-EJ.…”

APOBEC3 proteins and genomic stability

“…Similar observations have been made when B cells deficient for homologous recombination (HR) were induced to express AID. 29 Although it is unclear whether endogenous A3A can affect cell cycle progression, our results suggest that the effects of A3A expression on the host cell could have been a contributing factor in previous experiments focused on understanding A3A function as a restriction factor.…”

“…27,28 In this regard, it has been shown that homologous recombination repair proteins act as safeguards to prevent genomic instability by the widespread genomic breaks that can be induced by off-target activity of AID in B cells. 29 Interestingly, it has been hypothesized that over the course of evolution, AID activity has been restricted to minimize the risk of genomic stability.…”

Molecular Mechanisms Underlying Genomic Instability in Brca-Deficient Cells

“…Deep sequencing analyses of AID localization in stimulated B cells have also provided unmatched views of AID-induced DNA damage at non-Igh loci . It has been proposed that off-target AID sites are repaired by HR (Hasham et al, 2010); however, significant association of RPA and RAD51, as markers of HR-mediated repair, was found only at the Igh switch regions undergoing class switching and not at other loci, suggesting that AID recruitment is not sufficient for AID-induced DSB resection . Thus, while AID promotes somatic hypermutation at many genes (Liu et al, 2008;Muramatsu et al, 2007;Robbiani et al, 2009;, AIDinduced DSBs do not appear to occur frequently or to load significant levels of RPA .…”