Reviewer #2 (Public Review):
Here, the authors aim to address the role of R loops in CSR. Though implicated in CSR since decades, R loops remain enigmatic regarding their true function at the Igh locus during CSR. In particular, its role in AID targeting to S regions remains debated with no direct evidence supporting this claim. In this study, the Barlow lab sheds interesting new light on what R loops may be doing during CSR. They study the response to elevated R loop levels which they achieve using single or double KO of SETX (a helicase that can unwind R loops) and RNaseH2 (which can cleave R loops). In this system, R loop removal is deficient and the effect on CSR and genome instability can be assessed. This is a fresh approach which allows the authors to draw new insights into R loop biology. Overall, the results support the conclusions that the timely removal of R loops is not necessary for optimal CSR but is necessary to maintain genome stability. But there are some experiments that need to be done to solidify this conclusion.
The major findings are that the increase in steady-state R loops in dKO cells does not appear to affect CSR frequency although small increase in mutation is observed. However, in dKO cells, there is a significant increase in gross chromosomal aberrations (translocations and fusions) as well as increased usage of alternative end-joining during CSR. Thus, surprisingly, increased DNA damage and increased reliance on alternative end-joining do not appear to reduce CSR which would have been expected based on many previous studies. Thus, they conclude that R loop removal by SETX and RNaseH2 is necessary to enhance the usage of classical end-joining repair pathways that are more efficient and less prone to genome instability.
The major weakness here is the lack of a proper characterization of B cell development in the mice. They use Cd19-cre which acts earlier in B cell development in the bone marrow and hence it is important to know whether B cell populations were skewed or otherwise influenced by the early KO of Setx and Rnaseh2. Along these lines, gene expression analysis is necessary to know whether the single and double KO (both naïve and activated) splenic B cells have undergone differential expression in DNA repair pathways or other pathways that could impinge upon CSR and contribute to the DNA repair phenotype they observe.
There is no western blot analysis to show how well RNASEH2 is depleted. Cd19-cre is known to have variable effects hence it is unclear whether efficient deletion was obtained in mature B cells.
One puzzling finding is that R loops were increased only in the S-mu but not the S-gamma1 region although both form R loops. Some thoughts on this would be useful for the readers since this implies that R loop resolution at S-gamma1 is independent of both enzymes.