Title: The Role of CtIP in Lymphocyte Development and Lymphomagenesis
Description: Chromosomal translocation is a characteristic feature of human lymphoid malignancies and a driver of the initiation and progression of the disease. They arise from the mis-repair of physiological DNA double-strand breaks (DSBs) generated during the assembly and subsequent modifications of the antigen receptor gene loci, namely V(D)J recombination and class switch recombination (CSR). Mammalian cells have three DSB repair pathways –classical non-homologous end-joining (cNHEJ), alternative end-joining (A-EJ), and homologous recombination. DNA end-resection that generates a single-strand 3’ overhang is a critical regulator for the repair pathway choice. Specifically, localized end-resection prevents cNHEJ and exposes flanking microhomology (MH) to promote error-prone A-EJ. In addition to DNA repair, DNA end-resection generates extended single-strand DNA, which activates the ATR-mediated cell cycle checkpoint and indirectly contributes to genomic integrity. The central goal of my thesis research is to investigate the physiological role of DNA end-resection initiation in lymphocyte development and lymphomagenesis.

DNA end-resection in mammalian cells is mostly initiated by the endonuclease activity of MRE11-RAD50-NBS1 (MRN) complex aided by CtIP. In addition, MRN protein also recruits EXO1 and DNA2 nucleases in combination with Top3 helicase complex for more extensive resection. The CtIP protein is essential for the endonuclease activity of the MRN complex that initiates DNA end-resection. CtIP is essential for embryonic development. Here I utilized B cell-specific conditional deletion models and loss-of-function mutations to investigate the role and regulation of CtIP and CtIP-mediated DNA end-resection in lymphocyte development and tumorigenesis. The level and extent of CtIP-mediated resection are tightly regulated. For the first aim, we applied the ATAC-Seq and EndSeq methods to test whether chromatin accessibility determines the level of DNA end-resection. Specially, we found that chromatin-bound DNA damage response factors – H2AX and 53BP1- reduced the accessibility of the DNA around the DSBs and antagonized end-resection.

Our data also suggest that during DNA damage response, the nucleosome-free or accessible regions are more prone to secondary DNA breakages. Mechanistically, the preferential vulnerability is correlated with the availability of chromatin-bound DNA damage response factor 53BP1, which protects the nucleosome covered region at the price of the nucleosome-free regions. The work provides one explanation for tissue and cell type-specific translocations in transcriptionally active regions and super-enhancers. For the second and third aims, I investigated the role of CtIP and CtIP-mediated end-resection in lymphocyte development and lymphomagenesis in vivo using the conditional deletional CtIP allele and a phosphorylation-deficient CtIP-T855A mutant. T855 phosphorylation promotes end-resection but is not essential for cellular viability. I identified a sequence-context-dependent role of CtIP and end-resection in A-EJ mediated repair. We found that the reduced level of end-resection did not alter the frequency of the A-EJ mediated joining during B cell CSR, nor the levels of micro-homology at the junction, a defining feature of A-EJ mediated repair.

These findings, for the first time, showed that DNA end-resection is not essential for A-EJ-mediated chromosomal DSBs repair nor for the generation of MH at the junction in vivo. This unexpected observation also highlights a tissue- and cell type-specific regulation of A-EJ and the importance of sequence context for A-EJ. Moreover, we found that ATM kinase suppresses A-EJ mediated translocation and reported the very first cell cycle-dependent analyses of CSR junctions. In cNHEJ-deficient B cells (e.g., Xrcc4- or DNA-PKcs- deficient), the A-EJ pathway is responsible for both the residual CSR events and the generation of the oncogenic IgH-Myc chromosomal translocations. In the last chapter, I determined how CtIP contributes to oncogenesis using the CtIP-T855A phospho-deficient mouse model. The result showed that CtIP T855 phosphorylation is critical for the neonatal development of Xrcc4-/-p53-/- mice and IgH-Myc translocation driven lymphomagenesis in DNA-PKcs-/-Tp53-/- mice. Mechanistically, phospho-deficient CtIP compromises the extent of end-resection without affecting the initiation.

Reduced end-resection in CtIP-T855A mice and cells attenuated G2/M checkpoints and reduced the tolerance to the oncogene-induced replication stress, thereby limit lymphomagenesis. Collectively, our data provided the first in vivo characterization for the role of CtIP and its related end-resection pathway in lymphocyte development and lymphomagenesis. The results highlight the importance of end-resection for checkpoint maintenance (§ 4) and the context-dependent regulation of A-EJ and DNA repair pathway choice in vivo (§ 3), explaining why A-EJ is more robust at the repetitive switch regions. Finally, the application of HTGTS, EndSeq, and ATAC-Seq technologies in lymphocyte-specific gene rearrangements markedly improved the analysis depth and sensitivity while reducing the cost of repair-junction sequencing, allowing the quantitative detection of subtle changes and additional mechanistic insights. Specifically, we showed that end-resection could be regulated at the level of chromatin accessibility, which is determined by both baseline chromatin occupancy and DNA damage-induced changes (§ 2).

These findings provide one explanation for the tissue and cell type-specificity of translocation targeting. These techniques can be used to analyze the impact of other DNA repair factors during lymphocyte development and lymphomagenesis and in translocation in general.