154 | Chapter 6 To further substantiate the role of FOXP1 in driving RAG expression in pre-B cells, we overexpressed the full-length (FL) FOXP1 and its shorter isoform (iso), which were previously shown to have similar functions15, in the A70 v-Abl transformed mouse pre-B cell line (Figure 2A). Interestingly, FOXP1 overexpression was sufficient to induce productive recombination Igk recombination, and partly overcame the development block imposed by v-Abl expression, as determined by the detection of productive by Vk6-23-Jk1 coding joints by PCR (Figure 2B). The FOXP1 isoform, truncated at the N-terminus, is overexpressed in subsets of B-cells lymphomas and has been suggested to have an oncogenic capacity15–17. Our results imply that the FOXP1 N-terminus is not required to partially override the v-Abl-induced developmental block in the A70 pre-B cell line. In addition, overexpression of BCL618 a negative regulator of p53, also resulted in Vk6-23-Jk1 recombination events without STI571 treatment in A70 cells (Figure 2A-B and Suppl Figure 3A-B). Previously, the regulation of FOXP1 in developing B cells was linked to the p53 pathway through miR-34a. The activation of p53 has been shown to drive the expression of miR-34a in cell lines of various origin19–21, and constitutive miR-34a expression resulted in a B-cell development blockade, where FOXP1 was identified as the key mediator of this effect22. Moreover, in chronic lymphocytic leukemia (CLL) cells, the p53/miR-34a pathway has been shown to be a negative regulator of FOXP1 expression, responding rapidly to DNA damage and limiting B-cell receptor (BCR) signaling in mature B cells23. We therefore investigated the possible ties of ATM/p53 signaling induced by DSBs to the mirR-34a/ FOXP1/RAG axis. Our data show that g-irradiation drives the miR-34a expression in in vitro expanded progenitor B cells from primary mouse bone marrow in a dose-dependent manner. In agreement, the level of miR-34a was significantly higher upon treatment with the MDM2 inhibitor Nutlin-3 (Figure 2E). Furthermore, in in vitro expanded progenitor B cells from primary bone marrow, depletion of miR-34a using a miR-34a-sponge construct resulted in an increase in B220+DX6-Ly6C-IgM+ B cells (2.19% in cells transduced with pCLECO-miR-34a-sponge vs. 0.80% in cells transduced with pCL-ECO-scrambled) following the IL-7 and Flt3L withdrawal from the cell culture, suggesting that depletion of miR-34a promotes successful Igk recombination (Figure 2C- D). In summary, we propose a negative-feedback regulatory mechanism involving p53, miR-34a, and FOXP1, where DNA breaks limit the expression of RAG1 and RAG2 (Figure 2F). In this regulatory mechanism, (RAG-dependent) DNA damage activates ATM, which in turn stabilizes p53 and activates the p53 effectors, including miR-34a, which affects Foxp1 levels and Igk recombination in pre-B cells. In agreement, the levels of miR-34a as well as FOXP1 respond to exogenous and RAG-instigated DNA damage and limit the RAG1/2 expression and the V(D)J recombination. This regulatory process in developing B cells may serve as a self-protective mechanism against excessive RAG-dependent DNA damage and thus guards the stability of the genome.
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