RAG enhances BCR‐ABL1‐positive leukemic cell growth through its endonuclease activity in vitro and in vivo

Abstract BCR‐ABL1 gene fusion associated with additional DNA lesions involves the pathogenesis of chronic myelogenous leukemia (CML) from a chronic phase (CP) to a blast crisis of B lymphoid (CML‐LBC) lineage and BCR‐ABL1 + acute lymphoblastic leukemia ( BCR‐ABL1 + ALL). The recombination‐activating gene RAG1 and RAG2 (collectively, RAG) proteins that assemble a diverse set of antigen receptor genes during lymphocyte development are abnormally expressed in CML‐LBC and BCR‐ABL1 + ALL. However, the direct involvement of dysregulated RAG in disease progression remains unclear. Here, we generate human wild‐type (WT) RAG and catalytically inactive RAG‐expressing BCR‐ABL1 + and BCR‐ABL1 − cell lines, respectively, and demonstrate that BCR‐ABL1 specifically collaborates with RAG recombinase to promote cell survival in vitro and in xenograft mice models. WT RAG‐expressing BCR‐ABL1 + cell lines and primary CD34 + bone marrow cells from CML‐LBC samples maintain more double‐strand breaks (DSB) compared to catalytically inactive RAG‐expressing BCR‐ABL1 + cell lines and RAG‐deficient CML‐CP samples, which are measured by γ‐H2AX. WT RAG‐expressing BCR‐ABL1 + cells are biased to repair RAG‐mediated DSB by the alternative non–homologous end joining pathway (a‐NHEJ), which could contribute genomic instability through increasing the expression of a‐NHEJ‐related MRE11 and RAD50 proteins. As a result, RAG‐expressing BCR‐ABL1 + cells decrease sensitivity to tyrosine kinase inhibitors (TKI) by activating BCR‐ABL1 signaling but independent of the levels of BCR‐ABL1 expression and mutations in the BCR‐ABL1 tyrosine kinase domain. These findings identify a surprising and novel role of RAG in the functional specialization of disease progression in BCR‐ABL1 + leukemia through its endonuclease activity.