Mcm5 mutation leads to silencing of Stat1-bcl2 which accelerating apoptosis of immature T lymphocytes with DNA damage

Genomic instability drives the initiation, metastasis, and progression of T cell acute lymphoblastic leukemia (T-ALL) [1, 2]. During tumorigenesis, genomic instability resulting from p16INK4A deletion[3], mutations in mini-chromosome maintenance proteins MCM4/2[4-6] and impaired nonhomologous end joining (NHEJ)[7] acts cooperatively with MYC to increase cell proliferation, drive tumor initiation and expand early transformed cells. Although promotion of myeloid differentiation has been reported to inhibit leukemic self-renewal and malignant hematopoiesis in myeloid stem cells [8], efficient clearance of lymphocytes with genomic defects is a common way to prevent lymphoblastic leukemia [9-11]. This principle is also supported by the observation that inhibition of apoptosis in p53 mutant cells or overexpression of antiapoptotic signaling mediators drives leukemia [12-16], including T-ALL [13]. Therefore, during T lymphocyte development, immature lymphocytes appear to sensitively undergo cell death after undergoing DNA damage caused by gene mutations (chronic DNA replication stress) [6, 17, 18], while the surviving lymphocytes may progress to thymomas at later developmental stages [6, 19,59 20]. On the other hand, DNA damage arises from both endogenous gene mutations and transient exposure to environmental stress [21, 22]. When DNA damage is caused by transient replication stress, anti-apoptotic signaling is activated to prevent rapid cell death to help preserve life and evolution [20, 23]. Thus, organisms must evolve mechanisms to differentiate chronic DNA damage induced by gene mutations from transient DNA replication stress. Using such mechanisms, organisms can eliminate lymphocytes with severe and unrepaired DNA damage to maintain a stable genome and prevent leukemia, preserve life and enhance evolution under 4 / 45 transient environmental replication stress. However, these 66 mechanisms are far from being
elucidated. Early studies have shown that the Mcm4(D573H) allele destabilizes the MCM2–7 complex, resulting in chromosome instability and the formation of spontaneous T cell lymphoblastic leukemia/lymphoma (T-ALL) [6]. Mice with reduced expression of MCM2 due to an MCM2 mutant allele die with lymphoma within the first few months after birth [5]. MCM5 is also a crucial component of the DNA replication licensing system (MCM2-7 complex), and in Steere’s early screening work, a MCM5 variant allele was identified with pathogenic potential [24]. MCM5 is highly expressed in T cells from patients with T-ALL as well as in T-ALL cell lines
(Fig. S1A-C Fig. S1B Gel Supplementary). These findings imply that MCM5 may play a pivotal
role in T lymphocytic leukemia and early T cell development. Here, we focused on studying the
role of Mcm5 in T cell development and found that in mice and zebrafish, Mcm5 loss disrupted
DNA replication in immature T lymphocytes and sequentially led to upregulation of Tp53
signaling, which resulted in apoptosis of immature T lymphocytes. Moreover, in mcm5 mutants, the absence of mcm5 inhibited the phosphorylation of Stat1 and sequentially inhibited the enhancement of bcl2 signaling, which accelerated the apoptosis of immature T lymphocytes. Our results provide a mechanism by which immature T cells with DNA damage resulting from gene mutations are rapidly cleared during T-cell development.