Toshihiro Matsukawa, Mianmian Yin, Ryan Bertoli, Timour Baslan, Peter D. Aplan

• Submitter: Toshihiro Matsukawa (Postdoc or fellow)
• email: toshihiro.matsukawa@nih.gov

DNA replicative stress is associated with malignant transformation. Mini-chromosome maintenance 2 (hereafter Mcm2) is a component of a DNA helicase complex and is required for normal DNA replication. Mice with a Cre cassette “knocked-into” the 3’UTR of the Mcm2 gene (designated Mcm2Cre) are Mcm2 hypomorphs and have decreased Mcm2 protein levels. We have previously shown that mice with 2 copies of this allele (Mcm2Cre/Cre) universally develop pre-T lymphoblastic leukemia/lymphoma (pre-T LBL) associated with 10-20 interstitial deletions scattered throughout the genome involving important tumor suppressor genes (such as Pten, Tcf3, and Cdkn2a). Thus, Mcm2Cre/Cre mice display a novel mutator (deletor) phenotype that leads to malignant transformation of thymocytes. However, despite the fact that the Mcm2 hypomorph is a germline defect, and that we could show markedly diminished Mcm2 expression in mouse embryo fibroblasts, the malignancies were restricted to thymocytes. One possible explanation for this observation is that the mice die of pre-T LBL at an early age (mean 3 months), and do not live long enough to develop other malignancies. To investigate this question, we “protected” the mice from developing pre-T LBL by crossing the Mcm2Cre allele onto a nude (nu/nu) background; since nude mice are athymic, we reasoned that they would not develop pre-T LBL. The survival curve showed that the Mcm2Cre/Cre nu/nu did not develop pre-T LBL, and lived over 9 months. However, most of these mice died at ~ 12 months of age, due to acquisition of B cell precursor ALL (BCP ALL) or erythroleukemia. In an attempt to model acute myeloid leukemia (AML), we crossed a NUP98-PHF23 (NP23) transgene onto the Mcm2cre/cre nu/nu background, as NP23 mice predominantly develop AML. Mcm2cre/cre nu/nu NP23 mice did not develop AML disease, but did however accelerate BCP-ALL latency with mice succumbing to disease at 4 months (compared to ~12 months). We used sparse whole-genome sequencing (WGS) to detect copy number aberration (CNA) for Mcm2Cre/Cre nu/nu and Mcm2cre/cre nu/nu NP23 leukemias. Preliminary analysis of the data identified recurrent deletions in candidate genes known to be involved in BCP ALL such as IL17a, IL7R, and Bcor. The unique deletor phenotype found in Mcm2cre/cre mice is a useful tool to identify tumor suppressor genes in the context of both pre-T LBL and BCP ALL.