Synergism Between HOXA9 and Mutant JAK3 (M511I) Leads to Rapid Leukemia Development within an in Vivo Murine Bone Marrow Transplant Model

Activation mutations in JAK3 occur in 16% of T-cell acute lymphoblastic leukemia (T-ALL) cases, and co-occur frequently with HOXA cluster rearrangement. Genomic rearrangement of the HOXA cluster results in increased expression of HOXA9 and HOXA10. However it remains unclear if either HOXA9 or HOXA10 can cooperate with activating JAK3 mutations during oncogenic transformation and leukemogenesis. We have previously shown that JAK3 mutations lead to cell transformation and cause a long latency T-ALL in vivo using a mouse bone marrow transplant model. In this study we demonstrate that co-expression of the activating JAK3(M511I) protein with HOXA9 cooperate to develop leukemia within 30 days of transplant using an in vivo bone marrow transplant model. In our cooperative model, murine hematopoietic stem / progenitor cells were co-transduced with either both retroviral vectors encoding JAK3(M511I)/GFP and HOXA9/mCherry or each individually and then injected into sub-lethally irradiated recipient mice. Mice transplanted with bone marrow cells expressing JAK3(M511I) mutant alone developed T-ALL in 120 to 150 days. In sharp contrast, mice transplanted with cells expressing both JAK3(M511I) and HOXA9 showed rapid leukemia development within 30 days after transplant. Leukemia development was characterized by the rapid and specific increase in GFP-mCherry double positive cells. These animals showed high WBC, and splenomegaly and accumulation of immature CD8 single positive cells in the thymus. Similar experiments with HOXA10 did not show cooperation suggesting that HOXA9 is the more important oncogene in HOXA rearranged leukemias when a JAK3 activating mutation is present. To determine the underlying genetic mechanism for cooperation between HOXA9 and JAK3(M511I) the single positive JAK3 and double positive JAK3/HOXA9 expressing cells were isolated from thymi of leukemic mice for both epigenomic profiling using ATAC-seq and gene expression profiling. These analyses identified genetic pathways activated by the co-expression of HOXA9 and JAK3(M511I) mutation and provide mechanistic insight into the synergistic interaction between these two factors in driving leukemia development. Treatment of the animals with a JAK kinase inhibitor resulted in delayed leukemia development, confirming that the leukemia cells remain sensitive to the JAK inhibitor. This mouse model provides insight in the cooperation between oncogenes in leukemia development and provides a model for the study of targeted agents in this setting.