Oncogenic tyrosine kinases (OTKs) such as FLT3(ITD) and JAK2(V617F) induce acute myeloid leukemia (AML) and myeloproliferative neoplasms (MPNs), respectively. OTKs may be accompanied by “additional” mutations (e.g., TET2, DNMT3A) complicating genetic/epigenetic signature. Selective OTK inhibitors (OTKi) are developed against FLT3(ITD)-positive AMLs and JAK2(V617F)-positive MPNs, but complete remissions were rare in OTKi-treated patients, and after initial response the disease progressed often in to more malignant stage. FLT3(ITD)-positive AMLs and JAK2(V617F)-positive MPNs accumulate lethal DNA double- strand breaks (DSBs). DSBs are repaired by two major mechanisms, BRCA-mediated homologous recombination (HR) and DNA-PK –mediated non-homologous end-joining (D- NHEJ). HR and D-NHEJ repair DSBs in proliferating cells and D-NHEJ plays a major role in quiescent cells. PARP1–dependent back-up NHEJ (B-NHEJ) works in proliferating and quiescent cells. FLT3(ITD) and JAK2(V617F)-positive AML/MPN stem cells are usually resistant to DSBs because these OTKs modulate DNA repair pathways to promote survival. Cancer-specific defects in DSB repair create the opportunity to employ synthetic lethality, e.g. elimination of BRCA1/2-mutated cancer cells by PARP inhibitor (PARPi). We reported that OTKis induce HR and D-NHEJ deficiencies, which sensitize quiescent and proliferating FLT3(ITD)/JAK2(V617F)-positive AML/MPN stem cells to synthetic lethality triggered by PARPi. However, our recent reports and preliminary data strongly suggest that “additional” mutations (e.g., in TET2, DNMT3A) can change FLT3(ITD) and JAK2(V617F)-positive AML/MPN cells sensitivity to OTKi + PARPi. Therefore in Specific Aim #1 we propose to identify “additional” mutations and mechanisms which affect sensitivity of FLT3(ITD) and JAK2(V617F)- positive cells to OTKi+PARPi-mediated synthetic lethality. We also reported and obtained preliminary data that bone marrow microenvironment (BMM) induces resistance to OTKi + PARPi treatment. Therefore we will pinpoint BMM-related obstacles for OTKi + PARPi-mediated synthetic lethality and apply BMM inhibitor (BMMi) in Specific Aim #2: Overcoming the protective effect of BMM against OTKi + PARPi treatment. Finally, we will test therapeutic potential of OTKi + PARPi +/- BMMi against AMLs/MPNs xenografts carrying mutations favoring synthetic lethal effect in Specific Aim #3. Therapeutic effect of OTKi + PARPi +/- BMMi against AMLs/MPNs carrying specific “driver” mutations [FLT3(ITD/TKD), JAK2(V617F)] and “additional” mutations (e.g., TET2).