Abstract The interferon-inducible, dsRNA-dependent protein kinase, PKR, is activated by cellular stresses to inhibit protein synthesis and initiate apoptosis. Recently, we reported that expressing human PKR (TgPKR) in mouse hematopoietic tissues induced peripheral blood cytopenias and dysplastic changes in the bone marrow. Additionally, these findings were associated with reduced numbers of hematopoietic stem/progenitor cells (HSPCs) consistent with a myelodysplastic syndrome (MDS)-like phenotype. We have now tested whether TgPKR expression in HSPCs may cooperate with the Nup98-HoxD13 (NHD13) driver transgene. This NHD13 transgene induces a highly penetrant form of MDS with progression to acute leukemia in mice. TgPKR or PKR knockout (PKRKO) mice were crossed with the NHD13 transgenic mouse to generate TgPKR-NHD13 and PKRKO-NHD13 mice that were aged. The development of pancytopenia/MDS and evolution to acute leukemia were compared to NHD13 mice or to normal WT control mice. Beginning at 5 months and persisting through 8 months of age, the TgPKR-NHD13 mice developed reduced WBC and platelet counts compared to NHD13 controls (WBC mean 2.6 vs. 3.5 x103 cells/µl, [p: < 0.01], and mean platelet counts of 576.9 vs. 814.3 x103 platelets/µl). Both cell types were significantly lower than WT controls. By contrast, PKRKO-NHD13 mice maintained higher WBC counts beginning at age 6 months (n=4) compared to NHD13 controls (n=13) (mean 7 vs. 2.5 x103 cells/µl, [6 mos.], 11.6 vs. 3.7 x103 cells/µl [7 mos.], 9.62 vs. 3.5 x103 cells/µl [8 mos.], p: <0.05). To date, 1 of 13 of the NHD13 mice, aged 7-10 months, has developed acute leukemia. Of the TgPKR-NHD13 mice, 7 of 18 have died due to either acute leukemia (4/7), MDS with anemia requiring euthanasia (1/7), or due to unknown causes (2/7). By contrast, none of the PKRKO-NHD13 (n = 19) or WT mice (n= 11), both aged up to 9 months, have developed symptomatic MDS requiring euthanasia or acute leukemia These data indicate that PKR may cooperate with the NHD13 driver oncogenic mutation to create a more aggressive MDS-like phenotype characterized by deeper peripheral blood cytopenias and an increased rate of evolution to AML. Alternatively, inhibition of PKR expression/activation may delay or forestall evolution of MDS/AML in the NHD13 background. These findings suggest that PKR may be a potential novel therapeutic target in the treatment of MDS and its evolution to acute leukemia. Citation Format: Michael T. Byrne, Richard L. Bennett, Xiaodong Cheng, W. Stratford May. PKR cooperates with an MDS driver mutation to worsen cytopenias and promote acute leukemia. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 75. doi:10.1158/1538-7445.AM2014-75