Abstract 5589: CBFB::MYH11 expression activates Trp53 in myeloid cells, which inhibits cell growth and sensitizes cells to cytarabine

Abstract CBFB::MYH11 acute myeloid leukemia (AML) is a subset of Core Binding Factor AML initiated by an oncofusion in which Core Binding Factor Beta (CBFB) is fused to the C-terminus of smooth muscle myosin heavy chain 11 (MYH11). CBFB::MYH11 AML has a high cure rate with conventional anthracycline/cytarabine induction and high-dose cytarabine consolidation in patients under the age of 60, and remains chemosensitive after multiple relapses; the mechanisms underlying chemosensitivity are unknown. TP53 is a key regulator of DNA damage response and apoptosis, and TP53-mutated AML is often chemoresistant. Large sequencing studies have revealed only rare co-occurrence of TP53 mutations with CBFB::MYH11. We initially used CBFB::MYH11 as a negative control for an unrelated study of TP53 function and unexpectedly found striking synergy. While expression of CBFB::MYH11 in WT murine hematopoietic stem and progenitor cells (HSPCs) induced a low-penetrance (~20%), long latency (~6 months) myelomonocytic AML, expression of CBFB::MYH11 in Trp53-/- HSPCs induced AML with 100% penetrance and a median latency of 8 weeks. CBFB::MYH11-induced AMLs arising in Trp53-/- cells were similar by morphology and flow cytometry phenotype. We validated these findings in a second model, using a conditional Trp53flox/flox x Vav1-Cre model in which Trp53 is inactivated in HSPCs, with nearly identical results. Trp53-/- cells transduced with Empty Vector did not develop AML, but did develop T cell lymphoma with a median latency of 16 weeks, similar to prior studies. Trp53 deficiency also cooperated with RUNX1::RUNX1T1, the other Core Binding Factor oncofusion, to induce a high-penetrance (100%) and short latency AML (median latency 10 weeks); consistent with prior data, RUNX1::RUNX1T1 expression alone in Trp53 WT cells was insufficient to induce AML. To determine whether Trp53 influences the chemosensitivity of CBFB::MYH11 expressing cells, we performed cytarabine dose-response experiments. While CBFB::MYH11-transduced WT HSPCs were very sensitive to cytarabine (relative to EV), sensitivity was reduced in CBFB::MYH11-transduced Trp53-/- cells, suggesting that CBFB::MYH11-induced chemosensitivity is, at least in part, mediated by the TP53 pathway. To determine whether CBFB::MYH11 expression results in upregulation of key TP53 pathway genes, we transduced WT HSPCs with EV or CBFB::MYH11, transplanted cells into WT mice, harvested bone marrow at 3 months, and performed RNA-Seq. Key TP53 pathway genes were upregulated, including Bbc3/Puma, Bax, Cdkn1a/p21, and Trp53 (all 2.6-4.6-fold, p≤0.05). These data suggest that the chemosensitivity and the favorable prognosis of CBFB::MYH11 AML may be due to the direct activation of the TP53 pathway by CBFB::MYH11. Similar studies are underway with RUNX1::RUNX1T1-transduced cells to determine whether TP53 is relevant for the chemosensitivity of that CBF AML as well. Citation Format: Ryan B. Day, Julia A. Hickman, Casey D. Katerndahl, Sai Mukund Ramakrishnan, Christopher A. Miller, Timothy J. Ley. CBFB::MYH11 expression activates Trp53 in myeloid cells, which inhibits cell growth and sensitizes cells to cytarabine [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 5589.