Abstract 6301: DNA methyltransferase inhibitors increase ERV reactivation and STING-dependent interferon/inflammasome signaling in TP53 mutant AML
Authors: Kogan Aksinija A. , Topper Michael J. , Stojanovic Lora , McLaughlin Lena J. , Kingsbury Tammy J. , Baer Maria R. , Kessler Michael , Baylin Stephen B. , Rassool Feyruz V. 
Published: June 2022

Abstract
DNA methyltransferase inhibitors (DNMTis), which transcriptionally activate hypermethylated genes in cancers and leukemias, also activate endogenous retroviruses (ERVs), leading to increased cytosolic double-stranded (ds) RNA and interferon (IFN) signaling, in a process termed viral mimicry. The tumor suppressor TP53 has been reported to cooperate with DNA methylation and IFN signaling to maintain transcriptional silencing of various repeat sequences, including ERVs. We now report that DNMTis, used to treat acute myeloid leukemia (AML) patients unfit for chemotherapy, robustly induce expression of ERVs and STING-dependent IFN/inflammasome signaling in AML with mutations in TP53(approximately 10%), a subset with poor prognosis.
First, our studies of ERV transcripts (N=13) levels in TP53 mutant (KASUMI, KG1-1A, U937) and wild-type (WT; MOLM-14, OCI/AML2, and OCI/AML3) AML cell lines and primary cells (N=6 mutant, N=6 WT) show that, while TP53 mutant cells have low baseline ERV expression, DNMTi treatment significantly increases ERV expression, compared with WT TP53 cells (p&lt;0.05). Moreover, inhibiting TP53 in WT TP53 AML cell lines with the specific TP53 inhibitor pifithrin significantly upregulates ERVs, validating the role of TP53 in suppressing ERVs in AML cells.
Second, while TCGA data analysis showed that baseline STING transcript are low in TP53 mutant vs WT AML, phospho-STING (activated) is actually increased in TP53 mutant AML cells and DNMTi treatment further activates STING. Increased expression of downstream IFN/inflammasome genes in TP53 mutant AML treated with DNMTi, compared with results in WT cells, validates these results. Moreover, DNMTi combination with poly (ADP-ribose) polymerase inhibitors (PARPis), which we previously reported to activate STING in breast and ovarian cancer though increasing cytosolic dsDNA and dsRNA, further augments STING activation and IFN/inflammasome signaling (p&lt;0.05).
Third, the IFN/inflammasome signaling response to DNMTI/PARPI signaling is directly linked with decreased expression of DNA ds break repair (DSBR) genes and activity, leading to homologous recombination defects (HRD).
Fourth, the mechanism for both IFN/inflammasome signaling and HRD induction is dependent on STING, since both CRISPR/CAS KO of STING or treatment with the STING inhibitor H-151 can rescue these effects in TP53 mutant and WT AML cells. Our findings increase understanding of the therapy potential for combining DNMTi+PARPi treatment in AML and suggest use of this paradigm to treat mutant TP53 AML. These results also suggest that activating STING may be a key strategy for increasing IFN/inflammasome signaling in treatment of TP53 mutant AML and that immune responses may be further augmented with immune therapy in AML and other cancers.
Citation Format: Aksinija A. Kogan, Michael J. Topper, Lora Stojanovic, Lena J. McLaughlin, Tammy J. Kingsbury, Maria R. Baer, Michael Kessler, Stephen B. Baylin, Feyruz V. Rassool. DNA methyltransferase inhibitors increase ERV reactivation and STING-dependent interferon/inflammasome signaling in TP53 mutant AML [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 6301.