DNA Methyltransferase Inhibitors Promote Homologous Recombination Deficiency through Induction of Immune Signaling, Sensitizing Acute Myeloid Leukemia Cells to PARP Inhibitors

Acute myeloid leukemia (AML) patients unfit for intensive chemotherapy are treated with DNA methyltransferase inhibitors (DNMTis). However, while many AML patients respond to DNMTis, responses are not durable. We previously reporteda novel treatment strategy for AML that combines DNMTis with poly (ADP-ribose) polymerase inhibitors (PARPis), drugs classically used to treat breast and ovarian cancer patients with BRCA mutations and homologous recombination defects (HRD) (Faraoni and Graziani, 2018). We found that combining low doses of the potent PARP-trapping PARPi talazoparib with DNMTis increases PARP trapping and cytotoxicityin vitroand increases therapeutic efficacy in vivo (Muvarak et al, 2016).

We have nowidentified a novel mechanism through which DNMTis may sensitize BRCA-proficient AML cells to PARPis. This mechanism is tied to the capacity of these drugs to reprogram cancer signaling networks, including altering DNA repair pathways (Tsai et al, 2012). In studies in AML cell lines (N=6) and peripheral blood mononuclear cells (PBMCs) from AML patients (N=4), we now show that treatment with the DNMTi decitabine (DAC) at a low concentration (10nM) can directly induce HRD, by significantly (p<0.01) down-regulating key genes central to HR activity, including multiple genes in the Fanconi anemia (FA) pathway, as a mechanism for enhanced PARPi sensitivity.

How do DNMTis downregulate HR gene expression? We show for the first time that immune signaling is linked to induction of HRD. We have previously shown that DNMTis activate innate immune pathways involving interferon (IFN) □ and tumor necrosis factor (TNF) □, a phenomenon known as viral mimicry (Chiappinelli et al, 2015).First, The Cancer Genome Atlas (TCGA) AML data sets show an inverse correlation between type 1 interferon (IFN)/pro-inflammatory response and HR-related genes. Second, we verified in BRCA-proficient AML cell lines (N=6) that immune signaling by exogenous TNF□□or IFN□□treatment decreases HR gene expression and activity by more than two-fold for the majority of genes tested (p<0.0001). Third, treatment of AML cells with IFN□and the signal transducer and activator of transcription (STAT) 1/3 inhibitor ruxolitinib can rescue DAC-induced HRD.

Importantly, we identified a common immune signaling pathway induced by both DNMTis and PARPis. PARPis have also been shown to activate type 1 IFN pathways via induction of cytoplasmic double-stranded DNA sensing through signaling of the cyclic GMP-AMP Synthase - Stimulator of Interferon Genes(cGAS-STING) pathway. We now find that inhibition of STING with inhibitor H-151 (500nM) not only rescues immune signaling induced by PARPi, but also by DAC and PARPi combination treatment. Moreover, the STING inhibitor also rescues DAC- and/or PARPi-induced HRD. These data suggest that STING may be a central signaling hub linked to HRD and also suggest ways in which epigenetic therapy, inhibitors of DNA damage response proteins, and targeted immune therapy can synergize to treat AML.