Developing a Novel Combination Therapy and Elucidating Mechanisms of Increased ALT-NHEJ in Acute Myeloid Leukemia

Abstract

Acute myeloid leukemia (AML) is a cancer of the blood and bone marrow that results from a differentiation block of hematopoietic stem/progenitor cells, leading to accumulation of myeloblasts in the bone marrow. Frontline chemotherapy has a 22% survival rate, with rapid relapse occurring particularly in poor-response subgroups such as those patients bearing the FLT3/ITD mutation. While DNA methyltransferase inhibitors (DNMTis) are now second-line therapy in AML patients who have relapsed or are refractory to chemotherapy, treatment responses are not durable. This emphasizes the need for devising novel therapies for AML. Poly (ADP-ribose) polymerase (PARP) has been shown to bind DNMTs in response to DNA damage, and we reasoned that PARP inhibitors (PARPis) that are in use clinically could potentially enhance the DNMTi response. We showed that combination of DNMTi (decitabine) and PARPi (talazoparib) resulted in enhanced cytotoxicity in AML cell lines and primary AML samples relative to each drug alone. We found that the increased efficacy of this combination was due to enhanced PARP-DNA complexes, resulting in increased cytotoxic double-strand breaks. The efficacy of this combination was corroborated in two in vivo mouse models of AML. In a second study, we investigated the regulation of a highly error-prone form of non-homologous end-joining (NHEJ), alternative-NHEJ (ALT-NHEJ), in FLT3/ITD-positive AML. Our laboratory had previously shown that DNA ligase 3α (LIG3) and PARP1 (components of ALT-NHEJ pathway) are highly expressed in FLT3/ITD-positive AML. In this study, we demonstrated that c-MYC bound the promoters of LIG3 and PARP1, leading to their increased expression. Furthermore, c-MYC repressed microRNAs (miR-150 and miR-22) that regulate these two proteins at the post-transcriptional level. Importantly, inhibition of c-MYC or overexpression of miR-22 and miR-150 lead to a significant decrease in ALT-NHEJ activity. Taken together, we elucidated a mechanism by which ALT-NHEJ activity is amplified in AML, and exploited the upregulation of PARP1 as a target that enhanced the efficacy of the DNMTi-PARPi combination. Notably, our pre-clinical studies led to the launch of a Phase I/II clinical trial for testing decitabine-talazoparib combination therapy in refractory/relapsed AML. Results from this trial could lay the groundwork for similar trials in other cancers.