Polθ activity modulates sensitivity to standard therapies in DNMT3A-deficient leukemia.

Myeloid malignancies carrying somatic DNMT3A mutations (DNMT3Amut) are refractory to standard therapy. DNMT3Amut leukemia cells accumulate toxic DNA double-strand breaks (DSBs) and stalled replication forks, rendering them dependent on DNA damage response (DDR). We report here that DNA polymerase theta (Polθ), a key element in DSB repair by end-joining (Polθ-mediated end-joining [TMEJ]) and in fork restarting, promotes survival and proliferation of DNMT3Amut leukemia cells. Polθ is overexpressed in DNMT3Amut leukemia cells due to abrogation of PARP1 PARylation-dependent UBE2O E3 ligase-mediated ubiquitination and proteasomal degradation of Polθ. In addition, PARP1-mediated recruitment of the SMARCAD1-MSH2/MSH3 repressive complex to DSBs is diminished in DNMT3Amut leukemia cells, which facilitates association of Polθ with DNA damage. Polθ inhibitors enhance the anti-leukemic effects of standard drugs such as FLT3 kinase inhibitor quizartinib, cytarabine ± doxorubicin, and etoposide in vitro and in mice with DNMT3Amut leukemia. Altogether, Polθ is an attractive target in DNMT3Amut hematological malignancies.