DNA polymerase θ (POLQ), double-strand break repair, and cancer

DNA polymerase theta (pol theta) is encoded in the genomes of many eukaryotes, though not in fungi. Pol theta is encoded by the POLQ gene in mammalian cells. The C-terminal third of the protein is a family A DNA polymerase with additional insertion elements relative to prokaryotic homologs. The N-terminal third is a helicase-like domain with DNA-dependent ATPase activity. Pol theta is important in the repair of genomic double-strand breaks (DSBs) from many sources. These include breaks formed by ionizing radiation and topoisomerase inhibitors, breaks arising at stalled DNA replication forks, breaks introduced during diversification steps of the mammalian immune system, and DSB induced by CRISPR-Cas9. Pol theta participates in a route of DSB repair termed "alternative end-joining" (altEJ). AItEJ is independent of the DNA binding Ku protein complex and requires DNA end resection. Pol theta is able to mediate joining of two resected 3' ends harboring DNA sequence microhomology. "Signatures" of Pol theta action during altEJ are the frequent utilization of longer microhomologies, and the insertion of additional sequences at joining sites. The mechanism of end-joining employs the ability of Pol theta to tightly grasp a 3' terminus through unique contacts in the active site, allowing extension from minimally paired primers. Pol theta is involved in controlling the frequency of chromosome translocations and preserves genome integrity by limiting large deletions. It may also play a backup role in DNA base excision repair. POLQ is a member of a cluster of similarly upregulated genes that are strongly correlated with poor clinical outcome for breast cancer, ovarian cancer and other cancer types. Inhibition of pol theta is a compelling approach for combination therapy of radiosensitization. (C) 2016 Elsevier B.V. All rights reserved.