Human polymerase θ helicase positions DNA microhomologies for double-strand break repair

DNA double-strand breaks occur daily in all human cells and must be repaired with high fidelity to minimize genomic instability. Deficiencies in high-fidelity DNA repair by homologous recombination lead to dependence on DNA polymerase theta, which identifies DNA microhomologies in 3 ' single-stranded DNA overhangs and anneals them to initiate error-prone double-strand break repair. The resulting genomic instability is associated with numerous cancers, thereby making this polymerase an attractive therapeutic target. However, despite the biomedical importance of polymerase theta, the molecular details of how it initiates DNA break repair remain unclear. Here, we present cryo-electron microscopy structures of the polymerase theta helicase domain bound to microhomology-containing DNA, revealing DNA-induced rearrangements of the helicase that enable DNA repair. Our structures show that DNA-bound helicase dimers facilitate a microhomology search that positions 3 ' single-stranded DNA ends in proximity to align complementary bases and anneal DNA microhomology. We characterize the molecular determinants that enable the helicase domain of polymerase theta to identify and pair DNA microhomologies to initiate mutagenic DNA repair, thereby providing insight into potentially targetable interactions for therapeutic interventions.