Human polymerase theta helicase positions DNA microhomologies for double-strand break repair
DNA double-strand breaks occur in all human cells on a daily basis 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, the molecular details of this error-prone mechanism are 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 base pairs and anneal