DNA double strand breaks (DSBs) are repaired through homology-directed repair (HDR) or nonhomologous end joining (NHEJ). BRCA1/2-deficient cancers cannot perform HDR, conferring sensitivity to poly-ADP-ribose polymerase (PARP) inhibitors. However, concomitant loss of the pro-NHEJ factors 53BP1, RIF1, REV7-Shieldin (SHLD1-3) or CST-Polα in BRCA1-deficient cells restores HDR and PARP inhibitor resistance. Here, we identify the TRIP13 ATPase as a negative regulator of REV7. We show that REV7 exists in active "closed" and inactive "open" conformations, and TRIP13 catalyzes the inactivating conformational change, thereby dissociating REV7-Shieldin to promote HDR. TRIP13 similarly disassembles the REV7-REV3 translesion synthesis (TLS) complex, a component of the Fanconi Anemia pathway, inhibiting error-prone replicative lesion bypass and interstrand crosslink repair. Importantly, TRIP13 overexpression is common in BRCA1-deficient cancers, confers PARPi resistance, and correlates with poor prognosis. Thus, TRIP13 emerges as an important regulator of DNA repair pathway choicepromoting HDR, while suppressing NHEJ and TLS.

Prime editors (PEs) mediate genome modification without utilizing double-stranded DNA breaks or exogenous donor DNA as a template. PEs facilitate nucleotide substitutions or local insertions or deletions within the genome based on the template sequence encoded within the prime editing guide RNA (pegRNA). However, the efficacy of prime editing in adult mice has not been established. Here we report an NLS-optimized SpCas9-based prime editor that improves genome editing efficiency in both fluorescent reporter cells and at endogenous loci in cultured cell lines. Using this genome modification system, we could also seed tumor formation through somatic cell editing in the adult mouse. Finally, we successfully utilize dual adeno-associated virus (AAVs) for the delivery of a split-intein prime editor and demonstrate that this system enables the correction of a pathogenic mutation in the mouse liver. Our findings further establish the broad potential of this genome editing technology for the directed installation of sequence modifications in vivo, with important implications for disease modeling and correction.

Type V CRISPR–Cas12a systems provide an alternate nuclease platform to Cas9, with potential advantages for specific genome editing applications. Here we describe improvements to the Cas12a system that facilitate efficient targeted mutagenesis in mammalian cells and zebrafish embryos. We show that engineered variants of Cas12a with two different nuclear localization sequences (NLS) on the C terminus provide increased editing efficiency in mammalian cells. Additionally, we find that pre-crRNAs comprising a full-length direct repeat (full-DR-crRNA) sequence with specific stem-loop G-C base substitutions exhibit increased editing efficiencies compared with the standard mature crRNA framework. Finally, we demonstrate in zebrafish embryos that the improved LbCas12a and FnoCas12a nucleases in combination with these modified crRNAs display high mutagenesis efficiencies and low toxicity when delivered as ribonucleoprotein complexes at high concentration. Together, these results define a set of enhanced Cas12a components with broad utility in vertebrate systems.

BRCA1 deficient tumor cells have defects in homologous-recombination repair and in replication fork stability, resulting in PARP inhibitor sensitivity. Here, we demonstrate that a deubiquitinase, USP1, is upregulated in tumors with mutations in BRCA1. Knockdown or inhibition of USP1 resulted in replication fork destabilization and decreased viability of BRCA1 deficient cells, revealing a synthetic lethal relationship. USP1 binds to and is stimulated by fork DNA. A truncated form of USP1, lacking its DNA binding region, was not stimulated by DNA and failed to localize and protect replication forks. Persistence of monoubiquitinated PCNA at the replication fork was the mechanism of cell death in the absence of USP1. Taken together, USP1 exhibits DNA-mediated activation at the replication fork, protects the fork, and promotes survival in BRCA1 deficient cells. Inhibition of USP1 may be a useful treatment for a subset of PARP inhibitor resistant BRCA1 deficient tumors with acquired replication fork stabilization.

Prime editors (PEs) mediate genome modification without utilizing double-stranded DNA breaks or exogenous donor DNA as a template. PEs facilitate nucleotide substitutions or local insertions or deletions within the genome based on the template sequence encoded within the prime editing guide RNA (pegRNA). However, the efficacy of prime editing in adult mice has not been established. Here we report an NLS-optimized SpCas9-based prime editor that improves genome editing efficiency in both fluorescent reporter cells and at endogenous loci in cultured cell lines. Using this genome modification system, we could also seed tumor formation through somatic cell editing in the adult mouse. Finally, we successfully utilize dual adeno-associated virus (AAVs) for the delivery of a split-intein prime editor and demonstrate that this system enables the correction of a pathogenic mutation in the mouse liver. Our findings further establish the broad potential of this genome editing technology for the directed installation of sequence modifications in vivo, with important implications for disease modeling and correction.