Ataxia telangiectasia mutated and RAD3 related (ATR) protein kinase plays critical roles in ensuring DNA replication, DNA repair, and cell cycle control in response to replication stress, making ATR inhibition a promising therapeutic strategy for cancer treatment. To identify genes whose loss makes tumor cells hypersensitive to ATR inhibition, we performed CRISPR/Cas9-based whole-genome screens in 3 independent cell lines treated with a highly selective ATR inhibitor, AZD6738. These screens uncovered a comprehensive genome-wide profile of ATR inhibitor sensitivity. From the candidate genes, we demonstrated that RNASEH2 deficiency is synthetic lethal with ATR inhibition both in vitro and in vivo . RNASEH2-deficient cells exhibited elevated levels of DNA damage and, when treated with AZD6738, underwent apoptosis (short-time treated) or senescence (long-time treated). Notably, RNASEH2 deficiency is frequently found in prostate adenocarcinoma; we found decreased RNASEH2B protein levels in prostate adenocarcinoma patient derived xenograft (PDX) samples. Our findings suggest that ATR inhibition may be beneficial for cancer patients with reduced levels of RNASEH2 and that RNASEH2 merits further exploration as a potential biomarker for ATR inhibitor-based therapy.

Therapeutic genome editing technology has been widely used as a powerful tool for directly correcting genetic mutations in target pathological tissues and cells to cure of diseases. The modification of specific genomic sequences can be achieved by utilizing programmable nucleases, such as Meganucleases, zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and the clustered regularly-interspaced short palindromic repeat-associated nuclease Cas9 (CRISPR/Cas9). However, given the properties, such as large size, negative charge, low membrane penetrating ability, as well as weak tolerance for serum, and low endosomal escape, of these nucleases genome editing cannot be successfully applied unless in vivo delivery of related programmable nucleases into target organisms or cells is achieved. Here, we look back at delivery strategies having been used in the in vivo delivery of three main genome editing nucleases, followed by methodologies currently undergoing testing in clinical trials, and potential delivery strategies provided by analyzing characteristics of nucleases and commonly used vectors.

The mitochondria DNA (mtDNA) editing tool, zinc finger nucleases (ZFNs), transcription activator-like effector nuclease (TALENs), and clustered regularly interspaced short palindromic repeats/CRISPR associated protein 9 (CRISPR/Cas9) system, is a promising approach for the treatment of mtDNA diseases by eliminating mutant mitochondrial genomes. However, there have been no reports of repairing the mutant mtDNA with homologous recombination strategy to date. Here, we show a mito-CRISPR/Cas9 system that mito-Cas9 protein can specifically target mtDNA and reduce mtDNA copy number in both human cells and zebrafish. An exogenous single-stranded DNA with short homologous arm was knocked into the targeting loci accurately, and this mutagenesis could be steadily transmitted to F1 generation of zebrafish. Moreover, we found some major factors involved in nuclear DNA repair were upregulated significantly by the mito-CRISPR/Cas9 system. Taken together, our data suggested that the mito-CRISPR/Cas9 system could be a useful method to edit mtDNA by knock-in strategy, providing a potential therapy for the treatment of inherited mitochondrial diseases.

Contemporary improvements in the type II clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 (CRISPR/Cas9) system offer a convenient way for genome editing in zebrafish. However, the low efficiencies of genome editing and germline transmission require a time-intensive and laborious screening work. Here, we reported a method based on in vitro oocyte storage by injecting oocytes in advance and incubating them in oocyte storage medium to significantly improve the efficiencies of genome editing and germline transmission by in vitro fertilization (IVF) in zebrafish. Compared to conventional methods, the prior micro-injection of zebrafish oocytes improved the efficiency of genome editing, especially for the sgRNAs with low targeting efficiency. Due to high throughputs, simplicity and flexible design, this novel strategy will provide an efficient alternative to increase the speed of generating heritable mutants in zebrafish by using CRISPR/Cas9 system.