CoCas9 is a compact nuclease from the human microbiome for efficient and precise genome editing.

The expansion of the CRISPR-Cas toolbox is highly needed to accelerate the development of therapies for genetic diseases. Here, through the interrogation of a massively expanded repository of metagenome-assembled genomes, mostly from human microbiomes, we uncover a large variety (n = 17,173) of type II CRISPR-Cas loci. Among these we identify CoCas9, a strongly active and high-fidelity nuclease with reduced molecular size (1004 amino acids) isolated from an uncultivated Collinsella species. CoCas9 is efficiently co-delivered with its sgRNA through adeno associated viral (AAV) vectors, obtaining efficient in vivo editing in the mouse retina. With this study we uncover a collection of previously uncharacterized Cas9 nucleases, including CoCas9, which enriches the genome editing toolbox.

Eukaryotic-driven directed evolution of Cas9 nucleases.

BACKGROUND: Further advancement of genome editing highly depends on the development of tools with higher compatibility with eukaryotes. A multitude of described Cas9s have great potential but require optimization for genome editing purposes. Among these, the Cas9 from Campylobacter jejuni, CjCas9, has a favorable small size, facilitating delivery in mammalian cells. Nonetheless, its full exploitation is limited by its poor editing activity. RESULTS: Here, we develop a Eukaryotic Platform to Improve Cas Activity (EPICA) to steer weakly active Cas9 nucleases into highly active enzymes by directed evolution. The EPICA platform is obtained by coupling Cas nuclease activity with yeast auxotrophic selection followed by mammalian cell selection through a sensitive reporter system. EPICA is validated with CjCas9, generating an enhanced variant, UltraCjCas9, following directed evolution rounds. UltraCjCas9 is up to 12-fold more active in mammalian endogenous genomic loci, while preserving high genome-wide specificity. CONCLUSIONS: We report a eukaryotic pipeline allowing enhancement of Cas9 systems, setting the ground to unlock the multitude of RNA-guided nucleases existing in nature.

An optimized SpCas9 high-fidelity variant for direct protein delivery.

Electroporation of the Cas9 ribonucleoprotein (RNP) complex offers the advantage of preventing off-target cleavages and potential immune responses produced by long-term expression of the nuclease. Nevertheless, the majority of engineered high-fidelity Streptococcus pyogenes Cas9 (SpCas9) variants are less active than the wild-type enzyme and are not compatible with RNP delivery. Building on our previous studies on evoCas9, we developed a high-fidelity SpCas9 variant suitable for RNP delivery. The editing efficacy and precision of the recombinant high-fidelity Cas9 (rCas9HF), characterized by the K526D substitution, was compared with the R691A mutant (HiFi Cas9), which is currently the only available high-fidelity Cas9 that can be used as an RNP. The comparative analysis was extended to gene substitution experiments where the two high fidelities were used in combination with a DNA donor template, generating different ratios of non-homologous end joining (NHEJ) versus homology-directed repair (HDR) for precise editing. The analyses revealed a heterogeneous efficacy and precision indicating different targeting capabilities between the two variants throughout the genome. The development of rCas9HF, characterized by an editing profile diverse from the currently used HiFi Cas9 in RNP electroporation, increases the genome editing solutions for the highest precision and efficient applications.

Functional restoration of a CFTR splicing mutation through RNA delivery of CRISPR adenine base editor.

Cystic fibrosis (CF) is a genetic disease caused by mutations in the CF transmembrane conductance regulator (CFTR) gene. The 2789+5G>A CFTR mutation is a quite frequent defect causing an aberrant splicing and a non-functional CFTR protein. Here we used a CRISPR adenine base editing (ABE) approach to correct the mutation in the absence of DNA double-strand breaks (DSB). To select the strategy, we developed a minigene cellular model reproducing the 2789+5G>A splicing defect. We obtained up to 70% editing in the minigene model by adapting the ABE to the PAM sequence optimal for targeting 2789+5G>A with a SpCas9-NG (NG-ABE). Nonetheless, the on-target base correction was accompanied by secondary (bystander) A-to-G conversions in nearby nucleotides, which affected the wild-type CFTR splicing. To decrease the bystander edits, we used a specific ABE (NG-ABEmax), which was delivered as mRNA. The NG-ABEmax RNA approach was validated in patient-derived rectal organoids and bronchial epithelial cells showing sufficient gene correction to recover the CFTR function. Finally, in-depth sequencing revealed high editing precision genome-wide and allele-specific correction. Here we report the development of a base editing strategy to precisely repair the 2789+5G>A mutation resulting in restoration of the CFTR function, while reducing bystander and off-target activities.