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Replacing the SpCas9 HNH domain by deaminases generates compact base editors with an alternative targeting scope.

ABSTRACT: Base editors are RNA-guided deaminases that enable site-specific nucleotide transitions. The targeting scope of these Cas-deaminase fusion proteins critically depends on the availability of a protospacer adjacent motif (PAM) at the target locus and is limited to a window within the CRISPR-Cas R-loop, where single-stranded DNA (ssDNA) is accessible to the deaminase. Here, we reason that the Cas9-HNH nuclease domain sterically constrains ssDNA accessibility and demonstrate that omission of this domain expands the editing window. By exchanging the HNH nuclease domain with a monomeric or heterodimeric adenosine deaminase, we furthermore engineer adenine base editor variants (HNHx-ABEs) with PAM-proximally shifted editing windows. This work expands the targeting scope of base editors and provides base editor variants that are substantially smaller. It moreover informs of potential future directions in Cas9 protein engineering, where the HNH domain could be replaced by other enzymes that act on ssDNA.

SUBMITTER: Villiger L

PROVIDER: S-EPMC8479293 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Replacing the SpCas9 HNH domain by deaminases generates compact base editors with an alternative targeting scope

Project description:Base editors are RNA-guided deaminases that enable site-specific nucleotide transitions. The targeting scope of these Cas-deaminase fusion proteins critically depends on the availability of a protospacer adjacent motif (PAM) at the selected genomic locus, and is limited to a window within the CRISPR-Cas R-loop where single stranded (ss)DNA is accessible to the deaminase. Here, we reason that the Cas9-HNH nuclease domain sterically constrains ssDNA accessibility, and demonstrate that omission of this domain expands the editing window. By exchanging the HNH nuclease domain with an adenosine deaminase, we furthermore engineer adenine base editor variants (HNHx-ABE) with PAM-proximally shifted editing windows. HNHx-ABEs are substantially reduced in size, and expand the targeting scope of base editors. Our finding that the HNH domain is replaceable could moreover benefit future protein engineering efforts, where Cas9 operates together with other enzyme domains.

2020-11-12 | GSE161293 | GEO

Replacing the SpCas9 HNH domain by deaminases generates compact base editors with an alternative targeting scope [DNA-seq]

2020-11-12 | GSE161380 | GEO

Replacing the SpCas9 HNH domain by deaminases generates compact base editors with an alternative targeting scope [RNA-seq]

2020-11-12 | GSE161379 | GEO

Replacing the SpCas9 HNH domain by deaminases generates compact base editors with an alternative targeting scope

Project description:Replacing the SpCas9 HNH domain by deaminases generates compact base editors with an alternative targeting scope

| PRJNA677983 | ENA

Replacing the SpCas9 HNH domain by deaminases generates compact base editors with an alternative targeting scope [DNA-seq]

Project description:Replacing the SpCas9 HNH domain by deaminases generates compact base editors with an alternative targeting scope [DNA-seq]

| PRJNA677990 | ENA

Replacing the SpCas9 HNH domain by deaminases generates compact base editors with an alternative targeting scope [RNA-seq]

Project description:Replacing the SpCas9 HNH domain by deaminases generates compact base editors with an alternative targeting scope [RNA-seq]

| PRJNA677989 | ENA

Circularly permuted and PAM-modified Cas9 variants broaden the targeting scope of base editors.

Project description:Base editing requires that the target sequence satisfy the protospacer adjacent motif requirement of the Cas9 domain and that the target nucleotide be located within the editing window of the base editor. To increase the targeting scope of base editors, we engineered six optimized adenine base editors (ABEmax variants) that use SpCas9 variants compatible with non-NGG protospacer adjacent motifs. To increase the range of target bases that can be modified within the protospacer, we use circularly permuted Cas9 variants to produce four cytosine and four adenine base editors with an editing window expanded from ~4-5 nucleotides to up to ~8-9 nucleotides and reduced byproduct formation. This set of base editors improves the targeting scope of cytosine and adenine base editing.

| S-EPMC6551276 | biostudies-literature

Inhibition of base editors with anti-deaminases derived from viruses.

Project description:Cytosine base editors (CBEs), combining cytidine deaminases with the Cas9 nickase (nCas9), enable targeted C-to-T conversions in genomic DNA and are powerful genome-editing tools used in biotechnology and medicine. However, the overexpression of cytidine deaminases in vivo leads to unexpected potential safety risks, such as Cas9-independent off-target effects. This risk makes the development of deaminase off switches for modulating CBE activity an urgent need. Here, we report the repurpose of four virus-derived anti-deaminases (Ades) that efficiently inhibit APOBEC3 deaminase-CBEs. We demonstrate that they antagonize CBEs by inhibiting the APOBEC3 catalytic domain, relocating the deaminases to the extranuclear region or degrading the whole CBE complex. By rationally engineering the deaminase domain, other frequently used base editors, such as CGBE, A&CBE, A&CGBE, rA1-CBE and ABE8e, can be moderately inhibited by Ades, expanding the scope of their applications. As a proof of concept, the Ades in this study dramatically decrease both Cas9-dependent and Cas9-independent off-target effects of CBEs better than traditional anti-CRISPRs (Acrs). Finally, we report the creation of a cell type-specific CBE-ON switch based on a microRNA-responsive Ade vector, showing its practicality. In summary, these natural deaminase-specific Ades are tools that can be used to regulate the genome-engineering functions of BEs.

| S-EPMC8807840 | biostudies-literature

BE-PIGS: a base-editing tool with deaminases inlaid into Cas9 PI domain significantly expanded the editing scope.

Project description: Not available

| S-EPMC6799832 | biostudies-literature

Exploring alternative catalytic mechanisms of the Cas9 HNH domain.

Project description:Understanding the reaction mechanism of CRISPR-associated protein 9 (Cas9) is crucial for the application of programmable gene editing. Despite the availability of the structures of Cas9 in apo- and substrate-bound forms, the catalytically active structure is still unclear. Our first attempt to explore the catalytic mechanism of Cas9 HNH domain has been based on the reasonable assumption that we are dealing with the same mechanism as endonuclease VII, including the assumption that the catalytic water is in the first shell of the Mg2+ . Trying this mechanism with the cryo-EM structure forced us to induce significant structural change driven by the movement of K848 (or other positively charged residue) close to the active site to facilitate the proton transfer step. In the present study, we explore a second reaction mechanism where the catalytic water is in the second shell of the Mg2+ and assume that the cryo-EM structure by itself is a suitable representation of a catalytic-ready structure. The alternative mechanism indicates that if the active water is from the second shell, then the calculated reaction barrier is lower compared with the corresponding barrier when the water comes from the first shell.

| S-EPMC6942198 | biostudies-literature

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