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Type V CRISPR-Cas interference proteins use a single RuvC active site to make RNA-guided breaks in double-stranded DNA substrates, an activity essential for both bacterial immunity and genome editing. The best-studied of these enzymes, Cas12a, initiates DNA cutting by forming a 20-nucleotide R-loop in which the guide RNA displaces one strand of a double-helical DNA substrate, positioning the DNase active site for first-strand cleavage. However, crystal structures and biochemical data have not explained how the second strand is cut to complete the double-strand break. Here, we detect intrinsic instability in DNA flanking the RNA-3′ side of R-loops, which Cas12a can exploit to expose second-strand DNA for cutting. Interestingly, DNA flanking the RNA-5′ side of R-loops is not intrinsically unstable. This asymmetry in R-loop structure may explain the uniformity of guide RNA architecture and the single-active-site cleavage mechanism that are fundamental features of all type V CRISPR-Cas systems.
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Improved genome editing by an engineered CRISPR-Cas12a
Abstract CRISPR-Cas12a is an RNA-guided, programmable genome editing enzyme found within bacterial adaptive immune pathways. Unlike CRISPR-Cas9, Cas12a uses only a single catalytic site to both cleave target double-stranded DNA (dsDNA) (cis-activity) and indiscriminately degrade single-stranded DNA (ssDNA) (trans-activity). To investigate how the relative potency of cis- versus trans-DNase activity affects Cas12a-mediated genome editing, we first used structure-guided engineering to generate variants of Lachnospiraceae bacterium Cas12a that selectively disrupt trans-activity. The resulting engineered mutant with the biggest differential between cis- and trans-DNase activity in vitro showed minimal genome editing activity in human cells, motivating a second set of experiments using directed evolution to generate additional mutants with robust genome editing activity. Notably, these engineered and evolved mutants had enhanced ability to induce homology-directed repair (HDR) editing by 2–18-fold compared to wild-type Cas12a when using HDR donors containing mismatches with crRNA at the PAM-distal region. Finally, a site-specific reversion mutation produced improved Cas12a (iCas12a) variants with superior genome editing efficiency at genomic sites that are difficult to edit using wild-type Cas12a. This strategy establishes a pipeline for creating improved genome editing tools by combining structural insights with randomization and selection. The available structures of other CRISPR-Cas enzymes will enable this strategy to be applied to improve the efficacy of other genome-editing proteins.
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- Award ID(s):
- 1817593
- PAR ID:
- 10473059
- Publisher / Repository:
- Nucleic Acids Research
- Date Published:
- Journal Name:
- Nucleic Acids Research
- Volume:
- 50
- Issue:
- 22
- ISSN:
- 0305-1048
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Summary Class 2 Type V‐A CRISPR‐Cas (Cas12a) nucleases are powerful genome editing tools, particularly effective in A/T‐rich genomic regions, complementing the widely used CRISPR‐Cas9 in plants. To enhance the utility of Cas12a, we investigate three Cas12a orthologs—Mb3Cas12a, PrCas12a, and HkCas12a—in plants. Protospacer adjacent motif (PAM) requirements, editing efficiencies, and editing profiles are compared in rice. Among these orthologs, Mb3Cas12a exhibits high editing efficiency at target sites with a simpler, relaxed TTV PAM which is less restrictive than the canonical TTTV PAM of LbCas12a and AsCas12a. To optimize Mb3Cas12a, we develop an efficient single transcription unit (STU) system by refining the linker between Mb3Cas12a and CRISPR RNA (crRNA), nuclear localization signal (NLS), and direct repeat (DR). This optimized system enables precise genome editing in rice, particularly for fine‐tuning target gene expression by editing promoter regions. Further, we introduced Arginine (R) substitutions at Aspartic acid (D) 172, Asparagine (N) 573, and Lysine (K) 579 of Mb3Cas12a, creating two temperature‐tolerant variants: Mb3Cas12a‐R (D172R) and Mb3Cas12a‐RRR (D172R/N573R/K579R). These variants demonstrate significantly improved editing efficiency at lower temperatures (22 °C and 28 °C) in rice cells, with Mb3Cas12a‐RRR showing the best performance. We extend this approach by developing efficient Mb3Cas12a‐RRR STU systems in maize and tomato, achieving biallelic mutants targeting single or multiple genes in T0lines cultivated at 28 °C and 25 °C, respectively. This study significantly expands Cas12a's targeting capabilities in plant genome editing, providing valuable tools for future research and practical applications.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1093/nar/gkac1192
