CRISPR RNA architecture steers Cas9 catalysis and fidelity via a guide repeat clasp motif

Sequence Engineering of Guide DNA for Precise RNA Targeting by Cas12a.

Cas12a is highly accommodative toward noncanonical activation pathways to the extent of flipping its identity to be a DNA-guided RNA-targeting effector. A sequence engineering approach was used to systematically identify desirable guide DNA (gDNA) sequence motifs to achieve comparable RNA targeting efficiency as the canonical RNA-guided Cas12a with good selectivity down to single-nucleotide mismatch. Importantly, we introduced a split gDNA design concept with greater energetic differences arising from subtle nucleotide changes to probe the key spacer features for effective Cas12a-gDNA activation. Similar to the canonical RNA-guided activation pathway, Cas12a was found to engage actively in the "seed-like" scaffold-proximal region while the scaffold-distal region was largely hybridization-driven. We further evolved the split gDNA design to enhance the sequence selectivity by up to 21-fold compared to a single gDNA design and achieve single-nucleotide discrimination among representative let-7 family members. This study has established a gDNA sequence design framework to reprogram Cas12a as a precise RNA targeting platform.

Chemical and Structural Engineering of Guide RNAs for Precision Genome Editing: From Design Principles to Clinical Applications

CRISPR–Cas9 has revolutionised genome editing by enabling efficient and programmable modification of defined DNA sequences, with guide RNAs (gRNAs) serving as indispensable elements that direct Cas9 to specific genomic loci. Initially regarded as auxiliary components, gRNAs are now recognized as critical determinants of editing efficiency and specificity and have attracted growing attention as independent targets for engineering. Chemical modification, sequence optimisation, and structural alteration of gRNAs have been shown to enhance on‐target activity, suppress off‐target effects and cytotoxicity, and even achieve allele‐selective precision editing in a programmable manner. Moreover, advances in artificial intelligence and machine learning have markedly improved the predictive accuracy of gRNA design through large‐scale data analysis. Despite rapid progress, a consolidated review that integrates chemical, structural, and computational advances in gRNA engineering and highlights their translational potential for therapeutic genome editing has been lacking. This review uniquely addresses that gap by presenting an integrated framework that connects molecular design principles with clinical applicability.

RAPID: evaluation of Cas12a protospacer nicking and chimeric reporters for PAM-independent RNA and DNA diagnostics

Abstract CRISPR–Cas nucleases have revolutionized diagnostics and biotechnology by providing programmable specificity. Here, we extend the understanding of Cas12a biology with a screen that, unexpectedly, finds that Cas12a trans-cleavage activity can be modulated by nicks in the protospacer in a position-dependent manner. Wanting to explore the impact of non-conventional trans-cleavage substrates, we subsequently find that non-specific Cas12a cleavage can be significantly reduced with RNA and chimeric (mixed RNA/DNA) reporter sequences. Exploiting these features and building on emerging protospacer adjacent motif (PAM)-independent Cas12a diagnostics that use engineered DNA activators and split-guide architectures, we introduce RAPID (RNA/DNA Advanced chimeric, PAM-independent, Integrated Nicking, Diagnostics), a nick-tuned, PAM-duplex-mediated platform for PAM-independent RNA and DNA detection. By strategically introducing a nick within the spacer region, RAPID expands Cas12a detection to include target RNAs, which can be ligated in situ to create a hybrid protospacer-target with trans-cleavage activity matching conventional Cas12a. We then apply RAPID to detect single-point mutations in ssDNA and RNA substrates, a challenge for traditional Cas12 and Cas13 systems. In combination with RT-LAMP, RAPID is used for PAM-independent RNA detection in clinical samples, achieving sensitivity down to ∼1 aM and 100% concordance with RT-qPCR for samples with Ct ≤ 33.

CRISPR RNA-independent activation of Cas12a

Structural basis for target discrimination and activation by Cas13d

CRISPR-Cas13d is increasingly used for RNA knockdowns due to its programmability, but off-target RNA binding and cleavage of near-cognate RNAs hinder its broader adoption. Here, we explore the mechanisms of nuclease activation by solving seven ternary cryo-electron mi-croscopy structures of wild-type Cas13d in complex with matched and mismatched targets. These structures reveal a series of active, intermediate, and inactive states that illustrate a detailed activation mechanism. The crRNA undergoes dramatic conformational changes upon target RNA binding, with the helical-1 domain transitioning from an initially docked state with the N-terminal domain to an allosterically switched conformation that stabilizes the RNA duplex. Quantitative kinetics reveal that a single proximal mismatch preserves nanomolar binding affinity but completely abolishes nuclease activity by trapping Cas13d in an inactive state. We identify an active site loop in the HEPN domains that regulates substrate accessibility, with alanine scanning mutagenesis revealing both hypo- and hyperactivated variants. These findings establish the structural basis for Cas13d’s exquisite mismatch surveillance and provide a mechanistic framework for engineering RNA-targeting specificity and activity across HEPN nuclease family members.

Evaluating high-fidelity CRISPR-Cas nucleases in nucleosomal contexts using a quantitative framework

It is demonstrated that local nucleosome sequence and structure profoundly influence Cas nuclease accessibility and specificity, with HIFIv1 emerging as the top-performing nuclease for nucleosomal targets, while evoSpCas9 excelled in exposed contexts.