A yeast selection platform is developed to engineer Cas9 with re-specified activity across multiple additional non-canonical PAMs in yeast, further demonstrating its utility as a general and programmable framework for expanding the therapeutic reach of precision genome editing.
Abstract
The therapeutic potential of CRISPR–Cas9 genome editing is fundamentally constrained by the requirement for specific short DNA sequences (PAMs) flanking the target site, limiting access to many clinically relevant genomic loci. This stringent PAM requirement is particularly problematic in applications which require precise positioning, such as base editing and allele-specific editing. Although PAM-relaxed variants have expanded the targetable genome, they incur trade-offs in on-target activity, off-target editing, and cleavage kinetics. This highlights an unmet need for variants that are re-targeted to alternative PAMs in order to maintain the specificity and enzymatic performance inherent to stringent dinucleotide PAM recognition. To overcome these limitations, we developed a yeast selection platform to engineering SpCas9 variants with re-specified PAM recognition. Using a clinically relevant Huntington’s disease gene (HTT) SNP as a proof-of-concept target, we engineered variants with reciprocal NGC and NGT PAM selectivity, as a step toward allele-specific editing in a large percentage of Huntington’s disease patients. These yeast-selected SpCas9 variants retained their modified activity across multiple endogenous HEK293T loci, demonstrating that this specificity is robust across diverse genomic contexts. The variants surpassed PAM-broadened variants on their respective on-target PAM while displaying broad loss of activity across alternative PAMs, effectively re-specifying PAM recognition toward a single dinucleotide sequence. Retargeted variants recovered on-target cleavage kinetics approaching that of wild-type SpCas9, even under competing substrate conditions, demonstrating that PAM re-specification can simultaneously restore catalytic efficiency and improve specificity. Beyond NGC and NGT, we leveraged our high-throughput platform to engineer Cas9 with re-specified activity across multiple additional non-canonical PAMs in yeast, further demonstrating its utility as a general and programmable framework for expanding the therapeutic reach of precision genome editing.
Five previously uncharacterized MG102-like Cas9d orthologs are identified that share the hallmark genomic, sequence, and structural features of type II-D Cas9 and establish compact MG102-like Cas9d orthologs as robust and specific genome editors and provide promising, single-AAV– compatible scaffolds for in vivo therapeutic genome editing.
Qiaochu Wang, Ahmed Saleh, G. S. Rao et al.· bioRxiv· 0 citations
Amongst the genome manipulation tools based on the versatile CRISPR-Cas system, prime editing is the most prominent one as a method that enables precise insertions, deletions, and substitutions without inducing double-strand breaks. Cas12a nucleases are widely used for genome editing and nucleic acid detection, owing to their unique properties; however, their relatively long PAM requirements limit their applicability.
We develop PAM-flexible Cas12a variants capable of functioning effectively within mammalian cells, thereby enabling the cleavage of targets previously inaccessible to Cas12a nucleases. Amongst the Lb-, As-, Mb-, and FnCas12a variants we develop, flexiAsCas12a (AsCas12a with flexible PAM recognition) is the most effective, expanding the range of recognized PAM sequences by Cas12a variants to include NATN, NCCN and GTCN sequences. Using the currently available Cas12a variants with relaxed PAM recognition (impLbCas12a, flexiAsCas12a, and enAsCas12a), we develop circular RNA-guided split prime editors and validate their functionality on non-canonical PAM sequences.
flexiAsCas12a joins the repertoire of Cas12a PAM variants, enabling access to an increasing number of target sequences by Cas12a nucleases.
A novel genome-wide CRISPR screening strategy that will facilitate the systematic engineering of novel nonviral genome editing delivery methods, where the identified novel gene hits can be further used to increase editing efficiency for other therapeutically relevant cell types.
Shivani Saxena, Meha Kabra, Amr A. Abdeen et al.· bioRxiv· 2 citations
To systematically map cellular factors constraining nonviral genome editing, influencing uptake and intracellular trafficking, we develop a genome-wide CRISPR screening platform linking perturbation of 19,114 genes to editing outcomes in human cells. We identify six negative regulators of delivery whose depletion increases editing efficiency by up to six-fold across diverse payloads, loci, and cell types. We test the top two factors, GJB2 and BET1L, in two distinct human models: correction of a pathogenic adenine base mutation in KCNJ13 and introduction of a cytosine base mutation in the GABAA receptor gene. Depletion of either improves base-editing outcomes by 6-fold, potentially through effects on delivery. In a patient-derived model of retinal channelopathy, knockdown of either gene improves lipid nanoparticle base editing efficiency by over 3.5-fold. This enables functional restoration of Kir7.1 ion channels in a subset of edited cells, highlighting cellular barriers as actionable targets to enhance the potency of genetic therapies. Low editing efficiency of nonviral delivery in post mitotic tissues presents a challenge to the field of gene therapy. Here, authors dissect the genetic regulators of nonviral delivery in post mitotic retinal epithelial cells describe strategies for improved base editor delivery and editing.
Shivani Saxena, Meha Kabra, Amr A. Abdeen et al.· Nature Communications· 0 citations
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.
Masaki Kawamata, S. Niwa, Atsushi Suzuki· Chemical Biology and Drug De...· 0 citations
Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) technology has revolutionized genetic medicine by enabling precise genome editing for therapeutic benefit. CRISPR nucleases are programmed to target genomic sites with sequence complementarity to the spacer region of an associated guide RNA. However, these nucleases may target genomic loci with sequences similar to the target site, which can lead to unintended disruption of off-target genes. The risks associated with these off-target editing events are critical to assess as CRISPR-based in vivo editing systems advance to clinical development. Nonhuman primates (NHPs) are common model species for evaluating the human safety of many therapeutic modalities, but their relevance for evaluating human CRISPR off-target activity has yet to be determined. In this study, 1,220,908 Cas12a and 6,159,066 Cas9 spacer sequences targeting human genes were designed, and off-target editing sites were computationally predicted in humans and five common NHP species. Of the 7,413 Cas12a and 570,754 Cas9 spacers meeting defined on- and off-target inclusion criteria, only 14-21% of Cas12a and 7-15% of Cas9 human off-targets per spacer are recapitulated in the genomes of NHPs commonly used in preclinical studies. These results highlight the limitations of NHPs for the study of human CRISPR spacer specificity and contextualize human risk informed by these studies.
Laura Blaha, M. Bosinger, D. Bodian et al.· Human Gene Therapy· 0 citations