Efficient gene knockout and genetic interaction screening using the in4mer CRISPR/Cas12a multiplex knockout platform.

Genetic interactions mediate the emergence of phenotype from genotype, but technologies for combinatorial genetic perturbation in mammalian cells are challenging to scale. Here, we identify background-independent paralog synthetic lethals from previous CRISPR genetic interaction screens, and find that the Cas12a platform provides superior sensitivity and assay replicability. We develop the in4mer Cas12a platform that uses arrays of four independent guide RNAs targeting the same or different genes. We construct a genome-scale library, Inzolia, that is ~30% smaller than a typical CRISPR/Cas9 library while also targeting ~4000 paralog pairs. Screens in cancer cells demonstrate discrimination of core and context-dependent essential genes similar to that of CRISPR/Cas9 libraries, as well as detection of synthetic lethal and masking/buffering genetic interactions between paralogs of various family sizes. Importantly, the in4mer platform offers a fivefold reduction in library size compared to other genetic interaction methods, substantially reducing the cost and effort required for these assays.

Intrinsic RNA Targeting Triggers Indiscriminate DNase Activity of CRISPR-Cas12a.

The CRISPR-Cas12a system has emerged as a powerful tool for next-generation nucleic acid-based molecular diagnostics. However, it has long been believed to be effective only on DNA targets. Here, we investigate the intrinsic RNA-enabled trans-cleavage activity of AsCas12a and LbCas12a and discover that they can be directly activated by full-size RNA targets, although LbCas12a exhibits weaker trans-cleavage activity than AsCas12a on both single-stranded DNA and RNA substrates. Remarkably, we find that the RNA-activated Cas12a possesses higher specificity in recognizing mutated target sequences compared to DNA activation. Based on these findings, we develop the "Universal Nuclease for Identification of Virus Empowered by RNA-Sensing" (UNIVERSE) assay for nucleic acid testing. We incorporate a T7 transcription step into this assay, thereby eliminating the requirement for a protospacer adjacent motif (PAM) sequence in the target. Additionally, we successfully detect multiple PAM-less targets in HIV clinical samples that are undetectable by the conventional Cas12a assay based on double-stranded DNA activation, demonstrating unrestricted target selection with the UNIVERSE assay. We further validate the clinical utility of the UNIVERSE assay by testing both HIV RNA and HPV 16 DNA in clinical samples. We envision that the intrinsic RNA targeting capability may bring a paradigm shift in Cas12a-based nucleic acid detection and further enhance the understanding of CRISPR-Cas biochemistry.

Type III-B CRISPR-Cas cascade of proteolytic cleavages.

The generation of cyclic oligoadenylates and subsequent allosteric activation of proteins that carry sensory domains is a distinctive feature of type III CRISPR-Cas systems. In this work, we characterize a set of associated genes of a type III-B system from Haliangium ochraceum that contains two caspase-like proteases, SAVED-CHAT and PCaspase (prokaryotic caspase), co-opted from a cyclic oligonucleotide-based antiphage signaling system (CBASS). Cyclic tri-adenosine monophosphate (AMP)-induced oligomerization of SAVED-CHAT activates proteolytic activity of the CHAT domains, which specifically cleave and activate PCaspase. Subsequently, activated PCaspase cleaves a multitude of proteins, which results in a strong interference phenotype in vivo in Escherichia coli. Taken together, our findings reveal how a CRISPR-Cas-based detection of a target RNA triggers a cascade of caspase-associated proteolytic activities.

Antiviral type III CRISPR signalling via conjugation of ATP and SAM.

CRISPR systems are widespread in the prokaryotic world, providing adaptive immunity against mobile genetic elements1,2. Type III CRISPR systems, with the signature gene cas10, use CRISPR RNA to detect non-self RNA, activating the enzymatic Cas10 subunit to defend the cell against mobile genetic elements either directly, via the integral histidine-aspartate (HD) nuclease domain3-5 or indirectly, via synthesis of cyclic oligoadenylate second messengers to activate diverse ancillary effectors6-9. A subset of type III CRISPR systems encode an uncharacterized CorA-family membrane protein and an associated NrN family phosphodiesterase that are predicted to function in antiviral defence. Here we demonstrate that the CorA-associated type III-B (Cmr) CRISPR system from Bacteroides fragilis provides immunity against mobile genetic elements when expressed in Escherichia coli. However, B. fragilis Cmr does not synthesize cyclic oligoadenylate species on activation, instead generating S-adenosyl methionine (SAM)-AMP (SAM is also known as AdoMet) by conjugating ATP to SAM via a phosphodiester bond. Once synthesized, SAM-AMP binds to the CorA effector, presumably leading to cell dormancy or death by disruption of the membrane integrity. SAM-AMP is degraded by CRISPR-associated phosphodiesterases or a SAM-AMP lyase, potentially providing an 'off switch' analogous to cyclic oligoadenylate-specific ring nucleases10. SAM-AMP thus represents a new class of second messenger for antiviral signalling, which may function in different roles in diverse cellular contexts.

Harnessing the LdCsm RNA Detection Platform for Efficient microRNA Detection.

In cancer diagnosis, diverse microRNAs (miRNAs) are used as biomarkers for carcinogenesis of distinctive human cancers. Thus, the detection of these miRNAs and their quantification are very important in prevention of cancer diseases in human beings. However, efficient RNA detection often requires RT-PCR, which is very complex for miRNAs. Recently, the development of CRISPR-based nucleic acid detection tools has brought new promises to efficient miRNA detection. Three CRISPR systems can be explored for miRNA detection, including type III, V, and VI, among which type III (CRISPR-Cas10) systems have a unique property as they recognize RNA directly and cleave DNA collaterally. In particular, a unique type III-A Csm system encoded by Lactobacillus delbrueckii subsp. bulgaricus (LdCsm) exhibits robust target RNA-activated DNase activity, which makes it a promising candidate for developing efficient miRNA diagnostic tools. Herein, LdCsm was tested for RNA detection using fluorescence-quenched DNA reporters. We found that the system is capable of specific detection of miR-155, a microRNA implicated in the carcinogenesis of human breast cancer. The RNA detection system was then improved by various approaches including assay conditions and modification of the 5'-repeat tag of LdCsm crRNAs. Due to its robustness, the resulting LdCsm detection platform has the potential to be further developed as a better point-of-care miRNA diagnostics relative to other CRISPR-based RNA detection tools.

In Vitro CRISPR-Cas12a-Based Detection of Cancer-Associated TP53 Hotspot Mutations Beyond the crRNA Seed Region.

Cost-effective and time-efficient detection of oncogenic mutations supports improved presymptomatic cancer diagnostics and post-treatment disease monitoring. Clustered regularly interspaced short palindromic repeats (CRISPR)-Cas12a is an RNA-guided endonuclease that, upon protospacer adjacent motif (PAM)-dependent recognition of target DNA in cis, exhibits indiscriminate ssDNase activity in trans, which can be harnessed for diagnostics. TP53, one of the most frequently mutated tumor suppressor genes in cancer, displays recurring point mutations at so-called "hotspots." In this study, we optimized Cas12a-based assay conditions for in vitro detection of six TP53 hotspot mutations at the codon for p.R273, located outside the Cas12a seed region, and evaluated the specificities of four commercial Cas12a variants. We found that nonengineered LbCas12a significantly outperformed the other tested nucleases specifically in distinguishing mutant p.R273 codons in synthetic DNA, mock cell-free DNA, and tissue biopsies, despite the suboptimal PAM-distal positioning of the corresponding mutations. Future clinical Cas12a-based applications may include point-of-care tumor analysis, cost-effective mutation screening, and improved monitoring of individual cancer patients.

Endonuclease-Assisted PAM-free Recombinase Polymerase Amplification Coupling with CRISPR/Cas12a (E-PfRPA/Cas) for Sensitive Detection of DNA Methylation.

DNA methylation is considered as a potential cancer biomarker. The evaluation of DNA methylation level will contribute to the prognosis and diagnosis of cancer. Herein, we propose a novel assay based on endonuclease-assisted protospacer adjacent motif (PAM)-free recombinase polymerase amplification coupling with CRISPR/Cas12a (E-PfRPA/Cas) for sensitive detection of DNA methylation. The methylation-sensitive restriction enzyme (MSRE) is first used to selectively digest unmethylated DNA, while the methylated target remains structurally intact. Therefore, the methylated target can initiate the RPA reaction to generate a large amount of double-stranded DNA (dsDNA). To avoid the dependence of PAM site of CRISPR/Cas12a, one of the RPA primers is designed with 5'-phosphate terminuses. After treating with Lambda, the sequence with 5'-phosphate modification will be degraded, leaving the single-stranded DNA (ssDNA). The CRISPR/Cas12a can accurately locate ssDNA without PAM, then initiating its trans-cleavage activity for further signal amplification. Meanwhile, non-specific amplification can be also avoided under Lambda, effectively filtering the detection background. Benefiting from the specificity of MSRE, the high amplification efficiency of Lambda-assisted RPA, and the self-amplification effect of CRISPR/Cas, the E-PfRPA/Cas assay shows outstanding sensitivity and selectivity, and as low as 0.05% of methylated DNA can be distinguished. Moreover, the lateral flow assay is also introduced to exploit the point-of-care diagnostic platform. Most importantly, the proposed method shows high sensitivity for determination of genomic DNA methylation from cancer cells, indicating its great potential for tumor-specific gene analysis.

Structures of an active type III-A CRISPR effector complex.

Clustered regularly interspaced short palindromic repeats (CRISPR) and their CRISPR-associated proteins (Cas) provide many prokaryotes with an adaptive immune system against invading genetic material. Type III CRISPR systems are unique in that they can degrade both RNA and DNA. In response to invading nucleic acids, they produce cyclic oligoadenylates that act as secondary messengers, activating cellular nucleases that aid in the immune response. Here, we present seven single-particle cryo-EM structures of the type III-A Staphylococcus epidermidis CRISPR effector complex. The structures reveal the intact S. epidermidis effector complex in an apo, ATP-bound, cognate target RNA-bound, and non-cognate target RNA-bound states and illustrate how the effector complex binds and presents crRNA. The complexes bound to target RNA capture the type III-A effector complex in a post-RNA cleavage state. The ATP-bound structures give details about how ATP binds to Cas10 to facilitate cyclic oligoadenylate production.

Structural and biochemical characterization of in vivo assembled Lactococcus lactis CRISPR-Csm complex.

The small RNA-mediated immunity in bacteria depends on foreign RNA-activated and self RNA-inhibited enzymatic activities. The multi-subunit Type III-A CRISPR-Cas effector complex (Csm) exemplifies this principle and is in addition regulated by cellular metabolites such as divalent metals and ATP. Recognition of the foreign or cognate target RNA (CTR) triggers its single-stranded deoxyribonuclease (DNase) and cyclic oligoadenylate (cOA) synthesis activities. The same activities remain dormant in the presence of the self or non-cognate target RNA (NTR) that differs from CTR only in its 3'-protospacer flanking sequence (3'-PFS). Here we employ electron cryomicroscopy (cryoEM), functional assays, and comparative cross-linking to study in vivo assembled mesophilic Lactococcus lactis Csm (LlCsm) at the three functional states: apo, the CTR- and the NTR-bound. Unlike previously studied Csm complexes, we observed binding of 3'-PFS to Csm in absence of bound ATP and analyzed the structures of the four RNA cleavage sites. Interestingly, comparative crosslinking results indicate a tightening of the Csm3-Csm4 interface as a result of CTR but not NTR binding, reflecting a possible role of protein dynamics change during activation.

Comparison of Cas9 and Cas12a CRISPR editing methods to correct the W1282X-CFTR mutation.

BACKGROUND: W1282X-CFTR variant (c.3846G>A) is the second most common nonsense cystic fibrosis (CF)-causing mutation in the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene. Even though remarkable breakthroughs have been done towards CF treatment with the approval of four CFTR protein modulators, none of these are approved for patients with nonsense mutations. CRISPR gene editing tools can be of great value to permanently correct the genetic defects caused by these mutations. METHODS: We compared the capacity of homology-directed repair (HDR) mediated by Cas9 or Cas12a to correct W1282X CFTR mutation in the CFF-16HBEge W1282X CFTR cell line (obtained from CFF), using Cas9/gRNA and Cas12a/gRNA ribonucleoproteins (RNPs) and single strand DNA (ssODN) oligonucleotide donors. RESULTS: Cas9 shows higher levels of correction than Cas12a as, by electroporating cells with Cas9 RNPs and ssODN donor, nearly 18% of precise editing was achieved compared to just 8% for Cas12a. Such levels of correction increase the abundance of CFTR mRNA and protein, and partially restore CFTR function in the pool of edited cells to 18% of WT CFTR function. Moreover, homozygous corrected clones produced levels of mRNA, protein, and function comparable to those of cells expressing WT CFTR. CONCLUSION: Altogether, this work demonstrates the potential of gene editing as a therapeutic strategy for CF directly correcting the root cause of the disease.

Structural basis of cyclic oligoadenylate degradation by ancillary Type III CRISPR-Cas ring nucleases.

Type III CRISPR-Cas effector systems detect foreign RNA triggering DNA and RNA cleavage and synthesizing cyclic oligoadenylate molecules (cA) in their Cas10 subunit. cAs act as a second messenger activating auxiliary nucleases, leading to an indiscriminate RNA degradation that can end in cell dormancy or death. Standalone ring nucleases are CRISPR ancillary proteins which downregulate the strong immune response of Type III systems by degrading cA. These enzymes contain a CRISPR-associated Rossman-fold (CARF) domain, which binds and cleaves the cA molecule. Here, we present the structures of the standalone ring nuclease from Sulfolobus islandicus (Sis) 0811 in its apo and post-catalytic states. This enzyme is composed by a N-terminal CARF and a C-terminal wHTH domain. Sis0811 presents a phosphodiester hydrolysis metal-independent mechanism, which cleaves cA4 rings to generate linear adenylate species, thus reducing the levels of the second messenger and switching off the cell antiviral state. The structural and biochemical analysis revealed the coupling of a cork-screw conformational change with the positioning of key catalytic residues to proceed with cA4 phosphodiester hydrolysis in a non-concerted manner.

Discovery of putative tumor suppressors from CRISPR screens reveals rewired lipid metabolism in acute myeloid leukemia cells.

CRISPR knockout fitness screens in cancer cell lines reveal many genes whose loss of function causes cell death or loss of fitness or, more rarely, the opposite phenotype of faster proliferation. Here we demonstrate a systematic approach to identify these proliferation suppressors, which are highly enriched for tumor suppressor genes, and define a network of 145 such genes in 22 modules. One module contains several elements of the glycerolipid biosynthesis pathway and operates exclusively in a subset of acute myeloid leukemia cell lines. The proliferation suppressor activity of genes involved in the synthesis of saturated fatty acids, coupled with a more severe loss of fitness phenotype for genes in the desaturation pathway, suggests that these cells operate at the limit of their carrying capacity for saturated fatty acids, which we confirm biochemically. Overexpression of this module is associated with a survival advantage in juvenile leukemias, suggesting a clinically relevant subtype.

Rapid and Visual Detection of Heterodera schachtii Using Recombinase Polymerase Amplification Combined with Cas12a-Mediated Technology.

Heterodera schachtii is a well-known cyst nematode that causes serious economic losses in sugar beet production every year. Rapid and visual detection of H. schachtii is essential for more effective prevention and control. In this study, a species-specific recombinase polymerase amplification (RPA) primer was designed from a specific H. schachtii sequence-characterized amplified region (SCAR) marker. A band was obtained in reactions with DNA from H. schachtii, but absent from nontarget cyst nematodes. The RPA results could be observed by the naked eye, using a lateral flow dipstick (LFD). Moreover, we combined CRISPR technology with RPA to identify positive samples by fluorescence detection. Sensitivity analysis indicated that 10-4 single cysts and single females, 4-3 single second-stage juveniles, and a 0.001 ng genomic DNA template could be detected. The sensitivity of the RPA method for H. schachtii detection is not only higher than that of PCR and qPCR, but can also provide results in <1 h. Consequently, the RPA assay is a practical and useful diagnostic tool for early diagnosis of plant tissues infested by H. schachtii. Sugar beet nematodes were successfully detected in seven of 15 field sugar beet root samples using the RPA assay. These results were consistent with those achieved by conventional PCR, indicating 100% accuracy of the RPA assay in field samples. The RPA assay developed in the present study has the potential for use in the direct detection of H. schachtii infestation in the field.

New Type III CRISPR variant and programmable RNA targeting tool: Oh, thank heaven for Cas7-11.

Özcan et al. (2021) and van Beljouw et al. (2021) characterize a novel Type III-E CRISPR-Cas subtype, composed of a single polypeptide with crRNA processing and sequence-specific RNA cleavage activities, that provides a new RNA knockdown tool for mammalian cells with fewer off-target effects than current technologies.

Programmed genome editing by a miniature CRISPR-Cas12f nuclease.

The RNA-guided CRISPR-associated (Cas) nucleases are versatile tools for genome editing in various organisms. The large sizes of the commonly used Cas9 and Cas12a nucleases restrict their flexibility in therapeutic applications that use the cargo-size-limited adeno-associated virus delivery vehicle. More compact systems would thus offer more therapeutic options and functionality for this field. Here, we report a miniature class 2 type V-F CRISPR-Cas genome-editing system from Acidibacillus sulfuroxidans (AsCas12f1, 422 amino acids). AsCas12f1 is an RNA-guided endonuclease that recognizes 5' T-rich protospacer adjacent motifs and creates staggered double-stranded breaks to target DNA. We show that AsCas12f1 functions as an effective genome-editing tool in both bacteria and human cells using various delivery methods, including plasmid, ribonucleoprotein and adeno-associated virus. The small size of AsCas12f1 offers advantages for cellular delivery, and characterizations of AsCas12f1 may facilitate engineering more compact genome-manipulation technologies.

Gene interfered-ferroptosis therapy for cancers.

Although some effective therapies have been available for cancer, it still poses a great threat to human health and life due to its drug resistance and low response in patients. Here, we develop a ferroptosis-based therapy by combining iron nanoparticles and cancer-specific gene interference. The expression of two iron metabolic genes (FPN and LCN2) was selectively knocked down in cancer cells by Cas13a or microRNA controlled by a NF-κB-specific promoter. Cells were simultaneously treated by iron nanoparticles. As a result, a significant ferroptosis was induced in a wide variety of cancer cells. However, the same treatment had little effect on normal cells. By transferring genes with adeno-associated virus and iron nanoparticles, the significant tumor growth inhibition and durable cure were obtained in mice with the therapy. In this work, we thus show a cancer therapy based on gene interference-enhanced ferroptosis.

CRISPR-Cas12a Genome Editing at the Whole-Plant Level Using Two Compatible RNA Virus Vectors.

The use of viral vectors that can replicate and move systemically through the host plant to deliver bacterial CRISPR components enables genome editing at the whole-plant level and avoids the requirement for labor-intensive stable transformation. However, this approach usually relies on previously transformed plants that stably express a CRISPR-Cas nuclease. Here, we describe successful DNA-free genome editing of Nicotiana benthamiana using two compatible RNA virus vectors derived from tobacco etch virus (TEV; genus Potyvirus) and potato virus X (PVX; genus Potexvirus), which replicate in the same cells. The TEV and PVX vectors respectively express a Cas12a nuclease and the corresponding guide RNA. This novel two-virus vector system improves the toolbox for transformation-free virus-induced genome editing in plants and will advance efforts to breed more nutritious, resistant, and productive crops.

Epitranscriptomic editing of the RNA N6-methyladenosine modification by dCasRx conjugated methyltransferase and demethylase.

N6-methyladenosine (m6A) is a common modification on endogenous RNA transcripts in mammalian cells. Technologies to precisely modify the RNA m6A levels at specific transcriptomic loci empower interrogation of biological functions of epitranscriptomic modifications. Here, we developed a bidirectional dCasRx epitranscriptome editing platform composed of a nuclear-localized dCasRx conjugated with either a methyltransferase, METTL3, or a demethylase, ALKBH5, to manipulate methylation events at targeted m6A sites. Leveraging this platform, we specifically and efficiently edited m6A modifications at targeted sites, reflected in gene expression and cell proliferation. We employed the dCasRx epitranscriptomic editor system to elucidate the molecular function of m6A-binding proteins YTHDF paralogs (YTHDF1, YTHDF2 and YTHDF3), revealing that YTHDFs promote m6A-mediated mRNA degradation. Collectively, our dCasRx epitranscriptome perturbation platform permits site-specific m6A editing for delineating of functional roles of individual m6A modifications in the mammalian epitranscriptome.

AsCas12a ultra nuclease facilitates the rapid generation of therapeutic cell medicines.

Though AsCas12a fills a crucial gap in the current genome editing toolbox, it exhibits relatively poor editing efficiency, restricting its overall utility. Here we isolate an engineered variant, "AsCas12a Ultra", that increased editing efficiency to nearly 100% at all sites examined in HSPCs, iPSCs, T cells, and NK cells. We show that AsCas12a Ultra maintains high on-target specificity thereby mitigating the risk for off-target editing and making it ideal for complex therapeutic genome editing applications. We achieved simultaneous targeting of three clinically relevant genes in T cells at >90% efficiency and demonstrated transgene knock-in efficiencies of up to 60%. We demonstrate site-specific knock-in of a CAR in NK cells, which afforded enhanced anti-tumor NK cell recognition, potentially enabling the next generation of allogeneic cell-based therapies in oncology. AsCas12a Ultra is an advanced CRISPR nuclease with significant advantages in basic research and in the production of gene edited cell medicines.

Low-cost CRISPR diagnostics for resource-limited settings.

Next-generation sequencing (NGS) has identified disease hallmarks and catalogued a vast reservoir of genetic information from humans and other species. Precise nucleotide-interrogation properties of clustered regularly interspaced short palindromic repeats (CRISPR) proteins have been harnessed to rapidly identify DNA-RNA signatures for diverse applications, bypassing the cost and turnaround times associated with diagnostic NGS.