Recent advances in CRISPR-based functional genomics for the study of disease-associated genetic variants.

Advances in sequencing technology have greatly increased our ability to gather genomic data, yet understanding the impact of genetic mutations, particularly variants of uncertain significance (VUSs), remains a challenge in precision medicine. The CRISPR‒Cas system has emerged as a pivotal tool for genome engineering, enabling the precise incorporation of specific genetic variations, including VUSs, into DNA to facilitate their functional characterization. Additionally, the integration of CRISPR‒Cas technology with sequencing tools allows the high-throughput evaluation of mutations, transforming uncertain genetic data into actionable insights. This allows researchers to comprehensively study the functional consequences of point mutations, paving the way for enhanced understanding and increasing application to precision medicine. This review summarizes the current genome editing tools utilizing CRISPR‒Cas systems and their combination with sequencing tools for functional genomics, with a focus on point mutations.

Highly efficient genome editing via CRISPR-Cas9 ribonucleoprotein (RNP) delivery in mesenchymal stem cells.

The CRISPR-Cas9 system has significantly advanced regenerative medicine research by enabling genome editing in stem cells. Due to their desirable properties, mesenchymal stem cells (MSCs) have recently emerged as highly promising therapeutic agents, which properties include differentiation ability and cytokine production. While CRISPR-Cas9 technology is applied to develop MSC-based therapeutics, MSCs exhibit inefficient genome editing, and susceptibility to plasmid DNA. In this study, we compared and optimized plasmid DNA and RNP approaches for efficient genome engineering in MSCs. The RNP-mediated approach enabled genome editing with high indel frequency and low cytotoxicity in MSCs. By utilizing Cas9 RNPs, we successfully generated B2M-knockout MSCs, which reduced T-cell differentiation, and improved MSC survival. Furthermore, this approach enhanced the immunomodulatory effect of IFN-r priming. These findings indicate that the RNP-mediated engineering of MSC genomes can achieve high efficiency, and engineered MSCs offer potential as a promising therapeutic strategy. [BMB Reports 2024; 57(1): 60-65].

CRISPR prime editing for unconstrained correction of oncogenic KRAS variants.

KRAS is the most commonly mutated RAS family gene and is a primary cause of the occurrence of several types of cancer. However, KRAS mutations have several unique and diverse molecular identities, making it difficult to find specific treatments. Here, we developed universal pegRNAs which can correct all types of G12 and G13 oncogenic KRAS mutations with CRISPR-mediated prime editors (PEs). The universal pegRNA successfully corrected 12 types of KRAS mutations, accounting for 94% of all known KRAS mutations, by up to 54.8% correction frequency in HEK293T/17 cells. We also applied the universal pegRNA to correct endogenous KRAS mutations in human cancer cells and found that G13D KRAS mutation was successfully corrected to wild-type KRAS sequences with up to 40.6% correction frequency without indel mutations. We propose prime editing with the universal pegRNA as a 'one-to-many' potential therapeutic strategy for KRAS oncogene variants.

Therapeutic gene correction for Lesch-Nyhan syndrome using CRISPR-mediated base and prime editing.

Lesch-Nyhan syndrome (LNS) is inherited as an X-linked recessive genetic disorder caused by mutations in hypoxanthine-guanine phosphoribosyl transferase 1 (HPRT1). Patients with LNS show various clinical phenotypes, including hyperuricemia, gout, devastating behavioral abnormality, intellectual disability, and self-harm. Although uric acid overproduction can be modulated with the xanthine oxidase inhibitor allopurinol, there exists no treatment for behavioral and neurological manifestations of LNS. In the current study, CRISPR-mediated base editors (BEs) and prime editors (PEs) were utilized to generate LNS-associated disease models and correct the disease models for therapeutic approach. Cytosine BEs (CBEs) were used to induce c.430C>T and c.508C>T mutations in HAP1 cells, and then adenine BEs (ABEs) were used to correct these mutations without DNA cleavage. PEs induced a c.333_334ins(A) mutation, identified in a Korean patient with LNS, in HAP1 cells, which was corrected in turn by PEs. Furthermore, improved PEs corrected the same mutation in LNS patient-derived fibroblasts by up to 14% without any unwanted mutations. These results suggest that CRISPR-mediated BEs and PEs would be suggested as a potential therapeutic strategy of this extremely rare, devastating genetic disease.

Targeted dual base editing with Campylobacter jejuni Cas9 by single AAV-mediated delivery.

Various CRISPR‒Cas9 orthologs are used in genome engineering. One of the smallest Cas9 orthologs is cjCas9 derived from Campylobacter jejuni, which is a highly specific genome editing tool. Here, we developed cjCas9-based base editors including a cytosine base editor (cjCBEmax) and an adenine base editor (cjABE8e) that can successfully induce endogenous base substitutions by up to 91.2% at the HPD gene in HEK293T cells. Analysis of the base editing efficiency of 13 endogenous target sites showed that the active windows of cjCBEmax and cjABE8e are wider than those of spCas9-based base editors and that their specificities are slightly lower than that of cjCas9. Importantly, engineered cjCas9 and gRNA scaffolds can improve the base editing efficiency of cjABE8e by up to 6.4-fold at the HIF1A gene in HEK293T cells. Due to its small size, cjABE8e can be packaged in a single adeno-associated virus vector with two tandem arrays of gRNAs, and the delivery of the resulting AAV could introduce base substitutions at endogenous ANGPT2 and HPD target sites. Overall, our findings have expanded the potential of the use of base editors for in vivo or ex vivo therapeutic approaches.

Targeted genomic translocations and inversions generated using a paired prime editing strategy.

A variety of cancers have been found to have chromosomal rearrangements, and the genomic abnormalities often induced expression of fusion oncogenes. To date, a pair of engineered nucleases including ZFNs, TALENs, and CRISPR-Cas9 nucleases have been used to generate chromosomal rearrangement in living cells and organisms for disease modeling. However, these methods induce unwanted indel mutations at the DNA break junctions, resulting in incomplete disease modeling. Here, we developed prime editor nuclease-mediated translocation and inversion (PETI), a method for programmable chromosomal translocation and inversion using prime editor 2 nuclease (PE2 nuclease) and paired pegRNA. Using PETI method, we successfully introduced DNA recombination in episomal fluorescence reporters as well as precise chromosomal translocations in human cells. We applied PETI to create cancer-associated translocations and inversions such as NPM1-ALK and EML4-ALK in human cells. Our findings show that PETI generated chromosomal translocation and inversion in a programmable manner with efficiencies comparable of Cas9. PETI methods, we believe, could be used to create disease models or for gene therapy.

AWP1 Restrains the Aggressive Behavior of Breast Cancer Cells Induced by TNF-α.

TNF-α plays a crucial role in cancer initiation and progression by enhancing cancer cell proliferation, survival, and migration. Even though the known functional role of AWP1 (zinc finger AN1 type-6, ZFAND6) is as a key mediator of TNF-α signaling, its potential role in the TNF-α-dependent responses of cancer cells remains unclear. In our current study, we found that an AWP1 knockdown using short hairpin RNAs increases the migratory potential of non-aggressive MCF-7 breast cancer cells with no significant alteration of their proliferation in response to TNF-α. A CRISPR/Cas9-mediated AWP1 knockout in MCF-7 cells led to mesenchymal cell type morphological changes and an accelerated motility. TNF-α administration further increased this migratory capacity of these AWP1-depleted cells through the activation of NF-κB accompanied by increased epithelial-mesenchymal transition-related gene expression. In particular, an AWP1 depletion augmented the expression of Nox1, reactive oxygen species (ROS) generating enzymes, and ROS levels and subsequently promoted the migratory potential of MCF-7 cells mediated by TNF-α. These TNF-α-mediated increases in the chemotactic migration of AWP1 knockout cells were completely abrogated by an NF-κB inhibitor and a ROS scavenger. Our results suggest that a loss-of-function of AWP1 alters the TNF-α response of non-aggressive breast cancer cells by potentiating ROS-dependent NF-κB activation.

Functional Assessment of BRCA1 variants using CRISPR-Mediated Base Editors.

Recent studies have investigated the risks associated with BRCA1 gene mutations using various functional assessment methods such as fluorescent reporter assays, embryonic stem cell viability assays, and therapeutic drug-based sensitivity assays. Although they have clarified a lot of BRCA1 variants, these assays involving the use of exogenously expressed BRCA1 variants are associated with overexpression issues and cannot be applied to post-transcriptional regulation. To resolve these limitations, we previously reported a method for functional analysis of BRCA1 variants via CRISPR-mediated cytosine base editor that induce targeted nucleotide substitution in living cells. Using this method, we identified variants whose functions remain ambiguous, including c.-97C>T, c.154C>T, c.3847C>T, c.5056C>T, and c.4986+5G>A, and confirmed that CRISPR-mediated base editors are useful tools for reclassifying the variants of uncertain significance in BRCA1. Here, we describe a protocol for functional analysis of BRCA1 variants using CRISPR-based cytosine base editor. This protocol provides guidelines for the selection of target sites, functional analysis and evaluation of BRCA1 variants.

Engineered prime editors with PAM flexibility.

Although prime editors are a powerful tool for genome editing, which can generate various types of mutations such as nucleotide substitutions, insertions, and deletions in the genome without double-strand breaks or donor DNA, the conventional prime editors are still limited to their target scopes because of the PAM preference of the Streptococcus pyogenes Cas9 (spCas9) protein. Here, we describe the engineered prime editors to expand the range of their target sites using various PAM-flexible Cas9 variants. Using the engineered prime editors, we could successfully generate more than 50 types of mutations with up to 51.7% prime-editing activity in HEK293T cells. In addition, we successfully introduced the BRAF V600E mutation, which could not be induced by conventional prime editors. These variants of prime editors will broaden the applicability of CRISPR-based prime editing technologies in biological research.

Small-molecule inhibitors of histone deacetylase improve CRISPR-based adenine base editing.

CRISPR-based base editors (BEs) are widely used to induce nucleotide substitutions in living cells and organisms without causing the damaging DNA double-strand breaks and DNA donor templates. Cytosine BEs that induce C:G to T:A conversion and adenine BEs that induce A:T to G:C conversion have been developed. Various attempts have been made to increase the efficiency of both BEs; however, their activities need to be improved for further applications. Here, we describe a fluorescent reporter-based drug screening platform to identify novel chemicals with the goal of improving adenine base editing efficiency. The reporter system revealed that histone deacetylase inhibitors, particularly romidepsin, enhanced base editing efficiencies by up to 4.9-fold by increasing the expression levels of proteins and target accessibility. The results support the use of romidepsin as a viable option to improve base editing efficiency in biomedical research and therapeutic genome engineering.

Gene Manipulation Using Fusion Guide RNAs for Cas9 and Cas12a.

The clustered regularly interspaced short palindromic repeats (CRISPR)-associated (Cas) protein has emerged as a genome engineering tool for various organisms. Known as the CRISPR-Cas system, Cas endonucleases such as Cas9 and Cas12a (also known as Cpf1) and guide RNA (gRNA) complexes recognize and cleave the target DNA, allowing for targeted gene manipulation. Along with the Cas protein engineering, gRNA engineering has broadened the applications of the CRISPR-Cas system. Recently, we have developed fusion guide RNAs (fgRNAs) for orthogonal gene manipulation using Cas9 and Cas12a. Here, we describe the methods for designing and generating fgRNAs-expression constructs to achieve multiplex genome editing and gene manipulation in human cells.

A CRISPR-based base-editing screen for the functional assessment of BRCA1 variants.

Genetic mutations in BRCA1, which is crucial for the process of DNA repair and maintenance of genomic integrity, are known to increase markedly the risk of breast and ovarian cancers. Clinical genetic testing has been used to identify new BRCA1 variants; however, functional assessment and determination of their pathogenicity still poses challenges for clinical management. Here, we describe that CRISPR-mediated cytosine base editor, known as BE3, can be used for the functional analysis of BRCA1 variants. We performed CRISPR-mediated base-editing screening using 745 gRNAs targeting all exons in BRCA1 to identify loss-of-function variants and identified variants whose function has heretofore remained unknown, such as c.-97C>T, c.154C>T, c.3847C>T, c.5056C>T, and c.4986+5G>A. Our results show that CRISPR-mediated base editor is a powerful tool for the reclassification of variants of uncertain significance (VUSs) in BRCA1.

CRISPR/Cas-based customization of pooled CRISPR libraries.

Pooled CRISPR libraries are widely used in high-throughput screening to study various biological processes. Various pooled CRISPR libraries have been shared for CRISPR screens and useful tools have been developed to construct researcher's own libraries, however, many researchers are struggling to create their own pooled CRISPR libraries: it is a time-consuming, labor-intensive, and expensive process. In this study, we develop a simple method to customize conventional pooled CRISPR libraries using the CRISPR/Cas9 system. We show that conventional pooled CRISPR libraries can be modified by eliminating gRNAs that target positive genes, enabling the identification of unknown target genes in CRISPR screening. CRISPR/Cas9 system can be applied as a precise tool for customizing conventional pooled CRISPR libraries and will broaden the scope of high-throughput screening technology.

High-throughput genetic screens using CRISPR-Cas9 system.

The CRISPR-Cas9 system is a powerful tool for genome engineering, and its programmability and simplicity have enabled various types of gene manipulation such as gene disruption and transcriptional and epigenetic perturbation. Particularly, CRISPR-based pooled libraries facilitate high-throughput screening for functional regulatory elements in the human genome. In this review, we describe recent advances in CRISPR-Cas9 technology and its use in high-throughput genetic screening. We also discuss its potential for drug target discovery and current challenges of this technique in biomedical research.

Publisher Correction: Fusion guide RNAs for orthogonal gene manipulation with Cas9 and Cpf1.

The originally published version of this Article contained an error in the spelling of the author Da-eun Kim, which was incorrectly given as Da-Eun Kim. Furthermore, in Figure 1a, the Cas9 protein was positioned incorrectly during typesetting. These errors have now been corrected in both the PDF and HTML versions of the Article.

CRISPR/dCas9-mediated biosensor for detection of tick-borne diseases.

Rapid and highly sensitive detection of biomolecules is greatly needed for pathogen diagnosis in clinical samples, but the method needs to be significantly improved in terms of sensitivity and specificity for actual use in clinical settings. Here, we report the development of an improved molecular diagnostics tool that utilizes CRISPR/dCas9-mediated biosensor that couples a nuclease inactivated Cas9 (dCas9) and single microring resonator biosensor, enables label-free and real-time detection of pathogenic DNA and RNA. We addressed the clinical utility of this CRISPR/dCas9-mediated biosensor in tick-borne illnesses including scrub typhus (ST) and severe fever with thrombocytopenia syndrome (SFTS), whose clinical presentations are too similar to be easily differentiated. By using CRISPR/dCas9-mediated biosensor, we achieved single molecule sensitivity for the detection of ST (0.54 aM) and SFTS (0.63 aM); this detection sensitivity is 100 times more sensitive than that of RT-PCR assay. Finally, CRISPR/dCas9-mediated biosensor was able to clearly distinguish between ST and SFTS in serum samples within 20 min. We believe that CRISPR/dCas9-mediated biosensor will be useful for rapid and accurate molecular diagnostic tool that is suitable for immediate clinical applications.

Targeted Base Editing via RNA-Guided Cytidine Deaminases in Xenopus laevis Embryos.

Genome editing using programmable nucleases such as CRISPR/Cas9 or Cpf1 has emerged as powerful tools for gene knock-out or knock-in in various organisms. While most genetic diseases are caused by point mutations, these genome-editing approaches are inefficient in inducing single-nucleotide substitutions. Recently, Cas9-linked cytidine deaminases, named base editors (BEs), have been shown to convert cytidine to uridine efficiently, leading to targeted single-base pair substitutions in human cells and organisms. Here, we first report on the generation of Xenopus laevis mutants with targeted single-base pair substitutions using this RNA-guided programmable deaminase. Injection of base editor 3 (BE3) ribonucleoprotein targeting the tyrosinase (tyr) gene in early embryos can induce site-specific base conversions with the rates of up to 20.5%, resulting in oculocutaneous albinism phenotypes without off-target mutations. We further test this base-editing system by targeting the tp53 gene with the result that the expected single-base pair substitutions are observed at the target site. Collectively, these data establish that the programmable deaminases are efficient tools for creating targeted point mutations for human disease modeling in Xenopus.

Fusion guide RNAs for orthogonal gene manipulation with Cas9 and Cpf1.

The bacteria-derived clustered regularly interspaced short palindromic repeat (CRISPR)-Cas systems are powerful tools for genome engineering. Recently, in addition to Cas protein engineering, the improvement of guide RNAs are also performed, contributing to broadening the research area of CRISPR-Cas9 systems. Here we develop a fusion guide RNA (fgRNA) that functions with both Cas9 and Cpf1 proteins to induce mutations in human cells. Furthermore, we demonstrate that fgRNAs can be used in multiplex genome editing and orthogonal genome manipulation with two types of Cas proteins. Our results show that fgRNAs can be used as a tool for performing multiple gene manipulations.

A simple, flexible and high-throughput cloning system for plant genome editing via CRISPR-Cas system.

CRISPR-Cas9 system is now widely used to edit a target genome in animals and plants. Cas9 protein derived from Streptococcus pyogenes (SpCas9) cleaves double-stranded DNA targeted by a chimeric single-guide RNA (sgRNA). For plant genome editing, Agrobacterium-mediated T-DNA transformation has been broadly used to express Cas9 proteins and sgRNAs under the control of CaMV 35S and U6/U3 promoter, respectively. We here developed a simple and high-throughput binary vector system to clone a 19-20 bp of sgRNA, which binds to the reverse complement of a target locus, in a large T-DNA binary vector containing an SpCas9 expressing cassette. Two-step cloning procedures: (1) annealing two target-specific oligonucleotides with overhangs specific to the AarI restriction enzyme site of the binary vector; and (2) ligating the annealed oligonucleotides into the two AarI sites of the vector, facilitate the high-throughput production of the positive clones. In addition, Cas9-coding sequence and U6/U3 promoter can be easily exchanged via the Gateway(TM) system and unique EcoRI/XhoI sites on the vector, respectively. We examined the mutation ratio and patterns when we transformed these constructs into Arabidopsis thaliana and a wild tobacco, Nicotiana attenuata. Our vector system will be useful to generate targeted large-scale knock-out lines of model as well as non-model plant.

Targeted inversion and reversion of the blood coagulation factor 8 gene in human iPS cells using TALENs.

Hemophilia A, one of the most common genetic bleeding disorders, is caused by various mutations in the blood coagulation factor VIII (F8) gene. Among the genotypes that result in hemophilia A, two different types of chromosomal inversions that involve a portion of the F8 gene are most frequent, accounting for almost half of all severe hemophilia A cases. In this study, we used a transcription activator-like effector nuclease (TALEN) pair to invert a 140-kbp chromosomal segment that spans the portion of the F8 gene in human induced pluripotent stem cells (iPSCs) to create a hemophilia A model cell line. In addition, we reverted the inverted segment back to its normal orientation in the hemophilia model iPSCs using the same TALEN pair. Importantly, we detected the F8 mRNA in cells derived from the reverted iPSCs lines, but not in those derived from the clones with the inverted segment. Thus, we showed that TALENs can be used both for creating disease models associated with chromosomal rearrangements in iPSCs and for correcting genetic defects caused by chromosomal inversions. This strategy provides an iPSC-based novel therapeutic option for the treatment of hemophilia A and other genetic diseases caused by chromosomal inversions.