Cytosine base editing systems with minimized off-target effect and molecular size

Regulation, functional impact, and therapeutic targeting of APOBEC3A in cancer

Enzymes of the apolipoprotein B mRNA editing catalytic polypeptide like (APOBEC) family are cytosine deaminases that convert cytosine to uracil in DNA and RNA. Among these proteins, APOBEC3 sub-family members, APOBEC3A (A3A) and APOBEC3B (A3B), are prominent sources of mutagenesis in cancer cells. The aberrant expression of A3A and A3B in cancer cells leads to accumulation of mutations with specific single-base substitution (SBS) signatures, characterized by C→T and C→G changes, in a number of tumor types. In addition to fueling mutagenesis, A3A and A3B, particularly A3A, induce DNA replication stress, DNA damage, and chromosomal instability through their catalytic activities, triggering a range of cellular responses. Thus, A3A/B have emerged as key drivers of genome evolution during cancer development, contributing to tumorigenesis, tumor heterogeneity, and therapeutic resistance. Yet, the expression of A3A/B in cancer cells presents a cancer vulnerability that can be exploited therapeutically. In this review, we discuss the recent studies that shed light on the mechanisms regulating A3A expression and the impact of A3A in cancer. We also review recent advances in the development of A3A inhibitors and provide perspectives on the future directions of A3A research.

Navigating the CRISPR/Cas Landscape for Enhanced Diagnosis and Treatment of Wilson's Disease

The clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein (Cas) system continues to evolve, thereby enabling more precise detection and repair of mutagenesis. The development of CRISPR/Cas-based diagnosis holds promise for high-throughput, cost-effective, and portable nucleic acid screening and genetic disease diagnosis. In addition, advancements in transportation strategies such as adeno-associated virus (AAV), lentiviral vectors, nanoparticles, and virus-like vectors (VLPs) offer synergistic insights for gene therapeutics in vivo. Wilson's disease (WD), a copper metabolism disorder, is primarily caused by mutations in the ATPase copper transporting beta (ATP7B) gene. The condition is associated with the accumulation of copper in the body, leading to irreversible damage to various organs, including the liver, nervous system, kidneys, and eyes. However, the heterogeneous nature and individualized presentation of physical and neurological symptoms in WD patients pose significant challenges to accurate diagnosis. Furthermore, patients must consume copper-chelating medication throughout their lifetime. Herein, we provide a detailed description of WD and review the application of novel CRISPR-based strategies for its diagnosis and treatment, along with the challenges that need to be overcome.

Engineered cytosine base editor enabling broad-scope and high-fidelity gene editing in Streptomyces

Base editing (BE) faces protospacer adjacent motif (PAM) constraints and off-target effects in both eukaryotes and prokaryotes. For Streptomyces, renowned as one of the most prolific bacterial producers of antibiotics, the challenges are more pronounced due to its diverse genomic content and high GC content. Here, we develop a base editor named eSCBE3-NG-Hypa, tailored with both high efficiency and -fidelity for Streptomyces. Of note, eSCBE3-NG-Hypa recognizes NG PAM and exhibits high activity at challenging sites with high GC content or GC motifs, while displaying minimal off-target effects. To illustrate its practicability, we employ eSCBE3-NG-Hypa to achieve precise key amino acid conversion of the dehydratase (DH) domains within the modular polyketide synthase (PKS) responsible for the insecticide avermectins biosynthesis, achieving domains inactivation. The resulting DH-inactivated mutants, while ceasing avermectins production, produce a high yield of oligomycin, indicating competitive relationships among multiple biosynthetic gene clusters (BGCs) in Streptomyces avermitilis. Leveraging this insight, we use eSCBE3-NG-Hypa to introduce premature stop codons into competitor gene cluster of ave in an industrial S. avermitilis, with the mutant Δolm exhibiting the highest 4.45-fold increase in avermectin B1a compared to the control. This work provides a potent tool for modifying biosynthetic pathways and advancing metabolic engineering in Streptomyces.

Base editing strategies to convert CAG to CAA diminish the disease-causing mutation in Huntington's disease

An expanded CAG repeat in the huntingtin gene (HTT) causes Huntington's disease (HD). Since the length of uninterrupted CAG repeat, not polyglutamine, determines the age-at-onset in HD, base editing strategies to convert CAG to CAA are anticipated to delay onset by shortening the uninterrupted CAG repeat. Here, we developed base editing strategies to convert CAG in the repeat to CAA and determined their molecular outcomes and effects on relevant disease phenotypes. Base editing strategies employing combinations of cytosine base editors and guide RNAs (gRNAs) efficiently converted CAG to CAA at various sites in the CAG repeat without generating significant indels, off-target edits, or transcriptome alterations, demonstrating their feasibility and specificity. Candidate BE strategies converted CAG to CAA on both expanded and non-expanded CAG repeats without altering HTT mRNA and protein levels. In addition, somatic CAG repeat expansion, which is the major disease driver in HD, was significantly decreased in the liver by a candidate BE strategy treatment in HD knock-in mice carrying canonical CAG repeats. Notably, CAG repeat expansion was abolished entirely in HD knock-in mice carrying CAA-interrupted repeats, supporting the therapeutic potential of CAG-to-CAA conversion strategies in HD and potentially other repeat expansion disorders.

Multiplexed in-situ mutagenesis driven by a dCas12a-based dual-function base editor

Mutagenesis driving genetic diversity is vital for understanding and engineering biological systems. However, the lack of effective methods to generate in-situ mutagenesis in multiple genomic loci combinatorially limits the study of complex biological functions. Here, we design and construct MultiduBE, a dCas12a-based multiplexed dual-function base editor, in an all-in-one plasmid for performing combinatorial in-situ mutagenesis. Two synthetic effectors, duBE-1a and duBE-2b, are created by amalgamating the functionalities of cytosine deaminase (from hAPOBEC3A or hAID*Δ ), adenine deaminase (from TadA9), and crRNA array processing (from dCas12a). Furthermore, introducing the synthetic separator Sp4 minimizes interference in the crRNA array, thereby facilitating multiplexed in-situ mutagenesis in both Escherichia coli and Bacillus subtilis. Guided by the corresponding crRNA arrays, MultiduBE is successfully employed for cell physiology reprogramming and metabolic regulation. A novel mutation conferring streptomycin resistance has been identified in B. subtilis and incorporated into the mutant strains with multiple antibiotic resistance. Moreover, surfactin and riboflavin titers of the combinatorially mutant strains improved by 42% and 15-fold, respectively, compared with the control strains with single gene mutation. Overall, MultiduBE provides a convenient and efficient way to perform multiplexed in-situ mutagenesis.

A review on Gaucher disease: therapeutic potential of β-glucocerebrosidase-targeted mRNA/saRNA approach

Gaucher disease (GD), a rare hereditary lysosomal storage disorder, occurs due to a deficiency in the enzyme β-glucocerebrosidase (GCase). This deficiency leads to the buildup of substrate glucosylceramide (GlcCer) in macrophages, eventually resulting in various complications. Among its three types, GD2 is particularly severe with neurological involvements. Current treatments, such as enzyme replacement therapy (ERT), are not effective for GD2 and GD3 due to their inability to cross the blood-brain barrier (BBB). Other treatment approaches, such as gene or chaperone therapies are still in experimental stages. Additionally, GD treatments are costly and can have certain side effects. The successful use of messenger RNA (mRNA)-based vaccines for COVID-19 in 2020 has sparked interest in nucleic acid-based therapies. Remarkably, mRNA technology also offers a novel approach for protein replacement purposes. Additionally, self-amplifying RNA (saRNA) technology shows promise, potentially producing more protein at lower doses. This review aims to explore the potential of a cost-effective mRNA/saRNA-based approach for GD therapy. The use of GCase-mRNA/saRNA as a protein replacement therapy could offer a new and promising direction for improving the quality of life and extending the lifespan of individuals with GD.

Engineered deaminases as a key component of DNA and RNA editing tools

Over recent years, zinc-dependent deaminases have attracted increasing interest as key components of nucleic acid editing tools that can generate point mutations at specific sites in either DNA or RNA by combining a targeting module (such as a catalytically impaired CRISPR-Cas component) and an effector module (most often a deaminase). Deaminase-based molecular tools are already being utilized in a wide spectrum of therapeutic and research applications; however, their medical and biotechnological potential seems to be much greater. Recent reports indicate that the further development of nucleic acid editing systems depends largely on our ability to engineer the substrate specificity and catalytic activity of the editors themselves. In this review, we summarize the current trends and achievements in deaminase engineering. The presented data indicate that the potential of these enzymes has not yet been fully revealed or understood. Several examples show that even relatively minor changes in the structure of deaminases can give them completely new and unique properties.

CRISPR genetic toolkits of classical food microorganisms: Current state and future prospects

Production of food-related products using microorganisms in an environmentally friendly manner is a crucial solution to global food safety and environmental pollution issues. Traditional microbial modification methods rely on artificial selection or natural mutations, which require time for repeated screening and reproduction, leading to unstable results. Therefore, it is imperative to develop rapid, efficient, and precise microbial modification technologies. This review summarizes recent advances in the construction of gene editing and metabolic regulation toolkits based on the clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated proteins (CRISPR-Cas) systems and their applications in reconstructing food microorganism metabolic networks. The development and application of gene editing toolkits from single-site gene editing to multi-site and genome-scale gene editing was also introduced. Moreover, it presented a detailed introduction to CRISPR interference, CRISPR activation, and logic circuit toolkits for metabolic network regulation. Moreover, the current challenges and future prospects for developing CRISPR genetic toolkits were also discussed.

Directed evolution rice genes with randomly multiplexed sgRNAs assembly of base editors

CRISPR-based directed evolution is an effective breeding biotechnology to improve agronomic traits in plants. However, its gene diversification is still limited using individual single guide RNA. We described here a multiplexed orthogonal base editor (MoBE), and a randomly multiplexed sgRNAs assembly strategy to maximize gene diversification. MoBE could induce efficiently orthogonal ABE (<36.6%), CBE (<36.0%), and A&CBE (<37.6%) on different targets, while the sgRNA assembling strategy randomized base editing events on various targets. With respective 130 and 84 targets from each strand of the 34th exon of rice acetyl-coenzyme A carboxylase (OsACC), we observed the target-scaffold combination types up to 27 294 in randomly dual and randomly triple sgRNA libraries. We further performed directed evolution of OsACC using MoBE and randomly dual sgRNA libraries in rice, and obtained single or linked mutations of stronger herbicide resistance. These strategies are useful for in situ directed evolution of functional genes and may accelerate trait improvement in rice.

To Cut or not to Cut: Next-generation Genome Editors for Precision Genome Engineering

Since the original report of repurposing the CRISPR/Cas9 system for genome engineering, the past decade has witnessed profound improvement in our ability to efficiently manipulate the mammalian genome. However, significant challenges lie ahead that hinder the translation of CRISPR-based gene editing technologies into safe and effective therapeutics. The CRISPR systems often have a limited target scope due to PAM restrictions, and the off-target activity also poses serious risks for therapeutic applications. Moreover, the first-generation genome editors typically achieve desired genomic modifications by inducing double-strand breaks (DSBs) at target site(s). Despite being highly efficient, this "cut and fix" strategy is less favorable in clinical settings due to drawbacks associated with the nuclease-induced DSBs. In this review, we focus on recent advances that help address these challenges, including the engineering and discovery of novel CRISPR/Cas systems with improved functionalities and the development of DSB-free genome editors.

Applications of CRISPR-Cas9 for advancing precision medicine in oncology: from target discovery to disease modeling

The clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated protein 9 (Cas9) (CRISPR/Cas9) system is a powerful tool that enables precise and efficient gene manipulation. In a relatively short time, CRISPR has risen to become the preferred gene-editing system due to its high efficiency, simplicity, and programmability at low costs. Furthermore, in the recent years, the CRISPR toolkit has been rapidly expanding, and the emerging advancements have shown tremendous potential in uncovering molecular mechanisms and new therapeutic strategies for human diseases. In this review, we provide our perspectives on the recent advancements in CRISPR technology and its impact on precision medicine, ranging from target identification, disease modeling, and diagnostics. We also discuss the impact of novel approaches such as epigenome, base, and prime editing on preclinical cancer drug discovery.

Progress and Prospects of Gene Editing in Pluripotent Stem Cells

Applying programmable nucleases in gene editing has greatly shaped current research in basic biology and clinical translation. Gene editing in human pluripotent stem cells (PSCs), including embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), is highly relevant to clinical cell therapy and thus should be examined with particular caution. First, since all mutations in PSCs will be carried to all their progenies, off-target edits of editors will be amplified. Second, due to the hypersensitivity of PSCs to DNA damage, double-strand breaks (DSBs) made by gene editing could lead to low editing efficiency and the enrichment of cell populations with defective genomic safeguards. In this regard, DSB-independent gene editing tools, such as base editors and prime editors, are favored due to their nature to avoid these consequences. With more understanding of the microbial world, new systems, such as Cas-related nucleases, transposons, and recombinases, are also expanding the toolbox for gene editing. In this review, we discuss current applications of programmable nucleases in PSCs for gene editing, the efforts researchers have made to optimize these systems, as well as new tools that can be potentially employed for differentiation modeling and therapeutic applications.

Discovery of deaminase functions by structure-based protein clustering

The elucidation of protein function and its exploitation in bioengineering have greatly advanced the life sciences. Protein mining efforts generally rely on amino acid sequences rather than protein structures. We describe here the use of AlphaFold2 to predict and subsequently cluster an entire protein family based on predicted structure similarities. We selected deaminase proteins to analyze and identified many previously unknown properties. We were surprised to find that most proteins in the DddA-like clade were not double-stranded DNA deaminases. We engineered the smallest single-strand-specific cytidine deaminase, enabling efficient cytosine base editor (CBE) to be packaged into a single adeno-associated virus (AAV). Importantly, we profiled a deaminase from this clade that edits robustly in soybean plants, which previously was inaccessible to CBEs. These discovered deaminases, based on AI-assisted structural predictions, greatly expand the utility of base editors for therapeutic and agricultural applications.

A new compact adenine base editor generated through deletion of HNH and REC2 domain of SpCas9

BACKGROUND

Adenine base editors (ABEs) are promising therapeutic gene editing tools that can efficiently convert targeted A•T to G•C base pairs in the genome. However, the large size of commonly used ABEs based on SpCas9 hinders its delivery in vivo using certain vectors such as adeno-associated virus (AAV) during preclinical applications. Despite a number of approaches having previously been attempted to overcome that challenge, including split Cas9-derived and numerous domain-deleted versions of editors, whether base editor (BE) and prime editor (PE) systems can also allow deletion of those domains remains to be proven. In this study, we present a new small ABE (sABE) with significantly reduced size.

RESULTS

We discovered that ABE8e can tolerate large single deletions in the REC2 (Δ174-296) and HNH (Δ786-855) domains of SpCas9, and these deletions can be stacked together to create a new sABE. The sABE showed higher precision than the original ABE8e, with proximally shifted protospacer adjacent motif (PAM) editing windows (A3- A15), and comparable editing efficiencies to 8e-SaCas9-KKH. The sABE system efficiently generated A-G mutations at disease-relevant loci (T1214C in GAA and A494G in MFN2) in HEK293T cells and several canonical Pcsk9 splice sites in N2a cells. Moreover, the sABE enabled in vivo delivery in a single adeno-associated virus (AAV) vector with slight efficiency. Furthermore, we also successfully edited the genome of mouse embryos by microinjecting mRNA and sgRNA of sABE system into zygotes.

CONCLUSIONS

We have developed a substantially smaller sABE system that expands the targeting scope and offers higher precision of genome editing. Our findings suggest that the sABE system holds great therapeutic potential in preclinical applications.

Base editors: development and applications in biomedicine

Base editor (BE) is a gene-editing tool developed by combining the CRISPR/Cas system with an individual deaminase, enabling precise single-base substitution in DNA or RNA without generating a DNA double-strand break (DSB) or requiring donor DNA templates in living cells. Base editors offer more precise and secure genome-editing effects than other conventional artificial nuclease systems, such as CRISPR/Cas9, as the DSB induced by Cas9 will cause severe damage to the genome. Thus, base editors have important applications in the field of biomedicine, including gene function investigation, directed protein evolution, genetic lineage tracing, disease modeling, and gene therapy. Since the development of the two main base editors, cytosine base editors (CBEs) and adenine base editors (ABEs), scientists have developed more than 100 optimized base editors with improved editing efficiency, precision, specificity, targeting scope, and capacity to be delivered in vivo, greatly enhancing their application potential in biomedicine. Here, we review the recent development of base editors, summarize their applications in the biomedical field, and discuss future perspectives and challenges for therapeutic applications.

Challenges of Gene Editing Therapies for Genodermatoses

Genodermatoses encompass a wide range of inherited skin diseases, many of which are monogenic. Genodermatoses range in severity and result in early-onset cancers or life-threatening damage to the skin, and there are few curative options. As such, there is a clinical need for single-intervention treatments with curative potential. Here, we discuss the nascent field of gene editing for the treatment of genodermatoses, exploring CRISPR-Cas9 and homology-directed repair, base editing, and prime editing tools for correcting pathogenic mutations. We specifically focus on the optimisation of editing efficiency, the minimisation off-targets edits, and the tools for delivery for potential future therapies. Honing each of these factors is essential for translating gene editing therapies into the clinical setting. Therefore, the aim of this review article is to raise important considerations for investigators aiming to develop gene editing approaches for genodermatoses.

Assessing and advancing the safety of CRISPR-Cas tools: from DNA to RNA editing

CRISPR-Cas gene editing has revolutionized experimental molecular biology over the past decade and holds great promise for the treatment of human genetic diseases. Here we review the development of CRISPR-Cas9/Cas12/Cas13 nucleases, DNA base editors, prime editors, and RNA base editors, focusing on the assessment and improvement of their editing precision and safety, pushing the limit of editing specificity and efficiency. We summarize the capabilities and limitations of each CRISPR tool from DNA editing to RNA editing, and highlight the opportunities for future improvements and applications in basic research, as well as the therapeutic and clinical considerations for their use in patients.

CRISPR nuclease off-target activity and mitigation strategies

The discovery of CRISPR has allowed site-specific genomic modification to become a reality and this technology is now being applied in a number of human clinical trials. While this technology has demonstrated impressive efficacy in the clinic to date, there remains the potential for unintended on- and off-target effects of CRISPR nuclease activity. A variety of in silico-based prediction tools and empirically derived experimental methods have been developed to identify the most common unintended effect-small insertions and deletions at genomic sites with homology to the guide RNA. However, large-scale aberrations have recently been reported such as translocations, inversions, deletions, and even chromothripsis. These are more difficult to detect using current workflows indicating a major unmet need in the field. In this review we summarize potential sequencing-based solutions that may be able to detect these large-scale effects even at low frequencies of occurrence. In addition, many of the current clinical trials using CRISPR involve ex vivo isolation of a patient's own stem cells, modification, and re-transplantation. However, there is growing interest in direct, in vivo delivery of genome editing tools. While this strategy has the potential to address disease in cell types that are not amenable to ex vivo manipulation, in vivo editing has only one desired outcome-on-target editing in the cell type of interest. CRISPR activity in unintended cell types (both on- and off-target) is therefore a major safety as well as ethical concern in tissues that could enable germline transmission. In this review, we have summarized the strengths and weaknesses of current editing and delivery tools and potential improvements to off-target and off-tissue CRISPR activity detection. We have also outlined potential mitigation strategies that will ensure that the safety of CRISPR keeps pace with efficacy, a necessary requirement if this technology is to realize its full translational potential.

Site-specific genome editing in treatment of inherited diseases: possibility, progress, and perspectives

Advancements in genome editing enable permanent changes of DNA sequences in a site-specific manner, providing promising approaches for treating human genetic disorders caused by gene mutations. Recently, genome editing has been applied and achieved significant progress in treating inherited genetic disorders that remain incurable by conventional therapy. Here, we present a review of various programmable genome editing systems with their principles, advantages, and limitations. We introduce their recent applications for treating inherited diseases in the clinic, including sickle cell disease (SCD), β-thalassemia, Leber congenital amaurosis (LCA), heterozygous familial hypercholesterolemia (HeFH), etc. We also discuss the paradigm of ex vivo and in vivo editing and highlight the promise of somatic editing and the challenge of germline editing. Finally, we propose future directions in delivery, cutting, and repairing to improve the scope of clinical applications.

Small Molecules for Enhancing the Precision and Safety of Genome Editing

Clustered regularly interspaced short palindromic repeats (CRISPR)-based genome-editing technologies have revolutionized biology, biotechnology, and medicine, and have spurred the development of new therapeutic modalities. However, there remain several barriers to the safe use of CRISPR technologies, such as unintended off-target DNA cleavages. Small molecules are important resources to solve these problems, given their facile delivery and fast action to enable temporal control of the CRISPR systems. Here, we provide a comprehensive overview of small molecules that can precisely modulate CRISPR-associated (Cas) nucleases and guide RNAs (gRNAs). We also discuss the small-molecule control of emerging genome editors (e.g., base editors) and anti-CRISPR proteins. These molecules could be used for the precise investigation of biological systems and the development of safer therapeutic modalities.