BEAT: A Python Program to Quantify Base Editing from Sanger Sequencing

Enhancing prime editing in hematopoietic stem and progenitor cells by modulating nucleotide metabolism

Therapeutic prime editing of hematopoietic stem and progenitor cells (HSPCs) holds great potential to remedy blood disorders. Quiescent cells have low nucleotide levels and resist retroviral infection, and it is possible that nucleotide metabolism could limit reverse transcription-mediated prime editing in HSPCs. We demonstrate that deoxynucleoside supplementation and Vpx-mediated degradation of SAMHD1 improve prime editing efficiency in HSPCs, especially when coupled with editing approaches that evade mismatch repair.

FastAd: A versatile toolkit for rapid generation of single adenoviruses or diverse adenoviral vector libraries

Adenoviruses (Ads) are potent gene delivery vectors for in vitro and in vivo applications. However, current methods for their construction are time-consuming and inefficient, limiting their rapid production and utility in generating complex genetic libraries. Here, we introduce FastAd, a rapid and easy-to-use technology for inserting recombinant "donor" DNA directly into infectious "receiver" Ads in mammalian cells by the concerted action of two efficient recombinases: Cre and Bxb1. Subsequently, the resulting mixed recombinant Ad population is subjected to negative selections by flippase recombinase to remove viruses that missed the initial recombination. With this approach, recombinant Ad production time is reduced from 2 months to 10 days or less. FastAd can be applied for inserting complex genetic DNA libraries into Ad genomes, as demonstrated by the generation of barcode libraries with over 3 million unique clones from a T25 flask-scale transfection of 3 million cells. Furthermore, we leveraged FastAd to construct an Ad library containing a comprehensive genome-wide CRISPR-Cas9 guide RNA library and demonstrated its effectiveness in uncovering novel virus-host interactions. In summary, FastAd enables the rapid generation of single Ad vectors or complex genetic libraries, facilitating not only novel applications of Ad vectors but also research in foundamental virology.

Comprehensive evaluation and prediction of editing outcomes for near-PAMless adenine and cytosine base editors

Base editors enable the direct conversion of target bases without inducing double-strand breaks, showing great potential for disease modeling and gene therapy. Yet, their applicability has been constrained by the necessity for specific protospacer adjacent motif (PAM). We generate four versions of near-PAMless base editors and systematically evaluate their editing patterns and efficiencies using an sgRNA-target library of 45,747 sequences. Near-PAMless base editors significantly expanded the targeting scope, with both PAM and target flanking sequences as determinants for editing outcomes. We develop BEguider, a deep learning model, to accurately predict editing results for near-PAMless base editors. We also provide experimentally measured editing outcomes of 20,541 ClinVar sites, demonstrating that variants previously inaccessible by NGG PAM base editors can now be precisely generated or corrected. We make our predictive tool and data available online to facilitate development and application of near-PAMless base editors in both research and clinical settings.

Tailoring a CRISPR/Cas-based Epigenome Editor for Programmable Chromatin Acylation and Decreased Cytotoxicity

Engineering histone acylation states can inform mechanistic epigenetics and catalyze therapeutic epigenome editing opportunities. Here, we developed engineered lysine acyltransferases that enable the programmable deposition of acetylation and longer-chain acylations. We show that targeting an engineered lysine crotonyltransferase results in weak levels of endogenous enhancer activation yet retains potency when targeted to promoters. We further identify a single mutation within the catalytic core of human p300 that preserves enzymatic activity while substantially reducing cytotoxicity, enabling improved viral delivery. We leveraged these capabilities to perform single-cell CRISPR activation screening and map enhancers to the genes they regulate in situ. We also discover acylation-specific interactions and find that recruitment of p300, regardless of catalytic activity, to prime editing sites can improve editing efficiency. These new programmable epigenome editing tools and insights expand our ability to understand the mechanistic role of lysine acylation in epigenetic and cellular processes and perform functional genomic screens.

Evaluation of DNA minicircles for delivery of adenine and cytosine base editors using activatable gene on "GO" reporter imaging systems

Over 30,000 point mutations are associated with debilitating diseases, including many cancer types, underscoring a critical need for targeted genomic solutions. CRISPR base editors, like adenine base editors (ABEs) and cytosine base editors (CBEs), enable precise modifications by converting adenine to guanine and cytosine to thymine, respectively. Challenges in efficiency and safety concerns regarding viral vectors used in delivery limit the scope of base editing. This study introduces non-viral minicircles, bacterial-backbone-free plasmids, as a delivery vehicle for ABEs and CBEs. The research uses cells engineered with the "Gene On" (GO) reporter gene systems for tracking minicircle-delivered ABEs, CBEs, or Cas9 nickase (control), using green fluorescent protein (GFPGO), bioluminescence reporter firefly luciferase (LUCGO), or a highly sensitive Akaluciferase (AkalucGO) designed in this study. The results show that transfection of minicircles expressing CBE or ABE resulted in significantly higher GFP expression and luminescence signals over controls, with minicircles demonstrating the most substantial editing. This study presents minicircles as a new strategy for base editor delivery and develops an enhanced bioluminescence imaging reporter system for tracking ABE activity. Future studies aim to evaluate the use of minicircles in preclinical cancer models, facilitating potential clinical applications.

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.

Truncating NFKB1 variants cause combined NLRP3 inflammasome activation and type I interferon signaling and predispose to necrotizing fasciitis

In monogenic autoinflammatory diseases, mutations in genes regulating innate immune responses often lead to uncontrolled activation of inflammasome pathways or the type I interferon (IFN-I) response. We describe a mechanism of autoinflammation potentially predisposing patients to life-threatening necrotizing soft tissue inflammation. Six unrelated families are identified in which affected members present with necrotizing fasciitis or severe soft tissue inflammations. Exome sequencing reveals truncating monoallelic loss-of-function variants of nuclear factor κ light-chain enhancer of activated B cells (NFKB1) in affected patients. In patients' macrophages and in NFKB1-variant-bearing THP-1 cells, activation increases both interleukin (IL)-1β secretion and IFN-I signaling. Truncation of NF-κB1 impairs autophagy, accompanied by the accumulation of reactive oxygen species and reduced degradation of inflammasome receptor nucleotide-binding oligomerization domain, leucine-rich repeat-containing protein 3 (NLRP3), and Toll/IL-1 receptor domain-containing adaptor protein inducing IFN-β (TRIF), thus leading to combined excessive inflammasome and IFN-I activity. Many of the patients respond to anti-inflammatory treatment, and targeting IL-1β and/or IFN-I signaling could represent a therapeutic approach for these patients.

Targeting the IDH1 R132H mutation in gliomas by CRISPR/Cas precision base editing

Background

Gliomas, the most frequent malignant primary brain tumors, lack curative treatments. Understanding glioma-specific molecular alterations is crucial to develop novel therapies. Among them, the biological consequences of the isocitrate dehydrogenase 1 gene mutation (IDH1 R132H) remain inconclusive despite its early occurrence and widespread expression.

Methods

We thus employed CRISPR/Cas adenine base editors, which allow precise base pair alterations with minimal undesirable effects, to correct the IDH1 R132H mutation.

Results

Successful correction of the IDH1 R132H mutation in primary patient-derived cell models led to reduced IDH1 R132H protein levels and decreased production of 2-hydroxyglutarate, but increased proliferation. A dual adeno-associated virus split intein system was used to successfully deliver the base editor in vitro and in vivo.

Conclusions

Taken together, our study provides a strategy for a precise genetic intervention to target the IDH1 R132H mutation, enabling the development of accurate models to study its impact on glioma biology and serving as a framework for an in vivo gene therapy.

Precise Correction of Lhcgr Mutation in Stem Leydig Cells by Prime Editing Rescues Hereditary Primary Hypogonadism in Mice

Hereditary primary hypogonadism (HPH), caused by gene mutation related to testosterone synthesis in Leydig cells, usually impairs male sexual development and spermatogenesis. Genetically corrected stem Leydig cells (SLCs) transplantation may provide a new approach for treating HPH. Here, a novel nonsense-point-mutation mouse model (LhcgrW495X ) is first generated based on a gene mutation relative to HPH patients. To verify the efficacy and feasibility of SLCs transplantation in treating HPH, wild-type SLCs are transplanted into LhcgrW495X mice, in which SLCs obviously rescue HPH phenotypes. Through comparing several editing strategies, optimized PE2 protein (PEmax) system is identified as an efficient and precise approach to correct the pathogenic point mutation in Lhcgr. Furthermore, delivering intein-split PEmax system via lentivirus successfully corrects the mutation in SLCs from LhcgrW495X mice ex vivo. Gene-corrected SLCs from LhcgrW495X mice exert ability to differentiate into functional Leydig cells in vitro. Notably, the transplantation of gene-corrected SLCs effectively regenerates Leydig cells, recovers testosterone production, restarts sexual development, rescues spermatogenesis, and produces fertile offspring in LhcgrW495X mice. Altogether, these results suggest that PE-based gene editing in SLCs ex vivo is a promising strategy for HPH therapy and is potentially leveraged to address more hereditary diseases in reproductive system.

A temperature-tolerant CRISPR base editor mediates highly efficient and precise gene editing in Drosophila

CRISPR nucleases generate a broad spectrum of mutations that includes undesired editing outcomes. Here, we develop optimized C-to-T base editing systems for the generation of precise loss- or gain-of-function alleles in Drosophila and identify temperature as a crucial parameter for efficiency. We find that a variant of the widely used APOBEC1 deaminase has attenuated activity at 18° to 29°C and shows considerable dose-dependent toxicity. In contrast, the temperature-tolerant evoCDA1 domain mediates editing of typically more than 90% of alleles and is substantially better tolerated. Furthermore, formation of undesired mutations is exceptionally rare in Drosophila compared to other species. The predictable editing outcome, high efficiency, and product purity enables near homogeneous induction of STOP codons or alleles encoding protein variants in vivo. Last, we demonstrate how optimized expression enables conditional base editing in marked cell populations. This work substantially facilitates creation of precise alleles in Drosophila and provides key design parameters for developing efficient base editing systems in other ectothermic species.

Correction of DMD in human iPSC-derived cardiomyocytes by base-editing-induced exon skipping

Duchenne muscular dystrophy (DMD) is caused by mutations in the DMD gene. Previously, we showed that adenine base editing (ABE) can efficiently correct a nonsense point mutation in a DMD mouse model. Here, we explored the feasibility of base-editing-mediated exon skipping as a therapeutic strategy for DMD using cardiomyocytes derived from human induced pluripotent stem cells (hiPSCs). We first generated a DMD hiPSC line with a large deletion spanning exon 48 through 54 (ΔE48-54) using CRISPR-Cas9 gene editing. Dystrophin expression was disrupted in DMD hiPSC-derived cardiomyocytes (iCMs) as examined by RT-PCR, western blot, and immunofluorescence staining. Transfection of ABE and a guide RNA (gRNA) targeting the splice acceptor led to efficient conversion of AG to GG (35.9% ± 5.7%) and enabled exon 55 skipping. Complete AG to GG conversion in a single clone restored dystrophin expression (42.5% ± 11% of wild type [WT]) in DMD iCMs. Moreover, we designed gRNAs to target the splice sites of exons 6, 7, 8, 43, 44, 46, and 53 in the mutational hotspots and demonstrated their efficiency to induce exon skipping in iCMs. These results highlight the great promise of ABE-mediated exon skipping as a promising therapeutic approach for DMD.

Designing Guide-RNA for Generating Premature Stop Codons for Gene Knockout Using CRISPR-BETS

Cytosine base editors (CBEs) accurately modify target sites by mediating a C to T change (or a G to A change on the opposite strand). This allows us to install premature stop codons for gene knockout. However, highly specific sgRNAs (single-guide RNAs) are necessary for the CRISPR-Cas nuclease to work efficiently. In this study, we introduce a method of designing highly specific gRNA to generate premature stop codons and knock out a gene using CRISPR-BETS software.

Development of a universal antibiotic resistance screening reporter for improving efficiency of cytosine and adenine base editing

Base editing has emerged as a revolutionary technology for single nucleotide modifications. The cytosine and adenine base editors (CBEs and ABEs) have demonstrated great potential in clinical and fundamental research. However, screening and isolating target-edited cells remains challenging. In the current study, we developed a universal Adenine and Cytosine Base-Editing Antibiotic Resistance Screening Reporter (ACBE-ARSR) for improving the editing efficiency. To develop the reporter, the CBE-ARSR was first constructed and shown to be capable of enriching cells for those that had undergone CBE editing activity. Then, the ACBE-ARSR was constructed and was further validated in the editing assays by four different CBEs and two versions of ABE at several different genomic loci. Our results demonstrated that ACBE-ARSR, compared to the reporter of transfection (RoT) screening strategy, improved the editing efficiency of CBE and ABE by 4.6- and 1.9-fold on average, respectively. We found the highest CBE and ABE editing efficiencies as enriched by ACBE-ARSR reached 90% and 88.7%. Moreover, we also demonstrated ACBE-ARSR could be employed for enhancing simultaneous multiplexed genome editing. In conclusion, both CBE and ABE activity can be improved significantly using our novel ACBE-ARSR screening strategy, which we believe will facilitate the development of base editors and their application in biomedical and fundamental research studies.

CRISPR-BETS: a base-editing design tool for generating stop codons

Cytosine base editors (CBEs) can install a predefined stop codon at the target site, representing a more predictable and neater method for creating genetic knockouts without altering the genome size. Due to the enhanced predictability of the editing outcomes, it is also more efficient to obtain homozygous mutants in the first generation. With the recent advancement of CBEs on improved editing activity, purify and specificity in plants and animals, base editing has become a more appealing technology for generating knockouts. However, there is a lack of design tools that can aid the adoption of CBEs for achieving such a purpose, especially in plants. Here, we developed a user-friendly design tool named CRISPR-BETS (base editing to stop), which helps with guide RNA (gRNA) design for introducing stop codons in the protein-coding genes of interest. We demonstrated in rice and tomato that CRISPR-BETS is easy-to-use, and its generated gRNAs are highly specific and efficient for generating stop codons and obtaining homozygous knockout lines. While we tailored the tool for the plant research community, CRISPR-BETS can also serve non-plant species.

Precise plant genome editing using base editors and prime editors

The development of CRISPR-Cas systems has sparked a genome editing revolution in plant genetics and breeding. These sequence-specific RNA-guided nucleases can induce DNA double-stranded breaks, resulting in mutations by imprecise non-homologous end joining (NHEJ) repair or precise DNA sequence replacement by homology-directed repair (HDR). However, HDR is highly inefficient in many plant species, which has greatly limited precise genome editing in plants. To fill the vital gap in precision editing, base editing and prime editing technologies have recently been developed and demonstrated in numerous plant species. These technologies, which are mainly based on Cas9 nickases, can introduce precise changes into the target genome at a single-base resolution. This Review provides a timely overview of the current status of base editors and prime editors in plants, covering both technological developments and biological applications.

In vivo adenine base editing of PCSK9 in macaques reduces LDL cholesterol levels

Most known pathogenic point mutations in humans are C•G to T•A substitutions, which can be directly repaired by adenine base editors (ABEs). In this study, we investigated the efficacy and safety of ABEs in the livers of mice and cynomolgus macaques for the reduction of blood low-density lipoprotein (LDL) levels. Lipid nanoparticle-based delivery of mRNA encoding an ABE and a single-guide RNA targeting PCSK9, a negative regulator of LDL, induced up to 67% editing (on average, 61%) in mice and up to 34% editing (on average, 26%) in macaques. Plasma PCSK9 and LDL levels were stably reduced by 95% and 58% in mice and by 32% and 14% in macaques, respectively. ABE mRNA was cleared rapidly, and no off-target mutations in genomic DNA were found. Re-dosing in macaques did not increase editing, possibly owing to the detected humoral immune response to ABE upon treatment. These findings support further investigation of ABEs to treat patients with monogenic liver diseases.

Efficient precise in vivo base editing in adult dystrophic mice

Recent advances in base editing have created an exciting opportunity to precisely correct disease-causing mutations. However, the large size of base editors and their inherited off-target activities pose challenges for in vivo base editing. Moreover, the requirement of a protospacer adjacent motif (PAM) nearby the mutation site further limits the targeting feasibility. Here we modify the NG-targeting adenine base editor (iABE-NGA) to overcome these challenges and demonstrate the high efficiency to precisely edit a Duchenne muscular dystrophy (DMD) mutation in adult mice. Systemic delivery of AAV9-iABE-NGA results in dystrophin restoration and functional improvement. At 10 months after AAV9-iABE-NGA treatment, a near complete rescue of dystrophin is measured in mdx4cv mouse hearts with up to 15% rescue in skeletal muscle fibers. The off-target activities remains low and no obvious toxicity is detected. This study highlights the promise of permanent base editing using iABE-NGA for the treatment of monogenic diseases.

Identification of pathogenic variants in cancer genes using base editing screens with editing efficiency correction

BACKGROUND

Millions of nucleotide variants are identified through cancer genome sequencing and it is clinically important to identify the pathogenic variants among them. By introducing base substitutions at guide RNA target regions in the genome, CRISPR-Cas9-based base editors provide the possibility for evaluating a large number of variants in their genomic context. However, the variability in editing efficiency and the complexity of outcome mapping are two existing problems for assigning guide RNA effects to variants in base editing screens.

RESULTS

To improve the identification of pathogenic variants, we develop a framework to combine base editing screens with sgRNA efficiency and outcome mapping. We apply the method to evaluate more than 9000 variants across all the exons of BRCA1 and BRCA2 genes. Our efficiency-corrected scoring model identifies 910 loss-of-function variants for BRCA1/2, including 151 variants in the noncoding part of the genes such as the 5' untranslated regions. Many of them are identified in cancer patients and are reported as "benign/likely benign" or "variants of uncertain significance" by clinicians. Our data suggest a need to re-evaluate their clinical significance, which may be helpful for risk assessment and treatment of breast and ovarian cancer.

CONCLUSIONS

Our results suggest that base editing screens with efficiency correction is a powerful strategy to identify pathogenic variants in a high-throughput manner. Applying this strategy to assess variants in both coding and noncoding regions of the genome could have a direct impact on the interpretation of cancer variants.

Estimating Copy-Number Proportions: The Comeback of Sanger Sequencing

Determination of the relative copy numbers of mixed molecular species in nucleic acid samples is often the objective of biological experiments, including Single-Nucleotide Polymorphism (SNP), indel and gene copy-number characterization, and quantification of CRISPR-Cas9 base editing, cytosine methylation, and RNA editing. Standard dye-terminator chromatograms are a widely accessible, cost-effective information source from which copy-number proportions can be inferred. However, the rate of incorporation of dye terminators is dependent on the dye type, the adjacent sequence string, and the secondary structure of the sequenced strand. These variable rates complicate inferences and have driven scientists to resort to complex and costly quantification methods. Because these complex methods introduce their own biases, researchers are rethinking whether rectifying distortions in sequencing trace files and using direct sequencing for quantification will enable comparable accurate assessment. Indeed, recent developments in software tools (e.g., TIDE, ICE, EditR, BEEP and BEAT) indicate that quantification based on direct Sanger sequencing is gaining in scientific acceptance. This commentary reviews the common obstacles in quantification and the latest insights and developments relevant to estimating copy-number proportions based on direct Sanger sequencing, concluding that bidirectional sequencing and sophisticated base calling are the keys to identifying and avoiding sequence distortions.

Live-Cell Quantification of APOBEC1-Mediated RNA Editing: A Comparison of RNA Editing Assays

APOBEC1 is a member of the AID/APOBECs, a group of deaminases responsible for the editing of C>U in both DNA and RNA. APOBEC1 is physiologically involved in C>U RNA editing: while hundreds of targets have been discovered in mice, in humans the only well-characterized target of APOBEC1 is the apolipoprotein B (ApoB) transcript. APOBEC1 edits a CAA codon into a stop codon, which causes the translation of a truncated form of ApoB. A number of assays have been developed to investigate this process. Early assays, poisoned primer extension and Sanger sequencing, have focused on accuracy and sensitivity but rely on extraction of the RNA from tissues and cells. More recently, the need to visualize the RNA editing process directly in live cells have led to the development of fluorescence-based tools. These assays detect RNA editing through reporters whose editing causes a change in cellular localization or a change in fluorescent properties. Here we review the available assays to quantify RNA editing, and we present the protocol for cytofluorimetric analysis using a double-fluorescent reporter.

Microbial Base Editing: A Powerful Emerging Technology for Microbial Genome Engineering

Genome engineering is crucial for answering fundamental questions about, and exploring practical applications of, microorganisms. Various microbial genome-engineering tools, including CRISPR/Cas-enhanced homologous recombination (HR), have been developed, with ever-improving simplicity, efficiency, and applicability. Recently, a powerful emerging technology based on CRISPR/Cas-nucleobase deaminase fusions, known as base editing, opened new avenues for microbial genome engineering. Base editing enables nucleotide transition without inducing lethal double-stranded (ds)DNA cleavage, adding foreign donor DNA, or depending on inefficient HR. Here, we review ongoing efforts to develop and apply base editing to engineer industrially and clinically relevant microorganisms. We also summarize bioinformatics tools that would greatly facilitate guide (g)RNA design and sequencing data analysis and discuss the future challenges and prospects associated with this technology.

BEON: A Functional Fluorescence Reporter for Quantification and Enrichment of Adenine Base-Editing Activity

Adenine base editor (ABE) is a new generation of genome-editing technology through fusion of Cas9 nickase with an evolved E. coli TadA (TadA∗) and holds great promise as novel genome-editing therapeutics for treating genetic disorders. ABEs can directly convert A-T to G-C in specific genomic DNA targets without introducing double-strand breaks (DSBs). We recently showed that computer program-assisted analysis of Sanger sequencing traces can be used as a low-cost and rapid alternative of deep sequencing to assess base-editing outcomes. Here we developed a rapid fluorescence-based reporter assay (Base Editing ON [BEON]) to quantify ABE efficiency. The assay relies on the restoration of the downstream green fluorescent protein (GFP) in ABE-mediated editing of a stop codon located within the guide RNA (gRNA). We showed that this assay can be used to screen for effective ABE variants, characterize the protospacer adjacent motif (PAM) requirement of a novel NNG-targeting ABE based on ScCas9, and enrich for edited cells. Finally, we demonstrated that the reporter assay allowed us to assess the feasibility of ABE editing to correct point mutations associated with dysferlinopathy. Taken together, the BEON assay would facilitate and simplify the studies with ABEs.