Chemical modifications of adenine base editor mRNA and guide RNA expand its application scope

CRISPR-Cas Systems: A Functional Perspective and Innovations

Adaptation is a fundamental tenet of evolutionary biology and is essential for the survival of all organisms, including prokaryotes. The evolution of clustered regularity exemplifies this principle of interspaced short palindromic repeats (CRISPR) and associated proteins (Cas), an adaptive immune system that confers resistance to viral infections. By integrating short segments of viral genomes into their own, bacteria and archaea develop a molecular memory that enables them to mount a rapid and targeted response upon subsequent viral challenges. The fortuitous discovery of this immune mechanism prompted many studies and introduced researchers to novel tools that could potentially be developed from CRISPR-Cas and become clinically relevant as biotechnology rapidly advances in this area. Thus, a deeper understanding of the underpinnings of CRISPR-Cas and its possible therapeutic applications is required. This review analyses the mechanism of action of the CRISPR-Cas systems in detail and summarises the advances in developing biotechnological tools based on CRISPR, opening the field for further research.

CRISPR-dependent base editing as a therapeutic strategy for rare monogenic disorders

Rare monogenic disorders are caused by mutations in single genes and have an incidence rate of less than 0.5%. Due to their low prevalence, these diseases often attract limited research and commercial interest, leading to significant unmet medical needs. In a therapeutic landscape where treatments are targeted to manage symptoms, gene editing therapy emerges as a promising approach to craft curative and lasting treatments for these patients, often referred to as "one-and-done" therapeutics. CRISPR-dependent base editing enables the precise correction of genetic mutations by direct modification of DNA bases without creating potentially deleterious DNA double-strand breaks. Base editors combine a nickase version of Cas9 with cytosine or adenine deaminases to convert C·G to T·A and A·T to G·C, respectively. Together, cytosine (CBE) and adenine (ABE) base editors can theoretically correct ∼95% of pathogenic transition mutations cataloged in ClinVar. This mini-review explores the application of base editing as a therapeutic approach for rare monogenic disorders. It provides an overview of the state of gene therapies and a comprehensive compilation of preclinical studies using base editing to treat rare monogenic disorders. Key considerations for designing base editing-driven therapeutics are summarized in a user-friendly guide for researchers interested in applying this technology to a specific rare monogenic disorder. Finally, we discuss the prospects and challenges for bench-to-bedside translation of base editing therapies for rare monogenic disorders.

Porous Silicon Nanoneedles Efficiently Deliver Adenine Base Editor to Correct a Recurrent Pathogenic COL7A1 Variant in Recessive Dystrophic Epidermolysis Bullosa

Base editing, a CRISPR-based genome editing technology, enables precise correction of single-nucleotide variants, promising resolutive treatment for monogenic genetic disorders like recessive dystrophic epidermolysis bullosa (RDEB). However, the application of base editors in cell manufacturing is hindered by inconsistent efficiency and high costs, contributed by suboptimal delivery methods. Nanoneedles have emerged as an effective delivery approach, enabling highly efficient, non-perturbing gene therapies both in vitro and in vivo. Here we demonstrate that nanoneedle delivery of an adenine base editor corrects a heterozygous single-nucleotide pathogenic variant in COL7A1 in primary RDEB fibroblasts in vitro with 96.5% efficiency, without inducing off-target variants. The nanoneedle delivery maintains cell viability and displays modest phenotypical alterations unlike conventional cationic lipid transfection. The nanoneedle-mediated editing significantly increases the production and secretion of full-length type VII collagen protein, contributing to restore functional fibroblasts phenotype by improving cell adhesion. These findings underscore the suitability and safety of nanoneedles for gene editing in a clinically relevant context of cell manufacturing, establishing a foundation for their use in cell therapies.

Progress in Gene Therapy for Hereditary Tyrosinemia Type 1

Hereditary Tyrosinemia Type-1 (HT1), an inherited error of metabolism caused by a mutation in the fumarylacetoacetate hydrolase gene, is associated with liver disease, severe morbidity, and early mortality. The use of NTBC (2-(2-nitro-4-fluoromethylbenzoyl)-1,3-cyclohexanedione) has almost eradicated the acute HT1 symptoms and childhood mortality. However, patient outcomes remain unsatisfactory due to the neurocognitive effects of NTBC and the requirement for a strict low-protein diet. Gene therapy (GT) offers a potential single-dose cure for HT1, and there is now abundant preclinical data showing how a range of vector-nucleotide payload combinations could be used with curative intent, rather than continued reliance on amelioration. Unfortunately, there have been no HT1-directed clinical trials reported, and so it is unclear which promising pre-clinical approach has the greatest chance of successful translation. Here, to fill this knowledge gap, available HT1 preclinical data and available clinical trial data pertaining to liver-directed GT for other diseases are reviewed. The aim is to establish which vector-payload combination has the most potential as a one-dose HT1 cure. Analysis provides a strong case for progressing lentiviral-based approaches into clinical trials. However, other vector-payload combinations may be more scientifically and commercially viable, but these options require additional investigation.

Chemical engineering of CRISPR-Cas systems for therapeutic application

Clustered regularly interspaced short palindromic repeats (CRISPR) technology has transformed molecular biology and the future of gene-targeted therapeutics. CRISPR systems comprise a CRISPR-associated (Cas) endonuclease and a guide RNA (gRNA) that can be programmed to guide sequence-specific binding, cleavage, or modification of complementary DNA or RNA. However, the application of CRISPR-based therapeutics is challenged by factors such as molecular size, prokaryotic or phage origins, and an essential gRNA cofactor requirement, which impact efficacy, delivery and safety. This Review focuses on chemical modification and engineering approaches for gRNAs to enhance or enable CRISPR-based therapeutics, emphasizing Cas9 and Cas12a as therapeutic paradigms. Issues that chemically modified gRNAs seek to address, including drug delivery, physiological stability, editing efficiency and off-target effects, as well as challenges that remain, are discussed.

Principles of CRISPR-Cas13 mismatch intolerance enable selective silencing of point-mutated oncogenic RNA with single-base precision

Single-nucleotide variants (SNVs) are extremely prevalent in human cancers, although most of these remain clinically unactionable. The programmable RNA nuclease CRISPR-Cas13 has been deployed to specifically target oncogenic RNAs. However, silencing oncogenic SNVs with single-base precision remains extremely challenging due to the intrinsic mismatch tolerance of Cas13. Here, we show that introducing synthetic mismatches at precise positions of the spacer sequence enables de novo design of guide RNAs [CRISPR RNAs (crRNAs)] with strong preferential silencing of point-mutated transcripts. We applied these design principles to effectively silence the oncogenic KRAS G12 hotspot, NRAS G12D and BRAF V600E transcripts with minimal off-target silencing of the wild-type transcripts, underscoring the adaptability of this platform to silence various SNVs. Unexpectedly, the SNV-selective crRNAs harboring mismatched nucleotides reduce the promiscuous collateral activity of the RfxCas13d ortholog. These findings demonstrate that the CRISPR-Cas13 system can be reprogrammed to target mutant transcripts with single-base precision, showcasing the tremendous potential of this tool in personalized transcriptome editing.

Lipid nanoparticle-mediated base-editing of the Hao1 gene achieves sustainable primary hyperoxaluria type 1 therapy in rats

Primary hyperoxaluria type 1 (PH1) is a severe hereditary disease, leading to the accumulation of oxalate in multiple organs, particularly the kidney. Hydroxyacid oxidase 1 (HAO1), a pivotal gene involved in oxalate production, is an approved target for the treatment of PH1. In this study, we demonstrated the discovery of several novel therapeutic sites of the Hao1 gene and the efficient editing of Hao1 c.290-2 A in vivo with lipid nanoparticles (LNP) delivered adenine base editing (ABE) mRNA. A single infusion of LNP-ABE resulted in a near-complete knockout of Hao1 in the liver, leading to the sustainable normalization of urinary oxalate (for at least 6 months) and complete rescue of the patho-physiology in PH1 rats. Additionally, a significant correlation between Hao1 editing efficiency and urinary oxalate levels was observed and over 60% Hao1 editing efficiency was required to achieve the normalization of urinary oxalate in PH1 rats. These findings suggest that the LNP-mediated base-editing of Hao1 c.290-2 A is an efficient and safe approach to PH1 therapy, highlighting its potential utility in clinical settings.

Amphiphilic shuttle peptide delivers base editor ribonucleoprotein to correct the CFTR R553X mutation in well-differentiated airway epithelial cells

Base editing could correct nonsense mutations that cause cystic fibrosis (CF), but clinical development is limited by the lack of delivery methods that efficiently breach the barriers presented by airway epithelia. Here, we present a novel amphiphilic shuttle peptide based on the previously reported S10 peptide that substantially improved base editor ribonucleoprotein (RNP) delivery. Studies of the S10 secondary structure revealed that the alpha-helix formed by the endosomal leakage domain (ELD), but not the cell penetrating peptide (CPP), was functionally important for delivery. By isolating and extending the ELD, we created a novel shuttle peptide, termed S237. While S237 achieved lower delivery of green fluorescent protein, it outperformed S10 at Cas9 RNP delivery to cultured human airway epithelial cells and to pig airway epithelia in vivo, possibly due to its lower net charge. In well-differentiated primary human airway epithelial cell cultures, S237 achieved a 4.6-fold increase in base editor RNP delivery, correcting up to 9.4% of the cystic fibrosis transmembrane conductance regulator (CFTR) R553X allele and restoring CFTR channel function close to non-CF levels. These findings deepen the understanding of peptide-mediated delivery and offer a translational approach for base editor RNP delivery for CF airway disease.

Gene editing in liver diseases

The deliberate and precise modification of the host genome using engineered nucleases represents a groundbreaking advancement in modern medicine. Several clinical trials employing these approaches to address metabolic liver disorders have been initiated, with recent remarkable outcomes observed in patients with transthyretin amyloidosis, highlighting the potential of these therapies. Recent technological improvements, particularly CRISPR Cas9-based technology, have revolutionized gene editing, enabling in vivo modification of the cellular genome for therapeutic purposes. These modifications include gene supplementation, correction, or silencing, offering a wide range of therapeutic possibilities. Moving forward, we anticipate witnessing the unfolding therapeutic potential of these strategies in the coming years. The aim of our review is to summarize preclinical data on gene editing in animal models of inherited liver diseases and the clinical data obtained thus far, emphasizing both therapeutic efficacy and potential limitations of these medical interventions.

Branched-Tail Lipid Nanoparticles for Intravenous mRNA Delivery to Lung Immune, Endothelial, and Alveolar Cells in Mice

Lipid nanoparticles (LNPs) are proven safe and effective delivery systems on a global scale. However, their efficacy has been limited primarily to liver and immune cell targets. To extend the applicability of mRNA drugs, 580 ionizable lipidoids are synthesized and tested for delivery to extrahepatocellular targets. Of these, over 40 enabled protein expression in mice, with the majority transfecting the liver. Beyond the liver, several LNPs containing new, branched-tail ionizable lipidoids potently delivered mRNA to the lungs, with cell-level specificity depending on helper lipid chemistry. Incorporation of the neutral helper lipid 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) at 16 mol% enabled highly specific delivery to natural killer and dendritic cells within the lung. Although inclusion of the cationic lipid 1,2-di-(9Z-octadecenoyl)-3-trimethylammonium-propane (DOTAP) improved lung tropism, it decreased cell specificity, resulting in equal transfection of endothelial and lymphoid cells. DOTAP formulations are also less favorable than DOPE formulations because they elevated liver enzyme and cytokine levels. Together, these data identify a new branched-tailed LNP with a unique ability to selectively transfect lung immune cell populations without the use of toxicity-prone cationic helper lipids. This novel vehicle may unlock RNA therapies for lung diseases associated with immune cell dysregulation, including cancer, viral infections, and autoimmune disorders.

Lipid-nanoparticle-enabled nucleic acid therapeutics for liver disorders

Inherited genetic disorders of the liver pose a significant public health burden. Liver transplantation is often limited by the availability of donor livers and the exorbitant costs of immunosuppressive therapy. To overcome these limitations, nucleic acid therapy provides a hopeful alternative that enables gene repair, gene supplementation, and gene silencing with suitable vectors. Though viral vectors are the most efficient and preferred for gene therapy, pre-existing immunity debilitating immune responses limit their use. As a potential alternative, lipid nanoparticle-mediated vectors are being explored to deliver multiple nucleic acid forms, including pDNA, mRNA, siRNA, and proteins. Herein, we discuss the broader applications of lipid nanoparticles, from protein replacement therapy to restoring the disease mechanism through nucleic acid delivery and gene editing, as well as multiple preclinical and clinical studies as a potential alternative to liver transplantation.

Targeted genome editing with a DNA-dependent DNA polymerase and exogenous DNA-containing templates

Reverse transcriptases, used in prime editing systems, exhibit lower fidelity, processivity and dNTP affinity than many DNA-dependent DNA polymerases. We report that a DNA-dependent DNA polymerase (phi29), untethered from Cas9, enables editing from a synthetic, end-stabilized DNA-containing template at up to 60% efficiency in human cells. Compared to prime editing, DNA polymerase editing avoids autoinhibitory intramolecular base pairing of the template, facilitates template synthesis and supports larger insertions (>100 nucleotides).

Development of Lipid Nanoparticle Formulation for the Repeated Administration of mRNA Therapeutics

During the COVID-19 pandemic, mRNA vaccines emerged as a rapid and effective solution for global immunization. The success of COVID-19 mRNA vaccines has increased interest in the use of lipid nanoparticles (LNPs) for the in vivo delivery of mRNA therapeutics. Although mRNA exhibits robust expression profiles, transient protein expression is often observed, raising uncertainty regarding the frequency of its administration. Additionally, various RNA therapeutics may necessitate repeated dosing to achieve optimal therapeutic outcomes. Nevertheless, the impact of repeated administrations of mRNA/LNP on immune responses and protein expression efficacy remains unclear. In this study, we investigated the influence of the formulation parameters, specifically ionizable lipids and polyethylene glycol (PEG) lipids, on the repeat administration of mRNA/LNP. Our findings revealed that ionizable lipids had no discernible impact on the dose-responsive efficacy of repeat administrations, whereas the lipid structure and molar ratio of PEG lipids were primary factors that affected mRNA/LNP performance. The optimization of the LNP formulation with PEG lipid confirmed the sustained dose-responsive efficacy of mRNA after repeated administrations. This study highlights the critical importance of optimizing LNP formulations for mRNA therapeutics requiring repeated administrations.

Precise genome-editing in human diseases: mechanisms, strategies and applications

Precise genome-editing platforms are versatile tools for generating specific, site-directed DNA insertions, deletions, and substitutions. The continuous enhancement of these tools has led to a revolution in the life sciences, which promises to deliver novel therapies for genetic disease. Precise genome-editing can be traced back to the 1950s with the discovery of DNA's double-helix and, after 70 years of development, has evolved from crude in vitro applications to a wide range of sophisticated capabilities, including in vivo applications. Nonetheless, precise genome-editing faces constraints such as modest efficiency, delivery challenges, and off-target effects. In this review, we explore precise genome-editing, with a focus on introduction of the landmark events in its history, various platforms, delivery systems, and applications. First, we discuss the landmark events in the history of precise genome-editing. Second, we describe the current state of precise genome-editing strategies and explain how these techniques offer unprecedented precision and versatility for modifying the human genome. Third, we introduce the current delivery systems used to deploy precise genome-editing components through DNA, RNA, and RNPs. Finally, we summarize the current applications of precise genome-editing in labeling endogenous genes, screening genetic variants, molecular recording, generating disease models, and gene therapy, including ex vivo therapy and in vivo therapy, and discuss potential future advances.

Advances in functional lipid nanoparticles: from drug delivery platforms to clinical applications

Since Doxil's first clinical approval in 1995, lipid nanoparticles have garnered great interest and shown exceptional therapeutic efficacy. It is clear from the licensure of two RNA treatments and the mRNA-COVID-19 vaccination that lipid nanoparticles have immense potential for delivering nucleic acids. The review begins with a list of lipid nanoparticle types, such as liposomes and solid lipid nanoparticles. Then it moves on to the earliest lipid nanoparticle forms, outlining how lipid is used in a variety of industries and how it is used as a versatile nanocarrier platform. Lipid nanoparticles must then be functionally modified. Various approaches have been proposed for the synthesis of lipid nanoparticles, such as High-Pressure Homogenization (HPH), microemulsion methods, solvent-based emulsification techniques, solvent injection, phase reversal, and membrane contractors. High-pressure homogenization is the most commonly used method. All of the methods listed above follow four basic steps, as depicted in the flowchart below. Out of these four steps, the process of dispersing lipids in an aqueous medium to produce liposomes is the most unpredictable step. A short outline of the characterization of lipid nanoparticles follows discussions of applications for the trapping and transporting of various small molecules. It highlights the use of rapamycin-coated lipid nanoparticles in glioblastoma and how lipid nanoparticles function as a conjugator in the delivery of anticancer-targeting nucleic acids. High biocompatibility, ease of production, scalability, non-toxicity, and tailored distribution are just a meager of the enticing allowances of using lipid nanoparticles as drug delivery vehicles. Due to the present constraints in drug delivery, more research is required to utterly realize the potential of lipid nanoparticles for possible clinical and therapeutic purposes.

Self-delivering, chemically modified CRISPR RNAs for AAV co-delivery and genome editing in vivo

Guide RNAs offer programmability for CRISPR-Cas9 genome editing but also add challenges for delivery. Chemical modification, which has been key to the success of oligonucleotide therapeutics, can enhance the stability, distribution, cellular uptake, and safety of nucleic acids. Previously, we engineered heavily and fully modified SpyCas9 crRNA and tracrRNA, which showed enhanced stability and retained activity when delivered to cultured cells in the form of the ribonucleoprotein complex. In this study, we report that a short, fully stabilized oligonucleotide (a 'protecting oligo'), which can be displaced by tracrRNA annealing, can significantly enhance the potency and stability of a heavily modified crRNA. Furthermore, protecting oligos allow various bioconjugates to be appended, thereby improving cellular uptake and biodistribution of crRNA in vivo. Finally, we achieved in vivo genome editing in adult mouse liver and central nervous system via co-delivery of unformulated, chemically modified crRNAs with protecting oligos and AAV vectors that express tracrRNA and either SpyCas9 or a base editor derivative. Our proof-of-concept establishment of AAV/crRNA co-delivery offers a route towards transient editing activity, target multiplexing, guide redosing, and vector inactivation.

Base-editing mutagenesis maps alleles to tune human T cell functions

CRISPR-enabled screening is a powerful tool for the discovery of genes that control T cell function and has nominated candidate targets for immunotherapies1-6. However, new approaches are required to probe specific nucleotide sequences within key genes. Systematic mutagenesis in primary human T cells could reveal alleles that tune specific phenotypes. DNA base editors are powerful tools for introducing targeted mutations with high efficiency7,8. Here we develop a large-scale base-editing mutagenesis platform with the goal of pinpointing nucleotides that encode amino acid residues that tune primary human T cell activation responses. We generated a library of around 117,000 single guide RNA molecules targeting base editors to protein-coding sites across 385 genes implicated in T cell function and systematically identified protein domains and specific amino acid residues that regulate T cell activation and cytokine production. We found a broad spectrum of alleles with variants encoding critical residues in proteins including PIK3CD, VAV1, LCP2, PLCG1 and DGKZ, including both gain-of-function and loss-of-function mutations. We validated the functional effects of many alleles and further demonstrated that base-editing hits could positively and negatively tune T cell cytotoxic function. Finally, higher-resolution screening using a base editor with relaxed protospacer-adjacent motif requirements9 (NG versus NGG) revealed specific structural domains and protein-protein interaction sites that can be targeted to tune T cell functions. Base-editing screens in primary immune cells thus provide biochemical insights with the potential to accelerate immunotherapy design.

Shuttle peptide delivers base editor RNPs to rhesus monkey airway epithelial cells in vivo

Gene editing strategies for cystic fibrosis are challenged by the complex barrier properties of airway epithelia. We previously reported that the amphiphilic S10 shuttle peptide non-covalently combined with CRISPR-associated (Cas) ribonucleoprotein (RNP) enabled editing of human and mouse airway epithelial cells. Here, we derive the S315 peptide as an improvement over S10 in delivering base editor RNP. Following intratracheal aerosol delivery of Cy5-labeled peptide in rhesus macaques, we confirm delivery throughout the respiratory tract. Subsequently, we target CCR5 with co-administration of ABE8e-Cas9 RNP and S315. We achieve editing efficiencies of up-to 5.3% in rhesus airway epithelia. Moreover, we document persistence of edited epithelia for up to 12 months in mice. Finally, delivery of ABE8e-Cas9 targeting the CFTR R553X mutation restores anion channel function in cultured human airway epithelia. These results demonstrate the therapeutic potential of base editor delivery with S315 to functionally correct the CFTR R553X mutation in respiratory epithelia.

Use of adenine base editing and homology-independent targeted integration strategies to correct the cystic fibrosis causing variant, W1282X

Small molecule drugs known as modulators can treat ~90% of people with cystic fibrosis (CF), but do not work for premature termination codon variants such as W1282X (c.3846G>A). Here we evaluated two gene editing strategies, Adenine Base Editing (ABE) to correct W1282X, and Homology-Independent Targeted Integration (HITI) of a CFTR superexon comprising exons 23-27 (SE23-27) to enable expression of a CFTR mRNA without W1282X. In Flp-In-293 cells stably expressing a CFTR expression minigene bearing W1282X, ABE corrected 24% of W1282X alleles, rescued CFTR mRNA from nonsense mediated decay and restored protein expression. However, bystander editing at the adjacent adenine (c.3847A>G), caused an amino acid change (R1283G) that affects CFTR maturation and ablates ion channel activity. In primary human nasal epithelial cells homozygous for W1282X, ABE corrected 27% of alleles, but with a notably lower level of bystander editing, and CFTR channel function was restored to 16% of wild-type levels. Using the HITI approach, correct integration of a SE23-27 in intron 22 of the CFTR locus in 16HBEge W1282X cells was detected in 5.8% of alleles, resulting in 7.8% of CFTR transcripts containing the SE23-27 sequence. Analysis of a clonal line homozygous for the HITI-SE23-27 produced full-length mature protein and restored CFTR anion channel activity to 10% of wild-type levels, which could be increased three-fold upon treatment with the triple combination of CF modulators. Overall, these data demonstrate two different editing strategies can successfully correct W1282X, the second most common class I variant, with a concomitant restoration of CFTR function.

Precision RNA base editing with engineered and endogenous effectors

RNA base editing refers to the rewriting of genetic information within an intact RNA molecule and serves various functions, such as evasion of the endogenous immune system and regulation of protein function. To achieve this, certain enzymes have been discovered in human cells that catalyze the conversion of one nucleobase into another. This natural process could be exploited to manipulate and recode any base in a target transcript. In contrast to DNA base editing, analogous changes introduced in RNA are not permanent or inheritable but rather allow reversible and doseable effects that appeal to various therapeutic applications. The current practice of RNA base editing involves the deamination of adenosines and cytidines, which are converted to inosines and uridines, respectively. In this Review, we summarize current site-directed RNA base-editing strategies and highlight recent achievements to improve editing efficiency, precision, codon-targeting scope and in vivo delivery into disease-relevant tissues. Besides engineered editing effectors, we focus on strategies to harness endogenous adenosine deaminases acting on RNA (ADAR) enzymes and discuss limitations and future perspectives to apply the tools in basic research and as a therapeutic modality. We expect the field to realize the first RNA base-editing drug soon, likely on a well-defined genetic disease. However, the long-term challenge will be to carve out the sweet spot of the technology where its unique ability is exploited to modulate signaling cues, metabolism or other clinically relevant processes in a safe and doseable manner.

The mixing method used to formulate lipid nanoparticles affects mRNA delivery efficacy and organ tropism

mRNA is a versatile drug molecule with therapeutic applications ranging from protein replacement therapies to in vivo gene engineering. mRNA delivery is often accomplished using lipid nanoparticles, which are formulated via mixing of aqueous and organic solutions. Although this has historically been accomplished by manual mixing for bench scale science, microfluidic mixing is required for scalable continuous manufacturing and batch to batch control. Currently, there is limited understanding on how the mixing process affects mRNA delivery efficacy, particularly in regard to tropism. To address this knowledge gap, we examined the influence of the type of mixing and microfluidic mixing parameters on the performance of lipid nanoparticles in mice. This was accomplished with a Design of Experiment approach using four nanoparticle formulations with varied ionizable lipid chemistry. We found that each formulation required unique optimization of mixing parameters, with the total delivery efficacy of each lipid nanoparticle generated with microfluidics ranging from 100-fold less to 4-fold more than manually mixed LNPs. Further, mixing parameters influenced organ tropism, with the most efficacious formulations disproportionately increasing liver delivery compared to other organs. These data suggest that mixing parameters for lipid nanoparticle production may require optimization for each unique chemical formulation, complicating translational efforts. Further, microfluidic parameters must be chosen carefully to balance overall mRNA delivery efficacy with application-specific tropism requirements.

RNA Nanomedicine: Delivery Strategies and Applications

Delivery of RNA using nanomaterials has emerged as a new modality to expand therapeutic applications in biomedical research. However, the delivery of RNA presents unique challenges due to its susceptibility to degradation and the requirement for efficient intracellular delivery. The integration of nanotechnologies with RNA delivery has addressed many of these challenges. In this review, we discuss different strategies employed in the design and development of nanomaterials for RNA delivery. We also highlight recent advances in the pharmaceutical applications of RNA delivered via nanomaterials. Various nanomaterials, such as lipids, polymers, peptides, nucleic acids, and inorganic nanomaterials, have been utilized for delivering functional RNAs, including messenger RNA (mRNA), small interfering RNA, single guide RNA, and microRNA. Furthermore, the utilization of nanomaterials has expanded the applications of functional RNA as active pharmaceutical ingredients. For instance, the delivery of antigen-encoding mRNA using nanomaterials enables the transient expression of vaccine antigens, leading to immunogenicity and prevention against infectious diseases. Additionally, nanomaterial-mediated RNA delivery has been investigated for engineering cells to express exogenous functional proteins. Nanomaterials have also been employed for co-delivering single guide RNA and mRNA to facilitate gene editing of genetic diseases. Apart from the progress made in RNA medicine, we discuss the current challenges and future directions in this field.

Current approaches and potential challenges in the delivery of gene editing cargos into hematopoietic stem and progenitor cells

Advancements in gene delivery and editing have expanded the applications of autologous hematopoietic stem and progenitor cells (HSPCs) for the treatment of monogenic and acquired diseases. The gene editing toolbox is growing, and the ability to achieve gene editing with mRNA or protein delivered intracellularly by vehicles, such as electroporation and nanoparticles, has highlighted the potential of gene editing in HSPCs. Ongoing phase I/II clinical trials with gene-edited HSPCs for β-hemoglobinopathies provide hope for treating monogenic diseases. The development of safe and efficient gene editing reagents and their delivery into hard-to-transfect HSPCs have been critical drivers in the rapid translation of HSPC gene editing into clinical studies. This review article summarizes the available payloads and delivery vehicles for gene editing HSPCs and their potential impact on therapeutic applications.

Gene editing therapeutics based on mRNA delivery

The field of gene editing has received much attention in recent years due to its immense therapeutic potential. In particular, gene editing therapeutics, such as the CRISPR-Cas systems, base editors, and other emerging gene editors, offer the opportunity to address previously untreatable disorders. This review aims to summarize the therapeutic applications of gene editing based on mRNA delivery. We introduce gene editing therapeutics using mRNA and focus on engineering and improvement of gene editing technology. We subsequently examine ex vivo and in vivo gene editing techniques and conclude with an exploration of the next generation of CRISPR and base editing systems.

Lipid nanoparticles (LNPs) for in vivo RNA delivery and their breakthrough technology for future applications

RNA therapeutics show a significant breakthrough for the treatment of otherwise incurable diseases and genetic disorders by regulating disease-related gene expression. The successful development of COVID-19 mRNA vaccines further emphasizes the potential of RNA therapeutics in the prevention of infectious diseases as well as in the treatment of chronic diseases. However, the efficient delivery of RNA into cells remains a challenge, and nanoparticle delivery systems such as lipid nanoparticles (LNPs) are necessary to fully realize the potential of RNA therapeutics. While LNPs provide a highly efficient platform for the in vivo delivery of RNA by overcoming various biological barriers, several challenges remain to be resolved for further development and regulatory approval. These include a lack of targeted delivery to extrahepatic organs and a gradual loss of therapeutic potency with repeated doses. In this review, we highlight the fundamental aspects of LNPs and their uses in the development of novel RNA therapeutics. Recent advances in LNP-based therapeutics and preclinical/clinical studies are overviewed. Lastly, we discuss the current limitations of LNPs and introduce breakthrough technologies that might overcome these challenges in future applications.

Liver-specific in vivo base editing of Angptl3 via AAV delivery efficiently lowers blood lipid levels in mice

BACKGROUND

Gene editing has emerged as an exciting therapeutic development platform for numerous genetic and nongenetic diseases. Targeting lipid-modulating genes such as angiopoietin-related protein 3 (ANGPTL3) with gene editing offers hope for a permanent solution to lower cardiovascular disease risks associated with hypercholesterolemia.

RESULTS

In this study, we developed a hepatocyte-specific base editing therapeutic approach delivered by dual adeno-associated virus (AAV) to enable hepatocyte-specific targeting of Angptl3 to lower blood lipid levels. Systemic AAV9-mediated delivery of AncBE4max, a cytosine base editor (CBE), targeting mouse Angptl3 resulted in the installation of a premature stop codon in Angptl3 with an average efficiency of 63.3 ± 2.3% in the bulk liver tissue. A near-complete knockout of the ANGPTL3 protein in the circulation were observed within 2-4 weeks following AAV administration. Furthermore, the serum levels of triglyceride (TG) and total cholesterol (TC) were decreased by approximately 58% and 61%, respectively, at 4 weeks after treatment.

CONCLUSIONS

These results highlight the promise of liver-targeted Angptl3 base editing for blood lipid control.

Functional restoration of a CFTR splicing mutation through RNA delivery of CRISPR adenine base editor

Cystic fibrosis (CF) is a genetic disease caused by mutations in the CF transmembrane conductance regulator (CFTR) gene. The 2789+5G>A CFTR mutation is a quite frequent defect causing an aberrant splicing and a non-functional CFTR protein. Here we used a CRISPR adenine base editing (ABE) approach to correct the mutation in the absence of DNA double-strand breaks (DSB). To select the strategy, we developed a minigene cellular model reproducing the 2789+5G>A splicing defect. We obtained up to 70% editing in the minigene model by adapting the ABE to the PAM sequence optimal for targeting 2789+5G>A with a SpCas9-NG (NG-ABE). Nonetheless, the on-target base correction was accompanied by secondary (bystander) A-to-G conversions in nearby nucleotides, which affected the wild-type CFTR splicing. To decrease the bystander edits, we used a specific ABE (NG-ABEmax), which was delivered as mRNA. The NG-ABEmax RNA approach was validated in patient-derived rectal organoids and bronchial epithelial cells showing sufficient gene correction to recover the CFTR function. Finally, in-depth sequencing revealed high editing precision genome-wide and allele-specific correction. Here we report the development of a base editing strategy to precisely repair the 2789+5G>A mutation resulting in restoration of the CFTR function, while reducing bystander and off-target activities.

Gene editing innovations and their applications in cardiomyopathy research

Cardiomyopathies are among the major triggers of heart failure, but their clinical and genetic complexity have hampered our understanding of these disorders and delayed the development of effective treatments. Alongside the recent identification of multiple cardiomyopathy-associated genetic variants, advances in genome editing are providing new opportunities for cardiac disease modeling and therapeutic intervention, both in vitro and in vivo. Two recent innovations in this field, prime and base editors, have improved editing precision and efficiency, and are opening up new possibilities for gene editing of postmitotic tissues, such as the heart. Here, we review recent advances in prime and base editors, the methods to optimize their delivery and targeting efficiency, their strengths and limitations, and the challenges that remain to be addressed to improve the application of these tools to the heart and their translation to the clinic.

Recent advances in therapeutic CRISPR-Cas9 genome editing: mechanisms and applications

Recently, clustered regularly interspaced palindromic repeats (CRISPR)-Cas9 derived editing tools had significantly improved our ability to make desired changes in the genome. Wild-type Cas9 protein recognizes the target genomic loci and induced local double strand breaks (DSBs) in the guidance of small RNA molecule. In mammalian cells, the DSBs are mainly repaired by endogenous non-homologous end joining (NHEJ) pathway, which is error prone and results in the formation of indels. The indels can be harnessed to interrupt gene coding sequences or regulation elements. The DSBs can also be fixed by homology directed repair (HDR) pathway to introduce desired changes, such as base substitution and fragment insertion, when proper donor templates are provided, albeit in a less efficient manner. Besides making DSBs, Cas9 protein can be mutated to serve as a DNA binding platform to recruit functional modulators to the target loci, performing local transcriptional regulation, epigenetic remolding, base editing or prime editing. These Cas9 derived editing tools, especially base editors and prime editors, can introduce precise changes into the target loci at a single-base resolution and in an efficient and irreversible manner. Such features make these editing tools very promising for therapeutic applications. This review focuses on the evolution and mechanisms of CRISPR-Cas9 derived editing tools and their applications in the field of gene therapy.

Self-delivering CRISPR RNAs for AAV Co-delivery and Genome Editing in vivo

Guide RNAs offer programmability for CRISPR-Cas9 genome editing but also add challenges for delivery. Chemical modification, which has been key to the success of oligonucleotide therapeutics, can enhance the stability, distribution, cellular uptake, and safety of nucleic acids. Previously, we engineered heavily and fully modified SpyCas9 crRNA and tracrRNA, which showed enhanced stability and retained activity when delivered to cultured cells in the form of the ribonucleoprotein complex. In this study, we report that a short, fully stabilized oligonucleotide (a "protecting oligo"), which can be displaced by tracrRNA annealing, can significantly enhance the potency and stability of a heavily modified crRNA. Furthermore, protecting oligos allow various bioconjugates to be appended, thereby improving cellular uptake and biodistribution of crRNA in vivo. Finally, we achieved in vivo genome editing in adult mouse liver and central nervous system via co-delivery of unformulated, chemically modified crRNAs with protecting oligos and AAV vectors that express tracrRNA and either SpyCas9 or a base editor derivative. Our proof-of-concept establishment of AAV/crRNA co-delivery offers a route towards transient editing activity, target multiplexing, guide redosing, and vector inactivation.

A Brief Review of Duchenne Muscular Dystrophy Treatment Options, with an Emphasis on Two Novel Strategies

Despite the full cloning of the Dystrophin cDNA 35 years ago, no effective treatment exists for the Duchenne Muscular Dystrophy (DMD) patients who have a mutation in this gene. Many treatment options have been considered, investigated preclinically and some clinically, but none have circumvented all barriers and effectively treated the disease without burdening the patients with severe side-effects. However, currently, many novel therapies are in the pipelines of research labs and pharmaceutical companies and many of these have progressed to clinical trials. A brief review of these promising therapies is presented, followed by a description of two novel technologies that when utilized together effectively treat the disease in the mdx mouse model. One novel technology is to generate chimeric cells from the patient’s own cells and a normal donor. The other technology is to systemically transplant these cells into the femur via the intraosseous route.

Development of novel therapeutics for all individuals with CF (the future goes on)

Despite the major advances and successes in finding and establishing new treatments that tackle the basic defect in Cystic Fibrosis (CF), there is still an unmet need to bring these potentially curative therapies to all individuals with CF. Here, we review aspects of what is still missing to treat all individuals with CF by such approaches. On the one hand, we discuss novel holistic (high-throughput) approaches to elucidate mechanistic defects caused by distinct classes of mutations to identify novel drug targets. On the other hand, we examine therapeutic approaches to correct the gene in its own environment, i.e., in the genome.

CRISPR-Cas9 base editors and their current role in human therapeutics

BACKGROUND

Consistent progress has been made to create more efficient and useful CRISPR-Cas9-based molecular toolsfor genomic modification.

METHODS

This review focuses on recent articles that have employed base editors (BEs) for both clinical and research purposes.

RESULTS

CRISPR-Cas9 BEs are a useful system because of their highefficiency and broad applicability to gene correction and disruption. In addition, base editing has beensuggested as a safer approach than other CRISPR-Cas9-based systems, as it limits double-strand breaksduring multiplex gene knockout and does not require a toxic DNA donor molecule for genetic correction.

CONCLUSION

As such, numerous industry and academic groups are currently developing base editing strategies withclinical applications in cancer immunotherapy and gene therapy, which this review will discuss, with a focuson current and future applications of in vivo BE delivery.

Shuttle Peptide Delivers Base Editor RNPs to Rhesus Monkey Airway Epithelial Cells In Vivo

Gene editing strategies for cystic fibrosis are challenged by the complex barrier properties of airway epithelia. We previously reported that the amphiphilic S10 shuttle peptide non-covalently combined with CRISPR-associated (Cas) ribonucleoprotein (RNP) enabled editing of human and mouse airway epithelial cells. Here, to improve base editor RNP delivery, we optimized S10 to derive the S315 peptide. Following intratracheal aerosol of Cy5-labeled peptide cargo in rhesus macaques, we confirmed delivery throughout the respiratory tract. Subsequently, we targeted CCR5 with co-administration of ABE8e-Cas9 RNP and S315. We achieved editing efficiencies of up to 5.3% in rhesus airway epithelia. Moreover, we documented persistence of edited epithelia for up to 12 months in mice. Finally, delivery of ABE8e-Cas9 targeting the CFTR R553X mutation restored anion channel function in cultured human airway epithelial cells. These results demonstrate the therapeutic potential of base editor delivery with S315 to functionally correct the CFTR R553X mutation in respiratory epithelia.

Base Editing of Human Hematopoietic Stem Cells

Base editing by nucleotide deaminases linked to programmable DNA-binding proteins represents a promising approach to remedy blood disorders. Here we describe the ex vivo base editing of human CD34+ hematopoietic stem and progenitor cells (HSPCs) by electroporation of base editor mRNA or protein.

Genetics of Cystic Fibrosis: Clinical Implications

Cystic fibrosis (CF) is a multiorgan disease caused by a wide variety of mutations in the cystic fibrosis transmembrane conductance regulator gene. As treatment has progressed from symptom mitigation to targeting of specific molecular defects, genetics has played an important role in identifying the proper precision therapies for each individual. Novel therapeutic approaches are focused on expanding treatment to a greater number of individuals as well as working toward a cure. This review discusses the role of genetics in our understanding of CF with a particular emphasis on how genetics informs the exciting landscape of current and novel CF therapies.

ABE8e adenine base editor precisely and efficiently corrects a recurrent COL7A1 nonsense mutation

Base editing introduces precise single-nucleotide edits in genomic DNA and has the potential to treat genetic diseases such as the blistering skin disease recessive dystrophic epidermolysis bullosa (RDEB), which is characterized by mutations in the COL7A1 gene and type VII collagen (C7) deficiency. Adenine base editors (ABEs) convert A-T base pairs to G-C base pairs without requiring double-stranded DNA breaks or donor DNA templates. Here, we use ABE8e, a recently evolved ABE, to correct primary RDEB patient fibroblasts harboring the recurrent RDEB nonsense mutation c.5047 C > T (p.Arg1683Ter) in exon 54 of COL7A1 and use a next generation sequencing workflow to interrogate post-treatment outcomes. Electroporation of ABE8e mRNA into a bulk population of RDEB patient fibroblasts resulted in remarkably efficient (94.6%) correction of the pathogenic allele, restoring COL7A1 mRNA and expression of C7 protein in western blots and in 3D skin constructs. Off-target DNA analysis did not detect off-target editing in treated patient-derived fibroblasts and there was no detectable increase in A-to-I changes in the RNA. Taken together, we have established a highly efficient pipeline for gene correction in primary fibroblasts with a favorable safety profile. This work lays a foundation for developing therapies for RDEB patients using ex vivo or in vivo base editing strategies.

Base-editing-mediated dissection of a γ-globin cis-regulatory element for the therapeutic reactivation of fetal hemoglobin expression

Sickle cell disease and β-thalassemia affect the production of the adult β-hemoglobin chain. The clinical severity is lessened by mutations that cause fetal γ-globin expression in adult life (i.e., the hereditary persistence of fetal hemoglobin). Mutations clustering ~200 nucleotides upstream of the HBG transcriptional start sites either reduce binding of the LRF repressor or recruit the KLF1 activator. Here, we use base editing to generate a variety of mutations in the -200 region of the HBG promoters, including potent combinations of four to eight γ-globin-inducing mutations. Editing of patient hematopoietic stem/progenitor cells is safe, leads to fetal hemoglobin reactivation and rescues the pathological phenotype. Creation of a KLF1 activator binding site is the most potent strategy - even in long-term repopulating hematopoietic stem/progenitor cells. Compared with a Cas9-nuclease approach, base editing avoids the generation of insertions, deletions and large genomic rearrangements and results in higher γ-globin levels. Our results demonstrate that base editing of HBG promoters is a safe, universal strategy for treating β-hemoglobinopathies.

Lipid Nanoparticles: A Novel Gene Delivery Technique for Clinical Application

Lipid nanoparticles (LNPs) are an emerging vehicle for gene delivery that accommodate both nucleic acid and protein. Based on the experience of therapeutic liposomes, current LNPs have been developed based on the chemistry of lipids and RNA and on the biology of human disease. LNPs have been used for the development of Onpattro, an siRNA drug for transthyretin-mediated amyloidosis, in 2018. The subsequent outbreak of COVID-19 required a vaccine for its suppression. LNP-based vaccine production received much attention for this and resulted in great success. In this review, the essential technology of LNP gene delivery has been described according to the chemistry for LNP production and biology for its clinical application.

Robust genome editing via modRNA-based Cas9 or base editor in human pluripotent stem cells

CRISPR systems have revolutionized biomedical research because they offer an unprecedented opportunity for genome editing. However, a bottleneck of applying CRISPR systems in human pluripotent stem cells (hPSCs) is how to deliver CRISPR effectors easily and efficiently. Here, we developed modified mRNA (modRNA)-based CRIPSR systems that utilized Cas9 and p53DD or a base editor (ABE8e) modRNA for the purposes of knocking out genes in hPSCs via simple lipid-based transfection. ABE8e modRNA was employed to disrupt the splice donor site, resulting in defective splicing of the target transcript and ultimately leading to gene knockout. Using our modRNA CRISPR systems, we achieved 73.3% ± 11.2% and 69.6 ± 3.8% knockout efficiency with Cas9 plus p53DD modRNA and ABE8e modRNA, respectively, which was significantly higher than the plasmid-based systems. In summary, we demonstrate that our non-integrating modRNA-based CRISPR methods hold great promise as more efficient and accessible techniques for genome editing of hPSCs.

A precise and efficient adenine base editor

Adenine base editors (ABEs) are novel genome-editing tools, and their activity has been greatly enhanced by eight additional mutations, thus named ABE8e. However, elevated catalytic activity was concomitant with frequent generation of bystander mutations. This bystander effect precludes its safe applications required in human gene therapy. To develop next-generation ABEs that are both catalytically efficient and positionally precise, we performed combinatorial engineering of NG-ABE8e. We identify a novel variant (NG-ABE9e), which harbors nine mutations. NG-ABE9e exhibits robust and precise base-editing activity in human cells, with more than 7-fold bystander editing reduction at some sites, compared with NG-ABE8e. To demonstrate its practical utility, we used NG-ABE9e to correct the frequent T17M mutation in Rhodopsin for autosomal dominant retinitis pigmentosa. It reduces bystander editing by ∼4-fold while maintaining comparable efficiency. NG-ABE9e possesses substantially higher activity than NG-ABEmax and significantly lower bystander editing than NG-ABE8e in rice. Therefore, this study provides a versatile and improved adenine base editor for genome editing.

Programmable Genome-Editing Technologies as Single-Course Therapeutics for Atherosclerotic Cardiovascular Disease

PURPOSE OF REVIEW

To establish genome editing as a promising therapeutic approach for the treatment and prevention of atherosclerotic cardiovascular disease.

RECENT FINDINGS

Systemic delivery of a CRISPR adenine base editor using lipid nanoparticles demonstrated a near 90% reduction in circulating PCSK9 and over 60% reduction in blood LDL-C in nonhuman primates with the effects remaining durable at least 8 months following a single course. Preclinical proof-of-concept studies have elucidated the superior therapeutic potential of genome-editing approaches for the treatment of hyperlipidemia, thus substantiating their progression to clinical studies.

Advances in Preclinical In Vitro Models for the Translation of Precision Medicine for Cystic Fibrosis

The development of preclinical in vitro models has provided significant progress to the studies of cystic fibrosis (CF), a frequently fatal monogenic disease caused by mutations in the gene encoding the CF transmembrane conductance regulator (CFTR) protein. Numerous cell lines were generated over the last 30 years and they have been instrumental not only in enhancing the understanding of CF pathological mechanisms but also in developing therapies targeting the underlying defects in CFTR mutations with further validation in patient-derived samples. Furthermore, recent advances toward precision medicine in CF have been made possible by optimizing protocols and establishing novel assays using human bronchial, nasal and rectal tissues, and by progressing from two-dimensional monocultures to more complex three-dimensional culture platforms. These models also enable to potentially predict clinical efficacy and responsiveness to CFTR modulator therapies at an individual level. In parallel, advanced systems, such as induced pluripotent stem cells and organ-on-a-chip, continue to be developed in order to more closely recapitulate human physiology for disease modeling and drug testing. In this review, we have highlighted novel and optimized cell models that are being used in CF research to develop novel CFTR-directed therapies (or alternative therapeutic interventions) and to expand the usage of existing modulator drugs to common and rare CF-causing mutations.

Therapeutic in vivo delivery of gene editing agents

In vivo gene editing therapies offer the potential to treat the root causes of many genetic diseases. Realizing the promise of therapeutic in vivo gene editing requires the ability to safely and efficiently deliver gene editing agents to relevant organs and tissues in vivo. Here, we review current delivery technologies that have been used to enable therapeutic in vivo gene editing, including viral vectors, lipid nanoparticles, and virus-like particles. Since no single delivery modality is likely to be appropriate for every possible application, we compare the benefits and drawbacks of each method and highlight opportunities for future improvements.

Chemical Modifications of CRISPR RNAs to Improve Gene-Editing Activity and Specificity

CRISPR (clustered, regularly interspaced, short palindromic repeats) has become a cutting-edge research method and holds great potential to revolutionize biotechnology and medicine. However, like other nucleic acid technologies, CRISPR will greatly benefit from chemical innovation to improve activity and specificity for critical in vivo applications. Chemists have started optimizing various components of the CRISPR system; the present Perspective focuses on chemical modifications of CRISPR RNAs (crRNAs). As with other nucleic acid-based technologies, early efforts focused on well-established sugar and backbone modifications (2'-deoxy, 2'-F, 2'-OMe, and phosphorothioates). Some more significant alterations of crRNAs have been done using bicyclic (locked) riboses and phosphate backbone replacements (phosphonoacetates and amides); however, the range of chemical innovation applied to crRNAs remains limited to modifications that have been successful in RNA interference and antisense technologies. The encouraging results given by these tried-and-true modifications suggest that, going forward, chemists should take a bolder approach─research must aim to investigate what chemistry will have the most impact on maturing CRISPR as therapeutic and other in vivo technologies. With an eye to the future, this Perspective argues that the complexity of CRISPR presents rich unprecedented opportunities for chemists to synergize advances in synthetic methodology and structural biochemistry to rationally optimize crRNA-protein interactions.

Adenine Base Editing In Vivo with a Single Adeno-Associated Virus Vector

Base editors (BEs) have opened new avenues for the treatment of genetic diseases. However, advances in delivery approaches are needed to enable disease targeting of a broad range of tissues and cell types. Adeno-associated virus (AAV) vectors remain one of the most promising delivery vehicles for gene therapies. Currently, most BE/guide combinations and their promoters exceed the packaging limit (∼5 kb) of AAVs. Dual-AAV delivery strategies often require high viral doses that impose safety concerns. In this study, we engineered an adenine base editor (ABE) using a compact Cas9 from Neisseria meningitidis (Nme2Cas9). Compared with the well-characterized Streptococcus pyogenes Cas9-containing ABEs, ABEs using Nme2Cas9 (Nme2-ABE) possess a distinct protospacer adjacent motif (N₄CC) and editing window, exhibit fewer off-target effects, and can efficiently install therapeutically relevant mutations in both human and mouse genomes. Importantly, we show that in vivo delivery of Nme2-ABE and its guide RNA by a single AAV vector can efficiently edit mouse genomic loci and revert the disease mutation and phenotype in an adult mouse model of tyrosinemia. We anticipate that Nme2-ABE, by virtue of its compact size and broad targeting range, will enable a range of therapeutic applications with improved safety and efficacy due in part to packaging in a single-vector system.

Modern therapeutic approaches to liver-related disorders

The liver is a key production and processing site that is essential for health. Liver dysfunction can result in both systemic and liver-specific diseases. To combat these diseases, genetic approaches have been developed that have high liver tropism and are based on gene addition/editing or gene silencing. The gene addition/editing approach has yielded encouraging clinical data on the use of viral vectors in patients with haemophilia, as well as neuromuscular diseases, and has led to trials for liver-related disorders. However, the immune response and the long-term stability of exogenous expression remain important challenges. Gene editing and mRNA therapy have yielded first in-human proof-of-concept therapeutics and vaccines, but the road to the treatment of liver-related disorders remains long. Gene silencing is accomplished primarily via antisense oligonucleotides and small-interfering RNAs (siRNAs). siRNA modification with N-acetyl galactosamine results in hepatocellular-specific targeting and catapulted the liver to the centre of siRNA research. Several siRNA drugs for liver-related disorders have recently been approved, and the pipeline of drugs under investigation is crowded. Loss-of-function mutations might also be treated with enzyme substitution therapy. This review summarises current genetic approaches as well as key enzyme substitution therapies, focusing on recently approved compounds, potential adverse effects, and future challenges. Collectively, these recent advances place the liver at the forefront of precision medicine for metabolic and genetic diseases and are expected to transform the care and treatment of patients with both liver-specific and systemic diseases.

Intracellular RNase activity dampens zinc finger nuclease-mediated gene editing in hematopoietic stem and progenitor cells

Over the past decade, numerous gene-editing platforms which alter host DNA in a highly specific and targeted fashion have been described. Two notable examples are zinc finger nucleases (ZFNs), the first gene-editing platform to be tested in clinical trials, and more recently, CRISPR/Cas9. Although CRISPR/Cas9 approaches have become arguably the most popular platform in the field, the therapeutic advantages and disadvantages of each strategy are only beginning to emerge. We have established a nonhuman primate (NHP) model that serves as a strong predictor of successful gene therapy and gene-editing approaches in humans; our recent work shows that ZFN-edited hematopoietic stem and progenitor cells (HSPCs) engraft at lower levels than CRISPR/Cas9-edited cells. Here, we investigate the mechanisms underlying this difference. We show that optimized culture conditions, including defined serum-free media, augment engraftment of gene-edited NHP HSPCs in a mouse xenograft model. Furthermore, we identify intracellular RNases as major barriers for mRNA-encoded nucleases relative to preformed enzymatically active CRISPR/Cas9 ribonucleoprotein (RNP) complexes. We conclude that CRISPR/Cas9 RNP gene editing is more stable and efficient than ZFN mRNA-based delivery and identify co-delivered RNase inhibitors as a strategy to enhance the expression of gene-editing proteins from mRNA intermediates.