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In Vivo Base Editing of PCSK9 (Proprotein Convertase Subtilisin/Kexin Type 9) as a Therapeutic Alternative to Genome Editing.
OBJECTIVE: High-efficiency genome editing to disrupt therapeutic target genes, such as PCSK9 (proprotein convertase subtilisin/kexin type 9), has been demonstrated in preclinical animal models, but there are safety concerns because of the unpredictable nature of cellular repair of double-strand breaks, as well as off-target mutagenesis. Moreover, precise knock-in of specific nucleotide changes-whether to introduce or to correct gene mutations-has proven to be inefficient in nonproliferating cells in vivo. Base editors comprising CRISPR-Cas9 (clustered regularly interspaced short palindromic repeats [CRISPR]-CRISPR-associated 9) fused to a cytosine deaminase domain can effect the alteration of cytosine bases to thymine bases in genomic DNA in a sequence-specific fashion, without the need for double-strand DNA breaks. The efficacy of base editing has not been established in vivo. The goal of this study was to assess whether in vivo base editing could be used to modify the mouse Pcsk9 gene in a sequence-specific fashion in the liver in adult mice. APPROACH AND RESULTS: We screened base editors for activity in cultured cells, including human-induced pluripotent stem cells. We then delivered a base editor into the livers of adult mice to assess whether it could introduce site-specific nonsense mutations into the Pcsk9 gene. In adult mice, this resulted in substantially reduced plasma PCSK9 protein levels (>50%), as well as reduced plasma cholesterol levels (≈30%). There was no evidence of off-target mutagenesis, either cytosine-to-thymine edits or indels. CONCLUSIONS: These results demonstrate the ability to precisely introduce therapeutically relevant nucleotide variants into the genome in somatic tissues in adult mammals, as well as highlighting a potentially safer alternative to therapeutic genome editing.
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Clarifying CRISPR: Why Repeats Identified in the Human Genome Should Not Be Considered CRISPRs.
Clustered regularly interspaced short palindromic repeats (CRISPRs) and their associated genes (cas) are essential components of adaptive immune systems that protect bacteria and archaea from viral infection. CRISPR-Cas systems are found in about 40% of bacterial and 85% of archaeal genomes, but not in eukaryotic genomes. Recently, an article published in Communications Biology reported the identification of 12,572 putative CRISPRs in the human genome, which they call "hCRISPR." In this study, we attempt to reproduce this analysis and show that repetitive elements identified as putative CRISPR loci in the human genome contain neither the repeat-spacer-repeat architecture nor the cas genes characteristic of functional CRISPR systems.
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Introducing Point Mutations into Human Pluripotent Stem Cells using Seamless Genome Editing.
Custom designed endonucleases, such as RNA-guided Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)-Cas9, enable efficient genome editing in mammalian cells. Here we describe detailed procedures to seamlessly genome edit the hepatocyte nuclear factor 4 alpha (HNF4α) locus as an example in human pluripotent stem cells. Combining a piggyBac-based donor plasmid and the CRISPR-Cas9 nickase mutant in a two-step genetic selection, we demonstrate correct and efficient targeting of the HNF4α locus.
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Programmable DNA cleavage in vitro by Cas9.
The ternary Cas9-crRNA-tracrRNA complex (Cas9t) of the Type II CRISPR (clustered regularly interspaced short palindromic repeats)-Cas (CRISPR-associated) system functions as an Mg2+-dependent RNA-directed DNA endonuclease that locates its DNA target guided by the crRNA (CRISPR RNA) in the tracrRNA-crRNA structure and introduces a double-strand break at a specific site in DNA. The simple modular organization of Cas9t, where specificity for the DNA target is encoded by a small crRNA and the cleavage reaction is executed by the Cas9 endonuclease, provides a versatile platform for the engineering of universal RNA-directed DNA endonucleases. By altering the crRNA sequence within the Cas9t complex, programmable endonucleases can be designed for both in vitro and in vivo applications. Cas9t has been recently employed as a gene-editing tool in various eukaryotic cell types. Using Streptococcus thermophilus Cas9t as a model system, we demonstrate the feasibility of Cas9t as a programmable molecular tool for in vitro DNA manipulations.
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Nucleic Acid-Based Therapies - The Next Frontier in Treating Dyslipidemias.
Genetic determinants play a central role in the development of dyslipidemias, which are major contributors to atherosclerotic cardiovascular disease (ASCVD), aortic valve disease, and acute pancreatitis. With conventional lipid-lowering therapies, such as statins, many patients with genetic dyslipidemias remain inadequately controlled while other patients are unable to tolerate them, necessitating ongoing research and development of innovative therapies. Several therapeutic lipid targets-including proprotein convertase subtilisin/kexin type 9 (PCSK9), lipoprotein(a) [Lp(a)], apolipoprotein (apo) C-III (APOC3), and angiopoietin-like protein 3 (ANGPTL3)-have been identified and validated through human genetic and epidemiologic studies. Emerging ribonucleic acid (RNA) targeting and gene-editing therapies now offer the potential for durable correction of these metabolic disturbances. Small interfering RNAs (siRNAs) and antisense oligonucleotides (ASOs) targeting PCSK9, Lp(a), apo C-III and ANGPTL3 have shown marked efficacy in lowering atherogenic lipoproteins and triglycerides, while DNA base-editing approaches including clustered regularly interspaced short palindromic repeats (CRISPR), CRISPR associated protein 9 (Cas9) and related base-editing techniques, aim to provide long-term or even permanent gene silencing to control atherogenic lipids. Together, these innovations mark a paradigm shift toward precision, gene-based lipid management, expanding therapeutic options, improving clinical outcomes, and addressing unmet medical needs in patients with severe or refractory dyslipidemias or in those unable to tolerate standard therapies.
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The therapeutic application of CRISPR/Cas9 technologies for HIV.
INTRODUCTION: The use of antiretroviral therapy has led to a significant decrease in morbidity and mortality in HIV-infected individuals. Nevertheless, gene-based therapies represent a promising therapeutic paradigm for HIV-1, as they have the potential for sustained viral inhibition and reduced treatment interventions. One new method amendable to a gene-based therapy is the clustered regularly interspaced short palindromic repeats (CRISPR)-associated protein-9 nuclease (Cas9) gene editing system. AREAS COVERED: CRISPR/Cas9 can be engineered to successfully modulate an array of disease-causing genetic elements. We discuss the diverse roles that CRISPR/Cas9 may play in targeting HIV and eradicating infection. The Cas9 nuclease coupled with one or more small guide RNAs can target the provirus to mediate excision of the integrated viral genome. Moreover, a modified nuclease-deficient Cas9 fused to transcription activation domains may induce targeted activation of proviral gene expression allowing for the purging of the latent reservoirs. These technologies can also be exploited to target host dependency factors such as the co-receptor CCR5, thus preventing cellular entry of the virus. EXPERT OPINION: The diversity of the CRISPR/Cas9 technologies offers great promise for targeting different stages of the viral life cycle, and have the capacity for mediating an effective and sustained genetic therapy against HIV.
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Application of the CRISPR/Cas System in Pathogen Detection: A Review.
Early and rapid diagnosis of pathogens is important for the prevention and control of epidemic disease. The polymerase chain reaction (PCR) technique requires expensive instrument control, a special test site, complex solution treatment steps and professional operation, which can limit its application in practice. The pathogen detection method based on the clustered regularly interspaced short palindromic repeats (CRISPRs) and CRISPR-associated protein (CRISPR/Cas) system is characterized by strong specificity, high sensitivity and convenience for detection, which is more suitable for practical applications. This article first reviews the CRISPR/Cas system, and then introduces the application of the two types of systems represented by Type II (cas9), Type V (cas12a, cas12b, cas14a) and Type VI (cas13a) in pathogen detection. Finally, challenges and prospects are proposed.
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Detection of CRISPR adaptation.
Prokaryotic adaptive immunity is built when short DNA fragments called spacers are acquired into CRISPR (clustered regularly interspaced short palindromic repeats) arrays. CRISPR adaptation is a multistep process which comprises selection, generation, and incorporation of prespacers into arrays. Once adapted, spacers provide immunity through the recognition of complementary nucleic acid sequences, channeling them for destruction. To prevent deleterious autoimmunity, CRISPR adaptation must therefore be a highly regulated and infrequent process, at least in the absence of genetic invaders. Over the years, ingenious methods to study CRISPR adaptation have been developed. In this paper, we discuss and compare methods that detect CRISPR adaptation and its intermediates in vivo and propose suppressing PCR as a simple modification of a popular assay to monitor spacer acquisition with increased sensitivity.
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Cytosine base editing enables quadruple-edited allogeneic CART cells for T-ALL.
Allogeneic chimeric antigen receptor T-cell (CART) therapies require multiple gene edits to be clinically tractable. Most allogeneic CARTs have been created using gene editing techniques that induce DNA double-stranded breaks (DSBs), resulting in unintended on-target editing outcomes with potentially unforeseen consequences. Cytosine base editors (CBEs) install C•G to T•A point mutations in T cells, with between 90% and 99% efficiency to silence gene expression without creating DSBs, greatly reducing or eliminating undesired editing outcomes following multiplexed editing as compared with clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9). Using CBE, we developed 7CAR8, a CD7-directed allogeneic CART created using 4 simultaneous base edits. We show that CBE, unlike CRISPR-Cas9, does not impact T-cell proliferation, lead to aberrant DNA damage response pathway activation, or result in karyotypic abnormalities following multiplexed editing. We demonstrate 7CAR8 to be highly efficacious against T-cell acute lymphoblastic leukemia (T-ALL) using multiple in vitro and in vivo models. Thus, CBE is a promising technology for applications requiring multiplexed gene editing and can be used to manufacture quadruple-edited 7CAR8 cells, with high potential for clinical translation for relapsed and refractory T-ALL.
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Functional correction of the untreatable CFTR 1717-1G>A mutation through mRNA- and sgRNA-optimized base editing.
The 1717-1G>A is a prevalent splicing mutation causing cystic fibrosis (CF) for which no pharmacological treatments have been approved. This mutation disrupts a canonical 3' AG splice acceptor site in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, leading to severe RNA missplicing, which prevents the correct synthesis of the encoded protein. In this study, we developed an adenine base editing (ABE) strategy to efficiently correct the 1717-1G>A mutation. By using the ABE9 base editor with the protospacer adjacent motif-relaxed Streptococcus pyogenes clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR-associated protein 9 (Cas9) variant SpRY, we obtained up to 30% editing with limited bystander effects in a human embryonic kidney (HEK) 293-based cellular model. Through systematic optimizations of the ABE system, delivered by electroporation of base editor messenger RNA (mRNA) and single guide RNA (sgRNA), we demonstrated genetic repair of the 1717-1G>A mutation in airway epithelial cells and intestinal organoids derived from people with CF. Functional analysis was performed by measuring short-circuit current in air-liquid interface (ALI) culture and by assessing forskolin-induced swelling (FIS) in intestinal organoids, which revealed restoration of CFTR channel activity. These results highlight SpRY-ABE9 as a potential genome editing strategy to permanently correct the CFTR 1717-1G>A mutation and restore CFTR function.
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