1
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Zhu H, Wang L, Wang Y, Jiang X, Qin Q, Song M, Huang Q. Directed-evolution mutations enhance DNA-binding affinity and protein stability of the adenine base editor ABE8e. Cell Mol Life Sci 2024; 81:257. [PMID: 38874784 DOI: 10.1007/s00018-024-05263-7] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/01/2024] [Revised: 04/28/2024] [Accepted: 05/02/2024] [Indexed: 06/15/2024]
Abstract
Adenine base editors (ABEs), consisting of CRISPR Cas nickase and deaminase, can chemically convert the A:T base pair to G:C. ABE8e, an evolved variant of the base editor ABE7.10, contains eight directed evolution mutations in its deaminase TadA8e that significantly increase its base editing activity. However, the functional implications of these mutations remain unclear. Here, we combined molecular dynamics (MD) simulations and experimental measurements to investigate the role of the directed-evolution mutations in the base editing catalysis. MD simulations showed that the DNA-binding affinity of TadA8e is higher than that of the original deaminase TadA7.10 in ABE7.10 and is mainly driven by electrostatic interactions. The directed-evolution mutations increase the positive charge density in the DNA-binding region, thereby enhancing the electrostatic attraction of TadA8e to DNA. We identified R111, N119 and N167 as the key mutations for the enhanced DNA binding and confirmed them by microscale thermophoresis (MST) and in vivo reversion mutation experiments. Unexpectedly, we also found that the directed mutations improved the thermal stability of TadA8e by ~ 12 °C (Tm, melting temperature) and that of ABE8e by ~ 9 °C, respectively. Our results demonstrate that the directed-evolution mutations improve the substrate-binding ability and protein stability of ABE8e, thus providing a rational basis for further editing optimisation of the system.
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Affiliation(s)
- Haixia Zhu
- State Key Laboratory of Genetic Engineering, Shanghai Engineering Research Center of Industrial Microorganisms, MOE Engineering Research Center of Gene Technology, School of Life Sciences, Fudan University, Shanghai, 200438, China
| | - Lei Wang
- State Key Laboratory of Genetic Engineering, Shanghai Engineering Research Center of Industrial Microorganisms, MOE Engineering Research Center of Gene Technology, School of Life Sciences, Fudan University, Shanghai, 200438, China
| | - Ying Wang
- State Key Laboratory of Genetic Engineering, Shanghai Engineering Research Center of Industrial Microorganisms, MOE Engineering Research Center of Gene Technology, School of Life Sciences, Fudan University, Shanghai, 200438, China
| | - Xinyi Jiang
- State Key Laboratory of Genetic Engineering, Shanghai Engineering Research Center of Industrial Microorganisms, MOE Engineering Research Center of Gene Technology, School of Life Sciences, Fudan University, Shanghai, 200438, China
| | - Qin Qin
- State Key Laboratory of Genetic Engineering, Shanghai Engineering Research Center of Industrial Microorganisms, MOE Engineering Research Center of Gene Technology, School of Life Sciences, Fudan University, Shanghai, 200438, China
| | - Menghua Song
- State Key Laboratory of Genetic Engineering, Shanghai Engineering Research Center of Industrial Microorganisms, MOE Engineering Research Center of Gene Technology, School of Life Sciences, Fudan University, Shanghai, 200438, China
| | - Qiang Huang
- State Key Laboratory of Genetic Engineering, Shanghai Engineering Research Center of Industrial Microorganisms, MOE Engineering Research Center of Gene Technology, School of Life Sciences, Fudan University, Shanghai, 200438, China.
- Multiscale Research Institute of Complex Systems, Fudan University, Shanghai, 201203, China.
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2
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Medina-Munoz HC, Kofman E, Jagannatha P, Boyle EA, Yu T, Jones KL, Mueller JR, Lykins GD, Doudna AT, Park SS, Blue SM, Ranzau BL, Kohli RM, Komor AC, Yeo GW. Expanded palette of RNA base editors for comprehensive RBP-RNA interactome studies. Nat Commun 2024; 15:875. [PMID: 38287010 PMCID: PMC10825223 DOI: 10.1038/s41467-024-45009-4] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/03/2023] [Accepted: 01/03/2024] [Indexed: 01/31/2024] Open
Abstract
RNA binding proteins (RBPs) are key regulators of RNA processing and cellular function. Technologies to discover RNA targets of RBPs such as TRIBE (targets of RNA binding proteins identified by editing) and STAMP (surveying targets by APOBEC1 mediated profiling) utilize fusions of RNA base-editors (rBEs) to RBPs to circumvent the limitations of immunoprecipitation (CLIP)-based methods that require enzymatic digestion and large amounts of input material. To broaden the repertoire of rBEs suitable for editing-based RBP-RNA interaction studies, we have devised experimental and computational assays in a framework called PRINTER (protein-RNA interaction-based triaging of enzymes that edit RNA) to assess over thirty A-to-I and C-to-U rBEs, allowing us to identify rBEs that expand the characterization of binding patterns for both sequence-specific and broad-binding RBPs. We also propose specific rBEs suitable for dual-RBP applications. We show that the choice between single or multiple rBEs to fuse with a given RBP or pair of RBPs hinges on the editing biases of the rBEs and the binding preferences of the RBPs themselves. We believe our study streamlines and enhances the selection of rBEs for the next generation of RBP-RNA target discovery.
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Affiliation(s)
- Hugo C Medina-Munoz
- Department of Cellular and Molecular Medicine, University of California San Diego, La Jolla, CA, USA
- Stem Cell Program, University of California San Diego, La Jolla, CA, USA
- Institute for Genomic Medicine, University of California San Diego, La Jolla, CA, USA
| | - Eric Kofman
- Department of Cellular and Molecular Medicine, University of California San Diego, La Jolla, CA, USA
- Stem Cell Program, University of California San Diego, La Jolla, CA, USA
- Institute for Genomic Medicine, University of California San Diego, La Jolla, CA, USA
- Bioinformatics and Systems Biology Program, University of California San Diego, La Jolla, CA, USA
| | - Pratibha Jagannatha
- Department of Cellular and Molecular Medicine, University of California San Diego, La Jolla, CA, USA
- Stem Cell Program, University of California San Diego, La Jolla, CA, USA
- Institute for Genomic Medicine, University of California San Diego, La Jolla, CA, USA
- Bioinformatics and Systems Biology Program, University of California San Diego, La Jolla, CA, USA
| | - Evan A Boyle
- Department of Cellular and Molecular Medicine, University of California San Diego, La Jolla, CA, USA
- Stem Cell Program, University of California San Diego, La Jolla, CA, USA
- Institute for Genomic Medicine, University of California San Diego, La Jolla, CA, USA
| | - Tao Yu
- Department of Cellular and Molecular Medicine, University of California San Diego, La Jolla, CA, USA
- Stem Cell Program, University of California San Diego, La Jolla, CA, USA
- Institute for Genomic Medicine, University of California San Diego, La Jolla, CA, USA
| | - Krysten L Jones
- Department of Cellular and Molecular Medicine, University of California San Diego, La Jolla, CA, USA
- Stem Cell Program, University of California San Diego, La Jolla, CA, USA
- Institute for Genomic Medicine, University of California San Diego, La Jolla, CA, USA
| | - Jasmine R Mueller
- Department of Cellular and Molecular Medicine, University of California San Diego, La Jolla, CA, USA
- Stem Cell Program, University of California San Diego, La Jolla, CA, USA
- Institute for Genomic Medicine, University of California San Diego, La Jolla, CA, USA
| | - Grace D Lykins
- Department of Cellular and Molecular Medicine, University of California San Diego, La Jolla, CA, USA
- Stem Cell Program, University of California San Diego, La Jolla, CA, USA
- Institute for Genomic Medicine, University of California San Diego, La Jolla, CA, USA
| | - Andrew T Doudna
- Department of Cellular and Molecular Medicine, University of California San Diego, La Jolla, CA, USA
- Stem Cell Program, University of California San Diego, La Jolla, CA, USA
- Institute for Genomic Medicine, University of California San Diego, La Jolla, CA, USA
| | - Samuel S Park
- Department of Cellular and Molecular Medicine, University of California San Diego, La Jolla, CA, USA
- Stem Cell Program, University of California San Diego, La Jolla, CA, USA
- Institute for Genomic Medicine, University of California San Diego, La Jolla, CA, USA
| | - Steven M Blue
- Department of Cellular and Molecular Medicine, University of California San Diego, La Jolla, CA, USA
- Stem Cell Program, University of California San Diego, La Jolla, CA, USA
- Institute for Genomic Medicine, University of California San Diego, La Jolla, CA, USA
| | - Brodie L Ranzau
- Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, CA, 92093, USA
| | - Rahul M Kohli
- Department of Medicine, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA, USA
- Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA, USA
| | - Alexis C Komor
- Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, CA, 92093, USA
| | - Gene W Yeo
- Department of Cellular and Molecular Medicine, University of California San Diego, La Jolla, CA, USA.
- Stem Cell Program, University of California San Diego, La Jolla, CA, USA.
- Institute for Genomic Medicine, University of California San Diego, La Jolla, CA, USA.
- Bioinformatics and Systems Biology Program, University of California San Diego, La Jolla, CA, USA.
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3
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Medina-Munoz HC, Kofman E, Jagannatha P, Boyle EA, Yu T, Jones KL, Mueller JR, Lykins GD, Doudna AT, Park SS, Blue SM, Ranzau BL, Kohli RM, Komor AC, Yeo GW. Expanded palette of RNA base editors for comprehensive RBP-RNA interactome studies. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2023:2023.09.25.558915. [PMID: 37808757 PMCID: PMC10557582 DOI: 10.1101/2023.09.25.558915] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/10/2023]
Abstract
RNA binding proteins (RBPs) are key regulators of RNA processing and cellular function. Technologies to discover RNA targets of RBPs such as TRIBE (targets of RNA binding proteins identified by editing) and STAMP (surveying targets by APOBEC1 mediated profiling) utilize fusions of RNA base-editors (rBEs) to RBPs to circumvent the limitations of immunoprecipitation (CLIP)-based methods that require enzymatic digestion and large amounts of input material. To broaden the repertoire of rBEs suitable for editing-based RBP-RNA interaction studies, we have devised experimental and computational assays in a framework called PRINTER (protein-RNA interaction-based triaging of enzymes that edit RNA) to assess over thirty A-to-I and C-to-U rBEs, allowing us to identify rBEs that expand the characterization of binding patterns for both sequence-specific and broad-binding RBPs. We also propose specific rBEs suitable for dual-RBP applications. We show that the choice between single or multiple rBEs to fuse with a given RBP or pair of RBPs hinges on the editing biases of the rBEs and the binding preferences of the RBPs themselves. We believe our study streamlines and enhances the selection of rBEs for the next generation of RBP-RNA target discovery.
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Affiliation(s)
- Hugo C. Medina-Munoz
- Department of Cellular and Molecular Medicine, University of California San Diego, La Jolla, CA, USA
- Stem Cell Program, University of California San Diego, La Jolla, CA, USA
- Institute for Genomic Medicine, University of California San Diego, La Jolla, CA, USA
| | - Eric Kofman
- Department of Cellular and Molecular Medicine, University of California San Diego, La Jolla, CA, USA
- Stem Cell Program, University of California San Diego, La Jolla, CA, USA
- Institute for Genomic Medicine, University of California San Diego, La Jolla, CA, USA
- Bioinformatics and Systems Biology Program, University of California San Diego, La Jolla, CA, USA
| | - Pratibha Jagannatha
- Department of Cellular and Molecular Medicine, University of California San Diego, La Jolla, CA, USA
- Stem Cell Program, University of California San Diego, La Jolla, CA, USA
- Institute for Genomic Medicine, University of California San Diego, La Jolla, CA, USA
- Bioinformatics and Systems Biology Program, University of California San Diego, La Jolla, CA, USA
| | - Evan A. Boyle
- Department of Cellular and Molecular Medicine, University of California San Diego, La Jolla, CA, USA
- Stem Cell Program, University of California San Diego, La Jolla, CA, USA
- Institute for Genomic Medicine, University of California San Diego, La Jolla, CA, USA
| | - Tao Yu
- Department of Cellular and Molecular Medicine, University of California San Diego, La Jolla, CA, USA
- Stem Cell Program, University of California San Diego, La Jolla, CA, USA
- Institute for Genomic Medicine, University of California San Diego, La Jolla, CA, USA
| | - Krysten L. Jones
- Department of Cellular and Molecular Medicine, University of California San Diego, La Jolla, CA, USA
- Stem Cell Program, University of California San Diego, La Jolla, CA, USA
- Institute for Genomic Medicine, University of California San Diego, La Jolla, CA, USA
| | - Jasmine R. Mueller
- Department of Cellular and Molecular Medicine, University of California San Diego, La Jolla, CA, USA
- Stem Cell Program, University of California San Diego, La Jolla, CA, USA
- Institute for Genomic Medicine, University of California San Diego, La Jolla, CA, USA
| | - Grace D. Lykins
- Department of Cellular and Molecular Medicine, University of California San Diego, La Jolla, CA, USA
- Stem Cell Program, University of California San Diego, La Jolla, CA, USA
- Institute for Genomic Medicine, University of California San Diego, La Jolla, CA, USA
| | - Andrew T. Doudna
- Department of Cellular and Molecular Medicine, University of California San Diego, La Jolla, CA, USA
- Stem Cell Program, University of California San Diego, La Jolla, CA, USA
- Institute for Genomic Medicine, University of California San Diego, La Jolla, CA, USA
| | - Samuel S. Park
- Department of Cellular and Molecular Medicine, University of California San Diego, La Jolla, CA, USA
- Stem Cell Program, University of California San Diego, La Jolla, CA, USA
- Institute for Genomic Medicine, University of California San Diego, La Jolla, CA, USA
| | - Steven M. Blue
- Department of Cellular and Molecular Medicine, University of California San Diego, La Jolla, CA, USA
- Stem Cell Program, University of California San Diego, La Jolla, CA, USA
- Institute for Genomic Medicine, University of California San Diego, La Jolla, CA, USA
| | - Brodie L. Ranzau
- Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, CA 92093, USA
| | - Rahul M. Kohli
- Department of Medicine, Perelman School of Medicine, University of Pennsylvania Philadelphia, PA, USA
- Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA, USA
| | - Alexis C. Komor
- Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, CA 92093, USA
| | - Gene W. Yeo
- Department of Cellular and Molecular Medicine, University of California San Diego, La Jolla, CA, USA
- Stem Cell Program, University of California San Diego, La Jolla, CA, USA
- Institute for Genomic Medicine, University of California San Diego, La Jolla, CA, USA
- Bioinformatics and Systems Biology Program, University of California San Diego, La Jolla, CA, USA
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4
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Chen X, McAndrew MJ, Lapinaite A. Unlocking the secrets of ABEs: the molecular mechanism behind their specificity. Biochem Soc Trans 2023; 51:1635-1646. [PMID: 37526140 PMCID: PMC10586758 DOI: 10.1042/bst20221508] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/13/2023] [Revised: 07/13/2023] [Accepted: 07/18/2023] [Indexed: 08/02/2023]
Abstract
CRISPR-Cas, the bacterial immune systems, have transformed the field of genome editing by providing efficient, easily programmable, and accessible tools for targeted genome editing. DNA base editors (BE) are state-of-the-art CRISPR-based technology, allowing for targeted modifications of individual nucleobases within the genome. Among the BEs, adenine base editors (ABEs) have shown great potential due to their ability to convert A-to-G with high efficiency. However, current ABEs have limitations in terms of their specificity and targeting range. In this review, we provide an overview of the molecular mechanism of ABEs, with a focus on the mechanism of deoxyadenosine deamination by evolved tRNA-specific adenosine deaminase (TadA). We discuss how mutations and adjustments introduced via both directed evolution as well as rational design have improved ABE efficiency and specificity. This review offers insights into the molecular mechanism of ABEs, providing a roadmap for future developments in the precision genome editing field.
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Affiliation(s)
- Xiaoyu Chen
- School of Molecular Sciences, Arizona State University, Tempe, AZ, U.S.A
| | | | - Audrone Lapinaite
- School of Molecular Sciences, Arizona State University, Tempe, AZ, U.S.A
- Arizona State University-Banner Neurodegenerative Disease Research Center at the Biodesign Institute, Arizona State University, Tempe, AZ, U.S.A
- Center for Molecular Design and Biomimetics, The Biodesign Institute, Arizona State University, Tempe, AZ, U.S.A
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5
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Ranzau BL, Rallapalli KL, Evanoff M, Paesani F, Komor AC. The Wild-Type tRNA Adenosine Deaminase Enzyme TadA Is Capable of Sequence-Specific DNA Base Editing. Chembiochem 2023; 24:e202200788. [PMID: 36947856 PMCID: PMC10514239 DOI: 10.1002/cbic.202200788] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/28/2022] [Revised: 03/11/2023] [Accepted: 03/22/2023] [Indexed: 03/24/2023]
Abstract
Base editors are genome editing tools that enable site-specific base conversions through the chemical modification of nucleobases in DNA. Adenine base editors (ABEs) convert A ⋅ T to G ⋅ C base pairs in DNA by using an adenosine deaminase enzyme to modify target adenosines to inosine intermediates. Due to the lack of a naturally occurring adenosine deaminase that can modify DNA, ABEs were evolved from a tRNA-deaminating enzyme, TadA. Previous experiments with an ABE comprising a wild-type (wt) TadA showed no detectable activity on DNA, and directed evolution was therefore required to enable this enzyme to accept DNA as a substrate. Here we show that wtTadA can perform base editing in DNA in both bacterial and mammalian cells, with a strict sequence motif requirement of TAC. We leveraged this discovery to optimize a reporter assay to detect base editing levels as low as 0.01 %. Finally, we used this assay along with molecular dynamics simulations of full ABE:DNA complexes to better understand how the sequence recognition of mutant TadA variants change as they accumulate mutations to better edit DNA substrates.
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Affiliation(s)
- Brodie L. Ranzau
- Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, CA 92093, USA
| | - Kartik L. Rallapalli
- Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, CA 92093, USA
| | - Mallory Evanoff
- Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, CA 92093, USA
| | - Francesco Paesani
- Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, CA 92093, USA
- Halıcıoğlu Data Science Institute, University of California San Diego, La Jolla, California 92093, USA
- Materials Science and Engineering, University of California San Diego, La Jolla, California 92093, USA
- San Diego Supercomputer Center, University of California San Diego, La Jolla, California 92093, USA
| | - Alexis C. Komor
- Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, CA 92093, USA
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6
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Gao Z, Jiang W, Zhang Y, Zhang L, Yi M, Wang H, Ma Z, Qu B, Ji X, Long H, Zhang S. Amphioxus adenosine-to-inosine tRNA-editing enzyme that can perform C-to-U and A-to-I deamination of DNA. Commun Biol 2023; 6:744. [PMID: 37464027 PMCID: PMC10354150 DOI: 10.1038/s42003-023-05134-0] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/04/2022] [Accepted: 07/11/2023] [Indexed: 07/20/2023] Open
Abstract
Adenosine-to-inosine tRNA-editing enzyme has been identified for more than two decades, but the study on its DNA editing activity is rather scarce. We show that amphioxus (Branchiostoma japonicum) ADAT2 (BjADAT2) contains the active site 'HxE-PCxxC' and the key residues for target-base-binding, and amphioxus ADAT3 (BjADAT3) harbors both the N-terminal positively charged region and the C-terminal pseudo-catalytic domain important for recognition of substrates. The sequencing of BjADAT2-transformed Escherichia coli genome suggests that BjADAT2 has the potential to target E. coli DNA and can deaminate at TCG and GAA sites in the E. coli genome. Biochemical analyses further demonstrate that BjADAT2, in complex with BjADAT3, can perform A-to-I editing of tRNA and convert C-to-U and A-to-I deamination of DNA. We also show that BjADAT2 preferentially deaminates adenosines and cytidines in the loop of DNA hairpin structures of substrates, and BjADAT3 also affects the type of DNA substrate targeted by BjADAT2. Finally, we find that C89, N113, C148 and Y156 play critical roles in the DNA editing activity of BjADAT2. Collectively, our study indicates that BjADAT2/3 is the sole naturally occurring deaminase with both tRNA and DNA editing capacity identified so far in Metazoa.
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Affiliation(s)
- Zhan Gao
- Institute of Evolution & Marine Biodiversity and Department of Marine Biology, Ocean University of China, 266003, Qingdao, China.
| | - Wanyue Jiang
- Institute of Evolution & Marine Biodiversity, KLMME, Ocean University of China, 266003, Qingdao, China
| | - Yu Zhang
- Institute of Evolution & Marine Biodiversity and Department of Marine Biology, Ocean University of China, 266003, Qingdao, China
| | - Liping Zhang
- Institute of Evolution & Marine Biodiversity and Department of Marine Biology, Ocean University of China, 266003, Qingdao, China
| | - Mengmeng Yi
- Institute of Evolution & Marine Biodiversity and Department of Marine Biology, Ocean University of China, 266003, Qingdao, China
| | - Haitao Wang
- Institute of Evolution & Marine Biodiversity and Department of Marine Biology, Ocean University of China, 266003, Qingdao, China
| | - Zengyu Ma
- Institute of Evolution & Marine Biodiversity and Department of Marine Biology, Ocean University of China, 266003, Qingdao, China
| | - Baozhen Qu
- Institute of Evolution & Marine Biodiversity and Department of Marine Biology, Ocean University of China, 266003, Qingdao, China
| | - Xiaohan Ji
- Institute of Evolution & Marine Biodiversity and Department of Marine Biology, Ocean University of China, 266003, Qingdao, China
| | - Hongan Long
- Institute of Evolution & Marine Biodiversity, KLMME, Ocean University of China, 266003, Qingdao, China
- Laboratory for Marine Biology and Biotechnology, Laoshan Laboratory, 266237, Qingdao, China
| | - Shicui Zhang
- Institute of Evolution & Marine Biodiversity and Department of Marine Biology, Ocean University of China, 266003, Qingdao, China.
- Laboratory for Marine Biology and Biotechnology, Laoshan Laboratory, 266237, Qingdao, China.
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7
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Abstract
DNA-editing enzymes perform chemical reactions on DNA nucleobases. These reactions can change the genetic identity of the modified base or modulate gene expression. Interest in DNA-editing enzymes has burgeoned in recent years due to the advent of clustered regularly interspaced short palindromic repeat-associated (CRISPR-Cas) systems, which can be used to direct their DNA-editing activity to specific genomic loci of interest. In this review, we showcase DNA-editing enzymes that have been repurposed or redesigned and developed into programmable base editors. These include deaminases, glycosylases, methyltransferases, and demethylases. We highlight the astounding degree to which these enzymes have been redesigned, evolved, and refined and present these collective engineering efforts as a paragon for future efforts to repurpose and engineer other families of enzymes. Collectively, base editors derived from these DNA-editing enzymes facilitate programmable point mutation introduction and gene expression modulation by targeted chemical modification of nucleobases.
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Affiliation(s)
- Kartik L Rallapalli
- Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California, USA;
| | - Alexis C Komor
- Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California, USA;
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8
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Mahdizadeh SJ, Pålsson E, Carlesso A, Chevet E, Eriksson LA. QM/MM Well-Tempered Metadynamics Study of the Mechanism of XBP1 mRNA Cleavage by Inositol Requiring Enzyme 1α RNase. J Chem Inf Model 2022; 62:4247-4260. [PMID: 35960929 PMCID: PMC9472280 DOI: 10.1021/acs.jcim.2c00735] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
A range of in silico methodologies were herein employed to study the unconventional XBP1 mRNA cleavage mechanism performed by the unfolded protein response (UPR) mediator Inositol Requiring Enzyme 1α (IRE1). Using Protein-RNA molecular docking along with a series of extensive restrained/unrestrained atomistic molecular dynamics (MD) simulations, the dynamical behavior of the system was evaluated and a reliable model of the IRE1/XBP1 mRNA complex was constructed. From a series of well-converged quantum mechanics molecular mechanics well-tempered metadynamics (QM/MM WT-MetaD) simulations using the Grimme dispersion interaction corrected semiempirical parametrization method 6 level of theory (PM6-D3) and the AMBER14SB-OL3 force field, the free energy profile of the cleavage mechanism was determined, along with intermediates and transition state structures. The results show two distinct reaction paths based on general acid-general base type mechanisms, with different activation energies that perfectly match observations from experimental mutagenesis data. The study brings unique atomistic insights into the cleavage mechanism of XBP1 mRNA by IRE1 and clarifies the roles of the catalytic residues H910 and Y892. Increased understanding of the details in UPR signaling can assist in the development of new therapeutic agents for its modulation.
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Affiliation(s)
- Sayyed Jalil Mahdizadeh
- Department of Chemistry and Molecular Biology, University of Gothenburg, 405 30 Göteborg, Sweden
| | - Emil Pålsson
- Department of Chemistry and Molecular Biology, University of Gothenburg, 405 30 Göteborg, Sweden
| | - Antonio Carlesso
- Department of Chemistry and Molecular Biology, University of Gothenburg, 405 30 Göteborg, Sweden.,Faculty of Biomedical Sciences, Euler Institute, Università della Svizzera Italiana (USI),, Lugano 6904, Switzerland.,Department of Pharmacology, Sahlgrenska Academy, University of Gothenburg, 405 30 Göteborg, Sweden
| | - Eric Chevet
- INSERM U1242, University of Rennes 1, 35000 Rennes, France.,Centre Eugène Marquis, 35000 Rennes, France
| | - Leif A Eriksson
- Department of Chemistry and Molecular Biology, University of Gothenburg, 405 30 Göteborg, Sweden
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9
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Rallapalli KL, Ranzau BL, Ganapathy KR, Paesani F, Komor AC. Combined Theoretical, Bioinformatic, and Biochemical Analyses of RNA Editing by Adenine Base Editors. CRISPR J 2022; 5:294-310. [PMID: 35353638 DOI: 10.1089/crispr.2021.0131] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
Adenine base editors (ABEs) have been subjected to multiple rounds of mutagenesis with the goal of optimizing their function as efficient and precise genome editing agents. Despite an ever-expanding data set of ABE mutants and their corresponding DNA or RNA-editing activity, the molecular mechanisms defining these changes remain to be elucidated. In this study, we provide a systematic interpretation of the nature of these mutations using an entropy-based classification model that relies on evolutionary data from extant protein sequences. Using this model in conjunction with experimental analyses, we identify two previously reported mutations that form an epistatic pair in the RNA-editing functional landscape of ABEs. Molecular dynamics simulations reveal the atomistic details of how these two mutations affect substrate-binding and catalytic activity, via both individual and cooperative effects, hence providing insights into the mechanisms through which these two mutations are epistatically coupled.
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Affiliation(s)
- Kartik L Rallapalli
- Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, California, USA; University of California San Diego, La Jolla, California, USA
| | - Brodie L Ranzau
- Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, California, USA; University of California San Diego, La Jolla, California, USA
| | - Kaushik R Ganapathy
- Halıcıoğlu Data Science Institute, University of California San Diego, La Jolla, California, USA; University of California San Diego, La Jolla, California, USA
| | - Francesco Paesani
- Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, California, USA; University of California San Diego, La Jolla, California, USA.,Materials Science and Engineering, University of California San Diego, La Jolla, California, USA; and University of California San Diego, La Jolla, California, USA.,San Diego Supercomputer Center, University of California San Diego, La Jolla, California, USA
| | - Alexis C Komor
- Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, California, USA; University of California San Diego, La Jolla, California, USA
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10
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Wang Q, Yang J, Zhong Z, Vanegas JA, Gao X, Kolomeisky AB. A general theoretical framework to design base editors with reduced bystander effects. Nat Commun 2021; 12:6529. [PMID: 34764246 PMCID: PMC8586357 DOI: 10.1038/s41467-021-26789-5] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/05/2021] [Accepted: 10/22/2021] [Indexed: 11/09/2022] Open
Abstract
Base editors (BEs) hold great potential for medical applications of gene therapy. However, high precision base editing requires BEs that can discriminate between the target base and multiple bystander bases within a narrow active window (4 - 10 nucleotides). Here, to assist in the design of these optimized editors, we propose a discrete-state stochastic approach to build an analytical model that explicitly evaluates the probabilities of editing the target base and bystanders. Combined with all-atom molecular dynamic simulations, our model reproduces the experimental data of A3A-BE3 and its variants for targeting the "TC" motif and bystander editing. Analyzing this approach, we propose several general principles that can guide the design of BEs with a reduced bystander effect. These principles are then applied to design a series of point mutations at T218 position of A3G-BEs to further reduce its bystander editing. We verify experimentally that the new mutations provide different levels of stringency on reducing the bystander editing at different genomic loci, which is consistent with our theoretical model. Thus, our study provides a computational-aided platform to assist in the scientifically-based design of BEs with reduced bystander effects.
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Affiliation(s)
- Qian Wang
- Hefei National Laboratory for Physical Sciences at the Microscale and Department of Physics, University of Science and Technology of China, Hefei, 230026, Anhui, China.
- Center for Theoretical Biological Physics, Rice University, Houston, TX, 77005, USA.
| | - Jie Yang
- Department of Chemical and Biomolecular Engineering, Rice University, Houston, TX, 77005, USA
| | - Zhicheng Zhong
- Hefei National Laboratory for Physical Sciences at the Microscale and Department of Physics, University of Science and Technology of China, Hefei, 230026, Anhui, China
| | - Jeffrey A Vanegas
- Department of Chemical and Biomolecular Engineering, Rice University, Houston, TX, 77005, USA
| | - Xue Gao
- Department of Chemical and Biomolecular Engineering, Rice University, Houston, TX, 77005, USA.
- Department of Bioengineering, Rice University, Houston, TX, 77005, USA.
- Department of Chemistry, Rice University, Houston, TX, 77005, USA.
| | - Anatoly B Kolomeisky
- Center for Theoretical Biological Physics, Rice University, Houston, TX, 77005, USA.
- Department of Chemical and Biomolecular Engineering, Rice University, Houston, TX, 77005, USA.
- Department of Chemistry, Rice University, Houston, TX, 77005, USA.
- Department of Physics and Astronomy, Rice University, Houston, TX, 77005, USA.
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11
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Pujol-Navarro N, Kubiak-Ossowska K, Ferro V, Mulheran P. Simulating Peptide Monolayer Formation: GnRH-I on Silica. Int J Mol Sci 2021; 22:ijms22115523. [PMID: 34073815 PMCID: PMC8197186 DOI: 10.3390/ijms22115523] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/16/2021] [Revised: 05/14/2021] [Accepted: 05/14/2021] [Indexed: 11/16/2022] Open
Abstract
Molecular dynamics (MD) simulations can provide a detailed view of molecule behaviour at an atomic level, which can be useful when attempting to interpret experiments or design new systems. The decapeptide gonadotrophin-releasing hormone I (GnRH-I) is known to control fertility in mammals for both sexes. It was previously shown that inoculation with silica nanoparticles (SiNPs) coated with GnRH-I makes an effective anti-fertility vaccine due to how the peptide adsorbs to the nanoparticle and is presented to the immune system. In this paper, we develop and employ a protocol to simulate the development of a GnRH-I peptide adlayer by allowing peptides to diffuse and adsorb in a staged series of trajectories. The peptides start the simulation in an immobile state in solution above the model silica surface, and are then released sequentially. This facile approach allows the adlayer to develop in a natural manner and appears to be quite versatile. We find that the GnRH-I adlayer tends to be sparse, with electrostatics dominating the interactions. The peptides are collapsed to the surface and are seemingly free to interact with additional solutes, supporting the interpretations of the GNRH-I/SiNP vaccine system.
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Affiliation(s)
- Neret Pujol-Navarro
- Department of Chemical and Process Engineering, University of Strathclyde, 75 Montrose Street, Glasgow G1 1XJ, UK;
- Strathclyde Institute of Pharmacy and Biomedical Sciences, University of Strathclyde, 161 Cathedral Street, Glasgow G4 0RE, UK;
- Correspondence:
| | - Karina Kubiak-Ossowska
- ARCHIE-WeSt, Department of Physics, University of Strathclyde, 107 Rottenrow East, Glasgow G4 0NG, UK;
| | - Valerie Ferro
- Strathclyde Institute of Pharmacy and Biomedical Sciences, University of Strathclyde, 161 Cathedral Street, Glasgow G4 0RE, UK;
| | - Paul Mulheran
- Department of Chemical and Process Engineering, University of Strathclyde, 75 Montrose Street, Glasgow G1 1XJ, UK;
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12
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Wilson CJ, Chang M, Karttunen M, Choy WY. KEAP1 Cancer Mutants: A Large-Scale Molecular Dynamics Study of Protein Stability. Int J Mol Sci 2021; 22:5408. [PMID: 34065616 PMCID: PMC8161161 DOI: 10.3390/ijms22105408] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/21/2021] [Revised: 05/11/2021] [Accepted: 05/13/2021] [Indexed: 12/30/2022] Open
Abstract
We have performed 280 μs of unbiased molecular dynamics (MD) simulations to investigate the effects of 12 different cancer mutations on Kelch-like ECH-associated protein 1 (KEAP1) (G333C, G350S, G364C, G379D, R413L, R415G, A427V, G430C, R470C, R470H, R470S and G476R), one of the frequently mutated proteins in lung cancer. The aim was to provide structural insight into the effects of these mutants, including a new class of ANCHOR (additionally NRF2-complexed hypomorph) mutant variants. Our work provides additional insight into the structural dynamics of mutants that could not be analyzed experimentally, painting a more complete picture of their mutagenic effects. Notably, blade-wise analysis of the Kelch domain points to stability as a possible target of cancer in KEAP1. Interestingly, structural analysis of the R470C ANCHOR mutant, the most prevalent missense mutation in KEAP1, revealed no significant change in structural stability or NRF2 binding site dynamics, possibly indicating an covalent modification as this mutant's mode of action.
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Affiliation(s)
- Carter J. Wilson
- Department of Biochemistry, The University of Western Ontario, 1151 Richmond Street, London, ON N6A 5C1, Canada; (C.J.W.); (M.C.)
- Department of Applied Mathematics, The University of Western Ontario, 1151 Richmond Street, London, ON N6A 5B7, Canada
| | - Megan Chang
- Department of Biochemistry, The University of Western Ontario, 1151 Richmond Street, London, ON N6A 5C1, Canada; (C.J.W.); (M.C.)
| | - Mikko Karttunen
- Department of Applied Mathematics, The University of Western Ontario, 1151 Richmond Street, London, ON N6A 5B7, Canada
- Department of Chemistry, The University of Western Ontario, 1151 Richmond Street, London, ON N6A 3K7, Canada
- Centre for Advanced Materials and Biomaterials Research, The University of Western Ontario, 1151 Richmond Street, London, ON N6A 5B7, Canada
| | - Wing-Yiu Choy
- Department of Biochemistry, The University of Western Ontario, 1151 Richmond Street, London, ON N6A 5C1, Canada; (C.J.W.); (M.C.)
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13
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Porto EM, Komor AC, Slaymaker IM, Yeo GW. Base editing: advances and therapeutic opportunities. Nat Rev Drug Discov 2020; 19:839-859. [PMID: 33077937 PMCID: PMC7721651 DOI: 10.1038/s41573-020-0084-6] [Citation(s) in RCA: 196] [Impact Index Per Article: 49.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 08/20/2020] [Indexed: 12/19/2022]
Abstract
Base editing - the introduction of single-nucleotide variants (SNVs) into DNA or RNA in living cells - is one of the most recent advances in the field of genome editing. As around half of known pathogenic genetic variants are due to SNVs, base editing holds great potential for the treatment of numerous genetic diseases, through either temporary RNA or permanent DNA base alterations. Recent advances in the specificity, efficiency, precision and delivery of DNA and RNA base editors are revealing exciting therapeutic opportunities for these technologies. We expect the correction of single point mutations will be a major focus of future precision medicine.
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Affiliation(s)
- Elizabeth M Porto
- Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA, USA
| | - Alexis C Komor
- Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA, USA.
| | - Ian M Slaymaker
- Synthetic Biology Department, Beam Therapeutics, Cambridge, MA, USA
| | - Gene W Yeo
- Department of Cellular and Molecular Medicine, University of California, San Diego, La Jolla, CA, USA
- Biomedical Sciences and Bioinformatics and Systems Biology Graduate Programs, University of California, San Diego, La Jolla, CA, USA
- Institute for Genomic Medicine, University of California, San Diego, La Jolla, CA, USA
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14
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Nguyen Tran MT, Mohd Khalid MKN, Wang Q, Walker JKR, Lidgerwood GE, Dilworth KL, Lisowski L, Pébay A, Hewitt AW. Engineering domain-inlaid SaCas9 adenine base editors with reduced RNA off-targets and increased on-target DNA editing. Nat Commun 2020; 11:4871. [PMID: 32978399 PMCID: PMC7519688 DOI: 10.1038/s41467-020-18715-y] [Citation(s) in RCA: 39] [Impact Index Per Article: 9.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/08/2020] [Accepted: 09/10/2020] [Indexed: 12/31/2022] Open
Abstract
Precision genome engineering has dramatically advanced with the development of CRISPR/Cas base editing systems that include cytosine base editors and adenine base editors (ABEs). Herein, we compare the editing profile of circularly permuted and domain-inlaid Cas9 base editors, and find that on-target editing is largely maintained following their intradomain insertion, but that structural permutation of the ABE can affect differing RNA off-target events. With this insight, structure-guided design was used to engineer an SaCas9 ABE variant (microABE I744) that has dramatically improved on-target editing efficiency and a reduced RNA-off target footprint compared to current N-terminal linked SaCas9 ABE variants. This represents one of the smallest AAV-deliverable Cas9-ABEs available, which has been optimized for robust on-target activity and RNA-fidelity based upon its stereochemistry. Off-target effects and the feasibility for AAV-mediated delivery are the major barriers impeding the clinical in vivo application of base editors. Here, the authors report the small size AAV-deliverable Cas9-ABE variant that has improved on-target editing efficiency and reduced RNA-off target footprint.
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Affiliation(s)
- Minh Thuan Nguyen Tran
- Menzies Institute for Medical Research, School of Medicine, University of Tasmania, Tasmania, Australia.
| | | | - Qi Wang
- Menzies Institute for Medical Research, School of Medicine, University of Tasmania, Tasmania, Australia
| | - Jacqueline K R Walker
- Menzies Institute for Medical Research, School of Medicine, University of Tasmania, Tasmania, Australia
| | - Grace E Lidgerwood
- Department of Surgery, The University of Melbourne, Victoria, Australia.,Department of Anatomy and Neuroscience, The University of Melbourne, Victoria, Australia
| | - Kimberley L Dilworth
- Translational Vectorology Research Unit, Children's Medical Research Institute, Faculty of Medicine and Health, The University of Sydney, Westmead, Australia
| | - Leszek Lisowski
- Translational Vectorology Research Unit, Children's Medical Research Institute, Faculty of Medicine and Health, The University of Sydney, Westmead, Australia.,Military Institute of Hygiene and Epidemiology, The Biological Threats Identification and Countermeasure Centre, Puławy, Poland
| | - Alice Pébay
- Department of Surgery, The University of Melbourne, Victoria, Australia.,Department of Anatomy and Neuroscience, The University of Melbourne, Victoria, Australia
| | - Alex W Hewitt
- Menzies Institute for Medical Research, School of Medicine, University of Tasmania, Tasmania, Australia.,Centre for Eye Research Australia, The University of Melbourne, Victoria, Australia
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15
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Lapinaite A, Knott GJ, Palumbo CM, Lin-Shiao E, Richter MF, Zhao KT, Beal PA, Liu DR, Doudna JA. DNA capture by a CRISPR-Cas9-guided adenine base editor. Science 2020; 369:566-571. [PMID: 32732424 DOI: 10.1126/science.abb1390] [Citation(s) in RCA: 90] [Impact Index Per Article: 22.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/03/2020] [Accepted: 05/27/2020] [Indexed: 12/22/2022]
Abstract
CRISPR-Cas-guided base editors convert A•T to G•C, or C•G to T•A, in cellular DNA for precision genome editing. To understand the molecular basis for DNA adenosine deamination by adenine base editors (ABEs), we determined a 3.2-angstrom resolution cryo-electron microscopy structure of ABE8e in a substrate-bound state in which the deaminase domain engages DNA exposed within the CRISPR-Cas9 R-loop complex. Kinetic and structural data suggest that ABE8e catalyzes DNA deamination up to ~1100-fold faster than earlier ABEs because of mutations that stabilize DNA substrates in a constrained, transfer RNA-like conformation. Furthermore, ABE8e's accelerated DNA deamination suggests a previously unobserved transient DNA melting that may occur during double-stranded DNA surveillance by CRISPR-Cas9. These results explain ABE8e-mediated base-editing outcomes and inform the future design of base editors.
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Affiliation(s)
- Audrone Lapinaite
- Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720, USA
| | - Gavin J Knott
- Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720, USA.,Monash Biomedicine Discovery Institute, Department of Biochemistry & Molecular Biology, Monash University, Victoria 3800, Australia
| | - Cody M Palumbo
- Department of Chemistry, University of California, Davis, CA 95616, USA
| | - Enrique Lin-Shiao
- Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720, USA
| | - Michelle F Richter
- Merkin Institute of Transformative Technologies in Healthcare, Broad Institute of Harvard and MIT, Cambridge, MA 02141, USA.,Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, USA.,Howard Hughes Medical Institute, Harvard University, Cambridge, MA 02138, USA
| | - Kevin T Zhao
- Merkin Institute of Transformative Technologies in Healthcare, Broad Institute of Harvard and MIT, Cambridge, MA 02141, USA.,Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, USA.,Howard Hughes Medical Institute, Harvard University, Cambridge, MA 02138, USA
| | - Peter A Beal
- Department of Chemistry, University of California, Davis, CA 95616, USA
| | - David R Liu
- Merkin Institute of Transformative Technologies in Healthcare, Broad Institute of Harvard and MIT, Cambridge, MA 02141, USA.,Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, USA.,Howard Hughes Medical Institute, Harvard University, Cambridge, MA 02138, USA
| | - Jennifer A Doudna
- Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720, USA. .,Innovative Genomics Institute, University of California, Berkeley, CA 94720, USA.,MBIB Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.,Department of Chemistry, University of California, Berkeley, CA 94720, USA.,Howard Hughes Medical Institute, University of California, Berkeley, CA 94720, USA.,Gladstone Institute of Data Science and Biotechnology, Gladstone Institutes, San Francisco, CA 94158, USA
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16
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Anzalone AV, Koblan LW, Liu DR. Genome editing with CRISPR-Cas nucleases, base editors, transposases and prime editors. Nat Biotechnol 2020; 38:824-844. [PMID: 32572269 DOI: 10.1038/s41587-020-0561-9] [Citation(s) in RCA: 1061] [Impact Index Per Article: 265.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/03/2020] [Accepted: 05/15/2020] [Indexed: 12/14/2022]
Abstract
The development of new CRISPR-Cas genome editing tools continues to drive major advances in the life sciences. Four classes of CRISPR-Cas-derived genome editing agents-nucleases, base editors, transposases/recombinases and prime editors-are currently available for modifying genomes in experimental systems. Some of these agents have also moved rapidly into the clinic. Each tool comes with its own capabilities and limitations, and major efforts have broadened their editing capabilities, expanded their targeting scope and improved editing specificity. We analyze key considerations when choosing genome editing agents and identify opportunities for future improvements and applications in basic research and therapeutics.
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Affiliation(s)
- Andrew V Anzalone
- Merkin Institute of Transformative Technologies in Healthcare, Broad Institute of Harvard and MIT, Cambridge, MA, USA.,Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA, USA.,Howard Hughes Medical Institute, Harvard University, Cambridge, MA, USA
| | - Luke W Koblan
- Merkin Institute of Transformative Technologies in Healthcare, Broad Institute of Harvard and MIT, Cambridge, MA, USA.,Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA, USA.,Howard Hughes Medical Institute, Harvard University, Cambridge, MA, USA
| | - David R Liu
- Merkin Institute of Transformative Technologies in Healthcare, Broad Institute of Harvard and MIT, Cambridge, MA, USA. .,Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA, USA. .,Howard Hughes Medical Institute, Harvard University, Cambridge, MA, USA.
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