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Prime editing for precise and highly versatile genome manipulation
Programmable gene-editing tools have transformed the life sciences and have shown potential for the treatment of genetic disease. Among the CRISPR–Cas technologies that can currently make targeted DNA changes in mammalian cells, prime editors offer an unusual combination of versatility, specificity and precision. Prime editors do not require double-strand DNA breaks and can make virtually any substitution, small insertion and small deletion within the DNA of living cells. Prime editing minimally requires a programmable nickase fused to a polymerase enzyme, and an extended guide RNA that both specifies the target site and templates the desired genome edit. In this Review, we summarize prime editing strategies to generate programmed genomic changes, highlight their limitations and recent developments that circumvent some of these bottlenecks, and discuss applications and future directions.In this Review, Chen and Liu discuss the latest developments in prime editing systems, including improvements to their editing efficiency and capabilities, as well as diverse emerging applications in research and preclinical therapeutic studies.
Journal Article
Base editing: precision chemistry on the genome and transcriptome of living cells
RNA-guided programmable nucleases from CRISPR systems generate precise breaks in DNA or RNA at specified positions. In cells, this activity can lead to changes in DNA sequence or RNA transcript abundance. Base editing is a newer genome-editing approach that uses components from CRISPR systems together with other enzymes to directly install point mutations into cellular DNA or RNA without making double-stranded DNA breaks. DNA base editors comprise a catalytically disabled nuclease fused to a nucleobase deaminase enzyme and, in some cases, a DNA glycosylase inhibitor. RNA base editors achieve analogous changes using components that target RNA. Base editors directly convert one base or base pair into another, enabling the efficient installation of point mutations in non-dividing cells without generating excess undesired editing by-products. In this Review, we summarize base-editing strategies to generate specific and precise point mutations in genomic DNA and RNA, highlight recent developments that expand the scope, specificity, precision and in vivo delivery of base editors and discuss limitations and future directions of base editing for research and therapeutic applications.
Journal Article
Rewritable multi-event analog recording in bacterial and mammalian cells
Recording cellular events could advance our understanding of cellular history and responses to stimuli. The construction of intracellular memory devices, however, is challenging. Tang and Liu used Cas9 nucleases and base editors to record amplitude, duration, and order of stimuli as stable changes in both genomic and extrachromosomal DNA content (see the Perspective by Ho and Bennett). The recording of multiple stimuli—including exposure to antibiotics, nutrients, viruses, and light, as well as Wnt signaling—was achieved in living bacterial and human cells. Recorded memories could be erased and re-recorded over multiple cycles.
Science
, this issue p.
eaap8992
; see also p.
150
Base editors and CRISPR nucleases generate “cell data recorders” that enable durable, analog, rewritable recording of multiple stimuli.
We present two CRISPR-mediated analog multi-event recording apparatus (CAMERA) systems that use base editors and Cas9 nucleases to record cellular events in bacteria and mammalian cells. The devices record signal amplitude or duration as changes in the ratio of mutually exclusive DNA sequences (CAMERA 1) or as single-base modifications (CAMERA 2). We achieved recording of multiple stimuli in bacteria or mammalian cells, including exposure to antibiotics, nutrients, viruses, light, and changes in Wnt signaling. When recording to multicopy plasmids, reliable readout requires as few as 10 to 100 cells. The order of stimuli can be recorded through an overlapping guide RNA design, and memories can be erased and re-recorded over multiple cycles. CAMERA systems serve as “cell data recorders” that write a history of endogenous or exogenous signaling events into permanent DNA sequence modifications in living cells.
Journal Article
Methods for the directed evolution of proteins
Key Points
Directed evolution is a cyclic process that alternates between gene diversification and screening for or selection of functional gene variants.
Library size limitations can be overcome by focusing library diversity on residues implicated by molecular structures, computational models or phylogenetic data. In cases in which there is limited information, random mutagenesis can be used to interrogate the uncertain determinants of protein function.
Recombination methodologies access new combinations of functional variation and can shuffle disparate genetic elements to yield new chimeric proteins.
Low-throughput screens can directly measure individual phenotypes and thus accurately isolate desired subpopulations. Screen throughput can be increased using indirect visible reporters that are strongly coupled to the desired phenotypes.
Selections isolate functional variants through selective replication schemes or physical segregation. Selections operate simultaneously on entire populations and thus offer unparalleled throughput.
Directed evolution uses laboratory-based evolution to enhance the properties of biomolecules, primarily to generate proteins with optimized or novel activities. This Review discusses the diverse range of technologies for the directed evolution of proteins, particularly methods for generating diversity in the gene library and approaches for screening and selecting for variants with desired properties. The relative strengths and limitations of these approaches are highlighted to guide readers to appropriate strategies.
Directed evolution has proved to be an effective strategy for improving or altering the activity of biomolecules for industrial, research and therapeutic applications. The evolution of proteins in the laboratory requires methods for generating genetic diversity and for identifying protein variants with desired properties. This Review describes some of the tools used to diversify genes, as well as informative examples of screening and selection methods that identify or isolate evolved proteins. We highlight recent cases in which directed evolution generated enzymatic activities and substrate specificities not known to exist in nature.
Journal Article
Genome editing with CRISPR–Cas nucleases, base editors, transposases and prime editors
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.A growing arsenal of CRISPR-based tools enables increasingly sophisticated genome editing applications.
Journal Article
Small-molecule discovery through DNA-encoded libraries
The development of bioactive small molecules as probes or drug candidates requires discovery platforms that enable access to chemical diversity and can quickly reveal new ligands for a target of interest. Within the past 15 years, DNA-encoded library (DEL) technology has matured into a widely used platform for small-molecule discovery, yielding a wide variety of bioactive ligands for many therapeutically relevant targets. DELs offer many advantages compared with traditional screening methods, including efficiency of screening, easily multiplexed targets and library selections, minimized resources needed to evaluate an entire DEL and large library sizes. This Review provides accounts of recently described small molecules discovered from DELs, including their initial identification, optimization and validation of biological properties including suitability for clinical applications.DNA-encoded library (DEL) technology is a powerful small-molecule discovery platform, offering many advantages over traditional screening methods. Here, Peterson and Liu provide an in-depth review of recent small molecules discovered through DELs, illustrating the versatility, efficiency and broad impact of this technology.
Journal Article
Fusion of catalytically inactive Cas9 to FokI nuclease improves the specificity of genome modification
A fusion of the FokI nuclease and a catalytically inactive Cas9 is a highly specific genome editing tool.
Genome editing by Cas9, which cleaves double-stranded DNA at a sequence programmed by a short single-guide RNA (sgRNA), can result in off-target DNA modification that may be detrimental in some applications. To improve DNA cleavage specificity, we generated fusions of catalytically inactive Cas9 and FokI nuclease (fCas9). DNA cleavage by fCas9 requires association of two fCas9 monomers that simultaneously bind target sites ∼15 or 25 base pairs apart. In human cells, fCas9 modified target DNA sites with >140-fold higher specificity than wild-type Cas9 and with an efficiency similar to that of paired Cas9 'nickases', recently engineered variants that cleave only one DNA strand per monomer. The specificity of fCas9 was at least fourfold higher than that of paired nickases at loci with highly similar off-target sites. Target sites that conform to the substrate requirements of fCas9 occur on average every 34 bp in the human genome, suggesting the versatility of this approach for highly specific genome-wide editing.
Journal Article