Duration: 3 min
Yue Zhao, Ph.D.
Senior Scientist
Dr. Yue Zhao earned his Ph.D. in Structural Biology from the University of
Science and Technology of China. He has been working as a Senior Protein Scientist at
GenScript for the past 3 years. His research primarily focuses on the structure and function
of proteins, including GPCRs and ion channels. Dr. Zhao has published his work in
prestigious journals, such as
Cell Research, PNAS, and Cell Discovery.
Background
Base editing, a revolutionary gene-editing
technology introduced by David Liu in 2016,
integrates CRISPR/Cas9 ’s DNA-targeting capability with a deaminase enzyme [1]. This
technique
allows for the precise conversion of one DNA base pair to another without generating double-strand
breaks (DSBs). Cytosine base editors (CBEs) enable C•G-to-T•A editing, while adenine base editors
(ABEs) facilitate A•T-to-G•C editing. Base editors unwind double-stranded DNA (dsDNA) to permit a
single-strand-specific deaminase to act on the DNA strand not paired with the guide RNA, leading to
the deamination of C or A nucleobases and subsequent base changes after DNA repair or replication.
The base editors have great potential as therapeutics for genetic diseases especially those caused
by a single point mutation [2]. There are still some limitations of base editors
including
off-target editing and the creation of unwanted indel formations. The lab-evolved TadA* in ABEs
exhibits high on-target activity, low off-target editing, and a compact size (166 amino acids). In
contrast, CBEs utilize larger cytidine deaminases (227 amino acids for rAPOBEC1), resulting in
higher off-target activity and lower efficiency [3]. Currently, no CBE matches the peak
activity of
leading ABEs like ABE8e, which limits the applications of CBEs.
Figure 1 DNA base editors for genome editing (from: biotech.ucdavis.edu)
Recent Progress
David R. Liu has previously described TadA-derived CBEs developed through directed evolution or
rational protein engineering. Although these TadA-derived CBEs exhibit low off-target editing, they may
still retain some A•T-to-G•C editing at certain positions, limiting their utility for therapeutic stop codon
installation [4]. Recently, David R. Liu reported the evolution and characterization of new CBE6
variants
with significantly improved C•G-to-T•A editing activity and virtually no residual A•T-to-G•C activity
[5].
This advancement involved evolving M13 bacteriophages in E. coli hosts, where fitness was linked to the
expression of the phage protein gIII. A selection circuit was used to promote C•G-to-T•A editing while
suppressing A•T-to-G•C editing. Multiple rounds of PACE and subsequent screening identified key mutations at
positions N46 and Y73, enhancing C•G-to-T•A editing efficiency and eliminating A•T-to-G•C activity. The
evolved CBE6 variants demonstrated high editing efficiency, specificity, and low off-target effects in both
bacterial and human cells. These editors offer highly efficient, cytosine-selective on-target editing with
minimal sequence context bias and low off-target editing, which is advantageous for applications such as
installing stop codons to reduce proteins associated with increased disease risk.
Future Directions and Challenges
Looking ahead, optimizing delivery systems to enhance specificity and reduce off-target
effects remains critical for translating these advancements into clinical applications. For
instance, developing novel delivery vectors based on nanoparticles or viral vectors tailored for
specific cell types can enhance the efficiency and safety of gene editing therapies. Additionally,
refining genome editing techniques to achieve higher editing efficiencies and lower rates of
off-target mutations is essential. Techniques such as base editing and prime editing continue to
evolve, offering more precise control over genomic modifications.
Moreover, navigating regulatory landscapes globally presents challenges in ensuring the ethical and
safe application of genome editing technologies. Developing
standardized protocols and guidelines
for clinical trials and therapeutic applications is crucial to gaining regulatory approval and
public acceptance.
Figure 2 Schematic of the evolution of a cytosine base editor from a TadA-derived dual
base editor [3]
Application
The evolved cytosine base editor (CBE6) holds significant potential in gene therapy and genetic
research. It offers precise C • G - to - T • A base editing with high efficiency and specificity, making it
valuable for various applications:
- Correction of genetic mutations: Fixing point mutations in the HBB gene responsible for
beta-thalassemia.
- Gene knockout studies: Introducing stop codons into target genes to effectively knock
out gene function.
- Antiviral strategies: Exploring CBE6 as a potential tool to disrupt the HIV genome
integrated into host cells.
- Cancer research: Using CBE6 to introduce specific mutations into cancer cell lines to
study their role in cancer.
Prospect
The future of Cytosine Base Editors (CBEs) and Adenine Base Editors (ABEs) is promising for
both research and therapeutic applications. These tools provide precise genome editing to correct point
mutations, making them crucial for treating genetic disorders. Advances in delivery methods and editing
efficiency are expected to expand their clinical use and facilitate personalized medicine. Additionally,
their application in agriculture could lead to more resilient and nutritionally enhanced crops. Ongoing
research to minimize off-target effects will further enhance the safety and efficacy of these technologies,
driving transformative breakthroughs in medicine and biotechnology.
GenScript offers expert E. coli protein expression and purification services, delivering rapid and high-quality production of base editors to accelerate research and downstream applications.
References
[1] Komor AC, Kim YB, Packer MS, Zuris JA, Liu DR. Programmable editing of a target base in genomic DNA without double-stranded DNA cleavage. Nature. 2016 May 19;533(7603):420-4. doi: 10.1038/nature17946.
[2] Gaudelli, N. M. et al. Programmable base editing of A•T to G•C in genomic DNA without DNA cleavage. Nature. 2017 Nov 23;551(7681):464-471. doi: 10.1038/nature24644.
[3] Neugebauer, M. E. et al. Evolution of an adenine base editor into a small, efficient cytosine base editor with low off-target activity. Nat Biotechnol. 2023 May;41(5):673-685. doi: 10.1038/s41587-022-01533-6.
[4] Chen, L. et al. Re-engineering the adenine deaminase TadA-8e for efficient and specific CRISPR-based cytosine base editing. Nat Biotechnol. 2023 May;41(5):663-672. doi: 10.1038/s41587-022-01532-7.
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