Comprehensive Notes: Targeted Genome Editing & DNA Repair (Transcript Summary)

Targeted Genome Editing (Overview)

  • Targeted genome editing is a powerful tool for studying gene function, correcting defective genes, or introducing new functionality.
  • Core mechanism: sequence-specific breaks in target DNA, with edits incorporated during the repair process.
  • Applications include:
    • Gene knockout
    • Repression or activation control of gene expression
    • Drug discovery screens
    • Imaging genomic loci
  • All genome editing systems rely on sequence-specific cleavage by endonucleases.
  • Two mechanisms for sequence targeting:
    • DNA-guided systems
    • RNA-guided systems
  • Common DNA-guided systems: Meganucleases (MNs), Zinc Finger Nucleases (ZFNs), TALENs (Transcription Activator-Like Effector Nucleases).
  • Common RNA-guided system: CRISPR/Cas9 (Cas9 is the endonuclease).
  • Overall takeaway: editing relies on creating breaks and leveraging cellular repair to incorporate edits.

DNA Damage and DNA Repair

  • DNA is the basic unit of inheritance and must remain intact; however, it is constantly exposed to damaging agents and replication/repair errors.
  • Cells possess dedicated repair mechanisms to recognize and fix errors, restoring original sequence and protecting genetic integrity.
  • Efficient DNA repair is essential to prevent mutations and maintain healthy cells.

Sources of DNA Damage

  • Internal and external factors damage DNA; main sources include:
    • Replication stress leading to base mismatches during cell division
    • Reactive oxygen species (ROS), ionizing radiation, and chemotherapeutics causing single- or double-strand breaks
    • Ionizing radiation and chemotherapeutics can also create crosslinks between DNA strands
    • UV light and chemical pollutants cause bulky adducts and crosslinks within strands
  • These damages disrupt DNA structure and genetic information.

DNA Damage Response and Repair Pathways

  • Cells employ multiple systems to cope with DNA damage:
    • DNA repair mechanisms
    • Damage tolerance pathways
    • Cell cycle checkpoints
    • Cell death pathways
  • Understanding DNA damage and repair mechanisms is essential for linking DNA damage to cellular function, disease development, and therapeutic strategies.

What is DNA Damage?

  • DNA damage refers to changes/disruptions in the DNA molecule caused by environmental factors or normal cellular processes.
  • DNA strand breaks come in two main types:
    • Single-strand breaks (SSBs): one strand cut
    • Double-strand breaks (DSBs): both strands cut
  • Causes include ionizing radiation (X-rays, gamma rays) and certain chemicals.

What is DNA Repair?

  • DNA damage is common and can disrupt cellular processes; cells have evolved repair mechanisms to maintain genome integrity.
  • Repair pathways are chosen according to damage type:
    • Base Excision Repair (BER): fixes small, single-base lesions
    • Nucleotide Excision Repair (NER): removes bulky lesions and crosslinks
    • Mismatch Repair (MMR): corrects replication errors
    • Homologous Recombination (HR) and Non-Homologous End Joining (NHEJ): repair DSBs

Double-Stranded Breaks (DSBs)

  • DSBs are the most cytotoxic DNA damage.
  • If unrepaired, DSBs have deleterious consequences, including loss of chromosome segments, apoptosis, and blocking replication.

Cell Cycle Context for DSB Repair

  • Homologous recombination (HR) is possible in later cell-cycle phases when a sister chromatid is available (S and G2, and M).
  • In G1, homologous recombination is not possible due to absence of a sister chromatid; NHEJ is the main repair pathway.
  • Go/Quiescent phase: repair may be limited; chromosomal maintenance still essential.

Non-Homologous End Joining (NHEJ)

  • Key proteins: Ku70/Ku80, DNA-PKcs, LIG4/XRCC4 (and XLF).
    • Ku70/Ku80 bind DNA ends, protecting them and preventing recombination.
    • DNA-PKcs and LIG4/XRCC4 promote end joining.
  • Features:
    • Fast repair
    • Often error-prone, can introduce insertions/deletions (indels)
    • Direct ligation of broken ends without a homologous template
    • Can occur throughout the cell cycle

NHEJ Repair Mechanism (Step-by-Step)

1) Ku and DNA-PK binding:

  • Ku heterodimer binds broken DNA ends.
  • DNA-PKcs is recruited to form the DNA-PK complex.
    2) Approximation:
  • DNA-PK bridges ends to bring them into proximity for repair.
    3) Unwinding and Processing:
  • Ends are unwound and processed to create compatible ends; damaged nucleotides may be removed.
    4) Alignment and Base Pairing:
  • Processed ends are aligned via microhomology to enable base pairing.
    5) Ligation:
  • DNA ligase IV (with XRCC4 and XLF) seals the break and restores the backbone.

Recombinational Repair (HR)

  • HR uses a homologous sequence as a template to restore the correct genetic information after DSBs.
  • HR is precise and typically uses the sister chromatid as the repair template.
  • Key protein players include Rad51, Rad52, Rad54, Rad55, Rad57, Rad59; BRCA1/BRCA2 interact with Rad51 and regulate it.
  • HR is facilitated by47 RecA-like recombinases in eukaryotes and prokaryotes (e.g., Rad51 corresponds to RecA function).

HR Process (General Outline)

1) Nucleolytic processing and nucleoprotein filament formation:

  • MRN complex (MRE11-RAD50-NBS1) initiates resection at DSB ends.
  • ATM kinase is activated and coordinates the DNA damage response.
  • RPA coats ssDNA; BRCA1/2, RAD52 assist; RAD51 replaces RPA to form a nucleoprotein filament on 3' overhangs.
    2) Homology search and joint molecule formation:
  • RAD51-ssDNA filament invades a homologous duplex (usually the sister chromatid), forming a D-loop; RAD54 and RAD52 aid.
    3) Strand elongation:
  • DNA polymerase extends the invading strand using the homologous template.
    4) Base pairing with the other damaged strand:
  • Extended strand pairs with the other 3' overhang.
    5) Gap filling and ligation:
  • DNA polymerase fills remaining gaps; DNA ligase seals nicks to complete repair.

Comparison: NHEJ vs HR (Summary)

  • NHEJ: No template required; often error-prone; active throughout cell cycle; uses Ku70/80, DNA-PKcs, XRCC4, Ligase IV; direct ligation of ends; faster.
  • HR: Requires homologous DNA template; high-fidelity repair; active mainly in S and G2 with sister chromatid; backbone involves Rad51, BRCA1/2, BRCA-related mediators; invasion and template-guided repair.
  • Overall: NHEJ is quick and versatile but riskier for indels; HR is precise but restricted to phases with a sister chromatid and longer repair time.

Zinc Finger Nucleases (ZFNs)

  • ZFNs are chimeric proteins: a DNA-binding domain of zinc finger motifs fused to a FokI endonuclease domain.
  • Each zinc finger recognizes a 3-nucleotide sequence; typically 3–6 fingers guide specificity.
  • FokI nuclease requires dimerization to cleave DNA.
  • ZFN specificity depends on finger sequence, finger number, and nuclease interaction.
  • Endonucleases cleave to induce site-specific DSBs.
  • Common endonucleases in genome editing include ZFNs, TALENs, and CRISPR/Cas9.

Transcription Activator-Like Effector Nucleases (TALENs)

  • TALENs are chimeric proteins with a TALE DNA-binding domain fused to a FokI nuclease domain.
  • DNA-binding domain consists of repeats (33–35 aa); each repeat recognizes a single base pair via a Repeat Variable Di-residue (RVD).
  • FokI is a nuclease that must dimerize to cleave DNA.
  • TALENs are easier to engineer than ZFNs because specificity is determined by only two amino acids in each repeat.
  • TALENs and ZFNs both rely on two arms (left and right) to create a DSB in the spacer between binding sites.

FokI Endonuclease (Background)

  • FokI is a Type IIS restriction endonuclease originally from Flavobacterium okeanokoites.
  • Recognizes the asymmetric sequence 5'-GGATG-3' and cuts at a defined distance from the recognition site.
  • Has two domains: a DNA-binding domain and a nuclease domain that cleaves DNA.
  • Cleavage yields 4-base 5' overhangs.
  • Nuclease activity is activated upon dimerization of two FokI monomers.
  • Used as the nuclease domain in ZFN and TALEN architectures.

TALENs: Mechanism, Construction, and Design

  • Target sequence recognition:
    • Two TALENs (left and right) bind opposite DNA strands, each recognizing ~15–20 bp via TALE repeats.
    • The repeats recognize bases through RVDs.
  • FokI nuclease binding and dimerization:
    • Each TALEN arm provides a FokI nuclease domain.
    • When both arms bind, FokI dimerizes and becomes active, cleaving the spacer region.
  • DSB induction:
    • The dimerized FokI cleaves to produce a DSB within the spacer.
  • DNA repair pathway activation:
    • DSB triggers cellular repair mechanisms, predominantly NHEJ (for knockouts) or HDR (with a donor template for precise edits).

TALENs: Pros, Cons, and Design Rules

  • Pros:
    • High specificity due to modular recognition; relatively easy design compared to ZFNs.
    • Smaller size makes delivery easier (e.g., viral vectors).
  • Cons:
    • Repeats are technically challenging to clone; large arrays can impede delivery.
    • Off-target activity can occur.
    • Delivery methods include plasmids, mRNA, or purified proteins to minimize insertional mutagenesis.
  • Design rules (First-Generation):
    • TAL effector fused to FokI with a target sequence that starts with a 5' T at the left target end.
    • Requires a pair of TALENs to bind opposite strands with a spacer of 13–18 bp for efficient dimerization and DSB formation.
  • Improved TALENs (Engineered Versions):
    • 5' T requirement removed in later engineering
    • Spacer length optimized to ~15–16 bp for better FokI pairing and cleavage efficiency
    • Expanded target range and greater design flexibility

TALENs: Applications and Challenges

  • Applications across model organisms and crops:
    • Zebrafish: precise insertion/knockout via HDR; study development and disease models
    • Mice and rats: gene knockouts/knock-ins for studying gene function and disease mechanisms
    • Livestock (pigs, cows): disease resistance, improved traits
    • Rice (OsSWEET14): promoter targeting to enhance bacterial blight resistance
    • Herbicide resistance in crops via HDR-mediated edits
  • Challenges:
    • Size limits for packaging into viral vectors
    • Sequence biases (e.g., 5' T preference)
  • Advantages:
    • High target specificity due to one-to-one base targeting
    • Lower off-target effects compared to ZFNs and some CRISPR systems
    • Flexible target design for nearly any genomic site

TALEN Design and Construction (Practical Rules)

  • First-Generation TALEN design: left and right TALENs with a spacer of 13–18 bp; FokI dimerization yields DSB.
  • Improved TALENs: removal of 5' T requirement; spacer optimization to 15–16 bp; broader targeting possibilities.

Nobel Prize and CRISPR-Cas Overview

  • 2020 Nobel Prize in Chemistry awarded to Emmanuelle Charpentier and Jennifer Doudna for CRISPR-Cas9 genome editing.
  • CRISPR-Cas systems provide adaptive immunity in bacteria and archaea against phage and foreign DNA.
  • CRISPR-Cas9 is a widely used RNA-guided genome editing tool with programmable targeting guided by gRNA and PAM recognition.

CRISPR-Cas Systems: Components and Mechanism

  • Core components:
    • Cas9 endonuclease
    • Guide RNA (gRNA, often termed sgRNA when combined with tracrRNA)
    • PAM (Protospacer Adjacent Motif), e.g., NGG for SpCas9
  • How it works:
    • The gRNA directs Cas9 to a complementary DNA sequence adjacent to a PAM.
    • Cas9-gRNA complex scans DNA, binds, and induces a double-strand break (DSB) near the PAM.
    • DSB is repaired by the cell via NHEJ (often causing indels) or HDR (with a donor template for precise edits).
  • Multiplexing: Cas9 enables editing of multiple genes simultaneously by using multiple gRNAs.
  • Editing targets: coding, non-coding, regulatory, and epigenetic regions.

CRISPR-Cas9 Immunity and Targeting Details

  • CRISPR loci store spacer sequences as a memory of past infections.
  • Cas9 is activated by binding to gRNA and recognizing a PAM near the target; it creates a site-specific DSB.
  • crRNA guides Cas9 to the target; tracrRNA is part of the natural system; in practice, sgRNA combines crRNA and tracrRNA into a single RNA guide.
  • Targeting requires a PAM next to the protospacer sequence; cleavage occurs just upstream of the PAM.

CRISPR Repair Pathways and Outcomes

  • Non-Homologous End Joining (NHEJ) leads to knockout—indels disrupt gene function.
  • Homology-Directed Repair (HDR) uses a donor template to insert or correct sequences, enabling knockins, precise edits, or point mutations.
  • Two major CRISPR repair pathways summarized:
    • NHEJ: faster, error-prone, no donor template required
    • HDR: precise, donor-template dependent, generally more restricted to S/G2 phases

CRISPR Applications: Real-World Examples

  • Rice yield improvement: CRISPR-Cas9 used to delete parts of DEP1 promoter to increase yield traits (denser panicles, sturdier plants).
  • Citrus disease resistance: editing CsLOB1 promoter to reduce susceptibility to citrus canker, improving resistance.
  • Soybean flowering control: editing GmFT2a to delay flowering and extend vegetative growth, increasing biomass with stability across generations.
  • Human gene therapy (β-thalassemia): correcting HBB mutations in patient-derived iPSCs to restore hemoglobin production.
  • Note: These exemplify gene therapy, functional genomics, agriculture improvements, and disease modeling.

CRISPR-Cas9: Advantages and Limitations

  • Advantages:
    • Simplicity: no protein redesign required—only RNA guide synthesis
    • High efficiency across many cell types and species
    • Versatility: knockout, knock-in, gene activation, base editing, epigenetic editing
    • Cost-effective and rapidly deployable with online design tools and kits
  • Limitations:
    • Off-target effects: mismatches in gRNA can cause unintended cuts
    • PAM dependency: targeting limited to sequences near PAMs (e.g., NGG for SpCas9)
    • Delivery challenges: viral or non-viral delivery methods affect efficiency
    • Ethical concerns: germline editing implications

Base Editors (BE)

  • Concept: programmable conversion of one DNA base to another without creating DSBs, reducing indels.
  • Composition: Cas proteins fused to nucleobase deaminases; no donor DNA required; lower DNA damage.
  • Types:
    • Cytosine Base Editors (CBEs): C → T (or G → A)
    • Adenine Base Editors (ABEs): A → G (or T → C)
  • Core idea: deaminase converts a base within a defined editing window, followed by replication/repair to fix the base change.

Base Editing Mechanisms and Windows

  • Base Editor Structure: Cas9 variant (nCas9 or dCas9) + deaminase + UGI (for CBEs) to inhibit base excision repair.
  • CBE mechanism (example):
    • Cas9-sgRNA binds target; cytidine deaminase converts C to U in the single-stranded DNA bubble; during replication, U is read as T, giving C•G to T•A conversion.
  • ABE mechanism: TadA deaminates A to inosine (I), read as G, yielding A•T to G•C changes.
  • Editing window: typically about ~5 bp within the protospacer where deamination occurs.
  • Variants and improvements include BE3, BE4, evoBE, and high-fidelity versions; ABEs include ABE7.10, ABE8e, with ongoing enhancements.

Types of Base Editors and Related Tools

  • CBEs: cytidine deaminases; convert C to T within a defined window; examples BE3, BE4, evoBE3
  • ABEs: adenine deaminases; convert A to G; examples ABE7.10, ABE8e
  • Dual-function editors: combine ABE and CBE capabilities
  • RNA base editors: REPAIR and RESCUE (Cas13-based) for RNA edits
  • Fidelity and editing-range improvements across BE generations

Cas9 Variants and Their Functions

  • Various Cas9 enzymes with different PAM requirements and sizes:
    • SpCas9 (S. pyogenes): PAM NGG; size ~4.2 kb
    • SaCas9 (S. aureus): PAM NNGRRT; size ~3.2 kb; smaller, AAV-friendly
    • CjCas9 (C. jejuni): PAM NNNNACAC; size ~2.95 kb; very small with strict PAM
    • NmCas9 (N. meningitidis): PAM NNNNGATT; size ~3.2 kb
    • Engineered variants: eSpCas9/SpCas9-HF1 (high fidelity); xCas9 with expanded PAM compatibility
  • Cas9 nickase (nCas9): a Cas9 variant that makes single-strand nicks instead of DSBs; enables base editing and reduced off-target effects
  • dCas9: nuclease-dead Cas9 used for binding and gene modulation without cutting

Other CRISPR-Associated Systems (Cas Family)

  • Cas12 (Cpf1): Class 2, Type V; targets dsDNA with different PAMs; creates DSBs with staggered cuts; potential for collateral cleavage in some variants
  • Cas13 (C2c2): Class 2, Type VI; RNA-targeting nuclease; used for RNA editing and diagnostics
  • Cas14: Class 2, Type V-F; ssDNA-targeting nuclease; smallest Cas with diagnostic utility
  • Cas3, Cas10: Class 1 systems with DNA or RNA targeting and signaling roles
  • These variants expand PAM compatibility, target types, and delivery strategies

Base Editors: Advantages, Limitations, and Challenges

  • Advantages:
    • No double-strand breaks (DSBs) → reduced indels
    • Single-nucleotide precision
    • Lower risk of large deletions or chromosomal rearrangements
    • Suited for point mutation corrections, non-dividing cells, and gene modulation
  • Limitations/Challenges:
    • Editing window constraints limit which bases can be targeted
    • Off-target deamination on DNA and RNA
    • Delivery challenges, especially in vivo
    • Immune response to bacterial proteins like Cas9
    • Bystander activity: multiple cytidines/adenines within the editing window may be affected
    • Sequence context bias (e.g., APOBEC motifs) affects targetability

Advances and Future Tools in Base Editing

  • New Cas variants (e.g., Cas12a, SaCas9) broaden PAM compatibility and editing possibilities
  • Base editors with reduced off-target RNA editing
  • Narrow-window editors to limit bystander effects
  • Dual editors enabling simultaneous A and C edits
  • RNA base editing for reversible, non-permanent changes
  • Machine learning to improve gRNA design and predict outcomes
  • In vivo delivery improvements (lipid nanoparticles, AAVs, etc.)

Summary and Real-World Potential

  • Base editors are game-changing for precision editing with scarless outcomes.
  • They address key limitations of traditional CRISPR by avoiding DSBs and enabling single-base edits.
  • Suitable for single-base mutations, editing in non-dividing cells, and gene modulation.
  • Ongoing improvements in generations, delivery, and safety widen the potential for gene therapy, agriculture, and disease modeling.

β-thalassemia Example (Base Editing Context)

  • β-thalassemia is caused by mutations in the HBB gene, reducing β-globin production.
  • Many mutations are point mutations or small deletions affecting splicing or coding sequence.
  • Base editors (CBEs and ABEs) enable precise single-base changes without DSBs.
  • Example: CD17 A>T mutation (codon 17) can be corrected with ABEs; such edits can restore proper β-globin production.
  • Therapeutic potential: ex vivo editing of patient-derived hematopoietic stem cells followed by reinfusion.
  • CD17 = codon 17; A>T refers to an A-to-T substitution, potentially introducing a nonsense mutation if not corrected; ABEs can revert pathogenic variants.

Herbicide Tolerance in Maize (Illustrative Example)

  • A schematic shows acetolactate synthase (ALS) gene variants and mutations that confer herbicide tolerance.
  • Editing at specific positions (e.g., Pro165 substitutions) can alter enzyme activity to confer tolerance.
  • Editing window and specific sgRNA/tALEN settings determine the precise nucleotide substitutions.
  • The figure highlights WT vs edited genotypes and allele-specific edits relevant to herbicide resistance.

Prime Editing: An All-in-One Genome Editing Tool

  • Discovery: 2019 by Andrew Anzalone and David Liu; Nature publication.
  • Core idea: combines a Cas9 nickase with reverse transcriptase and a pegRNA to enable precise edits without requiring DSBs or donor DNA templates.
  • Key advantages: small insertions/deletions, all 12 types of base substitutions, and precise edits with reduced risk of DSB-induced damage.
  • Prime editing outperforms traditional base editing in some contexts due to broader editing capabilities.
  • Potential therapeutic applications and broad research utility for targeted edits.

Prime Editing Components

  • Prime Editor (PE): Cas9 nickase fused to reverse transcriptase
  • pegRNA (Prime Editing Guide RNA): guides Cas9 nickase and provides reverse transcription template
  • Reverse transcriptase: reads pegRNA template to synthesize edited DNA
  • 5' gRNA Target Sequence: guides PE to the intended site
  • PE:pegRNA complex and primer binding site (PBS) are integral parts of the editing process
  • red box in diagrams indicates the edited RNA sequence template within the pegRNA complex

pegRNA: Structure, Length, and Challenges

  • pegRNA is specialized for prime editing; it acts as both guide and template
  • Components:
    • Target Sequence (~20 nt): directs Cas9 nickase to the DNA site
    • Scaffold Sequence: structural component that binds Cas9 nickase
    • Reverse Transcription Template (25–40 nt): contains the desired DNA change and homology
    • Primer-Binding Site (PBS, 10–15 nt): starting point for DNA synthesis by reverse transcriptase
  • Typical pegRNA length: ~120–145 nt; extended versions can reach 170–190 nt or more
  • Challenges with pegRNA:
    • Synthesis difficulty due to long RNA sequences
    • Lower yield and purity from longer RNAs because of secondary structures
    • Delivery challenges: larger size complicates packaging into plasmids or nanoparticles
    • Stability issues inside cells: prone to degradation

How Prime Editing Works (Step-by-Step)

1) PE:pegRNA complex binds to target DNA.
2) Cas9 nickase makes a single-strand cut on one DNA strand, creating a nicked DNA with a 3' flap.
3) Reverse transcription uses the pegRNA as a template to synthesize the edited DNA sequence onto the nicked strand.
4) The edited strand is incorporated and the original DNA is cleaved/removed by cellular nucleases.
5) The opposite (unedited) strand is repaired to match the newly edited sequence; in PE3/PE3b designs, a second gRNA nick directs editing of the unedited strand to improve efficiency.
6) Final result: both DNA strands contain the desired edit.

  • PE3/PE3b strategies involve coordinating nicking on the unedited strand to bias repair toward the edited sequence.

Additional Notes on Prime Editing Mechanisms

  • Target recognition and binding:
    • PE uses a Cas9 nickase guided by pegRNA to a specific DNA target.
  • Nick generation:
    • Cas9 nickase makes a single-strand nick; creates a flexible 3' DNA flap.
  • Reverse transcription and insertion:
    • The pegRNA PBS anneals and reverse transcriptase extends the 3' end, incorporating the edited sequence into the genome.
  • Strand repair and final editing:
    • The unedited strand is repaired to mirror the edited strand; removal of residual DNA fragments completes editing.
  • PE3/PE3b strategies give a higher chance of achieving precise edits by nicking the non-edited strand to bias repair toward the edited sequence.

Practical Takeaways and Connections

  • NHEJ vs HDR and CRISPR-based editing reflect a spectrum of precision and efficiency trade-offs: speed vs accuracy, in vivo vs ex vivo applicability, and dependency on cell-cycle state.
  • ZFN/TALENs and CRISPR/Cas9 offer different design paradigms:
    • ZFNs/TALENs rely on engineered DNA-binding proteins; pros include specificity and modularity, but design and cloning can be challenging (especially ZFNs).
    • CRISPR/Cas9 relies on RNA guidance; easier to reprogram for new targets (just redesign gRNA).
  • Base editing and prime editing expand capabilities beyond conventional CRISPR/Cas9 cuts, enabling precise single-base substitutions and targeted sequence changes without creating DSBs.
  • Real-world applications span medicine, agriculture, and research; ethical and safety considerations remain central (off-target effects, delivery, immune responses, and germline editing concerns).

Appendix: Key Terms and Comparisons

  • DSB: Double-strand break
  • SSB: Single-strand break
  • NHEJ: Non-Homologous End Joining
  • HR: Homologous Recombination
  • HDR: Homology-Directed Repair
  • BE: Base Editor
  • CBE: Cytosine Base Editor (C to T)
  • ABE: Adenine Base Editor (A to G)
  • PE: Prime Editor (Cas9 nickase + reverse transcriptase)
  • pegRNA: Primer-Editing Guide RNA (guide + template)
  • PAM: Protospacer Adjacent Motif (e.g., NGG for SpCas9)
  • RVD: Repeat Variable Di-residue (component of TALEN DNA-binding specificity)
  • 5' T rule: early TALEN design rule requiring a thymine at the 5' end of the target binding site
  • Editing window: the span of bases within which base editing can occur, typically ~5 bp
  • Indel: insertion or deletion
  • D-loop: displacement loop formed during HR
  • DSB repair pathways may be targeted to achieve desired edits with context-appropriate tools and delivery methods