What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Prime Editing: Mechanism, Applications, and Experimental Design

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-08-11

Introduction to Prime Editing

Prime editing is a CRISPR-derived genome engineering technology that enables the introduction of precise small edits—single-nucleotide substitutions, small insertions, and small deletions—directly into the genome of living cells without requiring a double-strand break (DSB) or an exogenous donor DNA template. The system was first described in 2019 by Anzalone and colleagues, who demonstrated that a catalytically impaired Cas9 nickase fused to a reverse transcriptase (RT) could copy genetic information from an engineered guide RNA directly into the genomic target site.

The core innovation of prime editing lies in its use of a prime editing guide RNA (pegRNA), which serves dual functions: it specifies the genomic target site and carries the desired edit as an RNA template. This design bypasses the need for homology-directed repair (HDR), which is inefficient in most cell types, and avoids the stochastic outcomes of non-homologous end joining (NHEJ) that dominate after Cas9-induced DSBs. Prime editing is therefore particularly attractive for applications requiring precise correction of pathogenic mutations, introduction of reporter tags, or generation of subtle allelic series.

What Is Prime Editing?

Prime editing is a "search-and-replace" genome editing technology. The search function is provided by a Cas9 nickase (typically the H840A variant of Streptococcus pyogenes Cas9) that has been mutated to cut only one strand of the DNA duplex. The replace function is provided by a reverse transcriptase fused to the nickase, which uses an RNA template embedded in the pegRNA to synthesize new DNA directly at the target locus.

The pegRNA is a modified single-guide RNA (sgRNA) that contains, in addition to the standard guide sequence and scaffold, a 3' extension comprising two functional elements: a primer binding site (PBS) and a reverse transcription template (RTT). After the nickase creates a single-strand break on the non-target strand, the PBS hybridizes to the exposed 3' end of the nicked DNA strand, priming reverse transcription. The RT then copies the RTT sequence into DNA, generating a 3' flap that contains the desired edit. Cellular DNA repair machinery resolves the resulting single-stranded flaps, incorporating the edit into the genome.

A second, conventional sgRNA—the nick sgRNA—can be co-delivered to nick the non-edited strand, biasing repair toward the edited strand and improving editing efficiency. This three-component system (nickase-RT fusion, pegRNA, and nick sgRNA) constitutes the canonical prime editing architecture.

Prime Editing vs. Other CRISPR Systems

Prime editing occupies a distinct niche among CRISPR-based tools. Compared to conventional Cas9-mediated HDR, prime editing offers higher efficiency in non-dividing cells, lower rates of undesired indels, and no requirement for a donor template. Unlike Base Editing, which is restricted to transition mutations (C→T or A→G) within a narrow editing window, prime editing can install all 12 possible base substitutions, as well as insertions and deletions up to roughly 40–50 base pairs. Base editors also require the target base to lie within a specific protospacer adjacent motif (PAM) context and editing window, whereas prime editing has more flexible targeting constraints.

However, prime editing is generally less efficient than base editing for the specific edits base editors can perform, and it requires more complex design (two RNA components versus one). CRISPR knockout, which relies on NHEJ-mediated gene disruption, remains the method of choice for gene inactivation, but it cannot install precise edits. Prime editing thus fills the gap between "disrupt" and "correct," offering a programmable, scarless, and DSB-free approach to precise genome modification.

Molecular Mechanism of Prime Editing

The mechanism of prime editing proceeds through three coordinated phases: target recognition and nicking, reverse transcription and flap generation, and DNA repair with edit incorporation. Each phase involves specific molecular interactions that determine the efficiency and fidelity of the final edit.

Step 1: Target Recognition and Nicking

The process begins when the pegRNA guide sequence (typically 18–20 nucleotides) base-pairs with its complementary sequence in the target DNA, in a manner identical to standard CRISPR-Cas9 targeting. The Cas9 nickase (SpCas9 H840A) recognizes the protospacer adjacent motif (PAM, 5'-NGG-3' for SpCas9) immediately downstream of the target sequence. Upon guide-target hybridization and PAM recognition, the HNH nuclease domain of Cas9—which in the H840A variant retains activity—cleaves the target strand (the strand complementary to the guide RNA) 3 base pairs upstream of the PAM. The RuvC domain, which normally cleaves the non-target strand, is inactivated by the H840A mutation.

This produces a single-strand nick on the target strand, leaving the non-target strand intact. The nick exposes a free 3' hydroxyl group on the target strand, which is essential for the subsequent priming step. Importantly, the nicked strand is the one that will ultimately be replaced by newly synthesized DNA, while the intact non-target strand serves as a template for repair synthesis.

Step 2: Reverse Transcription and Flap Generation

Following nicking, the 3' end of the nicked target strand dissociates from the guide RNA and becomes available for hybridization with the primer binding site (PBS) located at the 3' end of the pegRNA. The PBS is typically 10–15 nucleotides long and is designed to be complementary to the sequence immediately upstream of the nick site on the target strand. This hybridization event positions the 3' hydroxyl group at the start of the reverse transcription template (RTT).

The reverse transcriptase domain of the fusion protein then initiates DNA synthesis, copying the RTT sequence into single-stranded DNA. The RTT is designed to contain the desired edit at its 5' end (relative to the pegRNA), followed by sequence homologous to the genomic region downstream of the nick. As reverse transcription proceeds, the newly synthesized DNA (the 3' flap) displaces the original target strand sequence, creating a single-stranded DNA flap that contains the edit.

The length of the RTT typically ranges from 10 to 40 nucleotides, with the edit positioned such that the PBS and RTT together span the region to be modified. The reverse transcriptase used is typically Moloney murine leukemia virus (M-MLV) RT, which has been engineered with mutations (e.g., D200N, T330P, L603W, and others) to improve processivity, thermostability, and template binding.

Step 3: DNA Repair and Edit Incorporation

The completion of prime editing requires resolution of the two single-stranded flaps generated during the process: the edited 3' flap (containing the new sequence) and the displaced 5' flap (containing the original sequence). This resolution is thought to occur through the action of structure-specific endonucleases and DNA repair factors.

In the most widely accepted model, the 5' flap is cleaved by a flap endonuclease (likely FEN1 or related nucleases), and the 3' flap is ligated into the genome by DNA ligase. This produces a heteroduplex where one strand contains the edit and the other contains the original sequence. The final step is DNA mismatch repair (MMR), which determines whether the edit is permanently incorporated.

Critically, MMR can either preserve or remove the edit depending on which strand is recognized as the template. In the absence of the nick sgRNA (PE2 configuration), the edited strand may be recognized as the mismatched strand and removed, reducing editing efficiency. The nick sgRNA (PE3 configuration) addresses this by introducing a second nick on the non-edited strand, directing MMR to use the edited strand as the template and thereby increasing the frequency of edit incorporation. However, the nick sgRNA must be positioned carefully to avoid simultaneous cleavage of both strands, which would create a DSB and trigger NHEJ.

Key Components and Design Considerations

The efficiency and specificity of prime editing depend critically on the design of its RNA components. Suboptimal pegRNA or nick sgRNA design is the most common cause of failed prime editing experiments.

pegRNA Architecture

A pegRNA consists of four functional modules arranged in a single transcript:

Guide sequence (18–20 nt): Complementary to the target DNA sequence, positioned immediately 5' of the PAM. This determines the nick site location.
sgRNA scaffold (approximately 80 nt): The conserved RNA structure that binds Cas9, identical to that in standard sgRNAs.
Primer binding site (PBS) (10–15 nt): Complementary to the sequence immediately 3' of the nick site on the target strand. This primes reverse transcription.
Reverse transcription template (RTT) (10–40 nt): Contains the desired edit at its 5' end (relative to the pegRNA), followed by sequence homologous to the genomic region downstream of the nick.

The pegRNA is typically expressed from a U6 promoter, which requires a guanine at the transcription start site. If the guide sequence does not naturally begin with G, an extra G is added, which can slightly reduce targeting efficiency but is generally tolerated.

Designing the Primer Binding Site and RT Template

The PBS length and GC content are critical parameters. A PBS that is too short (fewer than 8 nt) will hybridize weakly to the nicked DNA, reducing priming efficiency. A PBS that is too long (greater than 15 nt) can cause secondary structure formation in the pegRNA that interferes with reverse transcription. Empirically, PBS lengths of 10–14 nt with a GC content of 40–60% perform best.

The RTT must be designed so that the desired edit is positioned at its 5' end (the first nucleotides to be reverse transcribed). The sequence downstream of the edit in the RTT must be homologous to the genomic sequence immediately 3' of the nick site. The total RTT length (including the edit and homology region) typically ranges from 10 to 40 nt. For substitutions, an RTT of 15–25 nt is usually sufficient. For insertions, the RTT must include the inserted sequence plus flanking homology; insertions larger than 30–40 nt are generally inefficient.

The edit itself should be placed such that the PBS does not overlap with the edited region, as this would prevent the PBS from hybridizing to the unedited genomic sequence. A common design rule is to position the edit 3–6 nucleotides downstream of the PBS-RTT junction.

Nick sgRNA Design and Spacing

The nick sgRNA (used in PE3 and PE3b configurations) is a standard sgRNA that targets the non-edited strand. Its purpose is to create a nick on the strand opposite the pegRNA-induced nick, biasing MMR to preserve the edit. The nick sgRNA must be designed so that its target site is on the non-edited strand and positioned at an appropriate distance from the pegRNA nick site.

For PE3, the nick sgRNA is typically designed to target a site 50–100 base pairs away from the pegRNA nick, on the opposite strand. For PE3b, the nick sgRNA is designed to bind only to the edited sequence, so that it can nick only after the edit has been incorporated—this reduces the risk of concurrent nicks on both strands. The PE3b configuration is preferred when the edit creates a new PAM or protospacer sequence that can be specifically targeted.

Prime Editor Variants and Evolution

Since the initial description of prime editing, several generations of prime editor variants have been developed, each addressing specific limitations of the original system.

From PE1 to PE3b

PE1 is the foundational system: a SpCas9 H840A nickase fused to wild-type M-MLV reverse transcriptase, delivered with a pegRNA. PE1 exhibits low editing efficiency (typically 1–5% in mammalian cells) because the wild-type RT has poor processivity and thermostability at physiological temperatures.

PE2 incorporates an engineered M-MLV RT with five mutations (D200N, T330P, L603W, T306K, and W313F) that improve thermostability, processivity, and template binding. PE2 also removes the RNase H activity of the RT, preventing degradation of the pegRNA during reverse transcription. PE2 achieves roughly 2–5-fold higher editing efficiency than PE1.

PE3 adds the nick sgRNA to the PE2 system, introducing a second nick on the non-edited strand to bias MMR. PE3 typically achieves 2–5-fold higher efficiency than PE2, but at the cost of increased indel formation (due to the risk of concurrent nicks creating DSBs).

PE3b is a refinement of PE3 in which the nick sgRNA is designed to recognize only the edited sequence. This ensures that the second nick occurs only after the edit has been incorporated, minimizing the window for DSB formation and reducing indel rates.

Engineered Variants and Their Advantages

PE4 and PE5 incorporate dominant-negative MMR inhibitors. The MMR pathway is a major barrier to prime editing efficiency, as it frequently removes the edit during the heteroduplex repair step. PE4 co-expresses a dominant-negative form of MLH1 (MLH1dn), which suppresses MMR and increases editing efficiency 2–8-fold. PE5 combines PE4 with the PE3 nick sgRNA configuration, achieving the highest editing efficiencies of the standard variants.

PEmax is an architecture-optimized variant that uses a codon-optimized Cas9 nickase, an improved nuclear localization signal (NLS), and a more flexible linker between the nickase and RT domains. PEmax also incorporates the MLH1dn component. Together, these modifications increase editing efficiency by an additional 2–4-fold over PE2/PE3.

Additional engineered variants include PE6 variants with further optimized RT domains, and twin prime editing (twinPE) , which uses two pegRNAs to install large insertions or deletions (hundreds of base pairs) through sequential prime editing events. Prime editing with paired pegRNAs (PE-P3) and bi-PE are other dual-pegRNA strategies for larger edits.

Delivery Methods for Prime Editing

The delivery method for prime editing components significantly influences editing efficiency, toxicity, and the range of cell types that can be edited. Each approach has distinct advantages and limitations.

Non-Viral Delivery

Plasmid transfection is the most common delivery method for initial validation and in vitro experiments. Plasmids encoding the prime editor (nickase-RT fusion), pegRNA, and nick sgRNA are co-transfected using lipid-based reagents or electroporation. This approach is simple and inexpensive, but expression is transient (typically 2–5 days), and efficiency depends on transfection efficiency, which varies widely by cell type. For adherent cell lines such as HEK293T, lipofection with reagents like Lipofectamine 3000 achieves 70–90% transfection efficiency, yielding prime editing efficiencies of 20–50% for well-designed pegRNAs.

mRNA delivery involves in vitro transcription of the prime editor mRNA and co-delivery with synthetic pegRNA and nick sgRNA. This approach avoids the risk of plasmid integration and reduces the duration of editor expression, which can decrease off-target effects. mRNA delivery is particularly useful for primary cells and stem cells, where plasmid toxicity is a concern. However, mRNA is less stable than plasmid DNA and requires careful optimization of delivery conditions.

Ribonucleoprotein (RNP) delivery involves delivering the prime editor as a purified protein complexed with in vitro-transcribed pegRNA and nick sgRNA. RNPs offer the shortest duration of editor activity (hours), minimizing off-target effects and toxicity. RNP delivery is the method of choice for hard-to-transfect cells, including primary T cells and hematopoietic stem cells. However, RNP production is labor-intensive and expensive, and the efficiency of RNP-based prime editing is often lower than plasmid-based delivery due to reduced editor availability.

Viral Delivery and Packaging Constraints

Adeno-associated virus (AAV) is the most commonly used viral vector for prime editing in vivo. AAV has a packaging capacity of approximately 4.7 kb, which poses a significant challenge: the prime editor coding sequence alone is approximately 6.3 kb (SpCas9 nickase plus M-MLV RT), exceeding the AAV capacity. Strategies to overcome this limitation include splitting the prime editor into two AAV vectors (split-intein approach), using smaller Cas9 orthologs (e.g., Staphylococcus aureus Cas9 nickase, which is ~3.2 kb), or using dual-AAV systems where one vector carries the N-terminal half and the other carries the C-terminal half, with intein-mediated protein splicing reconstituting the full editor.

Lentiviral vectors have a larger packaging capacity (~8 kb) and can accommodate the full prime editor cassette. However, lentiviral delivery results in random integration into the host genome, which raises safety concerns for therapeutic applications. Integration-defective lentiviral vectors (IDLVs) can be used to avoid this issue, but they provide only transient expression.

Adenoviral vectors offer high packaging capacity (~8 kb) and do not integrate into the genome, but they elicit strong immune responses, limiting their use to ex vivo applications.

Measuring and Optimizing Prime Editing Efficiency

Accurate measurement of prime editing efficiency is essential for optimizing pegRNA design and delivery conditions. Several methods are available, each with different throughput and resolution.

Quantification Methods

Sanger sequencing is the simplest method for quantifying prime editing efficiency. Genomic DNA is harvested from edited cells, the target region is PCR-amplified, and the amplicon is subjected to Sanger sequencing. The editing efficiency is estimated by analyzing the sequencing chromatogram for mixed peaks at the edit site. Software tools such as EditR, ICE, or TIDE can quantify the proportion of edited alleles. This method is inexpensive and accessible but has limited sensitivity (typically cannot detect edits below 5–10%) and cannot resolve complex mixtures of alleles.

Next-generation sequencing (NGS) is the gold standard for precise quantification. Amplicon sequencing of the target locus with paired-end reads allows detection of edits at frequencies as low as 0.1%, and can distinguish between intended edits, unintended indels, and byproducts. NGS also enables analysis of editing outcomes at off-target sites. The cost and turnaround time are higher than Sanger sequencing, but the information content is substantially greater.

Reporter assays provide a rapid, high-throughput readout of prime editing activity. For example, a GFP reporter construct containing a premature stop codon can be designed such that successful prime editing restores GFP expression, allowing quantification by flow cytometry. These assays are useful for screening pegRNA designs or optimizing delivery conditions, but they require the construction of reporter cell lines and may not fully recapitulate editing at endogenous loci.

Factors Influencing Efficiency

Several biological factors influence prime editing efficiency:

Cell type and proliferation status: Prime editing is most efficient in rapidly dividing cells, where DNA replication and repair are active. In non-dividing cells (e.g., post-mitotic neurons), editing efficiency is significantly lower, likely due to reduced availability of DNA repair factors. However, prime editing is still more efficient than HDR in non-dividing cells.

Mismatch repair status: MMR is a major barrier to prime editing. Cells with defective MMR (e.g., HCT116 cells with MLH1 deficiency) show 2–10-fold higher editing efficiency than MMR-proficient cells. This is the basis for the PE4/PE5 variants that suppress MMR.

Chromatin context: The local chromatin environment affects pegRNA accessibility and editing efficiency. Heterochromatic regions are generally edited less efficiently than euchromatic regions. Histone deacetylase inhibitors (e.g., valproic acid) have been shown to modestly increase prime editing efficiency in some cell types.

pegRNA secondary structure: The 3' extension of the pegRNA (PBS + RTT) can form secondary structures that interfere with reverse transcription. Computational tools (e.g., PrimeDesign, pegFinder) can predict secondary structure and suggest design modifications.

Optimization Strategies

Several strategies can improve prime editing efficiency:

Optimize pegRNA design: Screen multiple PBS lengths (10–14 nt) and RTT lengths (10–30 nt) for each target site. Position the edit at the 5' end of the RTT, 3–6 nt downstream of the PBS-RTT junction.
Use PE4/PE5 or PEmax variants: These engineered variants consistently outperform PE2/PE3 across cell types.
Optimize delivery: Titrate plasmid amounts, use electroporation for hard-to-transfect cells, or switch to mRNA/RNP delivery if plasmid toxicity is an issue.
Add MMR inhibitors: Co-delivery of MLH1dn (as in PE4/PE5) or small-molecule MMR inhibitors can increase efficiency.
Enrich for edited cells: If the edit creates a restriction site or alters a fluorescent reporter, cells can be enriched by FACS or antibiotic selection.

Applications of Prime Editing in Research and Therapy

Prime editing has broad applications in basic research and translational medicine, owing to its precision, versatility, and relatively low off-target activity.

Disease Modeling and Gene Therapy

Prime editing is particularly well-suited for modeling and correcting point mutations that cause human genetic diseases. For example, prime editing has been used to correct the sickle cell disease mutation (a single A→T transversion in the HBB gene) in patient-derived hematopoietic stem cells, restoring functional hemoglobin expression. Similarly, prime editing has been applied to correct the ΔF508 mutation in CFTR (a 3-bp deletion) in cystic fibrosis patient-derived airway epithelial cells, restoring chloride channel function.

In the context of cancer research, prime editing enables the introduction of specific oncogenic mutations into cell lines or organoids to study their functional consequences. For example, introducing the KRAS G12D mutation into wild-type cells allows researchers to study the signaling pathways activated by this common cancer driver. Prime editing has also been used to generate isogenic cell line panels with different disease-associated alleles, enabling systematic genotype-phenotype correlation studies.

For therapeutic applications, prime editing offers several advantages over HDR: it does not require a DSB, reducing the risk of chromosomal rearrangements and p53 activation; it does not require a donor template, simplifying delivery; and it can be applied to non-dividing cells, expanding the range of target tissues. However, the delivery challenges and the risk of off-target edits remain significant hurdles for clinical translation.

High-Throughput Screens

Prime editing has been adapted for high-throughput functional genomics screens. Unlike CRISPR knockout screens, which only test loss-of-function, prime editing screens can test the functional consequences of specific alleles, including gain-of-function mutations, hypomorphic alleles, and non-coding variants. This is particularly valuable for interpreting the clinical significance of variants of uncertain significance (VUS) identified in genome-wide association studies.

The Prime Editing Guide RNA Library (PEGL) approach uses a library of pegRNAs targeting thousands of genomic sites to screen for variants that confer drug resistance, alter cell fitness, or affect gene expression. These screens require careful design to ensure that each pegRNA is specific and that the editing outcomes are accurately quantified by NGS.

Common Pitfalls and Troubleshooting in Prime Editing

Despite its power, prime editing is technically demanding, and several common pitfalls can derail experiments.

Design Pitfalls

Poor pegRNA design is the most frequent cause of low editing efficiency. Common errors include: PBS or RTT lengths outside the optimal range; the edit positioned too close to the PBS-RTT junction; secondary structure in the 3' extension; and guide sequences with high off-target similarity. Always use computational design tools and screen at least 2–3 pegRNA designs per target site.

Incorrect nick sgRNA design can reduce efficiency or increase indel rates. For PE3, the nick sgRNA must target the non-edited strand at an appropriate distance (50–100 bp) from the pegRNA nick. For PE3b, the nick sgRNA must be specific to the edited sequence—if it also binds the unedited sequence, it will behave like PE3 and increase indel rates.

PAM constraints: The SpCas9 nickase requires an NGG PAM adjacent to the target site. If no suitable PAM is available, consider using Cas9 variants with relaxed PAM requirements (e.g., SpCas9-NG, SpRY) or alternative Cas9 orthologs.

Efficiency Troubleshooting

If editing efficiency is low (<5%), consider the following:

Verify pegRNA expression: Check that the pegRNA is expressed at sufficient levels by RT-qPCR or Northern blot. The U6 promoter requires a G at the transcription start site; if the guide does not begin with G, the added G may reduce expression.
Check delivery efficiency: For plasmid transfection, co-transfect a fluorescent reporter to confirm transfection efficiency. If transfection is low, switch to electroporation or optimize lipid-to-DNA ratios.
Test different PE variants: If PE2/PE3 are inefficient, try PE4/PE5 or PEmax. The MLH1dn component can dramatically improve efficiency in MMR-proficient cells.
Optimize the ratio of pegRNA to nick sgRNA: For PE3, a 1:1 molar ratio is a reasonable starting point, but titrating the nick sgRNA (e.g., 1:2 or 2:1) can sometimes improve efficiency while reducing indels.
Increase the amount of pegRNA: In some cases, increasing the pegRNA plasmid concentration relative to the editor plasmid improves efficiency, as pegRNA is often limiting.

Off-Target Considerations

Prime editing has lower off-target activity than Cas9 nuclease because the nickase creates only single-strand breaks, which are less likely to cause mutations. However, off-target nicking can still occur at sites with partial complementarity to the guide sequence. To minimize off-target effects:

Use computational tools (e.g., Cas-OFFinder, GUIDE-seq) to identify potential off-target sites and select guide sequences with minimal off-target similarity.
Consider using high-fidelity Cas9 variants (e.g., SpCas9-HF1, eSpCas9) in the prime editor fusion, though these may slightly reduce on-target efficiency.
Use RNP delivery to limit the duration of editor activity, reducing the window for off-target nicking.
Perform whole-genome sequencing or targeted amplicon sequencing of predicted off-target sites to assess specificity.

Frequently Asked Questions

What is prime editing?

Prime editing is a genome editing technology that uses a Cas9 nickase-reverse transcriptase fusion protein and an engineered guide RNA (pegRNA) to introduce precise small edits—substitutions, insertions, or deletions—into the genome without creating a double-strand break or requiring a donor DNA template. The pegRNA both specifies the target site and carries the desired edit as an RNA template, which is reverse transcribed directly into the genome.

How does prime editing work?

Prime editing works through three steps: (1) the Cas9 nickase creates a single-strand nick at the target site specified by the pegRNA guide sequence; (2) the primer binding site on the pegRNA hybridizes to the nicked DNA, and the reverse transcriptase copies the reverse transcription template into DNA, generating a 3' flap containing the edit; (3) cellular DNA repair machinery resolves the flaps and incorporates the edit, with a second nick (from the nick sgRNA) biasing repair toward the edited strand.

What is a prime editing diagram?

A prime editing diagram typically illustrates the three components (nickase-RT fusion, pegRNA, and nick sgRNA) and their interactions with the target DNA. The diagram shows the pegRNA bound to the target strand, the nick site, the PBS hybridized to the nicked DNA, the RT synthesizing new DNA from the RTT, and the resulting 3' flap containing the edit. The nick sgRNA is shown binding to the non-edited strand at a distal site.

What is the prime editing protocol?

A typical prime editing protocol involves: (1) designing and cloning the pegRNA and nick sgRNA into expression plasmids; (2) co-transfecting the prime editor plasmid, pegRNA plasmid, and nick sgRNA plasmid into cells (e.g., using Lipofectamine 3000 for HEK293T cells); (3) harvesting genomic DNA 48–72 hours post-transfection; (4) PCR-amplifying the target region; and (5) quantifying editing efficiency by Sanger sequencing or NGS. For hard-to-transfect cells, electroporation or RNP delivery may be used.

What are the components of prime editing?

Prime editing requires three components: (1) a Cas9 nickase-reverse transcriptase fusion protein (e.g., SpCas9 H840A-M-MLV RT); (2) a prime editing guide RNA (pegRNA) containing the guide sequence, scaffold, primer binding site, and reverse transcription template; and (3) optionally, a nick sgRNA that targets the non-edited strand to improve efficiency (PE3 configuration).

What is the difference between PE2 and PE3?

PE2 uses only the pegRNA and the engineered nickase-RT fusion, relying on endogenous DNA repair to incorporate the edit. PE3 adds a nick sgRNA that creates a second nick on the non-edited strand, biasing mismatch repair to preserve the edit. PE3 is generally more efficient than PE2 (2–5-fold) but can produce more indels due to the risk of concurrent nicks creating double-strand breaks.

What are the limitations of prime editing?

The main limitations of prime editing are: (1) lower efficiency compared to base editing for the specific edits base editors can perform; (2) the complexity of pegRNA design, which requires optimization of PBS and RTT lengths; (3) the large size of the prime editor protein, which complicates viral delivery; (4) the barrier posed by mismatch repair, which can remove edits; and (5) limited ability to install large insertions or deletions (>50 bp) efficiently.

Key Takeaways

Prime editing enables precise genome edits—substitutions, insertions, and small deletions—without double-strand breaks or donor templates, filling a critical gap between CRISPR knockout and base editing.
The system comprises a Cas9 nickase-reverse transcriptase fusion, a pegRNA (guide + PBS + RTT), and an optional nick sgRNA; the pegRNA both targets the site and carries the edit template.
The mechanism involves target nicking, PBS-primed reverse transcription to generate an edited 3' flap, and flap resolution with edit incorporation; the nick sgRNA biases mismatch repair to preserve the edit.
Engineered variants (PE2, PE3, PE3b, PE4, PE5, PEmax) progressively improve efficiency by enhancing RT processivity, suppressing mismatch repair, and optimizing protein architecture.
Delivery options include plasmid transfection, mRNA, RNP complexes, and viral vectors; AAV delivery requires split-intein strategies due to the large editor size.
Editing efficiency is quantified by Sanger sequencing or NGS, and is influenced by cell type, MMR status, chromatin context, and pegRNA design; optimization strategies include screening pegRNA designs and using MMR-suppressing variants.
Prime editing has broad applications in disease modeling, gene therapy, and high-throughput functional genomics, but requires careful design and troubleshooting to achieve reliable results.