Practical Guidelines for Precise Genome Correction Encompassing Forward and Inverse Prime Editing
Background
Prime editing (PE) is a precise genome editing technology that records desired base substitutions and insertions/deletions without fully cutting the DNA double strand. Unlike traditional CRISPR-Cas9, which cuts target DNA, PE uses a fusion protein of Cas9 nickase and an engineered Moloney Murine Leukemia Virus reverse transcriptase (MLV-RT). A prime editing guide RNA (pegRNA) specifies both the editing location and the sequence to be synthesized. The pegRNA includes a guide sequence for target recognition, a primer binding site (PBS), and a reverse transcription template (RTT). This design allows for the introduction of point mutations and relatively short insertions/deletions without the need for external donor DNA. However, in practice, results vary significantly depending on the type of editor, the length of PBS and RTT, and transfection conditions. Predicting which pegRNA design will work among candidates is also challenging. Although various PE variants and auxiliary strand-cutting strategies have emerged, consistent procedural guidance for researchers with limited genome editing experience has been insufficient.
Key Findings
The research team presents a practical protocol for performing forward PE and inverse prime editing (iPE) in cultured cells and validating the results. Both methods use reverse transcriptase and pegRNA but differ in the direction of DNA synthesis. In standard PE, the RTT is complementary to the non-target strand, and new DNA is synthesized from the exposed 3′ end. In contrast, the RNA template in iPE is designed to be complementary to the target strand, extending the target-strand DNA in the opposite direction.
This difference is not merely a change in reaction direction. When the design of forward PE is constrained by the location or sequence structure of the protospacer-adjacent motif (PAM) near the target, iPE can provide an alternative approach. The research team has compiled criteria for selecting editors and designing pegRNA PBS and RTT for point mutations, as well as small- to medium-sized insertions and deletions. A notable feature of the experimental workflow is the emphasis on generating multiple candidates and comparing them directly in cells, rather than relying on a single design.
The protocol also addresses methods for evaluating editing outcomes. For reporter edits that result in the appearance or disappearance of fluorescence, flow cytometry is used to measure cell-level efficiency. For actual genomic loci, next-generation sequencing (NGS) is used to distinguish the proportion of intended mutations from unintended insertions/deletions. By bundling procedures from editor introduction to quantitative evaluation, the protocol focuses on reproducible experimental design rather than just operational principles.
Implications and Outlook
This protocol can serve as a starting point for researchers to choose between PE and iPE based on the constraints of the target sequence and to design and select candidate pegRNAs. It can be used to reproduce disease-associated single-nucleotide variants in cells to mimic pathogenesis or to create isogenic models by correcting existing mutations to normal sequences. Its application scope also extends to protein tagging and regulatory sequence analysis using insertions and deletions.
However, laboratory protocols do not immediately translate into therapeutic technologies. Editing efficiency is influenced by cell type, target locus, and DNA repair environment, and genome accuracy cannot be determined solely by flow cytometry results. Even with NGS to identify byproducts, off-target changes at distant sites, large structural variations, and long-term cellular dysfunction require separate analyses. In the future, cell-specific optimization, vector development, and broader safety validation will be necessary. The ability to compare forward and reverse editing within the same experimental system broadens the foundation for subsequent optimization.
Prime editing (PE) is a powerful method for introducing point mutations into the genomes of living organisms. PE utilizes a Cas9 nickase fused to an engineered Moloney Murine Leukemia Virus reverse transcriptase (MLV-RT), paired with an extended guide RNA known as pegRNA, which contains a primer binding site (PBS) and a reverse transcriptase template (RTT) complementary to the non-target strand DNA. Recently described inverse prime editing (iPE) also employs reverse transcriptase and pegRNAs; however, it utilizes an RNA template complementary to the target strand, resulting in the polymerization of target-strand DNA in the opposite direction. In this work, we provide a practical protocol for using either PE or iPE to introduce point mutation(s) or small-to-medium sized insertions and deletions in cell culture, and demonstrate how to assess editing efficiency via flow cytometry and next-generation sequencing. We include recommendations for prime editor selection and straightforward guidelines for pegRNA design, intended for researchers unfamiliar with genome editing technologies.
Pharmaceutical companies and disease model development firms can reproduce point mutations found in patients in normal cells and compare drug responses before and after correction. For example, when a suitable PAM for forward PE is absent at the target, iPE candidates can be designed in parallel, followed by primary screening using fluorescent reporters and precise correction rates and byproducts assessed via NGS. This process has the potential to reduce the time required for verifying the mechanism of action of candidate therapeutics and building cell models for precision medicine. However, in clinical manufacturing, transfection efficiency, off-target effects, large genomic rearrangements, and long-term cell function after prolonged culture must be validated separately.