Fluorescent Reporter System Boosts Prime Editing Efficiency in Stem Cells

Researchers have detailed a step-by-step protocol for a fluorescence-based enrichment method, called PINE-TREE, that substantially raises the recovery of prime-edited human pluripotent stem cells, a cell type long considered stubbornly resistant to precise genome modification. The protocol, published in BMC Methods, describes how laboratories can use a transient fluorescent reporter to single out the rare cells in which prime editing machinery has actually done its work, then expand those cells into clonal, isogenic lines suitable for disease modeling and regenerative medicine research.

Prime editing is a CRISPR-derived technique that can introduce precise insertions, deletions, and base substitutions without cutting both strands of the DNA double helix. Because it relies on a nickase-based mechanism paired with a reverse transcriptase rather than the double-stranded breaks produced by conventional Cas9 nucleases, it tends to minimize genomic instability and unintended insertions, deletions, or rearrangements. That precision makes it attractive for applications where genomic integrity is critical, including stem cell work. Yet the method has faced a persistent obstacle in human pluripotent stem cells, which are hypersensitive to DNA damage and carry robust mismatch repair mechanisms, resulting in low editing efficiency that has restricted broader use.

The PINE-TREE system, short for Prime-Induced Nucleotide Engineering using a Transient Reporter for Editing Enrichment, addresses that bottleneck by coupling editing activity to a visible signal. The approach uses a transient, episomal fluorescent reporter in which a blue fluorescent protein gene is engineered to convert into a green fluorescent protein gene upon a specific C-to-T base conversion. When human pluripotent stem cells are co-transfected with the reporter plasmid, a prime editor plasmid, and a guide RNA plasmid, only cells in which the prime editor and its pegRNA are functionally active display the green signal. That signal directly reflects editing at the co-transfected episomal construct within the same cell.

This design distinguishes PINE-TREE from enrichment strategies based on fluorescently labeled editor or reporter plasmids, which confirm only that DNA was delivered or expressed but say nothing about whether productive editing occurred. Because reporter activation is a direct readout of genome editing, the sorted green-positive population shows significantly higher frequencies of on-target genomic edits than unsorted populations or cells enriched by conventional transfection reporters. Forty-eight hours after transfection, dual blue-and-green positive cells are isolated by fluorescence-activated cell sorting, and the authors report that among green-positive cells enriched this way, editing efficiencies can reach 80 percent in bulk-sorted populations as validated by Sanger or next-generation sequencing of the target site.

The numbers reported in the protocol’s expected results section illustrate the practical workflow. In human pluripotent stem cells edited with a PE3b-based strategy, up to 5 to 10 percent of cells may show the dual blue-green signal by 72 hours after transfection. Following single-cell sorting into 96-well plates, colonies typically emerge by about day seven, and outgrowth can be expected from roughly 10 to 40 wells per plate depending on the cell line, transfection efficiency, and plating conditions. Most clones harbor the intended edit in either a heterozygous or homozygous state, consistent with the bulk-sorted results.

A notable feature of the system is that the reporter is transient. The PINE-TREE reporter is built on non-integrating episomal plasmids that are gradually lost over successive cell divisions. By 16 to 20 days after transfection, edited clones no longer show detectable fluorescence, consistent with plasmid loss and the establishment of stable, transgene-free edited lines. That matters for researchers who need edited stem cells free of lingering foreign genetic material before differentiation or downstream functional assays.

The protocol walks through four main stages: design and cloning of pegRNAs using a companion software tool called PINE-CONE, multiplexing the pegRNA and its nicking guide RNA into the reporter vector, transfection and fluorescence-based cell sorting, and clonal expansion with genotyping validation. PINE-CONE, a publicly available desktop application, generates validated pegRNA and nicking sgRNA sequences with the correct overhangs ready for cloning, and is the recommended design method. The authors also describe a two-step cloning strategy that inserts a U6 promoter upstream of both guide sequences in intermediate plasmids before final assembly, which they say improves cloning efficiency and allows sequence-validated guides to be tested in different combinations.

Quality control receives detailed attention. Edited clones are assessed for editing at the top three to five predicted off-target loci using tools such as CCTop, with validation by targeted sequencing. The protocol also includes confirmation of pluripotency marker expression for OCT4, NANOG, and SOX2 by immunofluorescence, tri-lineage spontaneous differentiation into ectoderm, mesoderm, and endoderm with lineage-specific marker staining, brightfield assessment of characteristic stem cell colony morphology, and conventional G-banding karyotype analysis to detect large chromosomal abnormalities. The authors note that they have previously shown the method does not increase the presence of indels in edited cell populations, and that they report minimal off-target effects in enriched populations, while cautioning that further investigation is warranted as additional prime editing modalities are applied across diverse genomic loci.

The system currently uses PE2 and PE3b editor architectures, both validated in human pluripotent stem cells, with PE3b favored for its higher efficiency. The authors suggest that newer editor variants such as PEmax, PE4, PE5, PE6, and PE7 could be incorporated to potentially improve performance, and that engineered pegRNAs with stabilizing 3′ motifs and optimized reverse transcriptase template and primer binding site lengths can further raise success rates. They also acknowledge limitations of the Lipofectamine-based chemical transfection at the heart of the protocol, including batch variability and cytotoxicity in sensitive cell types, and point to nucleofection, electroporation, or viral delivery as alternatives that would require their own optimization.

Although developed and validated in human pluripotent stem cells, the authors argue the approach holds promise for broader use across mammalian cell types, including immortalized lines and primary cells, and may extend to differentiated progeny such as neurons, cardiomyocytes, and hepatocytes for modeling tissue-specific variants. The method has already been demonstrated in both HEK293T cells and human pluripotent stem cells across multiple genomic loci. By enabling efficient generation of precisely edited clonal populations without stable integration of the editor or repeated transfection rounds, the protocol aims to make isogenic cell line production more accessible for developmental biology, disease modeling, and regenerative medicine research.

Subject of Research: A fluorescence-based enrichment protocol for improving prime editing efficiency in human pluripotent stem cells

Article Title: A simple and versatile fluorescence-based method to enhance prime editing in human pluripotent stem cells

Article References: Frisch, C., Leyasi, S., Kostes, W. W., Tekel, S. J., Standage-Beier, K., Srinivasan, G., Wang, X., & Brafman, D. A. (2026). A simple and versatile fluorescence-based method to enhance prime editing in human pluripotent stem cells. BMC Methods, 3(1), Article 33. https://doi.org/10.1186/s44330-026-00077-4

Image Credits: AI Generated

DOI: 10.1186/s44330-026-00077-4

Keywords: prime editing, human pluripotent stem cells, PINE-TREE, genome editing, CRISPR, fluorescence-activated cell sorting, pegRNA, PINE-CONE, isogenic cell lines, regenerative medicine, off-target effects, episomal reporter