Base editors represent one of the most elegant refinements of the original CRISPR-Cas9 concept. Rather than cutting both strands of DNA and relying on the cell’s error-prone repair machinery—a process that generates random insertions and deletions—base editors fuse a catalytically impaired Cas nickase to a deaminase enzyme that chemically converts one nucleobase into another. Cytosine base editors, the class at the center of the new work, convert C-G base pairs into T-A pairs without creating double-strand breaks or requiring a donor DNA template. This is a critical advantage given that more than half of known human hereditary diseases are caused by single point mutations, and that precision breeding in agriculture likewise demands exact nucleotide changes rather than gene knockouts.

The Achilles’ heel of conventional cytosine base editors has been precision. Most editors operate across a wide activity window spanning roughly four to seventeen nucleotides, meaning they can alter not just the intended target cytosine but also nearby ‘bystander’ cytosines. That is a serious problem in a clinical context: many disease-associated alleles, including those in the HBB locus linked to beta-thalassemia, the APOE4 gene associated with Alzheimer’s disease, and the TYR locus involved in oculocutaneous albinism, contain multiple cytosines within a typical editing window. Even synonymous substitutions, which do not change the encoded amino acid, have been shown to exert strong non-neutral effects, underscoring the risk of any unintended edit.

The research team, building on their earlier development of high-precision editors based on CDA1, a deaminase from the sea lamprey, tackled two intertwined limitations: targeting scope and efficiency. Previous work had shown that trimming thirteen to twenty amino acids from the C-terminus of CDA1 restricts the editing window so that the editor overwhelmingly modifies a single cytosine at position C-18 relative to the PAM sequence. But those editors still required a guanine within the PAM, leaving many genomic sites inaccessible. To eliminate this constraint, the researchers replaced the standard Cas9 nickase with nSpRY, an engineered variant that tolerates nearly any three-nucleotide sequence as a PAM, effectively enabling editing at almost any genomic location.

A series of experiments in yeast demonstrated that the combination worked, and in a surprisingly beneficial way. Seven editor variants were built, pairing nSpRY with full-length or truncated CDA1, and tested on twelve target sites lacking NGG PAM motifs, including demanding stretches of consecutive cytidines. The truncated variants produced editing windows of only one to two nucleotides, centered on C-18, and frequently converted the target cytosine two to three times more efficiently than its nearest neighbor. Even more strikingly, the truncated SpRY editors outperformed the full-length editor at position C-18 itself—by up to 6.11-fold at certain sites. Deep sequencing of the sgRNA expression plasmids revealed why: the full-length editor engaged in broad ‘self-editing’ of the guide RNA spacer, introducing multiple mismatches that cripple CRISPR targeting, whereas the truncated editor confined self-editing mostly to a single position, preserving a larger pool of functional guides.

The team then distilled their findings into a straightforward four-step design workflow that allows researchers to precisely edit any user-chosen cytidine in a genome: identify the target cytosine, design a guide RNA that places it at position -18, select the appropriate editor based on the PAM context—standard nCas9 editors for NGG PAMs, Cas9-NG or SpG editors for NGH PAMs, and SpRY editors for everything else—and pick the optimal CDA1 truncation. Validating the approach, the researchers targeted each of three consecutive cytidines at a randomly chosen yeast locus. Their best editor, CDA1Δ194-SpRY-BE3, converted the intended cytosine 1.05- to 7.41-fold more efficiently than its bystander neighbors, with single-target products making up 41.60 to 63.65 percent of all edited reads. The full-length control editors, by contrast, produced fewer than 8.7 percent of the desired single-target products, instead yielding doubly and triply edited molecules.

With precision and targeting scope secured, the remaining challenge was efficiency, which is often compromised in PAM-flexible Cas9 variants. The researchers systematically screened fifteen DNA-interacting proteins spanning four functional classes—transcription activators, chromatin-associated factors, pioneer transcription factors, and bacterial DNA-bending peptides—fusing each to the N-terminus of their best editor. Most fusions reduced activity, but two stood out: HMGN1, a nucleosome-binding chromatin protein, and VP64, a potent viral transcriptional activator domain. Individually they delivered modest gains, averaging 1.17-fold and 1.42-fold respectively. But combining both factors produced a dramatic synergistic effect: across thirty-six tested target sites, the dual fusion boosted average editing efficiency 4.24-fold, with improvements observed at every site examined. In multiplex editing experiments using two guide RNAs simultaneously, the fusion editor achieved 6.86-fold and 2.31-fold efficiency increases at two loci while retaining its narrow editing window and generating 40.61 percent and 26.12 percent C-18-only products, compared with just 3.2 and 7.98 percent for the control editor. The strategy also generalized: fusing HMGN1 and VP64 to an rAPOBEC1-based SpRY editor yielded an average 2.47-fold efficiency gain across eleven loci.

Safety analyses confirmed that the enhanced editors did not sacrifice specificity. Whole-genome sequencing of edited yeast showed that truncated-CDA1 editors produced fewer genome-wide single-nucleotide variants than full-length constructs, and although VP64 caused a slight uptick in SNVs—likely because the activator domain can expose single-stranded DNA by altering local transcription—the fusion editors remained well below the off-target levels of canonical editors. Off-target RNA editing was likewise negligible for truncated variants, and targeted sequencing of predicted guide-dependent off-target sites showed frequencies comparable to controls. The truncated CDA1Δ190 variant in particular displayed transition levels nearly indistinguishable from unedited controls.

Crucially, the platform extended beyond yeast. In rice cells, the optimized truncated editor improved editing efficiency at eighteen of twenty endogenous target sites relative to full-length controls, with gains ranging from 1.32-fold to an extraordinary 4,617.95-fold and a maximum efficiency of 61.09 percent. Adding the HMGN1-VP64 fusion pushed performance further still, with improvements of up to 5,938.93-fold and a peak efficiency of 78.57 percent. In a direct head-to-head comparison with ePPE, a plant-optimized prime editor, at seven non-NGG target sites, the fusion base editor outperformed prime editing at six of seven sites, achieving efficiencies 2.27- to 28.61-fold higher and averaging 18.53 percent versus 5.31 percent for ePPE, albeit with slightly more bystander editing.

The authors position their enhanced base editors and prime editors as complementary tools rather than competitors: prime editors excel at installing diverse edit types with high product purity, while the new high-precision base editors offer unmatched efficiency for C-to-T transitions at previously unreachable genomic sites. Remaining challenges include the lower absolute efficiency at NYN PAMs, where SpRY’s intrinsic DNA-binding affinity appears rate-limiting, suggesting that next-generation variants such as the chimeric SpRYc could push the system even further. For now, the work delivers a versatile toolkit—one that pairs surgical precision with near-universal targeting reach and robust efficiency across yeast and rice—whose implications extend from functional genomics and precision crop breeding toward the long-term goal of safely correcting the point mutations that underlie much of human genetic disease.

Subject of Research: Engineering synergistic HMGN1 and VP64 fusions with truncated CDA1 and PAM-flexible SpRY Cas9 for high-precision cytosine base editing

Article Title: Synergistic HMGN1 and VP64 Fusions Potentiate High‐Precision and PAM‐Flexible Base Editing

Article References: Luo, X., Qu, Y., Ye, Z., Li, Z., Zhang, Y., Luo, L., Li, S., Zhao, W., Wang, M., Bock, R., Wan, J., & Tan, J. (2026). Synergistic HMGN1 and VP64 Fusions Potentiate High‐Precision and PAM‐Flexible Base Editing. Advanced Science, 13(50), Article e76047. https://doi.org/10.1002/advs.76047

Image Credits: AI Generated

DOI: 10.1002/advs.76047

Keywords: base editing, CRISPR-Cas9, SpRY, CDA1 deaminase, HMGN1, VP64, PAM-flexible targeting, bystander editing, cytosine deamination, rice genome editing, prime editing, gene therapy