The editors under scrutiny all share the same basic architecture: a Cas9 nickase fused to an adenosine deaminase, guided to its target by a single guide RNA, or sgRNA. The deaminase chemically converts adenosine to inosine within a short editing window, and cellular machinery reads inosine as guanosine, effectively rewriting the genetic letter. Because the process avoids creating double-strand breaks, base editors sidestep many of the risks associated with conventional CRISPR nucleases, including unwanted indels and chromosomal rearrangements. That safety profile has made them attractive candidates for treating human genetic diseases, from retinal degeneration to metabolic disorders.

The catch lies in the targeting constraints of the underlying Cas9 scaffold. Wild-type SpCas9, the workhorse of the field, requires a protospacer adjacent motif, or PAM, with the sequence NGG immediately downstream of the target site. This strict requirement means that many disease-causing mutations simply sit out of reach. Two engineered variants changed the calculus. SpCas9-NG, developed through protein engineering, recognizes a relaxed NG PAM, dramatically expanding the pool of editable sites. SpRY, described as a near-PAMless variant, pushes this flexibility even further, approaching unconstrained genome targeting. Both variants have been incorporated into adenine base editor systems, but until now, no systematic head-to-head comparison existed to tell researchers when the added flexibility is worth the trade-off.

The research team constructed three editor systems, which they refer to as NGG-ABE8e, NG-ABE8e, and SpRY-ABE8e, pairing the high-activity ABE8e deaminase domain with each Cas9 scaffold. They then evaluated all three in human cells, measuring editing activity, the width of the editing window, and the editors’ ability to discriminate against mismatched target sequences. The results revealed a nuanced picture rather than a simple hierarchy. NGG-ABE8e, built on the conventional SpCas9 scaffold, delivered higher editing activity at most of the tested NGG-PAM sites, confirming that the original enzyme remains a potent choice when the target sequence happens to sit near a canonical PAM.

SpRY-ABE8e, by contrast, showed a distinct advantage in specificity. In targeted off-target analyses at selected predicted sites bearing NGG PAMs, the SpRY-based editor displayed stronger mismatch discrimination and lower detectable A-to-G editing than its counterparts. This finding carries real clinical weight. Off-target editing, in which the editor chemically alters unintended sites in the genome, remains one of the central safety concerns for any therapeutic application. An editor that tolerates a broader range of PAMs might be expected to be sloppier, yet the data suggest SpRY-based editors can be more discriminating at certain off-target loci, a property that could make them preferable for applications where precision outweighs raw efficiency.

Beyond the choice of scaffold, the study delved into a subtler variable that is often overlooked in guide RNA design: the length of the spacer sequence and the addition of an extra guanine at the 5′ end. Guide RNAs are typically transcribed from U6 promoters, which have a strong preference for starting with a guanine, and a common practice has been to append a 5′ G when the native spacer does not begin with one. The team found that both spacer length and 5′ G addition affected editing outcomes, but crucially, these effects were site- and editor-dependent. There was no universal recipe that worked across all targets.

Perhaps the most striking practical lesson concerns the added 5′ guanine itself. The conventional wisdom held that appending this nucleotide was generally harmless or even beneficial. The new data overturn that assumption. Adding an extra 5′ G was not uniformly helpful and could actually reduce editing efficiency, particularly when the native spacer already began with a guanine. In other words, the very practice intended to standardize guide design can silently undermine performance at some targets. For laboratories designing guides for therapeutic editing, this means the sequence context of each individual target must be considered rather than relying on blanket rules.

The team did not stop at synthetic test targets. They extended their comparison to endogenous genomic sites, including therapeutic targets of clinical interest. Among these was a mutation in the Rhodopsin gene, which is associated with autosomal dominant retinitis pigmentosa, a devastating inherited form of progressive blindness. The editors were tested for their ability to correct this disease-relevant mutation, providing a proof-of-concept demonstration that the selection principles established in the study translate to real therapeutic scenarios. Retinal diseases are a particularly compelling frontier for base editing because the eye is relatively accessible to local delivery and immune-privileged, and prior work has already shown that adenine base editors can correct patient-specific Rhodopsin mutations in mouse models.

The broader significance of the study lies in its reframing of editor selection as an engineering decision with quantifiable trade-offs. For a mutation located near an NGG PAM, the conventional NGG-ABE8e offers maximum activity. For targets where specificity is paramount, or where predicted off-target sites with NGG PAMs raise concerns, SpRY-ABE8e may be the safer bet despite potentially lower on-target efficiency. SpCas9-NG-derived editors occupy the middle ground, extending the reachable target space beyond the canonical PAM while retaining substantial activity. Layered on top of scaffold choice is the guide design dimension, where spacer length and 5′ nucleotide composition must be tuned per target. The authors also note that the field is moving toward predictive tools, including deep learning models that forecast base editing efficiencies in different cellular contexts, and empirical datasets like this one provide exactly the kind of ground truth such models need.

As base editors advance from cell culture experiments toward clinical trials, with in vivo editing of PCSK9 in macaques already demonstrating cholesterol reduction and approved CRISPR therapies for sickle cell disease establishing regulatory precedent, the questions this study addresses are no longer academic. Every therapeutic program must justify its choice of editor and guide design to regulators, and evidence that a given combination maximizes on-target correction while minimizing off-target risk is essential to that case. By systematically benchmarking three editor architectures across activity, specificity, and guide design variables, and by validating the findings at endogenous and therapeutic sites, the researchers have converted a confusing menu of options into a rational decision framework. For a field racing toward precision medicine, knowing which molecular tool to reach for, and how to load it correctly, may prove as important as the tools themselves.

Subject of Research: Comparative evaluation of SpCas9 and variant-derived adenine base editors and sgRNA design for human genome editing

Article Title: Rational selection of SpCas9 and variant-derived adenine base editors for human genome editing

Article References: Wang, J., He, L., Li, L., Zhang, Y., Wang, Y., Yang, Y., Song, Z., Liang, Y., Yang, J., & Gu, F. (2026). Rational selection of SpCas9 and variant-derived adenine base editors for human genome editing. Gene Therapy. https://doi.org/10.1038/s41434-026-00639-9

Image Credits: AI Generated

DOI: 10.1038/s41434-026-00639-9

Keywords: adenine base editors, SpCas9, SpCas9-NG, SpRY, ABE8e, CRISPR, sgRNA design, PAM, off-target editing, gene therapy, Rhodopsin mutation, retinitis pigmentosa