flexiAsCas12a and Other Variants Expand Cas12a Nuclease Applications in Prime Editing

A new generation of CRISPR genome-editing enzymes could make it possible to rewrite DNA sequences that have remained out of reach for one of the field’s most precise technologies. Researchers in Hungary have developed a series of Cas12a variants with more flexible target-recognition rules, including a version called flexiAsCas12a that can identify DNA sites previously excluded by the enzyme’s strict requirements. The advance is designed to expand the reach of prime editing, a technique capable of making carefully specified insertions, deletions and substitutions without cutting both strands of the DNA double helix. The findings, reported in Genome Biology, suggest that Cas12a-based editing may become useful across a broader fraction of mammalian genomes, although further work will be needed to assess efficiency, specificity and performance in disease-relevant cells and organisms.

CRISPR systems do not generally cut DNA at arbitrary locations. Their molecular targeting depends on two interacting components: a guide sequence that pairs with the chosen DNA site and a nearby short motif known as a protospacer adjacent motif, or PAM. The PAM acts as a molecular permission signal, allowing a CRISPR nuclease to bind and activate only when the correct neighboring sequence is present. Cas12a, a nuclease widely used for genome editing and nucleic-acid detection, has distinctive advantages, including its ability to recognize a PAM positioned differently from the one used by the more familiar Cas9 enzyme. But those advantages come with a limitation: commonly used Cas12a proteins generally require relatively long or restrictive PAM sequences. A desired genetic target may therefore be perfectly suited to the guide RNA yet remain inaccessible because the adjacent DNA does not satisfy the enzyme’s PAM rule.

Prime editing adds another layer of precision to this problem. Rather than relying on a conventional double-strand break followed by the cell’s repair machinery, prime editing typically uses a modified Cas protein fused to a reverse transcriptase. The Cas component nicks one DNA strand, while a specialized prime-editing guide RNA both directs the complex to the target and carries a template encoding the intended change. The reverse transcriptase copies that template into the DNA, creating a modified strand that the cell can incorporate into the genome. In principle, this arrangement allows researchers to install single-base substitutions, short insertions and deletions while avoiding the potentially disruptive double-strand breaks associated with standard CRISPR cutting. Yet prime editing is only as versatile as the nuclease that delivers it: if the Cas enzyme cannot recognize a nearby PAM, the edit may be impossible or require a less favorable target site.

To loosen that constraint, the team examined and engineered variants related to four Cas12a enzymes: LbCas12a, AsCas12a, MbCas12a and FnCas12a. Their goal was to create PAM-flexible proteins that remained active inside mammalian cells rather than merely showing altered biochemical behavior in a test tube. Among the variants tested, flexiAsCas12a—derived from AsCas12a—proved the most effective in the reported experiments. The researchers found that the variant could extend Cas12a’s recognized PAM repertoire to include sequences described as NATN, NCCN and GTCN. In this notation, N represents any nucleotide, while the specified letters impose only partial constraints on the four-base motif. Compared with a highly restrictive PAM requirement, these patterns make more sites across the genome potentially addressable by the nuclease.

The significance of the expanded recognition rules is not simply that three additional sequence patterns have been added to a catalog. PAM availability is distributed unevenly across DNA, and the position of a PAM relative to a disease-associated mutation, regulatory element or coding sequence can determine whether an edit is practical. A target may require the guide and editing machinery to approach from a particular direction, or the desired alteration may fall within a limited distance from the nick introduced by the nuclease. More permissive PAM recognition increases the odds that a usable site exists in the right genomic context. It could also reduce the need to redesign an experiment around a nearby, imperfectly positioned target. However, broader recognition must be balanced against the risk of unintended activity: an enzyme that accepts more PAMs may encounter more possible sites, making rigorous off-target testing essential before therapeutic applications can be considered.

The study also connects PAM flexibility to a less conventional form of prime editing. The researchers used currently available Cas12a variants with relaxed PAM recognition—impLbCas12a, flexiAsCas12a and enAsCas12a—to develop circular RNA-guided split prime editors. In a split editor, the molecular machinery is divided into separate components rather than delivered as one large protein or complex. This strategy can help address the size limitations that complicate the delivery of genome-editing systems, particularly with viral vectors, whose cargo capacity is restricted. Circular RNAs are RNA molecules whose ends are joined, a structure that can offer greater resistance to degradation than conventional linear RNA. By combining circular RNA guidance with split prime-editing architecture, the researchers sought to create systems that could function at targets carrying non-canonical PAMs—sequences that would not ordinarily be accepted by standard Cas12a tools.

The experiments validated the functionality of these circular RNA-guided split editors on non-canonical PAM sequences, according to the study. That result is important because it demonstrates more than a theoretical change in target recognition: the engineered enzymes could be incorporated into a working prime-editing system in mammalian cells. The work does not, however, establish that every newly recognized target will be edited with equal efficiency, nor does it show that the approach is ready for clinical use. Genome editing performance depends on many variables, including chromatin structure, guide-RNA design, the exact DNA change being attempted, cellular repair pathways and the concentration and duration of the editing components. The study’s central achievement is therefore an expansion of the addressable target space, rather than a universal solution to the technical and safety challenges facing prime editing.

Cas12a’s biology may make these variants attractive for applications beyond prime editing as well. Cas12a enzymes are already used in nucleic-acid detection because their activity can produce detectable signals after target recognition. In genome manipulation, their guide-RNA processing properties and targeting geometry differ from those of Cas9, providing alternative ways to design multiplexed or directionally constrained editing experiments. A broader PAM range could allow researchers to choose among more Cas12a configurations according to the genomic site and desired outcome. The new variants may also be useful when a Cas9-based tool has unfavorable off-target behavior or cannot reach a sequence in the required orientation. Whether flexiAsCas12a or related enzymes outperform established systems will depend on head-to-head measurements of editing yield, product purity, unintended edits, cellular toxicity and delivery efficiency.

The authors describe flexiAsCas12a as a new addition to the expanding collection of Cas12a PAM variants, with the broader objective of making more genomic sequences accessible to precision editing. The work was conducted by researchers affiliated with the HUN-REN Research Centre for Natural Sciences, the University of Szeged, the Biological Research Centre, Eötvös Loránd University, Semmelweis University and Hungarian biotechnology organizations. The article was made available as an early peer-reviewed, accepted version carrying a permanent DOI, with the publisher noting that it may later be replaced by a final version of record. As the field moves toward increasingly programmable genome modification, the practical importance of the advance will be determined by how reliably these flexible nucleases edit difficult targets while preserving the selectivity that makes prime editing appealing. For now, the study offers a molecular workaround to one of CRISPR’s most persistent limitations: the short sequence beside a target that can decide whether the target is editable at all.