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One of the biggest obstacles in targeting CRISPR therapy deliveries directly into the body isn’t the editing chemistry, it’s the size of the editors themselves. The field’s workhorse nucleases, including Cas9 and Cas12a, are considerably large (exceeding 1,300 amino acids) to fit inside adeno‑associated virus (AAV) vectors, the most widely used delivery vehicle for in vivo gene therapy. That size mismatch has forced most clinical applications to rely on ex vivo editing of blood or bone‑marrow‑derived cells, leaving many tissues out of reach. A smaller CRISPR system that can be packaged into AAV without sacrificing efficiency has long been a key missing piece.

A new study published in Nature Structural & Molecular Biology takes a major step toward that goal. Researchers at the University of Texas at Austin and collaborators report the discovery and engineering of a compact Cas12f nuclease that performs robustly in human cells, a notable advance for a class of miniature enzymes that have historically shown lower efficiencies in mammalian cells compared to larger systems. The paper is titled, “Comparative characterization of Cas12f orthologs reveals mechanistic features underlying enhanced genome editing efficiency.”

The team began by mining metagenomic datasets for naturally small CRISPR enzymes and identified a previously uncharacterized ortholog, Alistipes sp. Cas12f (Al3Cas12f). Despite its compact size—roughly one‑third that of Cas9—the nuclease showed unexpectedly strong activity in human cells. In initial screens, Al3Cas12f produced more than 50% editing at many genomic sites and exceeded 90% at several targets. The authors wrote, “Results from a gRNA screen targeting intron 1 of the ALB gene, exon 3 of the APOA1 gene and the AAVS1 site within PPP1R12C intron 1 showed that 27 target sites displayed >10% editing, 19 sites displayed >50% editing and 10 sites displayed >90% editing across AAVS1 and APOA1.”

Cryo‑EM structures revealed why this miniature enzyme punches above its weight. Compared with other Cas12f orthologs, Al3Cas12f forms a more extensive and interlocking dimer interface, creating a stable, preassembled complex that supports efficient R‑loop formation. The guide RNA scaffold also appears naturally streamlined: unlike other Cas12f gRNAs, it lacks an extraneous stem‑loop and adopts a compact conformation that docks cleanly into the protein. As the authors noted, Al3Cas12f achieves “efficient R‑loop formation through a stable dimer interface and a naturally optimized gRNA.”

Using these structural insights, the team engineered an enhanced variant, Al3Cas12f RKK, that dramatically boosts editing efficiency across genomic loci. In human cells, the variant increased editing from below 10% to more than 80% at many targets, with some sites reaching 90%. The researchers tested the system in a leukemia‑derived human cell line, focusing on genes implicated in cancer, atherosclerosis, and ALS.

The mechanistic comparisons were equally revealing. By solving the structures of two additional Cas12f orthologs—Oscillibacter sp. Cas12f and Ruminiclostridium herbifermentans Cas12f—the team noted “divergent architectures and regulatory features governing protospacer-adjacent motif recognition, gRNA binding, dimerization, and DNA cleavage.” Al3Cas12f’s extended helices and mortise‑and‑tenon‑like interactions appear to be lineage‑specific adaptations that stabilize the nuclease and support high activity.

The next step is to test whether the enzyme maintains its performance when packaged into AAV vectors. If successful, the system could offer a blueprint for engineering future generations of compact CRISPR tools.

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