The appeal of miniature CRISPR nucleases is straightforward. The most widely used editors, such as Streptococcus pyogenes Cas9, are large multidomain proteins whose coding sequences, together with their guide RNAs and regulatory elements, often exceed the packaging capacity of adeno-associated virus (AAV) vectors, the workhorses of in vivo gene therapy delivery. Compact type V effectors sidestep this bottleneck. Cas12n nucleases, classified as type V-U4, are among the smallest RNA-guided DNA endonucleases known, spanning roughly 400 to 700 amino acids, and are considered evolutionary intermediates between ancestral transposon-encoded TnpB proteins and the larger type V CRISPR effectors such as Cas12a.

Cas12n also brings a distinctive targeting preference. Unlike Cas12f nucleases, which must dimerize to cut double-stranded DNA and favor T-rich PAM sequences, Cas12n enzymes are presumed to function as monomers and recognize an A-rich protospacer-adjacent motif. That preference is orthogonal to the G-rich PAM of SpCas9 and the T-rich PAM of AsCas12a, meaning Cas12n can reach genomic sites inaccessible to existing tools. Yet the family’s performance in mammalian cells is strikingly uneven. While some orthologs, such as those from Actinomadura craniellae and Rothia dentocariosa, edit human genomes, CgCas12n from Corynebacterium glutamicum, despite its compact 537-amino-acid length and close structural similarity to active variants, showed negligible editing activity in prior studies.

To find out why, the research team took a structure-guided approach rooted in electrostatics. Inside the Cas12n enzyme’s active complex, the crRNA and target DNA form a highly negatively charged RNA-DNA heteroduplex. The team hypothesized that boosting local positive charge on the protein surface could stabilize this interaction. Comparing CgCas12n sequences with those of AsCas12a and the active AcCas12n, they identified positions where CgCas12n carried neutral or negatively charged residues while its more capable relatives carried positively charged ones. From this analysis they built 52 single-amino-acid substitution variants and screened them using a fluorescence reporter assay in HEK293T cells, in which nuclease cleavage restores the reading frame of an out-of-frame EGFP gene through repair of the break.

Eleven variants showed significantly improved activity, with gains ranging from about 1.2-fold to 5.2-fold. The team then combined beneficial substitutions into double, triple, and quadruple mutants. The standout was a quadruple mutant designated v4.6, carrying the substitutions D157R, Q170R, T184R, and T423R, which exceeded 20 percent editing efficiency in the reporter assay, an approximately 60-fold improvement over the wild-type enzyme. At endogenous genomic loci with a 5′-AAG PAM in HEK293T cells, the variant achieved indel frequencies of 58.7 percent and 71.4 percent at two target sites, and comparable activity in HeLa and K562 cells. AlphaFold2 structural modeling placed all four introduced arginines near the crRNA-target DNA duplex, consistent with the idea that enhanced electrostatic interactions underlie the improved performance.

Protein engineering was only half the story. The researchers also dismantled and rebuilt the single-guide RNA scaffold. RNAfold-based secondary structure prediction divided the scaffold into four stem-loop domains, and complete deletion of any one abolished activity, confirming that all four contribute to function. Partial truncation screening revealed that only cuts within the second stem-loop retained substantial activity. The team further shortened the scaffold by deleting single base pairs from the tracrRNA-crRNA pairing region, and the best variant, called D19, preserved roughly 92 percent of wild-type activity while shaving 38 base pairs off the guide. Combining that truncation with the optimal stem-loop-2 trim produced the T6D19 scaffold, just 131 base pairs long, which actually outperformed the wild-type guide. Pairing the v4.6 protein with T6D19 yielded the fully engineered system, eCgCas12n, which roughly doubled average editing efficiency at endogenous loci compared with protein optimization alone, lifting average indel frequencies from about 20 percent to about 40 percent.

Importantly, the gains did not come at the cost of accuracy. Deep sequencing of 18 predicted off-target sites, identified with Cas-OFFinder allowing up to four mismatches, detected no significant indel formation, and the engineered system showed no increase in off-target editing relative to the wild-type configuration. Mismatch tolerance testing showed that editing dropped sharply when the guide carried single- or double-nucleotide mismatches, especially in the PAM-proximal region, reflecting the stringent target recognition typical of type V nucleases. In head-to-head comparisons at 18 shared target sites, eCgCas12n matched the editing efficiencies of SpCas9 and AsCas12a, cementing its credentials as a compact but competitive alternative.

The team then converted the nuclease into a base editor. They introduced a D297A mutation in the conserved RuvC domain to create a catalytically dead version, but early fusions with the deaminases TadA-8e and rAPOBEC, both optimized for Cas9-based systems, failed to produce detectable base conversion, suggesting that Cas9-compatible deaminases do not transfer readily to the single-RuvC architecture of Cas12-family effectors. Switching to the eCDA1 cytidine deaminase, fused to the N-terminus through an XTEN linker and augmented with tandem uracil glycosylase inhibitor domains, solved the problem. The resulting eCgCas12n-CBE mediated efficient C-to-T conversion across a broad editing window spanning spacer positions 2 through 17, reached efficiencies up to about 60 percent at individual loci with an average near 15 percent, showed little sequence-context bias, and produced desired edited alleles with purity above 95 percent while generating virtually no unintended indels. RNA sequencing and an orthogonal R-loop assay using catalytically inactive SpCas9 indicated only minimal off-target deamination at the transcriptome and genome levels.

As a functional demonstration, the researchers aimed the base editor at the mouse Dmd gene, which encodes dystrophin, the membrane protein whose loss causes Duchenne muscular dystrophy. By converting cytosines within glutamine, arginine, and tryptophan codons, the editor can install premature stop codons that trigger nonsense-mediated mRNA decay. In N2a cells and C2C12 myoblasts, eCgCas12n-CBE achieved editing efficiencies above 26 percent at the best target sites, with stop codon induction averaging about 10 percent. Edited C2C12 cells showed markedly reduced dystrophin and myosin heavy chain protein, decreased Dmd mRNA, impaired myotube formation, and downregulation of key myogenic markers, faithfully recapitulating the differentiation defects seen in DMD models.

The most striking result came in vivo. Because of its compact size, the entire eCgCas12n-CBE cassette fit inside a single AAV9 vector. Intramuscular injection into the tibialis anterior muscles of C57BL/6 mice produced C-to-T conversion at the Dmd target site with an average efficiency of about 11 percent, alongside significantly reduced dystrophin-positive myofibers, lower Dmd mRNA, and diminished dystrophin protein four weeks after treatment. The study also showed that eCgCas12n-CBE outperformed TSmini-CBE, a cytosine base editor built on the engineered ultra-compact enUnCas12f scaffold, at most of twelve disease-relevant human loci drawn from ClinVar. Together, the findings establish a generalizable strategy, pairing arginine enrichment of the protein with rational guide RNA minimization, for converting naturally weak miniature nucleases into practical editing platforms, and position eCgCas12n as a promising candidate for therapeutic applications where AAV delivery constraints make every base pair count.

Subject of Research: Engineering the compact CRISPR-Cas12n nuclease for efficient mammalian genome and base editing

Article Title: Engineering a compact CgCas12n platform for efficient genome and base editing in mammalian cells

Article References: Hou, B., Liu, X., Fan, J., Wang, W., Jin, C., Li, G., Zhang, Y., Yu, W., Huang, L., Li, X., Huang, X., & Li, K. (2026). Engineering a compact CgCas12n platform for efficient genome and base editing in mammalian cells. Molecular Systems Biology. https://doi.org/10.1038/s44320-026-00248-z

Image Credits: AI Generated

DOI: 10.1038/s44320-026-00248-z

Keywords: CRISPR, Cas12n, CgCas12n, genome editing, base editing, cytosine base editor, AAV9 delivery, guide RNA engineering, Dmd gene, Duchenne muscular dystrophy, miniature CRISPR, off-target specificity

The research, led by Çağrı Doğan of the Department of Medical Genetics at Ordu University Training and Research Hospital together with pediatric neurologist Seren Aydın of Ondokuz Mayıs University Hospital, examined every consecutive individual tested for DMD variants at their centre between January 2022 and February 2026. Forty-seven individuals passed through the diagnostic pipeline, 29 female and 18 male, with a median age of 12.8 years. The team applied two complementary molecular techniques: multiplex ligation-dependent probe amplification, or MLPA, a copy-number detection method available for 45 of the 47 participants, and whole-gene sequencing, performed in 16 individuals and run in parallel with MLPA in 13. All variants were described against the reference transcript NM_004006.3 and classified according to the 2015 ACMG/AMP standards for sequence variants and the 2020 ACMG/ClinGen standards for copy-number variants.

The headline result is deceptively simple: pathogenic variants were identified in 19 of the 47 individuals. Ten of these were probands, the first affected or index individuals in their families to receive a molecular diagnosis, and nine were heterozygous female carriers discovered through cascade screening, the systematic testing of relatives once a familial variant is known. In index testing, the diagnostic yield was 30.3 percent, or ten of 33 individuals tested, with a 95 percent confidence interval of 17.4 to 47.3 percent. Once a familial variant was established, cascade screening detected carriers in nine of 14 relatives tested, a carrier-detection rate of 64.3 percent with a confidence interval of 38.8 to 83.7 percent. Those figures underline a point that geneticists have long emphasised: identifying one patient can protect an entire extended family.

The variant spectrum itself conformed closely to what has been reported across Türkiye and worldwide. Among the nine families with characterised variants, seven carried deletions and two carried single-nucleotide sequence variants: a nonsense change, NM_004006.3:c.701C>G producing p.(Ser234Ter), and a canonical splice-site variant, c.2803+1G>T, which disrupts the invariant guanine at the first position of an intron and is predicted to derail RNA splicing. Strikingly, six of the nine families carried deletions within the so-called distal hotspot spanning exons 45 to 55, a known deletion-prone region of this extraordinarily large gene. Three of the deletions were in-frame, meaning the reading frame of the dystrophin protein was preserved, and four were out-of-frame, consistent with the classical reading-frame rule that links out-of-frame disruptions to the severe Duchenne phenotype and in-frame disruptions to the milder Becker form. Each deletion was cross-verified against public resources, the Leiden Open Variation Database and the DOVE deletion-out-of-frame evaluation tool.

The study also captured a subtler and less frequently discussed facet of dystrophinopathy: female carriers with biochemical evidence of muscle damage. Four heterozygous women, three of whom were probands in their own right rather than relatives of an affected boy, presented with unexplained hyperCKemia, with creatine kinase values ranging from 776 to 2,816 U/L, but no documented muscle weakness. The one carrier who underwent complete evaluation had normal strength and a normal echocardiogram. This pattern reflects the stochastic nature of X-chromosome inactivation, the process by which female cells randomly silence one X chromosome. When inactivation happens to favour the chromosome carrying the mutant DMD allele, enough dystrophin-deficient muscle fibres can exist to leak creatine kinase into the blood, occasionally producing overt disease in women, including cardiomyopathy, decades before anyone suspects a condition classically associated with boys.

Two of the deletions were apparently de novo, arising fresh in the affected child rather than being inherited from a carrier mother, a well-recognised phenomenon in DMD given the enormous size of the gene and its high spontaneous mutation rate. Equally consequential for treatment planning, none of the probands in this series carried a variant amenable to an approved exon-skipping oligonucleotide. These antisense drugs, including eteplirsen, golodirsen, viltolarsen and casimersen, work by masking specific exons during RNA splicing so that the reading frame is restored and a shorter but partly functional dystrophin protein can be made. Eligibility depends entirely on the exact exon boundaries of the deletion or point mutation, which is why a precise molecular diagnosis is not merely academic but the gateway to genotype-matched therapy.

Technically, the study illustrates the standard two-step architecture of DMD molecular diagnostics. MLPA, first described by Schouten and colleagues in 2002, uses pairs of probes that ligate only when bound to their target sequences and are then amplified, allowing relative quantification of all 79 exons of the DMD gene in a single reaction; it remains the most efficient way to detect the exon deletions and duplications that account for the majority of pathogenic DMD variants. When MLPA is negative, sequencing of the gene captures the point mutations, small insertions and deletions, and splice-site changes that copy-number assays cannot see. Running both methods in parallel, as this centre did in 13 cases, accelerates turnaround. The authors’ adherence to Human Genome Variation Society nomenclature and to consensus ACMG classification frameworks ensures that their variants can be compared directly with entries in international databases such as the TREAT-NMD global mutation registry, which has catalogued more than 7,000 DMD mutations.

The regional significance of the work lies in establishing a baseline. Previous Turkish series, including a 2019 analysis of 260 dystrophinopathy patients and later single-centre surveys, had sketched the national mutation spectrum, but the Central-Eastern Black Sea provinces had remained a blank spot on that map. The new data suggest that the local spectrum is broadly congruent with national and global patterns, dominated by deletions concentrated in the distal hotspot. That congruence matters for genetic counselling: it means clinicians in the region can reasonably apply the established genotype-phenotype correlations and exon-skipping eligibility frameworks without needing region-specific correction, while still sequencing each family’s variant to confirm its exact boundaries.

The authors are careful about the limits of what such a small series can prove. With only ten probands, the reported proportions are descriptive rather than prevalence estimates, and genotype-based phenotype assignment in individual patients requires prospective confirmation with longitudinal follow-up. The retrospective, single-centre design also means the cohort reflects referral patterns and test availability at one hospital rather than systematic population screening. Still, the cascade-screening outcome, nine additional heterozygotes identified from fourteen relatives, is a concrete demonstration of why molecular diagnosis should trigger immediate family testing. Every carrier identified can be offered cardiac surveillance, reproductive counselling including prenatal and preimplantation genetic options, and the knowledge that ultimately saves the next generation of boys in the family from a delayed diagnosis.

For a disease in which early corticosteroid treatment, standardised cardiac and respiratory care, and emerging gene-targeted therapies all depend on catching patients early, the message from Türkiye’s Black Sea coast is clear: find the variant in one child, and the laboratory work has only just begun. The first regional map of DMD variants is now drawn, and its most important lines may be the family trees radiating outward from each proband.

Subject of Research: Mutational spectrum of DMD gene variants and cascade screening for Duchenne and Becker muscular dystrophy in Türkiye

Article Title: Mutational Spectrum of DMD Gene Variants and Cascade Screening Outcomes in the Central-Eastern Black Sea Region of Türkiye: A Single-Centre Retrospective Study

Article References: Doğan, Ç., & Aydın, S. (2026). Mutational Spectrum of DMD Gene Variants and Cascade Screening Outcomes in the Central-Eastern Black Sea Region of Türkiye: A Single-Centre Retrospective Study. Molecular Biology Reports, 53(1), Article 1600. https://doi.org/10.1007/s11033-026-12785-9

Image Credits: AI Generated

DOI: 10.1007/s11033-026-12785-9

Keywords: Duchenne muscular dystrophy, DMD gene, Becker muscular dystrophy, cascade screening, MLPA, female carriers, exon deletions, genetic testing, dystrophinopathy, hyperCKemia, Türkiye, X-chromosome inactivation