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Comprehensive assessment of activity, specificity, and safety of hypercompact TnpB systems for gene editing

ABSTRACT: As the ancestor of CRISPR-Cas12 nucleases, TnpB represents the most compact gene editing tool currently available. Recent studies have identified multiple TnpB systems with gene editing activity in mammalian cells, and the potential of TnpB in treating diseases has been demonstrated in animal models. However, the editing characteristics of various TnpB systems, comparable to CRISPR tools, require more extensive investigation. Using a standardized evaluation framework, we conducted a thorough analysis of the editing properties of four TnpB variants alongside representative Cas12 and Cas9 tools. Overall, TnpBs exhibit intermediate editing activity and safety profiles among all tested systems, with ISYmu1 TnpB demonstrating a good performance in both editing activity and specificity. Considering its compact size, potent editing efficiency and high specificity, ISYmu1 TnpB represents a promising candidate for in vivo gene therapy applications.

ORGANISM(S): Homo sapiens

PROVIDER: GSE264389 | GEO | 2025/01/01

REPOSITORIES: GEO

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Project description:Insertion sequences (IS) are compact and pervasive transposable elements found in bacteria, which encode only the genes necessary for their mobilization and maintenance. IS200/IS605 elements undergo ‘peel-and-paste’ transposition catalyzed by a TnpA transposase, but intriguingly, they also encode diverse, TnpB-family genes that are evolutionarily related to the CRISPR-associated effectors Cas9 and Cas12. Recent studies demonstrated that TnpB-family enzymes function as RNA-guided DNA endonucleases, but the broader biological role of this activity has remained enigmatic. Here we show that IscB and TnpB are essential to prevent loss of the donor IS element and potential transposon extinction as a consequence of the TnpA transposition mechanism. We first performed phylogenetic analysis of IscB/TnpB proteins and selected a family of related IS elements from Geobacillus stearothermophilus that we predicted would be mobilized by a common TnpA homolog. After reconstituting transposition using a heterologous expression system in E. coli, we found that IS elements were readily lost from the donor site due to the activity of TnpA in rejoining the flanking sequences back together upon excision. However, these IS elements also encode non-coding RNAs that guide TnpB and IscB nucleases to precisely recognize and cleave these excision products, leading either to elimination of the excision product or re-installation of the transposon through recombination. Indeed, under experimental conditions in which TnpA and TnpB-RNA complexes were co-expressed together with a genomically integrated IS element, transposon retention was significantly increased relative to conditions expressing TnpA alone. Remarkably, both TnpA and TnpB recognize the same AT-rich transposon-adjacent motif (TAM) during transposon excision and RNA-guided DNA cleavage, respectively, revealing a striking convergence in the evolution of DNA sequence specificity between transposase and nuclease. Collectively, our study reveals that RNA-guided DNA cleavage is a primal biochemical activity that arose to bias the selfish inheritance of transposable elements, which was later co-opted during the evolution of CRISPR-Cas adaptive immunity for antiviral defense.

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Comprehensive assessment of activity, specificity, and safety of hypercompact TnpB systems for gene editing

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