Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)-Cas9 enables induction of chromosomal inversions from hundreds of base pairs to millions of base pairs, but the factors influencing inversion frequency are not well understood. Prior reports differ in species, detection methods, and delivery strategies, making direct comparisons difficult. We addressed this by introducing a normalisation strategy based on a reference guide RNA (gRNA) as an internal standard and testing inversion sizes spanning kilobases to tens of megabases in tomato protoplasts. Tomato (Solanum lycopersicum) protoplasts were transfected with constructs encoding a fixed “reference” gRNA and a second “variable” gRNA positioned at increasing genomic distances, creating potential inversions from 1 kilobase to 37.5 megabases. Using the reference gRNA to normalise across samples, we found that up to ~ 1 megabase, inversion frequency tracked the cutting efficiency of the less efficient gRNA, consistent with gRNA activity being a major contributor within the chromosome tested. For these intervals, the inversion frequencies reached up to 1.24% when both gRNAs were efficient. Above ~ 1 megabase, inversion frequencies declined sharply despite efficient cutting, suggesting a size-dependent barrier to inversion formation; for example, 37.5 megabase inversions occurred at substantially lower frequency (up to 0.18%) despite efficient gRNAs. Because each interval corresponds to a distinct genomic location. Inversions were only observed when both gRNAs were active, and large deletions were more frequent than inversions when dual breaks were induced. In our experiments, the gRNA cutting efficiency was a major determinant of inversion frequency in our experiment up to ~ 1 megabase, while locus-specific genomic context may also contribute., Larger inversions may be limited by an additional, size-dependent constraint. These findings inform the design of edits aimed at reverting breeding-relevant inversions (for example, those linked to resistance loci) and suggest that achieving high efficiency for multi-megabase inversions will require strategies that overcome spatial or repair-related constraints. The internal reference gRNA normalises sample-to-sample variability in DNA delivery and Cas9 activity, enabling direct comparison of the performance of different gRNAs across samples on a shared, ratio-based scale. This ratio-to-reference strategy may likewise be used to benchmark gRNA performance and edit yields (inversions, deletions, translocations, and base/prime edits) across transfections in plant protoplasts, and may be extended to additional cell systems beyond plants.
Grubben et al. (Tue,) studied this question.