Identification of on-target mutagenesis during correction of a beta-thalassemia splice mutation in iPS cells with optimised CRISPR/Cas9-double nickase reveals potential safety concerns.

Precise and accurate gene correction is crucial for enabling iPSC-based therapies, and Cas9-Nickase based approaches are increasingly considered for correction of diseases such as beta-thalassemia. Here, we generate footprint-free induced pluripotent stem cells from a patient with a beta-thalassemia mutation IVSII-1 G > A) and employ a double Cas9nickase-mediated correction strategy combined with a transposon-modified donor vector for gene correction. Our approach further aimed to minimize the formation of adjacent single-strand breaks at the targeted allele through the destruction of the binding site for one guide and the use of a synonymous protospacer adjacent motif blocking mutation (canonical PAM sequence 5'-NGG-3' is changed to 5'-NCG-3', where N indicates any nucleobase) for the other guide. We show that this strategy indeed not only permits bi-allelic seamless repair of the beta-globin gene splice site mutation and negligible off-target mutagenesis or re-editing of the targeted allele but also results in unexpected on-target mutagenesis with some guide RNAs (gRNAs) in several targeted clones. This study thus not only validates a framework for seamless gene correction with enhanced specificity and accuracy but also highlights potential safety concerns associated with Cas9-nickase based gene correction.