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Background: Gene correction via homology directed repair (HDR) in patient-derived induced pluripotent stem (iPS) cells for regenerative medicine are becoming a more realistic approach to develop personalized and mutation-specific therapeutic strategies due to current developments in gene editing and iPSC technology. Cystic fibrosis (CF) is the most common inherited disease in the Caucasian population, caused by mutations in the CF transmembrane conductance regulator (CFTR) gene. Since CF causes significant multi-organ damage and with over 2,000 reported CFTR mutations, CF patients could be one prominent population benefiting from gene and cell therapies. When considering gene-editing techniques for clinical applications, seamless gene corrections of the responsible mutations, restoring native “wildtype” DNA sequence without remnants of drug selectable markers or unwanted DNA sequence changes, would be the most desirable approach.Result: The studies reported here describe the seamless correction of the W1282X CFTR mutation using CRISPR/Cas9 nickases (Cas9n) in iPS cells derived from a CF patient homozygous for the W1282X Class I CFTR mutation. In addition to the expected HDR vector replacement product, we discovered another class of HDR products resulting from vector insertion events that created partial duplications of the CFTR exon 23 region. These vector insertion events were removed via intrachromosomal homologous recombination (IHR) enhanced by double nicking with CRISPR/Cas9n which resulted in the seamless correction of CFTR exon 23 in CF-iPS cells.Conclusion: We show here the removal of the drug resistance cassette and generation of seamless gene corrected cell lines by two independent processes: by treatment with the PiggyBac (PB) transposase in vector replacements or by IHR between the tandemly duplicated CFTR gene sequences.

Cystic Fibrosis (CF) is caused by a diverse set of mutations distributed across the approximately 250 thousand base pairs of the CFTR gene locus, of which at least 382 are disease-causing (CFTR2.org). Although a variety of editing tools are now available for correction of individual mutations, a strong justification can be made for a more universal gene insertion approach, in principle capable of correcting virtually all CFTR mutations. Provided that such a methodology is capable of efficiently correcting relevant stem cells of the airway epithelium, this could potentially provide life-long correction for the lung. In this Perspective we highlight several requirements for efficient gene insertion into airway epithelial stem cells. In addition, we focus on specific features of the transgene construct and the endogenous CFTR locus that influence whether the inserted gene sequences will give rise to robust and physiologically relevant levels of CFTR function in airway epithelium. Finally, we consider how in vitro gene insertion methodologies may be adapted for direct in vivo editing.

Summary: Airway basal cells play an essential role in the maintenance of the airway epithelium. Here, we provide a detailed directed differentiation protocol to generate ‘‘induced basal cells (iBCs)’’ from human pluripotent stem cells. iBCs recapitulate biological and functional properties of airway basal cells including mucociliary differentiation in vitro or in vivo in tracheal xenografts, facilitating the study of inherited and acquired diseases of the airway, as well as potential use in regenerative medicine.For complete details on the use and execution of this protocol, please refer to Hawkins et al. (2021).