132 Introduction of F508del human mutation into the CFTR gene of sheep fetal fibroblasts using CRISPR/Cas9 ribonucleoprotein

Abstract

Cystic fibrosis (CF) is an autosomal recessive genetic disease that affects over 30 000 people in the United States and is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. The CFTR protein is a cAMP-regulated C− channel responsible for regulation of anion transport, primarily in the epithelial cells. We have previously generated a sheep model of CF by genetically inactivating the CFTR gene (Fan et al. 2018 JCI Insight 3, e123529). The newborn CFTR −/− sheep develops severe disease consistent with CF pathology in humans. The CF model is extremely valuable for understanding the developmental aspects of CF disease, as sheep have been used extensively in the study of human fetal growth and development. Sheep, like humans, typically give birth to only one or two offspring in each pregnancy, which make them more suitable than many other species for testing prenatal gene-editing treatments. Thus, in this new study, we are working on the generation of F508del sheep CF model. The F508del mutation was chosen because it is the most common mutation in the human CFTR gene (~70%). This mutation is characterised by the deletion of the CTT nucleotides, which ultimately deletes the phenylalanine residue at position 508. The F508del mutation causes misfolding of the CFTR protein, which is further degraded by proteases. Even though several CFTR modulators are available, they are not effective in all patients. Additionally, they cannot reverse deleterious prenatal CF manifestations. Hence, this model will be valuable for evaluating both prenatal drug and gene therapies. Here, we used a CRISPR/Cas9 gene-editing approach to introduce the F508del mutation into the sheep genome. We designed an sgRNA targeting exon 11 of the sheep CFTR gene using the Benchling software (https://benchling.com/academic). The sgRNA was synthesised by Synthego and Cas9 purchased from ThermoFisher. Using the Lonza-4D-Nucleofector system, Cas9/sgRNA ribonucleoprotein complex was transfected into sheep fetal fibroblasts (SFFs), along with 100bp single-stranded oligodeoxynucleotide, flanking the F508del mutation, for the homology-directed repair. The transfected cells were subsequently cultured in Dulbecco's modified Eagle's medium, supplemented with 15% fetal bovine serum and 1% penicillin, and incubated at 38.5°C. Two days post-transfection, SFFs were seeded individually into five 96-well plates by limited dilution. After seven days, the individual colonies were expanded into 24-well plates and cultured for three more days. A total of 56 single-cell-derived SFF colonies were isolated. The presence of F508del mutation was confirmed by amplifying the PCR products of the exon 11 flanking the mutation site and subjecting each amplicon to Sanger sequencing. The sequencing results indicated that the indels (insertion/deletion) were introduced in 49 out of 56 (87.5%) of the colonies, and four (7.14%) of them were confirmed to have biallelic F508del mutations based on sequencing peaks. Therefore, we successfully introduced the F508del mutation in SFFs that will be used for the production of F508del CF sheep by somatic cell nuclear transfer.