Adenine base editing of CFTR using receptor targeted nanoparticles restores function to G542X cystic fibrosis airway epithelial cells

The cystic fibrosis (CF) causing variant G542X harbours a premature translation stop signal in the cystic fibrosis transmembrane conductance regulator (CFTR) mRNA. This results in nonsense-mediated decay and loss of functional CFTR protein which leads to defective anion transport and the development of CF disease pathology. Currently available CF modulator therapies cannot be used to treat this variant. We used an adenine base editor (ABE8e Cas9) and guide RNA (sgRNA)/enhanced green fluorescent protein (EGFP) plasmids encapsulated in receptor targeted nanoparticles (RTN), delivered to Bmi-1 transduced basal human CF nasal epithelial cells harbouring the homozygous CFTR G542X variant, to convert the stop codon to G542R, a variant which is amenable to modulator therapy. ABE resulted in 17% of alleles edited to G542R and further selection of GFP fluorescent cells by FACS liberated a population with 52% G542R edited alleles with no editing of neighbouring adenines (A) and few off target edits using a gRNA homology-based approach. In cells differentiated at air-liquid-interface (ALI), 17% and 52% editing of CFTR G542X increased mRNA abundance. 52% editing alone or 17% and 52% editing of CFTR G542X plus treatment with CFTR modulators (VX-445/VX-661/VX-770; ETI/Trikafta/Kaftrio) increased epithelial CFTR protein expression, CFTR protein band C abundance, CFTR₁₇₂ inhibitable anion transport, and changes in airway surface liquid height and pH in response to vasoactive intestinal peptide (VIP) stimulation. Epithelial scratch repair speed and directionality was also improved. These data provide proof-of-concept that ABE of G542X to G542R in human CF airway epithelial cells could provide a feasible therapy for this variant.