Cystic Fibrosis Gene-Edited Cell Models for Drug Discovery

Cystic fibrosis (CF) is an autosomal recessive disorder caused by mutations in the CFTR gene. According to the World Health Organization, CF affects approximately 70,000 people worldwide, with an incidence of 1 in 2,500 to 1 in 6,000 live births in populations of European descent. The median predicted survival age has increased to about 50 years in developed countries due to improved therapies, but CF remains a life-limiting condition. The Cystic Fibrosis Foundation Patient Registry reports that lung disease is the primary cause of morbidity, with 90% of deaths attributed to respiratory failure. Key risk factors include family history and specific CFTR mutations, with F508del (c.1521_1523delCTT) being the most common, present in ~70% of CF alleles globally.

CF is an ideal model for studying epithelial ion transport, mucus biology, and chronic inflammation. The well-defined genetic basis (single gene, multiple mutations) allows precise genotype-phenotype correlations. Public datasets from the CFTR2 project, NCBI ClinVar, and the Cystic Fibrosis Foundation provide extensive mutation and clinical data. Open questions include the mechanisms of CFTR modulator resistance, the role of modifier genes, and the development of therapies for nonsense and rare mutations. Gene-edited cell models are essential for dissecting these mechanisms and for preclinical testing of new CFTR modulators.

CFTR dysfunction leads to defective chloride and bicarbonate transport across epithelial cell membranes. This results in dehydrated, acidic airway surface liquid, impaired mucociliary clearance, and chronic bacterial infection. The downstream consequences include:

• • Defective Ion Transport: Loss of CFTR-mediated Cl- secretion and ENaC-mediated Na+ hyperabsorption leads to airway surface liquid depletion.
• • Mucus Hyperconcentration: Reduced bicarbonate secretion alters mucin expansion, producing thick, adherent mucus.
• • Chronic Inflammation: Persistent neutrophil-dominated inflammation, driven by IL-8 and other cytokines, causes bronchiectasis and tissue destruction.
• • Infection Susceptibility: Impaired innate defense allows colonization by Pseudomonas aeruginosa, Staphylococcus aureus, and other pathogens.

Data from the CFTR2 database and NCBI ClinVar.

CFTR loss disrupts several signaling networks:

• • NF-kB Pathway: Chronic activation in CF airway epithelial cells leads to increased IL-8 production and inflammation.
• • MAPK/ERK Pathway: Altered in response to oxidative stress and bacterial products, contributing to epithelial remodeling.
• • PI3K/AKT Pathway: Dysregulated in CF, affecting cell survival and proliferation.
• • Autophagy Pathway: Impaired CFTR function leads to defective autophagy, contributing to protein aggregation and inflammation.
• • Wnt/beta-catenin Pathway: Altered in CF airway epithelium, affecting repair and regeneration.

Organoid models derived from patient rectal or nasal biopsies recapitulate CFTR function and are used for personalized drug testing. They retain the genetic background of the donor and can be expanded long-term.

• • CFTR knockout mice: First CF mouse model (CFTR-/-), shows intestinal obstruction but mild lung disease.
• • CFTR F508del mice: Knock-in model expressing human F508del mutation, more relevant for testing correctors.
• • CFTR G551D mice: Knock-in model for gating mutations, used for potentiator studies.
• • CFTR-/- ferrets: Develop spontaneous lung disease similar to humans, including infection and inflammation.
• • CFTR-/- pigs: Exhibit pancreatic insufficiency and lung disease, closely mimicking human CF.
• • Patient-derived xenografts (PDX): Not common for CF; organoids are preferred.

CRISPR-Cas9 technology enables the generation of isogenic cell lines with precise CFTR mutations. For example, CFTR F508del knock-in lines in 16HBE14o- or Caco-2 backgrounds allow direct comparison with wild-type controls. CFTR knockout lines (CFTR-/-) are used to study loss-of-function phenotypes. Commercially available, sequence-verified isogenic cell lines accelerate research by providing reproducible, validated models for drug screening, mechanistic studies, and target validation. These models eliminate the confounding effects of different genetic backgrounds seen in patient-derived cells.

CRISPR knockout and knock-in lines are used to validate the function of CFTR mutations and modifier genes. For example, CFTR F508del knock-in cells are used to study the effect of the mutation on protein trafficking and channel activity. CFTR knockout lines help identify compensatory ion channels (e.g., TMEM16A, SLC26A9) that could be therapeutic targets. Genome-wide CRISPR screens in CFTR-deficient cells have identified genes that modulate CFTR function or rescue the defect.

Isogenic CFTR mutant cell lines are used in high-throughput screens for CFTR modulators. For example, F508del knock-in cells are used to test correctors (e.g., lumacaftor, tezacaftor) and potentiators (e.g., ivacaftor). Resistance mechanisms to CFTR modulators can be studied by generating cells with secondary mutations or by long-term exposure to drugs. Isogenic pairs (wild-type vs. mutant) provide a clean system to assess drug efficacy and toxicity.

CRISPR-engineered CFTR mutant cells are used to identify biomarkers of CFTR function, such as changes in sweat chloride, nasal potential difference, or inflammatory markers. Synthetic lethality screens in CFTR-deficient cells can identify targets that are essential only in the absence of CFTR, providing new therapeutic avenues. For example, targeting the unfolded protein response or autophagy pathways may selectively kill CF cells.