Cystic Fibrosis (CF) Cell Models for Research

Cystic fibrosis (CF) is a life-limiting autosomal recessive disorder caused by mutations in the CFTR gene. According to the World Health Organization (WHO), the global incidence is estimated at 1 in 2,500 to 1 in 6,000 live births, with higher prevalence in populations of European descent. The median predicted survival for individuals with CF in the United States is now over 50 years, according to the Cystic Fibrosis Foundation Patient Registry (2021). However, significant morbidity and mortality remain due to progressive lung disease, pancreatic insufficiency, and other complications. The National Cancer Institute (NCI) does not track CF as a cancer, but CF patients have an increased risk of certain cancers, particularly gastrointestinal cancers, as noted in a 2015 study in the Journal of the National Cancer Institute. The disease burden is substantial, with lifelong management and reduced quality of life.

CF is an ideal model for studying ion transport, epithelial biology, and protein misfolding. The disease is monogenic, with over 2,000 CFTR mutations identified (ClinVar), providing a clear genotype-phenotype correlation. Research models are essential for understanding CFTR function, testing modulator therapies, and developing gene editing approaches. Public datasets, such as the CFTR2 database, provide comprehensive mutation information. Open questions include the role of modifier genes, the impact of CFTR mutations on immune responses, and the development of therapies for rare mutations.

CFTR dysfunction leads to defective chloride and bicarbonate transport across epithelial membranes, resulting in thick mucus, chronic inflammation, and infection. Key pathways include:

• • Defective ion transport: CFTR is a cAMP-regulated chloride channel. Mutations impair channel synthesis, trafficking, gating, or conductance.
• • Mucus hypersecretion: Impaired ion transport leads to dehydrated airway surface liquid, causing mucus accumulation and impaired mucociliary clearance.
• • Chronic inflammation: Persistent bacterial infections (e.g., Pseudomonas aeruginosa) trigger neutrophil-dominated inflammation, leading to tissue damage.
• • Fibrosis: Repeated cycles of infection and inflammation result in airway remodeling and fibrosis.

Data from CFTR2 database and ClinVar.

CFTR dysfunction affects multiple signaling networks:

• • Inflammatory signaling: NF-κB and MAPK pathways are hyperactivated due to chronic infection.
• • Autophagy: Impaired CFTR leads to defective autophagy, contributing to inflammation.
• • Epithelial-mesenchymal transition (EMT): CFTR loss may promote EMT, contributing to fibrosis.
• • Calcium signaling: CFTR interacts with calcium-activated chloride channels, affecting intracellular calcium homeostasis.

Organoids are particularly valuable as they recapitulate patient-specific physiology and allow drug testing in a 3D context.

• • CFTR knockout mice: First generation models, but they do not fully recapitulate human lung disease.
• • CFTR F508del mice: Show mild lung phenotype, useful for studying basic mechanisms.
• • CFTR G551D mice: Used for testing potentiators.
• • Ferret and pig models: More closely mimic human lung disease, but are expensive and difficult to handle.
• • Patient-derived xenografts (PDX): Not commonly used for CF, but organoid-based xenografts are emerging.

CRISPR-Cas9 gene editing enables the creation of isogenic cell lines with specific CFTR mutations, providing precise models for studying mutation-specific effects. For example, a CFTR knockout cell line can be generated in a wild-type background to study the loss-of-function phenotype. Conversely, a F508del knock-in cell line can be created in a wild-type background to study the mutation in a controlled genetic context. These models are commercially available from various sources, and sequence-verified clones ensure reliability. Such gene-edited cell models are essential for drug discovery, as they allow high-throughput screening of modulators and assessment of off-target effects.

Gene-edited cell lines enable functional validation of CFTR mutations. For instance, a CFTR knockout cell line can be used to confirm the role of CFTR in chloride transport. Knock-in lines with specific mutations allow the study of mutation-specific defects, such as F508del misfolding or G551D gating. These models are also used to identify modifier genes via CRISPR screens.

Isogenic pairs (e.g., wild-type vs. F508del) are used to screen for correctors and potentiators. High-throughput screening can identify compounds that rescue CFTR function. Resistance to CFTR modulators can be studied by exposing cells to increasing drug concentrations and selecting resistant clones.

CRISPR synthetic lethality screens can identify genes that are essential in CFTR-mutant cells but not in wild-type cells, providing potential therapeutic targets. For example, a screen in F508del cells might identify genes involved in protein degradation pathways that could be targeted to enhance corrector efficacy.