Cystic Fibrosis: Pathogenesis

Cystic fibrosis results from mutations in the CFTR gene on chromosome 7, with over 2,000 known mutations identified. The most common mutation is ΔF508. These mutations disrupt the CFTR protein, which normally functions as an ion channel regulating chloride and sodium transport in mucous-producing cells, leading to thick, sticky mucus production.

CFTR mutations cause decreased chloride secretion and increased sodium absorption in mucous-producing cells. This ion imbalance reduces water secretion into mucus, resulting in thicker, more viscous secretions. The abnormal mucus accumulates in airways and ducts, obstructing normal function across multiple organ systems.

Initially, CF lungs appear normal, but mutations trigger a cascade of infection and inflammation. Thick mucus plugs the bronchioles, causing obstructive lung disease. CF lung disease clinically resembles chronic bronchitis, bronchiectasis, and can involve colonization with Pseudomonas aeruginosa, leading to progressive airway damage.

CF primarily affects exocrine glands and multiple organ systems. The most commonly affected organs include sinuses, lungs, pancreas, liver and biliary systems, intestines, and sweat glands. Pancreatic involvement causes insufficiency and early-onset diabetes, while hepatobiliary manifestations include cirrhosis and portal hypertension.

CF diagnosis requires clinical symptoms, positive family history, and confirmatory tests. The sweat chloride test measures elevated chloride levels in sweat. DNA testing identifies two disease-causing CFTR mutations, and nasal potential difference testing measures abnormal ion transport. Pulmonary function tests monitor disease progression and lung involvement.

CF can clinically resemble chronic bronchitis, bronchiectasis, atypical asthma, and allergic bronchopulmonary aspergillosis. Mucus plugging in bronchioles causes obstructive airway disease and chronic infections. Abnormal pulmonary function tests reflect progressive lung damage, and bacterial colonization, particularly Pseudomonas aeruginosa, accelerates respiratory decline.

CF is autosomal recessive because the CFTR gene mutation on chromosome 7 requires two mutated copies—one from each parent—for disease expression. Individuals with one mutated copy are carriers but typically asymptomatic. This inheritance pattern means affected individuals have carrier parents, and siblings have a 25% chance of inheriting the disease.