What is cystic fibrosis?

What is Cystic Fibrosis

Cystic fibrosis is the most common lethal autosomal recessive genetic disorder in Caucasians, caused by mutations in the CFTR gene on chromosome 7, resulting in defective chloride and bicarbonate transport across epithelial cell membranes, leading to thick viscous secretions that obstruct multiple organ systems, particularly the lungs and pancreas. 1, 2

Genetic Basis and Epidemiology

• Inheritance pattern: Autosomal recessive disorder requiring mutations on both CFTR gene alleles 3
• Prevalence: Approximately 1 in 2,500-3,300 live births in Caucasians, with about 1 in 25 being heterozygous carriers 2
• Global burden: Affects approximately 89,000 people worldwide, with over 30,000 in the United States 4
• Racial variation: Birth prevalence is 1/2,500-3,500 in non-Hispanic whites, 1/4,000-10,000 in Hispanics, and 1/15,000-20,000 in non-Hispanic blacks 3

Molecular Pathophysiology

• CFTR protein: A 1,480 amino acid membrane-bound cyclic AMP-regulated chloride channel that also regulates bicarbonate transport and other ion channels 2
• Common mutation: The F508del mutation (deletion of phenylalanine at position 508) accounts for approximately 66% of all CF alleles worldwide and 85.5% of US cases 1, 4
• Mutation spectrum: Over 2,000 CFTR gene variants have been identified, classified into five functional classes based on their impact on protein production and function 5, 6
• Functional defect: Diminished chloride and water secretion leads to viscous secretions and impaired mucociliary clearance in affected organs 1

Clinical Manifestations

Respiratory System (Primary Cause of Morbidity and Mortality)

• Chronic infection: Recurrent and persistent pulmonary infections, particularly with Pseudomonas aeruginosa, which colonizes 29.8% of patients aged 2-5 years and 81.3% of those aged 26-30 years 1
• Structural damage: Progressive bronchiectasis, airway obstruction, and chronic neutrophil-dominated inflammation 3, 4
• Clinical symptoms: Chronic cough, wheezing, and progressive respiratory failure leading to premature death in 90% of patients 1

Gastrointestinal and Pancreatic Involvement

• Pancreatic insufficiency: Occurs in 85-90% of patients due to obstruction of intra-pancreatic ducts, causing fat and protein malabsorption 2, 3
• Neonatal presentation: Meconium ileus in 10-20% of newborns with CF 2
• Malabsorption symptoms: Steatorrhea (loose, foul-smelling fatty stools), fat-soluble vitamin deficiencies, and growth failure 3, 4
• Other manifestations: Chronic sinusitis, nasal polyps, liver disease, pancreatitis 2

Additional Organ Systems

• Sweat glands: Defective salt reabsorption leading to excessively salty sweat and risk of electrolyte imbalance and dehydration 3
• Reproductive tract: Congenital bilateral absence of the vas deferens (CBAVD) causing male infertility 2
• Metabolic complications: Cystic fibrosis-related diabetes and CF liver disease 7

Diagnosis

• Newborn screening: Measures immunoreactive trypsin, now universally available in many countries including the United States 2, 4
• Confirmatory testing: Sweat chloride testing with levels ≥60 mmol/L diagnostic for CF 2
• Genetic testing: CFTR mutation analysis to identify specific variants 2

Prognosis and Survival

• Historical survival: Median survival has dramatically improved from 36.3 years (95% CI, 35.1-37.9) in 2006 to 53.1 years (95% CI, 51.6-54.7) in 2021 4
• Earlier estimates: Average survival was approximately 30 years in the early 2000s 2
• Primary mortality factor: Pulmonary disease accounts for 90% of premature deaths 1

Treatment Paradigm

Traditional Therapies

• Mucolytics: Dornase alfa to improve lung function and reduce exacerbations 8, 4
• Anti-inflammatories: Azithromycin for chronic suppression of inflammation 8, 4
• Antibiotics: Inhaled tobramycin for chronic P. aeruginosa infection 8, 4
• Airway clearance: Inhaled hypertonic saline to improve mucus hydration and clearance 8
• Nutritional support: Pancreatic enzyme replacement therapy for malabsorption 2

CFTR Modulator Therapy (Transformative Treatment)

• Mechanism: Small molecule drugs that restore CFTR protein production and/or function at the cellular level 4, 7, 9
• Elexacaftor-tezacaftor-ivacaftor: The most effective combination, improving lung function by 13.8% (95% CI, 12.1%-15.4%) and reducing pulmonary exacerbations by 63% (rate ratio 0.37; 95% CI, 0.25-0.55) in patients with F508del variants 4
• Eligibility: Approximately 90% of people with CF aged 2 years or older may benefit from this combination therapy 4
• Additional benefits: Corrects salt-wasting, improves blood pressure and electrolyte balance, and significantly enhances quality of life, particularly in physical health domains 10, 11
• Paradigm shift: These modulators address the underlying genetic defect rather than just managing symptoms, fundamentally changing the disease trajectory 7, 9