Cystic Fibrosis: The Primary Genetic Disorder Affecting Mucus Secretion
Cystic fibrosis (CF) is the most common identifiable genetic disorder affecting mucus secretion, caused by mutations in the CFTR gene that disrupt chloride and fluid transport across epithelial cell membranes, resulting in thick, tenacious mucus throughout the respiratory tract, pancreas, and other organs. 1, 2, 3
Pathophysiology
- CF results from recessive mutations in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR) protein, which functions as an ion channel responsible for chloride exchange in epithelial cells 1, 4
- The CFTR gene is located on the long arm of chromosome 7, and over 2000 CFTR variants have been identified to date 4
- CFTR dysfunction disrupts normal fluid and electrolyte flow across cell membranes, altering secretion composition in the respiratory tract, pancreas, GI tract, and sweat glands 2
- In the lungs, this manifests as excessive and tenacious mucus secretion, impaired mucus transport, dilation and hypertrophy of bronchial glands, goblet cell hyperplasia, leading to proximal airway plugging and bronchiectasis 2
- The relationship between CFTR and mucins (MUC5AC and MUC5B) is closely integrated—CFTR-mediated ion transport and mucin secretion work together to maintain airway mucus homeostasis 5
Epidemiology and Clinical Significance
- CF occurs in approximately 1 per 2,000 to 3,000 live births and is likely the most common identifiable cause of bronchiectasis in the United States and Europe 2, 3
- CF-related diabetes (CFRD) is the most common comorbidity, occurring in about 20% of adolescents and 40-50% of adults with CF 1
- CFRD is associated with increased morbidity, mortality, worse nutritional status, more severe inflammatory lung disease, and greater patient burden compared to type 1 or type 2 diabetes 1
Diagnostic Approach
Initial Screening
- Newborn screening is performed using immunoreactive trypsinogen (IRT) with three main approaches: IRT/DNA (genetic analysis), IRT/IRT (repeat IRT), or IRT/pancreatitis-associated protein 4
- Newborns with positive CF newborn screen should have sweat chloride testing performed as soon as possible after 10 days of age, ideally by the end of the neonatal period (four weeks), when the infant weighs >2 kg and is at least 36 weeks corrected gestational age 4
Confirmatory Testing: Sweat Chloride Test
- The sweat test (pilocarpine iontophoresis) remains the most accurate diagnostic method, as it is positive in almost all cases when performed properly 2, 4
- Sweat chloride analysis should be performed within a few hours of sweat collection 4
- Diagnostic criteria:
- Sweat chloride ≥60 mmol/L confirms CF diagnosis in individuals with positive newborn screen, clinical features consistent with CF, or positive family history 4
- Sweat chloride <30 mmol/L indicates CF is unlikely (though may still be considered if clinical criteria and/or CFTR genotyping support CF) 4
- Sweat chloride 30-59 mmol/L (intermediate range) on two separate occasions may indicate CF and warrants extended CFTR gene analysis and/or CFTR functional analysis 4
Genetic Testing
- CFTR genetic testing should be performed if the genotype was not available through screening or is incomplete 4
- Commercial laboratories identify around 90% of CF mutations in the general population, though there are >700 identified mutations 2
- Genetic testing uses Sanger sequencing, Next Generation Sequencing, targeted deep sequencing (TDS), and Droplet Digital PCR technologies 4
Clinical Presentation Warranting Testing
- CF should be considered in children and younger adults with recurrent sinopulmonary infection and/or bronchiectasis, even in the absence of GI symptoms 2
- Weight loss or failure of expected weight gain is a risk factor for CFRD and should prompt screening regardless of age 1
Screening for CF-Related Diabetes
- Annual oral glucose tolerance test (OGTT) is the recommended screening test for CFRD 1
- A1C is not recommended for screening due to low sensitivity; however, a value ≥6.5% (≥48 mmol/mol) is consistent with CFRD diagnosis and reduces patient screening burden 1
- Screening for diabetes before age 10 years can identify risk for progression to CFRD in those with abnormal glucose tolerance 1
Management Implications
- In infants with presumptive CF identified through newborn screening, CF treatment should not be delayed while efforts to establish diagnosis are initiated 4
- Early diagnosis and treatment of CFRD is associated with preservation of lung function 1
- Patients with confirmed CF should be referred to a cystic fibrosis specialty clinic for comprehensive multidisciplinary care 2, 3
- Immunosuppressive therapy considerations: immunosuppressive plans shown to provide the best outcomes for individuals and graft survival should be used irrespective of CFRD risk in transplant patients 1
Other Genetic Disorders Affecting Mucus (For Differential Consideration)
Primary Ciliary Dyskinesia (PCD)
- PCD is a genetically heterogeneous autosomal-recessive motile ciliopathy associated with 32 genes, causing impaired mucociliary clearance 4, 6
- Clinical manifestations include situs inversus, neonatal respiratory distress, progressive bronchiectasis, and respiratory failure in young adulthood 4
- Diagnostic tests include nasal nitric oxide (nNO), high-speed video microscopy (HSVM), transmission electron microscopy (TEM), immunofluorescence of ciliary proteins, and genetic testing 6, 7, 6, 8
Hereditary Mucin Deficiency
- Biallelic loss-of-function mutations in MUC5B can cause congenital absence of MUC5B protein, defining a new category of genetic respiratory disease 9
- The phenotype includes bronchiectasis, impaired pulmonary function, repeated Staphylococcus aureus infection, impaired mucociliary clearance, and extensive sinus disease 9