Cystic Fibrosis - Complete USMLE-Oriented Guide

1. Overview

USMLE Pearl: CF is autosomal recessive. Both parents are carriers (each has a 1:25 chance in European-ancestry populations). Risk of two carriers having an affected child = 1:4.

2. Genetics and Molecular Biology

The CFTR Gene and Protein

• Gene: chromosome 7q31.2, 27 exons, spans ~190 kb
• Protein: ~170 kDa integral membrane protein
• Belongs to the ABC (ATP-binding cassette) transporter superfamily
• The protein has 5 domains:
  • 2 membrane-spanning domains (each with 6 transmembrane segments - these form the Cl⁻ channel pore)
  • 2 nucleotide (ATP)-binding domains (ATP hydrolysis opens/closes the channel)
  • 1 regulatory domain (phosphorylation by PKA activates the channel)

The ΔF508 Mutation

• >70% of all CF patients have at least one copy of F508del (deletion of phenylalanine at position 508)
• This is a Class II mutation - the protein misfolds and is prematurely degraded in the ER (never reaches the cell surface)
• Founder mutation ~52,000 years ago in the Middle East

USMLE Pearl: The most common CF mutation is ΔF508 (delta-F508), a 3-base-pair deletion causing loss of a phenylalanine residue. The protein misfolds and is tagged for destruction before it reaches the apical membrane.

CFTR Mutation Classes (High-Yield Table)

USMLE Pearl: Class I-III are severe ("pancreatic insufficient"). Class IV-V are milder ("pancreatic sufficient"). G551D is the classic Class III mutation treated by ivacaftor alone.

3. Pathophysiology

Sweat Glands

USMLE Pearl: "Salty baby" - parents notice salty taste when kissing child. This is a classic CF vignette opener.

Airways and Lungs

1. Loss of CFTR → reduced Cl⁻ and HCO₃⁻ secretion into the airway lumen
2. Simultaneously, Na⁺ absorption increases (via ENaC channels)
3. Net result: airway surface liquid dehydration → mucus becomes thick and viscous
4. Impaired mucociliary clearance → mucus plugging
5. Mucus creates a niche for bacterial biofilms (especially Pseudomonas aeruginosa and Staphylococcus aureus)
6. Biofilm organisms activate innate and adaptive immune responses
7. Neutrophil elastase causes airway wall destruction → bronchiectasis (predominantly upper lobes)

USMLE Sequence: CFTR mutation → ↓Cl⁻/HCO₃⁻ secretion + ↑Na⁺ absorption → airway dehydration → thick mucus → impaired clearance → chronic Pseudomonas infection → neutrophilic inflammation → bronchiectasis

Pancreas

• CFTR normally secretes bicarbonate- and chloride-rich fluid from pancreatic ductal cells
• Without CFTR → pancreatic duct obstruction → pancreatic autodigestion by retained enzymes
• Exocrine pancreas replaced by fat and fibrous tissue
• 85-90% of CF patients develop pancreatic insufficiency (fat + protein malabsorption)
• Fat malabsorption → deficiency of fat-soluble vitamins: A, D, E, K
• Chronic inflammation can also destroy islet cells → CF-related diabetes mellitus (CFRD) in ~1/3 of patients by age 30

USMLE Pearl: CF causes steatorrhea (bulky, foul-smelling, oily stools) from pancreatic insufficiency. Think fat-soluble vitamin deficiencies: night blindness (A), rickets/osteoporosis (D), hemolytic anemia/neuropathy (E), coagulopathy (K).

Liver

• CFTR absent in bile ducts → thick bile → obstruction of biliary canaliculi → focal biliary cirrhosis
• ~5% progress to hepatic failure requiring transplantation

Small Intestine

• Inadequate hydration of intestinal contents → obstipation, distal intestinal obstruction syndrome (DIOS), intussusception

Reproductive

• Males: >98% are infertile due to congenital bilateral absence of the vas deferens (CBAVD) - the vas deferens fails to develop (obstructive azoospermia, but spermatogenesis is normal)
• Females: Reduced fertility (thick cervical mucus, nutritional/hormonal issues), but some do become pregnant

USMLE Pearl: An otherwise healthy male with infertility and azoospermia on semen analysis but normal testosterone/LH/FSH should prompt CFTR gene testing for CBAVD. This is a forme fruste of CF - can occur with mild CFTR mutations even without pulmonary disease.

4. Clinical Features

Pulmonary

• Chronic productive cough, crackles, wheezing
• Recurrent pulmonary infections
• Bronchiectasis (predominantly upper lobes - differentiates CF from most other bronchiectasis causes which are lower lobe)
• Nasal polyps, chronic sinusitis
• Digital clubbing
• Exacerbations triggered by viral URI

1. Staphylococcus aureus (early childhood)
2. Pseudomonas aeruginosa (older children/adults - mucoid form = biofilm)
3. Burkholderia cepacia (late disease, very resistant, poor prognosis)
4. Stenotrophomonas, Aspergillus, atypical mycobacteria (MALDI)

USMLE Pearl: Mucoid Pseudomonas aeruginosa in the sputum of a young person with recurrent lung infections = think CF until proven otherwise.

Chest X-Ray Findings (Classic on USMLE)

GI/Nutritional

• Meconium ileus at birth (10-15% of CF newborns) - the classic neonatal presentation; highly specific for CF
• Distal intestinal obstruction syndrome (DIOS) - adult equivalent
• Steatorrhea, failure to thrive, rectal prolapse
• Fat-soluble vitamin deficiencies (A, D, E, K)
• Pancreatic insufficiency → short stature, poor weight gain

Other

• CFRD (CF-related diabetes) - unique insulin-deficient, ketoacidosis-resistant form
• Osteoporosis/osteopenia
• Hypochloremic, hypokalemic metabolic alkalosis (paradoxical: sweat loses NaCl, compensatory renal retention of H⁺ and K⁺ are excreted)
• Erythema nodosum
• Hemoptysis (from bronchiectasis)

USMLE Pearl: A newborn with intestinal obstruction (double bubble or microcolon on X-ray) and no meconium passed within 24-48 hours → meconium ileus → workup for CF.

5. Diagnosis

Newborn Screening

• Immunoreactive trypsinogen (IRT) from heel-stick blood sample - high sensitivity
• Elevated IRT → reflex to CFTR mutation analysis

Sweat Chloride Test (Gold Standard)

• Method: Pilocarpine iontophoresis (pilocarpine stimulates sweat)
• Diagnostic: Cl⁻ ≥ 60 mEq/L
• Normal: Cl⁻ < 30 mEq/L
• Borderline: 30-59 mEq/L (milder variants, "forme fruste")

• Nephrogenic diabetes insipidus
• Malnutrition
• Addison disease
• Hypoparathyroidism
• Atopic dermatitis (rarely)

USMLE Pearl: Sweat chloride ≥ 60 mEq/L + compatible clinical features = CF diagnosis confirmed. Two CFTR mutations found on genetic testing also establishes the diagnosis.

Genetic Testing

• Panel testing for ~40 most common CFTR mutations (initial screen)
• Full CFTR sequencing if clinical suspicion high and panel negative

Pulmonary Function Tests

• Obstructive pattern: ↓FEV₁, ↓FEV₁/FVC, ↑RV/TLC (air trapping)
• Progressive FEV₁ decline over time

6. Treatment

Airway Clearance (Cornerstone)

• Chest physiotherapy (postural drainage + percussion)
• Oscillating vest (HFCWO)
• Positive expiratory pressure (PEP) mask

Mucolytic/Secretion-Thinning Agents

• Dornase alfa (rhDNase) - recombinant human DNase breaks down neutrophil-derived DNA in mucus; given by inhalation once daily
• Hypertonic saline (7%) - osmotic hydration of airway surface
• N-acetylcysteine

USMLE Pearl: Dornase alfa (Pulmozyme) improves FEV₁ and reduces exacerbations. The mechanism - neutrophil-derived DNA makes CF mucus viscous; DNase breaks it down.

Infection Management

• Chronic Pseudomonas: Inhaled tobramycin (28 days on/off cycles), inhaled aztreonam, aerosolized colistin
• Exacerbations: IV antibiotics (anti-pseudomonal: piperacillin-tazobactam, ceftazidime, tobramycin, meropenem)
• MRSA: linezolid, vancomycin
• Oral ciprofloxacin for outpatient mild exacerbations

CFTR Modulators (Disease-Modifying - High-Yield)

USMLE Pearl: Ivacaftor = potentiator (opens the gate). Works for Class III gating mutations (G551D). Lumacaftor/tezacaftor = correctors (help the misfolded protein traffic to the cell surface). Trikafta (elexacaftor/tezacaftor/ivacaftor) = most effective therapy; suitable for patients with at least one ΔF508 allele.

Pancreatic Enzyme Replacement

• Pancrelipase (Creon) - oral lipase/protease/amylase with each meal and snack
• Fat-soluble vitamin supplementation (ADEK)

Other

• Bronchodilators (if airway hyperresponsiveness)
• Anti-inflammatory: ibuprofen (high-dose, slows lung decline in 6-17 year-olds), azithromycin (macrolide - anti-inflammatory + anti-biofilm)
• Management of CFRD: insulin therapy
• Lung transplantation - bilateral sequential transplant for end-stage disease (FEV₁ <30% predicted; hypercapnia; frequent exacerbations)

7. Complications Summary

8. High-Yield USMLE Correlations

Classic USMLE Scenarios

Genetics Pearl for USMLE

• If both parents are carriers (1:25 in whites): 1:4 chance of CF child
• Carrier testing: part of standard prenatal screening

Key Numbers

• Sweat Cl⁻ diagnostic: ≥ 60 mEq/L
• Sweat Cl⁻ normal: < 30 mEq/L
• Most common mutation: ΔF508 (~70% of alleles)
• Pancreatic insufficiency: 85-90% of patients
• Male infertility: >98% (CBAVD)
• Meconium ileus: 10-15% of newborns with CF
• CFRD by age 30: ~33%

9. Prognosis