Cystic Fibrosis — Examination Question Bank

Genetics, Molecular Defect & Pathophysiology

Question

Answer

1. Describe the genetic defect and CFTR functional classes in Cystic Fibrosis.

Cystic Fibrosis is an autosomal recessive disorder caused by mutations in the CFTR gene (Cystic Fibrosis Transmembrane Conductance Regulator) located on the long arm of chromosome $7q31.2$, encoding a cAMP-regulated ATP-binding cassette chloride and bicarbonate channel.
- Over 2,000 mutations identified, categorized into 6 functional classes:
- Class I (No synthesis): Nonsense, frameshift, or splice-site mutations (e.g., G542X, W1282X) producing premature stop codons.
- Class II (Defective trafficking / processing): Protein misfolded and degraded in endoplasmic reticulum. Phe508del ($\Delta F508$) is the most common mutation globally ($70\%$ in Caucasians, $20-40\%$ in Indians).
- Class III (Defective regulation / gating): Protein reaches apical membrane but channel gating fails (e.g., G551D).
- Class IV (Defective conductance): Channel opens but chloride transport rate is reduced (e.g., R117H).
- Class V (Reduced synthesis): Normal protein produced in markedly reduced quantities due to promoter or splice defects.
- Class VI (Accelerated turnover): Channel unstable at apical surface and rapidly degraded.
- Phenotype: Classes I, II, and III cause severe classical CF with pancreatic insufficiency; Classes IV, V, and VI typically produce milder disease with pancreatic sufficiency.

2. Explain the pathophysiological cascade in the respiratory tract.

In the airway epithelium, dysfunctional CFTR results in:
1) Decreased apical chloride and bicarbonate secretion.
2) Disinhibition of the epithelial sodium channel (ENaC), leading to hyperabsorption of sodium and water into the cell.
3) Dehydration of the airway surface liquid (ASL) and periciliary layer, causing ciliary collapse.
4) Production of hyperviscous, tenacious, acidic mucus that plugs the bronchioles.
5) Defective mucociliary clearance leads to chronic bacterial colonization (S. aureus, P. aeruginosa), triggering uninhibited neutrophil recruitment, release of neutrophil elastase and DNA, progressive bronchiectasis, and respiratory failure.

Diagnosis, Sweat Chloride & Pancreatic Assessment

Question	Answer
3. Detail the Sweat Chloride Test: Method, Standardization, and Interpretation.	- Gold Standard Technique: Quantitative Pilocarpine Iontophoresis (Gibson-Cooke method) or Macroduct collection. - Pilocarpine is driven into skin by low electric current; sweat is collected onto filter paper or microbore tubing for $\ge 30 ext{ minutes}$. - Minimum sweat weight/volume required: $\ge 75 ext{ mg}$ (Gibson-Cooke) or $\ge 15\mu ext{L}$ (Macroduct) to prevent false evaporation artifacts. - Diagnostic Interpretation (All Ages): - $\ge 60 ext{ mmol/L}$: Positive / Diagnostic of Cystic Fibrosis (when paired with clinical features or positive sibling history). - $30 - 59 ext{ mmol/L}$: Intermediate / Equivocal / Borderline (requires repeat sweat test, expanded CFTR sequencing, and CFTR physiological testing like nasal potential difference). - $<30 ext{ mmol/L}$: Normal / Unlikely CF.
4. What are the causes of False-Positive and False-Negative Sweat Chloride Tests?	- False-Positive Causes: Severe malnutrition / marasmus, untreated adrenal insufficiency (Addison disease), nephrogenic diabetes insipidus, ectodermal dysplasia, hypothyroidism, G6PD deficiency, fucosidosis, hypoparathyroidism. - False-Negative Causes: Technical collection error, severe edema / hypoalbuminemia (water dilutes sweat electrolytes), skin excoriations, or rare atypical Class IV/V mutations.
5. How do you assess Exocrine Pancreatic Insufficiency (EPI)?	- Fecal Elastase-1 (FE-1): The test of choice. Pancreatic elastase is resistant to intestinal degradation and specific to human pancreas. - $<100 ext{ mcg/g of stool}$: Severe exocrine pancreatic insufficiency. - $100 - 200 ext{ mcg/g of stool}$: Moderate insufficiency. - $>200 ext{ mcg/g of stool}$: Normal exocrine pancreatic function. - Major Advantage: Unaffected by exogenous Pancreatic Enzyme Replacement Therapy (PERT) and can be tested while on enzymes! (Stool must be formed/semi-formed; watery diarrhea can cause false-positive dilution). - 72-hour Fecal Fat Estimation: Historical gold standard (Coefficient of Fat Absorption $<93\%$).

Multidisciplinary Management, Modulators & Viva Traps

Question	Answer
6. Formulate the comprehensive management of Pancreatic Enzyme Replacement Therapy (PERT).	- Formulations: Enteric-coated microtablets/microspheres (Pancreatin / Creon) protected against gastric acid destruction. - Starting Dose: Infants: $2,000-4,000 ext{ lipase units}$ per $120 ext{ mL}$ formula or breastfeed; Children $<4$ years: $1,000 ext{ lipase units/kg/meal}$; Children $>4$ years: $500 ext{ lipase units/kg/meal}$ (snacks receive half-meal dose). - Maximum Dose Limit: Never exceed $2,500 ext{ lipase units/kg/meal}$ or $10,000 ext{ lipase units/kg/day}$. - Administration: Administer immediately before or throughout the meal. Never crush or chew capsules; never mix with alkaline foods ($pH >5.5$, e.g., dairy products). Co-administer PPI or H2 blocker if persistent steatorrhea despite adequate dosing.
7. VIVA TRAP: What is "Fibrosing Colonopathy" and what causes it?	Fibrosing Colonopathy is a severe, life-threatening complication characterized by transmural fibrosis, stricturing, and luminal narrowing predominantly in the ascending colon and ileocecal region, causing bowel obstruction. - Etiology: Strongly associated with prolonged high doses of PERT exceeding $>10,000 ext{ units of lipase/kg/day}$. - Prevention: Strictly restrict pancreatic enzyme dosing to $\le 10,000 ext{ units/kg/day}$ or $\le 2,500 ext{ units/kg/meal}$.
8. Outline the management of an Acute Pulmonary Exacerbation in CF.	1) Airway Clearance Therapy: Intensive chest physiotherapy, oscillating PEP flutter valve, postural drainage 3-4 times daily. 2) Inhaled Mucolytics: Nebulized Recombinant Human DNase (Dornase Alfa $2.5 ext{ mg}$ OD) to cleave extracellular neutrophil DNA, and Nebulized 7% Hypertonic Saline (induces osmotic hydration of ASL). 3) Intravenous Antibiotics: Dual anti-pseudomonal coverage for 14-21 days: - Anti-pseudomonal beta-lactam: IV Ceftazidime ($150-200 ext{ mg/kg/day}$ q8h) or Meropenem ($120 ext{ mg/kg/day}$ q8h). - PLUS Aminoglycoside: IV Amikacin ($30-35 ext{ mg/kg/day}$ once daily) or Tobramycin ($10-12 ext{ mg/kg/day}$ once daily). (Higher doses required in CF due to expanded volume of distribution and enhanced renal clearance). 4) Nutritional Support: High-calorie ($120-150\%$ RDA), high-fat ($35-40\%$), fat-soluble vitamins (ADEK in water-miscible form).
9. What are CFTR Modulator Therapies and what is Trikafta?	- Potentiators (Ivacaftor - VX-770): Binds to defective CFTR at the cell surface and holds the chloride channel gate open (effective in gating mutations like G551D). - Correctors (Lumacaftor, Tezacaftor, Elexacaftor): Act as molecular chaperones to facilitate proper folding and intracellular trafficking of $\Delta F508$ protein to the apical membrane. - Trikafta (Elexacaftor / Tezacaftor / Ivacaftor): Triple combination therapy approved for patients $\ge 2$ years carrying at least one Phe508del ($\Delta F508$) allele. It restores $pprox 50\%$ CFTR function, dramatically improving FEV1, reducing pulmonary exacerbations by $>60\%$, normalizing sweat chloride, and improving nutritional status!
10. VIVA TRAP: How do you diagnose CF-Related Diabetes (CFRD) and why is HbA1c unreliable?	- CFRD occurs due to progressive fibroadipose destruction of islet architecture with secondary insulinopenia. - Screening: Annual 2-hour 75g Oral Glucose Tolerance Test (OGTT) starting from age 10 years. - Diagnostic Criteria: Fasting plasma glucose $\ge 126 ext{ mg/dL}$ or 2-hour post-load glucose $\ge 200 ext{ mg/dL}$. - VIVA TRAP: HbA1c is notoriously unreliable and false-negative in CF due to increased red cell turnover, chronic inflammation, and recurrent hemoptysis. A normal HbA1c ($<6.5\%$) NEVER excludes CFRD!