A mutation in the gene for aquaporin channel leads to which of the following conditions?

Correct Answer: B. Nephrogenic DI

Aquaporin channels are water channel proteins essential for osmotic water transport across cell membranes. In the kidney, aquaporin-2 (AQP2) is the vasopressin-regulated water channel located on the apical membrane of collecting duct principal cells. Mutations in the AQP2 gene cause nephrogenic diabetes insipidus (NDI), a condition where the kidney fails to respond to antidiuretic hormone (ADH/vasopressin) despite normal or elevated ADH levels. This results in inability to concentrate urine, leading to polyuria (up to 10–15 L/day), polydipsia, and hypernatremia if fluid intake is inadequate. The defect is post-receptor—ADH signaling and cAMP generation are intact, but water reabsorption in the collecting duct is impaired due to absent or dysfunctional aquaporin-2 channels. Nephrogenic DI accounts for ~10% of all DI cases in India and can be congenital (genetic mutations in AQP2, AVPR2) or acquired (lithium toxicity, hypercalcemia, chronic kidney disease). The genetic form follows autosomal recessive or X-linked recessive inheritance depending on the gene involved. Management includes adequate hydration, NSAIDs (reduce prostaglandin-mediated polyuria), and thiazide diuretics (paradoxically reduce urine output by causing mild volume depletion and increased proximal tubular reabsorption).

Why the other options are wrong

A. Barter syndrome — Bartter syndrome results from mutations in genes encoding proteins of the thick ascending limb of the loop of Henle (NKCC2, ROMK, ClC-Kb, barttin, or CaSR), not aquaporin channels. It presents with hypokalemic metabolic alkalosis, hypercalciuria, and normal blood pressure. The pathophysiology involves defective salt reabsorption in the TAL, not water channel dysfunction. This is a common NBE distractor because both are inherited renal tubular disorders. C. Liddle's syndrome — Liddle syndrome is caused by gain-of-function mutations in genes encoding the epithelial sodium channel (ENaC) subunits in the collecting duct, leading to excessive sodium reabsorption and potassium wasting. It presents with hypertension, hypokalemia, and metabolic alkalosis—the opposite of Bartter syndrome. Aquaporin mutations do not affect sodium handling; they specifically impair water reabsorption. This option tests whether students confuse different collecting duct ion channel disorders. D. Cystic fibrosis — Cystic fibrosis is caused by mutations in the CFTR gene (cystic fibrosis transmembrane conductance regulator), a chloride channel, not an aquaporin water channel. CF presents with thick, viscous secretions in lungs, pancreas, and GI tract, leading to recurrent infections, pancreatic insufficiency, and meconium ileus. While both are ion/water channel disorders, CFTR dysfunction affects chloride and fluid secretion in epithelial tissues, not renal water reabsorption. This is a trap for students who confuse different channel protein families.

High-Yield Facts

Mnemonics

AQP2 → NDI (Aquaporin-2 → Nephrogenic DI) Aquaporin-2 in collecting duct → Nephrogenic Diabetes Insipidus. Remember: AQP2 mutations = kidney can't respond to ADH = water stays in urine = polyuria. Use this when you see 'aquaporin mutation' in the stem. NDI vs CDI: Desmopressin Challenge Nephrogenic DI = No response to desmopressin (post-receptor defect). Central DI = Corrects with desmopressin (ADH deficiency). If the question mentions aquaporin (water channel), it's nephrogenic—the kidney itself is broken.

NBE Trap

NBE pairs aquaporin mutations with other inherited renal tubular disorders (Bartter, Liddle, CF) to test whether students confuse different channel protein families. The trap is assuming "renal tubular disorder + genetic mutation" automatically means Bartter or Liddle; students must recognize that aquaporins specifically regulate water transport, not ion transport.

Clinical Pearl

In Indian clinical practice, nephrogenic DI from lithium toxicity (used for bipolar disorder) is more common than congenital aquaporin mutations. A patient on lithium presenting with polyuria and hypernatremia should raise suspicion for acquired nephrogenic DI; genetic testing for AQP2 mutations is reserved for congenital cases with family history and early-onset symptoms in childhood.

_Reference: Harrison Ch. 295 (Diabetes Insipidus); KD Tripathi Ch. 12 (Endocrine Pharmacology); Robbins Ch. 20 (Kidney pathology)_