Familial Hypomagnesemia with Secondary Hypocalcemia (TRPM6)
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smile Pediatrics
A 6-week-old female infant presents to the emergency department with generalized tonic-clonic seizures, irritability, and muscle spasms that do not respond to intravenous calcium gluconate boluses. Serum magnesium is 0.3 mg/dL (normal 1.7–2.4), serum calcium is 6.2 mg/dL, and PTH is inappropriately low. Genetic testing reveals a homozygous loss-of-function mutation in the locus marked **A** on the karyotype. Which of the following best explains the pathophysiology of this infant's secondary hypocalcemia?
A. Primary renal magnesium wasting due to defective paracellular reabsorption in the thick ascending limb of the loop of Henle
B. Defective intestinal calcium absorption due to vitamin D-dependent mechanisms independent of magnesium status
C. Defective active transcellular magnesium absorption in the small intestine and distal convoluted tubule leads to severe hypomagnesemia, which impairs PTH secretion and induces end-organ PTH resistance
D. Mutations in the SLC12A1 gene causing secondary magnesium depletion through excessive urinary losses
Explanation
Why option 1 is correct
The structure marked A is chromosome 9q22, the locus of the TRPM6 gene. TRPM6 encodes a transient receptor potential melastatin 6 channel that mediates active transcellular magnesium absorption in the small intestine and distal convoluted tubule. Loss-of-function mutations in TRPM6 cause selective intestinal magnesium malabsorption, resulting in profound hypomagnesemia. The severe hypomagnesemia then impairs PTH secretion and induces end-organ PTH resistance, producing the characteristic secondary hypocalcemia that is refractory to calcium supplementation alone. This is the defining pathophysiology of familial hypomagnesemia with secondary hypocalcemia (HSH, OMIM 602014), an autosomal recessive disorder (Nelson Textbook of Pediatrics 22e, Ch 70; Schlingmann et al., Nat Genet 2002).
Why each distractor is wrong
Option 2: While renal magnesium wasting does occur in some genetic disorders (e.g., Bartter syndrome), HSH is characterized by low urinary magnesium excretion (FE-Mg <2%), indicating intact renal conservation. The primary defect is intestinal malabsorption, not renal wasting. This distractor confuses HSH with renal tubular disorders.
Option 3: SLC12A1 (chromosome 15) encodes the Na-K-2Cl cotransporter in the thick ascending limb and is mutated in Bartter syndrome, which presents with hypokalemia, metabolic alkalosis, and secondary hypermagnesuria—a completely different phenotype from HSH. This is a common genetic confusion.
Option 4: Calcium absorption is not the primary defect in HSH; the disorder is specifically one of magnesium malabsorption. Hypocalcemia develops secondarily due to impaired PTH secretion and action from hypomagnesemia, not from a primary vitamin D-dependent calcium absorption defect. This distractor misidentifies the primary pathophysiology.
High-YieldNEET PG
TRPM6 mutations → intestinal Mg malabsorption → severe hypomagnesemia → impaired PTH secretion + end-organ resistance → secondary hypocalcemia refractory to calcium alone; diagnosis confirmed by low FE-Mg (<2%) and biallelic TRPM6 mutations.
Nelson Textbook of Pediatrics 22e Ch 70; Schlingmann TRPM6 Nat Genet 2002
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