Abstract
Introduction
The extracellular calcium-sensing receptor (CaSR) expressed mainly in the parathyroid gland and kidneys regulates calcium (Ca+2) homeostasis through parathyroid hormone (PTH) secretion. Activating mutations of CaSR can lead to autosomal dominant hypocalcemia and severe congenital hypoparathyroidism. Constitutively activated CaSR receptors blocks PTH release leading to hypocalcemia, hyperphosphatemia and decreased Ca+2 reabsorption from the kidney.
Case 1:
14 year old male presented for an evaluation of hypocalcemia and hyperphosphatemia found on routine blood work. He denied symptoms of hypocalcemia. He had normal vital signs, positive Chovstek sign but rest of exam was unremarkable. His lab results showed low Ca+2 8.1 mg/dl (8.6–10 mg/dl), high phosphorus 6 mg/dl (2.7–4.5 mg/dl) and inappropriately normal PTH 26.8 pg/ml (10–65 pg/ml). FISH was negative for DiGeorge. Genetic testing showed heterozygous CaSR gene mutation I822T, variant of uncertain significance. His father with primary hypoparathyroidism has the same CaSR gene mutation; mother is healthy and tested negative for this variant. Given the inheritance pattern of the mutation, it is likely a pathologic mutation. He is maintained on Calcium (1500 mg BID) and Calcitriol (0.5 mcg PO BID) and is doing well.
Case 2:
One day old premature 32-week old infant girl was found to have early onset neonatal hypocalcemia 6.1 mg/dl (6.2–11 mg/dl) during NICU admission for respiratory distress, inappropriately normal PTH 18.5 pg/ml and high phosphorus 8.8 mg/dl (4.6–7.9 mg/dl). She had no symptoms of hypocalcemia in the NICU or at home. She did not have any dysmorphic features. FISH was negative for DiGeorge. Genetic testing to sequence genes including AIRE, AP2S1, CASR, GNAS, HADHA, HADHB, PTH1R, SOX3, STX16, TBCE was done and revealed a novel heterozygous mutation in the CaSR gene for a missense variant c.2495T>C
(p.lle832Thr) and STX16 c.644A>T, possibly benign variant. Unfortunately, the parents have not consented to testing yet. Further familial and functional characterization of this new variant is necessary to confirm its possible pathogenetic role in this hypocalcemic patient. Currently she is maintained on ergocalciferol 800 IU, calcitriol 0.25 mcg and sevelamer 3 packets daily and is doing well.
Conclusion:
In the workup for primary hypoparathyroidism without dysmorphic features and tests negative for DiGeorge, CaSR mutations should be investigated as part of the differential as we have identified variants in the CaSR gene in 2 children with asymptomatic hypocalcemia, one of which is a novel mutation which has never been reported before.