Novel Heterozygous Calcium Sensing Receptor (CASR) Genetic Variant in Child with Unique Phenotype: Hypocalcemia, Mandibular Hypoplasia, Renal Cysts and Type E Brachydactyly

Abstract

Abstract Background: There are over 230 disease-causing variants in the calcium-sensing receptor gene (CaSR). Gain-of-function missense mutations in CaSR cause Autosomal Dominant Hypocalcemia (ADH) characterized by hypocalcemia (hCa), hypoparathyroidism (hPTH), and hypercalciuria. Patients with ADH are sensitive to fluctuations in serum calcium (Ca); and supplementation with Ca and vitamin D can cause inappropriate renal calcium retention leading to hypercalcemic events and long-term renal complications. Clinical Case: A 15-year-old adopted (at age 18 months) Korean female was initially diagnosed with hPTH and chronic hCa after presenting with hCa seizures. Laboratory values showed hCa (7.7 mg/dL), hyperphosphatemia (7.6 mg/dL) and hPTH (< 3 pg/mL.) Initially, she was treated with Ca supplementation (20 mg/kg/day elemental Ca), and calcitriol (0.01 mcg/kg/day). She presented at age 4 with hematuria and was found to have obstructive nephrolithiasis requiring operative intervention. Renal ultrasound (US) showed bilateral medullary nephrocalcinosis. She continued treatment with Ca and calcitriol. At age 6, a thiazide diuretic and potassium citrate supplement were added due to hypercalciuria. She had recurrent nephrolithiasis and persistent nephrocalcinosis. Follow-up renal US also showed bilateral renal cysts. Biweekly laboratory evaluation demonstrated an exuberant response to calcium supplementation. Serum Ca levels oscillated between 7.0 -10 mg/dL, but she showed minimal symptoms of hCa. At age 14, she was also recognized to have submandibular hypoplasia and brachydactyly of the 4th and 5th metacarpals and metatarsals bilaterally and genetic testing for CaSR gene mutation was requested. Sshe developed acute kidney injury and hypercalcemia, possibly precipitated by viral illness. However, 3 weeks before, calcitriol dose was increased to 1.25 mcg twice a day (0.07 mcg/kg/day). At admission, serum Ca was 12.7 mg/dL, iPTH 5.2 mg/dL, phosphorus 4.5 mg/dL, BUN 36 mg/dL, creatinine 1.85 mg/dL. Symptoms included headache, muscle spasm and throat spasm. She received intravenous fluids and recovered, but had an extended hospital stay. Targeted genetic analysis of the CaSR gene was completed, and identified a heterozygous variant (c.2506G>T, p.V836L) which is predicted to be likely pathogenic and cause ADH. After CaSR gene mutation identification, the calcitriol and also elemental Ca dosing were decreased to achieve a low Ca level (~7 mg/dL) with normal urine Ca/creatinine ratio. Patient remains asymptomatic. Conclusion: This is the first case of a novel mutation in the CaSR (c.2506G>T, p.V836L) associated with ADH, brachydactyly, renal cysts, and mandibular hypoplasia. Timely genetic testing for ADH in patients with newly diagnosed hPTH can lead to changes in therapy and improved prognosis.