SAT-361 Familial Hypocalciuric Hypercalcemia Type 3: AP2S1 Mutation

Abstract

Background: Familial hypocalciuric hypercalcemia (FHH) type 3 can appear similar to primary hyperparathyroidism and make the diagnosis of etiology of hypercalcemia challenging.Clinical Case: A 45-year-old man with hypertension and glaucoma was evaluated in clinic for hypercalcemia. His calcium was 12.3 mg/dL (8.4–10.2), PTH 41.9 pg/mL (15–65), Vitamin D 14.8 ng/mL (6.6–49) and phosphorus 1.7 mg/dL (2.7–4.5). He denied history of thiazide diuretic use, fragility fracture, nephrolithiasis and family history of calcium disorders. Further workup revealed normal kidney function, undetectable PTH related peptide and dual-energy x-ray absorptiometry (DEXA) scan with a T-score of -3.3 at spine L1-L4, -2.7 at femoral neck and -2.1 at distal one-third forearm. A 24-hour urine collection revealed a urinary calcium of 42.4 mg/24-hour (100–300) and calcium: creatinine clearance ratio of 0.003. Diagnosis of primary hyperparathyroidism was made despite low urinary calcium as this was thought to be due to vitamin D deficiency. Sestamibi scintigraphy and four-dimensional computed tomography did not localize a parathyroid adenoma, however, the patient was sent to surgery for four gland parathyroid exploration for primary hyperparathyroidism in setting of high calcium and young age with evidence of end-organ failure of osteoporosis. During surgery, three large abnormal parathyroid glands were identified and one normal parathyroid gland. Patient had a three-gland parathyroidectomy with intraoperative drop in PTH by 26.5%. Pathology returned as benign parathyroid tissues. After surgery, patient had persistently elevated calcium level of 12.6 mg/dL and an inappropriately non-suppressed PTH. He was then started on bisphosphonate and cinacalcet for osteoporosis and hypercalcemia, respectively and sent for genetic testing of FHH. His CASR gene was negative but his AP2S1 gene was positive which confirmed the diagnosis of FHH type 3. His calcium responded well to cinacalcet and repeat DEXA scan showed stability of bone mineral density in spine and hip after two years of treatment with bisphosphonate therapy.Conclusion: Familial hypocalciuric hypercalcemia type 3 is caused by an inactivating mutation in the AP2S1 gene. This gene encodes the adaptor-related protein complex 2, sigma 1 subunit which is located downstream from calcium-sensing receptor. This genetic mutation can appear similar to primary hyperparathyroidism in that it produces high levels of calcium and PTH and low phosphorus. Hypercalcemia, however, persists despite removal of parathyroid gland. This genetic mutation can be treated with cinacalcet in patients with high levels of calcium (>1 upper limit of normal) or symptoms of hypercalcemia.