7633 Novel CASR Gene Mutation in a patient with Familial Hypocalciuric Hypercalcemia and Papillary Thyroid Cancer

Abstract Disclosure: A. Parihar: None. E.S. Nylen: None. S.M. Gandhi: None. Familial Hypocalciuric Hypercalcemia (FHH) is predominantly caused by loss-of-function mutations in the calcium-sensing receptor (CaSR) characterized by a benign elevation in calcium and PTH which can be mistaken for primary hyperparathyroidism (PHPT). Missense mutations in the CaSR gene causing FHH have been documented but not much is known regarding their clinical significance. We describe a patient with FHH found to have a novel variant mutation. A 37-year-old male with hypogonadism and fatty liver was incidentally found to have hypercalcemia suggestive of PHPT with elevated PTH (102-114 pg/ml), normal vitamin D, and Fractional Excretion of Calcium (FECa) of 0.014. His father also had hypercalcemia and had undergone parathyroid surgery. The patient denied kidney stones and had a normal bone density. Subsequent Sestamibi and 4D CT scan suggested parathyroid adenoma on the left mid-lower pole of the thyroid. Neck ultrasound also showed a left 1.2 x 1 x 1 cm thyroid nodule as well as a left neck mass with calcifications. Fine needle aspiration was positive for metastatic Papillary thyroid cancer(PTC) in the left neck mass and patient underwent total thyroidectomy along with subtotal parathyroidectomy and lateral neck dissection. Thyroid pathology confirmed multifocal PTC with lymph node involvement that were BRAF V600 positive. Parathyroid pathology was benign hyperplasia. The PTH level decreased appropriately intraoperatively (69%), but postoperatively a rise was noted with levels remaining 70-90 pg/ml with normal calcium levels. Repeat FECa was 0.007, supporting the diagnosis of FHH. Given patient’s father also had hypercalcemia, genetic testing was pursued. This revealed CaSR gene missense mutation (c.1382T>C p.Leu461Pro, heterozygous). This variation is not present in population databases and has been classified as a variant of uncertain significance. It can be difficult to distinguish PHPT from FHH based on FECa as in this case. The patient also had PTC which is not known to be associated with FHH. Due to uncertain significance of this novel CaSR missense mutation, long term follow-up and further familial genetic testing will help better understand clinical implications. Presentation: 6/2/2024