OR23-02 Genotype-Phenotype Correlation In Calcium Sensing Receptor (Casr) Gene Gain-Of-Function Mutations: A Case Series And Report Of 2 Novel Mutations
Authors: Ali Dalal S , Marini Francesca , Alsarraf Farah , Alalwani Hatim H , Alamri Abdulrahman , Aziz Khan Aliya , Luisa Brandi Maria 
Published: October 2023

Abstract
Disclosure: D.S. Ali: None. F. Marini: None. F. Alsarraf: None. H.H. Alalwani: None. A. Alamri: None. A.A. Khan: Advisory Board Member; Self; Amgen Inc, Takeda. Grant Recipient; Self; Amgen Inc, Alexion Pharmaceuticals, Inc., Takeda, Ultragenyx, Radius Health, Inc. M. Brandi: Consulting Fee; Self; Alexion Pharmaceuticals, Inc. Speaker; Self; Amgen Inc, Alexion Pharmaceuticals, Inc., Eli Lilly & Company.
Background: Autosomal dominant hypocalcemia (ADH1) is a genetic disorder characterized by low serum calcium and low or inappropriately normal parathyroid hormone levels. It is caused by a heterozygous activating mutation of the calcium-sensing receptor (CaSR) gene. ADH1 has been linked to 113 unique germline mutations, of which 96% are missense mutations. There is a lack of clear genotype-phenotype correlation in the reported literature. Methods: We described a case series of six unrelated ADH1 probands, each with a gain-of-function CaSR mutation, and two children of one of these cases, comparing clinical, biochemical, and complication profiles and matching our identified mutations to those previously reported in the literature. Results: Of our cases 75% are familial and 25% are secondary to de novo mutation. Our 6 unrelated ADH1 probands showed distinct clinical presentations, while 3 cases within the same pedigree shared similar presentation and clinical characteristics including presentation in infancy, hypocalcemic seizures (2/3; 66.7%), hypomagnesemia, hyperphosphatemia, and hypercalciuria. The biochemical profile differed among the 6 cases in the degree of hypocalcemia, associated hypercalciuria, hypomagnesemia, and/or hypokalemia. Response to therapy also differed. When comparing our cases to previously published cases, we noted that mutations in the 5th transmembrane domain of the CaSR (TM5) carry a more severe form of the disease and may be associated with a higher rate of hypocalcemic seizures, basal ganglia calcifications as well as nephrocalcinosis, nephrolithiasis and renal insufficiency compared to mutations in the extracellular regions. Hypomagnesemia was present in 100% of the cases harboring mutations in TM5. We also describe two novel activating mutations of the CaSR gene; c.2468T&gt;A, p.lle823Asn and c.1756G&gt;A, p.Glu586Lys. Both of these were not reported either on the most common databases of human mutations or in the literature. In silico analysis of p.lle823Asn by PolyPhen-2 predicted the Ile to Asn substitution at position 823 as “probably damaging”, while the p.Glu586Lys substitution was suggested to have a “benign” nature. Our patient with the former mutation had a history of seizure and nephrolithiasis, while our patient with the latter mutation is mildly symptomatic, presented in her 40s, and is on low doses of active vitamin D and calcium supplements. Conclusion: As a result of these genetic and clinical comparisons, we propose that a genotype-phenotype correlation may exist because our cases showed similar presentation, characteristics, and severity, with respect to published cases with the same or similar mutations. We also contended that the severity of the presentation is highly influenced by the specific CaSR variant. These findings, however, require further evaluation and assessment with a systematic review.
Presentation: Saturday, June 17, 2023