RESUMO
A neonate affected by a novel inactivating mutation in the calcium-sensing receptor (CASR) gene is presented. This mutation is homozygously inherited and has not been previously described. A deletion in exon 5 (c.1392_1404del13) of the gene causes a loss of function of the receptor, which results in neonatal severe hyperparathyroidism and an ensuing extreme hypercalcemia. In a case of homozygosis of the CASR gene, the use of cinacalcet is the second reported calcimimetic treatment attempt and the first treatment attempt prior to surgery. However, because of the type of mutation, parathyroid surgery was necessary at 4 months of age after therapeutic failure. Because there are multiple mutations that affect the CASR gene in different ways, treatment with cinacalcet as an alternative to surgery may be valuable in homozygous cases that are caused by different mutations than the reported case.