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Objectives Hypomagnesemia 1, intestinal (HOMG1) is characterized by neurological symptoms that occur due to hypocalcemia and hypomagnesemia and caused by mutations in the TRPM6. Most of the identified variants in TRPM6 lead to premature termination: nonsense, frameshift, deletion, and splice site mutations. Case presentation Herein, we report a 1.5 month-old case who presented with convulsion due to hypocalcemia and hypomagnesemia in the early infancy. Sequencing of TRPM6 revealed a novel homozygous synonymous variant [c.2538G > A (p.Thr846Thr)] in the last codon of exon 19, which is most likely to affect the splicing. We report a novel homozygous synonymous variant in the TRPM6 leading to HOMG1, expanding the mutational spectrum. Conclusions Synonymous mutations that were previously considered as harmless should be evaluated at the nucleotide level, keeping in mind that they may affect splicing and cause to the disease.

UNLABELLED Activating calcium sensor receptor (CaSR) mutations often present with hypocalcemia and hypomagnesemia. Severe hypocalcemia with this mutation has been reported but severe hypomagnesemia has not been well described. AIM To identify the cause of severe hypocalcemia and hypomagnesemia in a young child, and explore the efficacy of continuous subcutaneous magnesium therapy as a safer alternative to intravenous magnesium. PATIENT A 2-8/12 year-old female with severe hypocalcemia and hypomagnesemia of unknown etiology. METHODS Genetic analysis was performed on the proband and both parents. The proband was treated with human parathyroid hormone (teriparatide) and a continuous infusion of subcutaneous magnesium sulfate initially using a Deltec insulin pump and subsequently a Curlin infusion pump. RESULTS The patient has a known de novo mutation in the CASR gene (A843E). She could not be adequately managed with enteral and intravenous electrolyte replacement even after adding teriparatide. She responded well to adjunctive therapy with continuous subcutaneous magnesium. CONCLUSIONS Severe hypomagnesemia can be part of the phenotype of activating CaSR mutations. Subcutaneous magnesium should be considered in patients with difficult to control hypomagnesemia.