mTOR-activating mutations in RRAGD cause kidney tubulopathy and cardiomyopathy (KICA) syndrome

Background Over the last decaces, advances in genetic techniques have resulted in the identification of rare hereditary disorders of renal magnesium and salt handling. Nevertheless, ±20% of all tubulopathy patients remain without genetic diagnosis. Here, we explore a large multicentric patient cohort with a novel inherited salt-losing tubulopathy, hypomagnesemia and dilated cardiomyopathy (DCM). Methods Whole exome and genome sequencings were performed with various subsequent functional analyses of identified RRAGD variants in vitro. Results In 8 children from unrelated families with a tubulopathy characterized by hypomagnesemia, hypokalemia, salt-wasting, and nephrocalcinosis, we identified heterozygous missense variants in RRAGD that mostly occurred de novo . Six of these patients additionally suffered from DCM requiring heart transplantation in 3 of them. An additional dominant variant in RRAGD was simultaneously identified in eight members of a large family with a similar renal phenotype. RRAGD encodes GTPase RagD mediating amino acid signaling to the mechanistic target of rapamycin complex 1 (mTORC1). RagD expression along the mammalian nephron include the thick ascending limb and the distal convoluted tubule. The identified RRAGD variants were shown to induce a constitutive activation of mTOR signaling in vitro , Conclusions Our findings establish a novel disease phenotype combining kidney tubulopathy and cardiomyopathy (KICA) caused by an activation of mTOR signaling suggesting a critical role of Rag GTPase D for renal electrolyte handling and cardiac function. SIGNIFICANCE STATEMENT Here, we report on heterozygous variants in RRAGD in patients with profound hypomagnesemia, renal salt wasting, nephrocalcinosis, and dilated cardiomyopathy. The identified RagD variants induce a constitutive activation of mTOR signaling in vitro . These findings not only establish a novel monogenic disorder of the kidney tubule, but demonstrate the essential role of mTOR signaling for distal tubular electrolyte handling and cardiac function. ### Competing Interest Statement M.L. has received honoraria for scientific lectures from Pfizer. Representing the University Hospital of Cologne, M.L. has been counselling Otsuka Pharmaceuticals in an advisory board. The other authors have no conflict of interest to disclose.