mTOR-Activating Mutations in RRAGD Are Causative for Kidney Tubulopathy and Cardiomyopathy.

BACKGROUND: Over the last decade, advances in genetic techniques have resulted in the identification of rare hereditary disorders of renal magnesium and salt handling. Nevertheless, approximately 20% of all patients with tubulopathy lack a genetic diagnosis. METHODS: We performed whole-exome and -genome sequencing of a patient cohort with a novel, inherited, salt-losing tubulopathy; hypomagnesemia; and dilated cardiomyopathy. We also conducted subsequent in vitro functional analyses of identified variants of RRAGD, a gene that encodes a small Rag guanosine triphosphatase (GTPase). RESULTS: In eight 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 also had dilated cardiomyopathy and three underwent heart transplantation. We identified a heterozygous variant in RRAGD that segregated with the phenotype in eight members of a large family with similar kidney manifestations. The GTPase RagD, encoded by RRAGD, plays a role in mediating amino acid signaling to the mechanistic target of rapamycin complex 1 (mTORC1). RagD expression along the mammalian nephron included 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, which we call autosomal dominant kidney hypomagnesemia (ADKH-RRAGD), that combines an electrolyte-losing tubulopathy and dilated cardiomyopathy. The condition is caused by variants in the RRAGD gene, which encodes Rag GTPase D; these variants lead to an activation of mTOR signaling, suggesting a critical role of Rag GTPase D for renal electrolyte handling and cardiac function.

Performance of an interstitial glucose monitoring device in patients with type 1 diabetes during haemodialysis.

Background: The use of interstitial glucose monitoring devices such as flash glucose monitoring has been shown to be beneficial in patients with type 1 diabetes mellitus (T1DM). However, these devices have been little studied in patients with diabetes treated by chronic haemodialysis (HD). Methods: The goal of this prospective, observational, multicentric study was to evaluate the analytical performance of the FreeStyle Libre 2 (FSL2) sensor in T1DM patients during HD sessions. During three HD sessions, interstitial fluid glucose (ISFG) concentrations given by the FSL2 were compared every 15 minutes with blood glucose (BG) concentrations obtained simultaneously. BG concentrations were measured by two different glucometers: the Accu-Chek Guide and StatStrip meters. Results: Twelve HD patients were included, with a mean age of 54 ± 11 years and a mean diabetes duration of 36.5 ± 11.6 years. Dialysis vintage was 35 ± 22 months. A total of 565 pairs of ISFG/BG values were available for analysis. The mean absolute relative difference, defined as the mean of the absolute relative differences between the ISFG and BG measurements, was 17.4% and 20.9% when the ISFG was compared with the StatStrip meter or Accu-Chek Guide, respectively. Interstitial results tend to underestimate blood results, but all values were classified as having clinically acceptable error. The differences observed remained stable during the dialysis session and were not associated with the ultrafiltration rate. Conclusion: Use of the FSL2 interstitial glucose monitoring device in HD patients with T1DM is clinically acceptable, even though the accuracy of the device is generally poorer than in studies including non-dialysis patients.