Tubular Disorders of Electrolyte Regulation

Mineralocorticoid receptor mutations and a severe recessive pseudohypoaldosteronism type 1

Pseudohypoaldosteronism type 1 (PHA1) is a rare genetic disease of mineralocorticoid resistance characterized by salt wasting and failure to thrive in infancy. Here we describe the first case of a newborn with severe recessive PHA1 caused by two heterozygous mutations in NR3C2, gene coding for the mineralocorticoid receptor (MR). Independent segregation of the mutations occurred in the family, with p.Ser166X being transmitted from the affected father and p.Trp806X from the asymptomatic mother Whereas the truncated MR(166X) protein was degraded, MR(806X) was expressed both at the mRNA and protein level. Functional studies demonstrated that despite its inability to bind aldosterone, MR(806X) had partial ligand-independent transcriptional activity. Partial nuclear localization of MR(806X) in the absence of hormone was identified as a prerequisite to initiate transcription. This exceptional case broadens the spectrum of clinical phenotypes of PHA1 and demonstrates that minimal residual activity of MR is compatible with life. It also suggests that rare hypomorphic NR3C2 alleles may be more common than expected from the prevalence of detected PHA1 cases. This might prove relevant for patient's care in neonatal salt losing disorders and may affect renal salt handling and blood pressure in the general population.

A novel CLCN5 mutation in a boy with Bartter-like syndrome and partial growth hormone deficiency

Dent disease is an X-linked recessive disorder affecting the proximal tubule and is characterized by low-molecular-weight proteinuria (LMWP), hypercalciuria, nephrocalcinosis/nephrolithiasis with a variable number of features of Fanconi syndrome. It is most often associated with mutations in CLCN5, which encodes the endosomal electrogenic chloride/proton exchanger ClC-5. Renal acidification abnormalities are only rarely seen in Dent disease, whereas the hypokalemic metabolic alkalosis associated with hyperreninemic hyperaldosteronism (Bartter-like syndrome) has been reported in only one patient so far. We report on a 5-year-old boy with Dent disease caused by mutation in CLCN5 gene, c.1073G>A, who presented with hypokalemic metabolic alkalosis and hyperreninemic hyperaldosteronism persisting over the entire follow-up. No mutations were found in NKCC2, ROMK, NCCT, or ClC-Kb genes. In addition, the patient exhibited growth failure associated with partial growth hormone (GH) deficiency. Coexistence of Bartter-like syndrome features with LMWP should prompt a clinician to search for Dent disease. The Bartter syndrome phenotype seen in Dent disease patients may represent a distinct form of Bartter syndrome, the exact mechanism of which has yet to be fully elucidated. Growth delay that persists in spite of appropriate therapy should raise suspicion of other causes, such as GH deficiency.