ESPE Abstracts

Background: PHA1 is a rare inherited disease characterized by resistance to aldosterone action and distinguished in two forms: the autosomal dominant renal form caused by mutations of the NR3C2 gene (MR) and the autosomal recessive systemic form caused by mutations of the subunit genes SCNN1A, SCNN1B, SCNN1G of the epithelial sodium channel (ENaC). The classic phenotype of the autosomal recessive form of PHA1 is usually severe, lifelong, and expressed with multi-organ symptoms, whereas the autosomal dominant form is milder, transient and restricted to the kidneys. A large consanguineous family with a mild and transient form of autosomal recessive PHA1 due to a novel homozygous mutation in the SCNN1A gene has been previously described.

Objective: To establish the effect of the p.F226C SCNN1A gene mutation on the PHA1 phenotype by functional studies.

Methods: Human αENaC wt, or αF226C or αF226S, together with human β and γ ENaC from synthetic cRNAs, were expressed in Xenopus laevis oocytes. F226S was generated to have a conservative substitution of the Cys226 lacking the reactive -SH side chain. ENaC activity was measured as amiloride-sensitive currents (INa) with the addition of trypsin during concurrent recording. The expression of α ENaC wt and mutants was determined by Western blot.

Results: Patients were diagnosed between 5 and 60 days of age presenting with failure to thrive or during the course of a respiratory illness, with hyperkalemia, hyponatremia, elevated renin and aldosterone levels and a positive sweat test. All patients responded well to sodium supplementation with decreasing requirements with age until discontinuation of treatment. All patients were found to be homozygotes, whereas their parents were heterozygotes for the mutation p.F226C of the SCNN1A gene. ENaC activity was significantly reduced for the F226C (83% reduction, n=40) and the F226S (94% reduction, n=13). Both mutants responded to trypsin by a robust increase in ENaC current, but under trypsin, the magnitude of ENaC current remained lower for the mutants than the wt. However, the trypsin-induced increase in ENaC current relative to baseline was greater for the mutants than the wt. Western blot revealed that a reduced amount of ENaC protein was expressed under both the full-length and the cleaved αENaC mutants, F226C or F226S, compared to wt.

Conclusions: The p.F226C SCNN1A gene mutation leads to a 50% decrease in the intrinsic ENaC activity, probably due to a lower channel open probability.