A mild and transient form of autosomal recessive Pseudohypoaldosteronism type 1 (PHA1) caused by a novel mutation in the SCNN1A gene

OBJECTIVE

To investigate the genetic etiology in a cohort of patients with a clinical, biochemical, and hormonal profile suggestive of a mild and transient form of PHA1. Patients - Methods: Twelve PHA1 patients with clinical and biochemical data were analyzed. The coding regions of NR3C2 and SCNN1A genes were sequenced. Human αENaC wt, αF226C and αF226S ENaC variants, were expressed in Xenopus laevis oocytes. to evaluate ENaC activity. The protein expression of α ENaC wt and mutants was determined by Western blot.

RESULTS

All patients were homozygotes for the p.Phe226Cys of the a subunit of ENaC, resulting in significantly reduced ENaC activity (83% reduction). Quantitative western blot analysis was consistent with a reduced ENaC protein expression for the αF226C compared to wt.

CONCLUSIONS

We present 12 patients from 4 different families with a mild and transient autosomal recessive PHA1 due to a novel homozygous missense mutation in the SCNN1A gene. Functional studies showed that the p.Phe226Cys substitution mutation in ENaC leads to a partial loss of function resulting mainly from both a decrease in the intrinsic ENaC activity, and a reduction in channel expression at the protein level. The partial loss of ENaC function could explain the mild phenotype, variable expressivity, and the transient course of the disorder in these patients.

The Human Acid-Sensing Ion Channel ASIC1a: Evidence for a Homotetrameric Assembly State at the Cell Surface

The chicken acid-sensing ion channel ASIC1 has been crystallized as a homotrimer. We address here the oligomeric state of the functional ASIC1 in situ at the cell surface. The oligomeric states of functional ASIC1a and mutants with additional cysteines introduced in the extracellular pore vestibule were resolved on SDS-PAGE. The functional ASIC1 complexes were stabilized at the cell surface of Xenopus laevis oocytes or CHO cells either using the sulfhydryl crosslinker BMOE, or sodium tetrathionate (NaTT). Under these different crosslinking conditions ASIC1a migrates as four distinct oligomeric states that correspond by mass to multiples of a single ASIC1a subunit. The relative importance of each of the four ASIC1a oligomers was critically dependent on the availability of cysteines in the transmembrane domain for crosslinking, consistent with the presence of ASIC1a homo-oligomers. The expression of ASIC1a monomers, trimeric or tetrameric concatemeric cDNA constructs resulted in functional channels. The resulting ASIC1a complexes are resolved as a predominant tetramer over the other oligomeric forms, after stabilization with BMOE or NaTT and SDS-PAGE/western blot analysis. Our data identify a major ASIC1a homotetramer at the surface membrane of the cell expressing functional ASIC1a channel.