A mutation of the epithelial sodium channel associated with atypical cystic fibrosis increases channel open probability and reduces Na⁺ self inhibition

Increased activity of the epithelial sodium channel (ENaC) in the respiratory airways contributes to the pathophysiology of cystic fibrosis (CF), a genetic disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. In some patients suffering from atypical CF a mutation can be identified in only one CFTR allele. We recently identified in this group of CF patients a heterozygous mutation (W493R) in the α-subunit of ENaC. Here, we investigate the functional effects of this mutation by expressing wild-type αβγENaC or mutant α_W493RβγENaC in Xenopus oocytes. The αW493R mutation stimulated amiloride-sensitive whole-cell currents (ΔI_ami) by ∼4-fold without altering the single-channel conductance or surface expression of ENaC. As these data suggest that the open probability (P_o) of the mutant channel is increased, we investigated the proteolytic activation of ENaC by chymotrypsin. Single-channel recordings revealed that chymotrypsin activated near-silent channels in outside-out membrane patches from oocytes expressing wild-type ENaC, but not in membrane patches from oocytes expressing the mutant channel. In addition, the αW493R mutation abolished Na⁺ self inhibition of ENaC, which might also contribute to its gain-of-function effects. We conclude that the αW493R mutation promotes constitutive activation of ENaC by reducing the inhibitory effect of extracellular Na⁺ and decreasing the pool of near-silent channels. The resulting gain-of-function phenotype of the mutant channel might contribute to the pathophysiology of CF in patients carrying this mutation.