Functional Characterization of a Partial Loss-of-Function Mutation of the Epithelial Sodium Channel (ENaC) Associated with Atypical Cystic Fibrosis
Loss-of-function mutations of the epithelial sodium channel (ENaC) may contribute to pulmonary symptoms resembling those of patients with atypical cystic fibrosis (CF). Recently, we identified a loss-of-function mutation in the α-subunit of ENaC (αF61L) in an atypical CF patient without mutations in CFTR. To investigate the functional effect of this mutation, we expressed human wild-type αβγ-ENaC or mutant α F61Lβγ-ENaC in Xenopus laevis oocytes. The αF61L mutation reduced the ENaC mediated whole-cell currents by ñ90%. In contrast, the mutation decreased channel surface expression only by ñ40% and did not alter the single-channel conductance. These findings indicate that the major effect of the mutation is a reduction of the average channel open probability (Po). This was confirmed by experiments using the βS520C mutant ENaC which can be converted to a channel with a Po of nearly one, and by experiments using chymotrypsin to proteolytically activate the channel. These experiments revealed that the mutation reduced the average Po of ENaC by ñ75%. Na+ self inhibition of the mutant channel was significantly enhanced, but the observed effect was too small to account for the large reduction in average channel Po. The ENaC-activator S3969 partially rescued the loss-of-function phenotype of the αF61L mutation. We conclude that the αF61L mutation may contribute to respiratory symptoms in atypical CF patients.