Pankaj B Agrawal, Ruobing Wang, Hongmei Lisa Li, Klaus Schmitz-Abe, Chantelle Simone-Roach, Jingxin Chen, Jiahai Shi, Tin Louie, Shaohu Sheng, Meghan C Towne, Christine F Brainson, Michael A Matthay, Carla F Kim, Michael Bamshad, Mary J Emond, Norma P Gerard, Thomas R Kleyman, Craig Gerard

Cystic Fibrosis (CF) remains the most lethal genetic disease in the Caucasian population. However, there is great variability of clinical phenotype and survival, even among patients harboring the same genotype. We identified five CF patients with a homozygous F508del mutation in the CFTR gene living in their 5th or 6th decade of life, all with minimal change in lung function over longitudinal period of more than 20 years. Because of the rarity of this long-term non-progressive phenotype, we hypothesized those individuals may carry rare genetic variants in modifier genes that ameliorate disease severity. Individuals at the extremes of survival time and lung function trajectory underwent whole exome sequencing, and the data was filtered for missense, stopgain, indel, and splicing variants present with mean allele frequency <0.2%, and their potential impact on protein function was evaluated. The mutations in δ-ENaC were generated via site-directed mutagenesis, and expressed to carry out Xenopus oocyte assay. Four of the five individuals carried extremely rare or never reported variants in the SCNN1D and SCNN1B genes of the epithelial sodium channel (ENaC). Separately, an independently enriched rare variant in SCNN1D was identified in the exome variant server database associated with a milder pulmonary disease phenotype. Functional analysis using Xenopus oocytes revealed two of the three variants in δ-ENaC encoded by SCNN1D exhibited hypomorphic channel activity. Our data suggest a potential role of δ-ENaC in control of the sodium reabsorption in the airways, and advance the plausibility of ENaC as a therapeutic target in CF.