Cystic Fibrosis-related Diabetes Is Associated with Worse Lung Function Trajectory despite Ivacaftor Use

Cystic fibrosis

Cystic fibrosis is a rare genetic disease caused by mutations in CFTR, the gene encoding cystic fibrosis transmembrane conductance regulator (CFTR). The discovery of CFTR in 1989 has enabled the unravelling of disease mechanisms and, more recently, the development of CFTR-directed therapeutics that target the underlying molecular defect. The CFTR protein functions as an ion channel that is crucial for correct ion and fluid transport across epithelial cells lining the airways and other organs. Consequently, CFTR dysfunction causes a complex multi-organ disease but, to date, most of the morbidity and mortality in people with cystic fibrosis is due to muco-obstructive lung disease. Cystic fibrosis care has long been limited to treating symptoms using nutritional support, airway clearance techniques and antibiotics to suppress airway infection. The widespread implementation of newborn screening for cystic fibrosis and the introduction of a highly effective triple combination CFTR modulator therapy that has unprecedented clinical benefits in up to 90% of genetically eligible people with cystic fibrosis has fundamentally changed the therapeutic landscape and improved prognosis. However, people with cystic fibrosis who are not eligible based on their CFTR genotype or who live in countries where they do not have access to this breakthrough therapy remain with a high unmet medical need.

Potentiation of BKCa channels by cystic fibrosis transmembrane conductance regulator correctors VX-445 and VX-121

Cystic fibrosis results from mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) anion channel, ultimately leading to diminished transepithelial anion secretion and mucociliary clearance. CFTR correctors are therapeutics that restore the folding/trafficking of mutated CFTR to the plasma membrane. The large-conductance calcium-activated potassium channel (BKCa, KCa1.1) is also critical for maintaining lung airway surface liquid (ASL) volume. Here, we show that the class 2 (C2) CFTR corrector VX-445 (elexacaftor) induces K+ secretion across WT and F508del CFTR primary human bronchial epithelial cells (HBEs), which was entirely inhibited by the BKCa antagonist paxilline. Similar results were observed with VX-121, a corrector under clinical evaluation. Whole-cell patch-clamp recordings verified that CFTR correctors potentiated BKCa activity from both primary HBEs and HEK cells stably expressing the α subunit (HEK-BK cells). Furthermore, excised patch-clamp recordings from HEK-BK cells verified direct action on the channel and demonstrated a significant increase in open probability. In mouse mesenteric artery, VX-445 induced a paxilline-sensitive vasorelaxation of preconstricted arteries. VX-445 also reduced firing frequency in primary rat hippocampal and cortical neurons. We raise the possibilities that C2 CFTR correctors gain additional clinical benefit by activation of BKCa in the lung yet may lead to adverse events through BKCa activation elsewhere.

Pleiotropic modifiers of age-related diabetes and neonatal intestinal obstruction in cystic fibrosis

Individuals with cystic fibrosis (CF) develop complications of the gastrointestinal tract influenced by genetic variants outside of CFTR. Cystic fibrosis-related diabetes (CFRD) is a distinct form of diabetes with a variable age of onset that occurs frequently in individuals with CF, while meconium ileus (MI) is a severe neonatal intestinal obstruction affecting ∼20% of newborns with CF. CFRD and MI are slightly correlated traits with previous evidence of overlap in their genetic architectures. To better understand the genetic commonality between CFRD and MI, we used whole-genome-sequencing data from the CF Genome Project to perform genome-wide association. These analyses revealed variants at 11 loci (6 not previously identified) that associated with MI and at 12 loci (5 not previously identified) that associated with CFRD. Of these, variants at SLC26A9, CEBPB, and PRSS1 associated with both traits; variants at SLC26A9 and CEBPB increased risk for both traits, while variants at PRSS1, the higher-risk alleles for CFRD, conferred lower risk for MI. Furthermore, common and rare variants within the SLC26A9 locus associated with MI only or CFRD only. As expected, different loci modify risk of CFRD and MI; however, a subset exhibit pleiotropic effects indicating etiologic and mechanistic overlap between these two otherwise distinct complications of CF.