Gain-of-Function Polymorphisms in Human Inflammasomes: Implications for Cystic Fibrosis

Abstract

Cystic fibrosis (CF) is an inherited disease involving chronic infection and inflammation of the lungs (1). CF results frommutations in the CFTR (CF transmembrane conductance regulator) gene (2) and is the most common inherited genetic disease in white individuals, affecting 1 in 3,000 newborns (3).Pseudomonas aeruginosa is a common cause of morbidity in patients with CF, triggering excessive inflammation, lung damage, and eventual respiratory failure (4). Several therapeutic approaches have been attempted or are currently in use, including antiinflammatory drugs, antioxidants, antibiotics, CFTRmodulators, and gene therapy (1, 5). However, management of infection and lung damagebecauseof chronic inflammation remains amajor challenge for patients with CF. CF lung disease involves elevated expression of proinflammatory cytokinesbecauseofdysregulationof the immune response (6).Central regulators of innate immunity are pattern recognition receptors, includingTLRs(Toll-likereceptors)andNLRs(nod-likereceptors) that recognize pathogen-associated molecular patterns and damageassociatedmolecularpatterns. SomeNLRs, suchas inflammasomes, are self-oligomerizingproteincomplexes thatconsistofnucleotide-binding and oligomerization domains, and the ASC protein, which recruits procaspase-1, leading to its activation (7–9). Active caspase-1 is responsible for proteolytic activation of proinflammatory cytokines, IL-1b and IL-18. The release of these proinflammatory cytokines and subsequentpyroptosis of the cell can lead to inflammatory lungdamage (6, 7). In a recent study of a murine CFmodel using P. aeruginosa, the inhibition of the NLRP3 inflammasome was linked to improved bacterial clearance from the lung (10). Unlike animal models, the complexity of inflammasome biology in the human population arises from SNPs of these genes. These SNPs can give rise to inflammasome variants, which alter their activity, leading to changes in proinflammatory cytokine expression. In fact, it has been known for some time that mutations in the gene encodingNLRP3 can induce a number of autoinflammatory disorders knownas cryopyrin-associated periodic syndromes (11). Studies are now beginning to delve into the mechanistic action of specific inflammasome genetic variants. For instance, a recent report on theNLRP3 (Q705K) variant demonstrated an increase in inflammasome activation for the variant over wild-type NLRP3 (12). However, a variant in theNLRC4 gene was found to correlate with decreased expression of IL-18 and improved lung function in patients with human immunodeficiency virus (HIV) and tuberculosis coinfection (13). In another study, a missense variant of NLRC4 was also associated with decreased expression of IL-18 (14). In this issueof the Journal,Grausteinandcolleagues (pp. 157–166) identify inflammasome SNPs associated with P. aeruginosa lung colonization and lung function in CF (15). For this study, the authors genotyped variants from patients in the EPIC (Early Pseudomonas Infection Control) observational study. Previous investigations have described SNPs forNLRP3 andNLRC4 that alter the activity of these inflammasomes (12–14, 16, 17). However, Graustein and colleagues now show for the first time a correlation between inflammasome SNPs and the clinical outcomes of patients with CF carrying these genetic variants. The authors demonstrate that the NLRP3 (Q705K) variant correlated with an increased rate of P. aeruginosa lung infection and decreased lung function over time. However, only children who had previously been infected with P. aeruginosa demonstrated this effect. Theauthorsnote that agroupofolder childrenwhowerenever infected with P. aeruginosa could exert a cohort bias against containing the NLRP3 (Q705K) variant, possibly explaining why the effects were only observed in children previously infected. The authors also identify the novelNLRC4 (A929S) variant as an SNP associated with protection of lung function for children with CF never infectedwithP.aeruginosa.However, becauseof the small sample size, the effect of theNLRC4 (A929S) SNP on P. aeruginosa lung colonization could not be directly investigated. Instead, the authors analyzedagroupofvariants frommultiplegenes intheNLRC4pathway asasinglevariableandfoundaprotectiveroleagainstP.aeruginosa lung colonization for these alterations. The lung colonization findings include only patients never infected with P. aeruginosa before enrollment. As with theNLRP3 SNP cohorts, this protectiveNLRC4 variant could reflect cohort bias against including children previously infected with P. aeruginosa. The authors also note that theNLRC4relatedvariants studiedwereonlyassociatedwithprotection inchildren with wild-typeCAV2 (caveolin 2), a gene associated with facilitating P. aeruginosa infection (18). This finding suggests that the NLRC4 pathway variants may antagonize the activity of CAV2. To better understand the importance ofNLRP3 andNLRC4 SNPs inchronic inflammation, the authorsusedhumanmacrophage-like cell lines expressing wild-type or variant inflammasome constructs for in vitro studies. Using these cell lines, the authors demonstrate that the NLRP3 (Q705K) variant-expressing cells are more responsive to