Genetic Variations in the NLRP3 Inflammasome : Susceptibility Factor for Chronic Inflammation

Sammanfattning: NLRP3 has been recognized as one of the key components of innate immunity. Upon activation, NLRP3 forms a multiprotein complex called as the ‘inflammasome’ which leads to the activation of pro-inflammatory caspase-1 which subsequently results in the formation of Interleukin (IL)-1β and IL-18. Mutations in the NLRP3 gene can lead to its constitutive activation resulting in an uncontrolled production of IL-1β. These mutations have been implicated in hereditary inflammatory diseases, often grouped under Cryopyrin associated periodic syndromes (CAPS, cryopyrin being an alternative name for NLRP3).Paper I in this thesis presents the case of a patient with a long history of arthritis and antibiotic resistant fever, but without the typical symptoms of CAPS. The patient was a heterozygous carrier of two common polymorphisms, Q705K in NLRP3 and C10X in CARD-8. Experimental studies indicated elevated activity of caspase-1 and IL-1β levels in the patient and a total clinical remission was achieved by IL-1β blockade. These two polymorphisms simultaneously occur in almost 4% of the control population, suggesting the possibility of a genetic predisposition for inflammation in these individuals. We, therefore, investigated a cohort of rheumatoid arthritis (RA) patients in paper II, and found that carrying the combined polymorphisms resulted in increased RA susceptibility and a more severe disease course. Hypothetically, this subgroup might benefit from IL-1β blockade. Paper III presents two patients: siblings, who did not fit into a typical CAPS phenotype. The inflammatory symptoms in both the patients appeared in adult life. A novel and functional M299V mutation in NLRP3 was detected in the siblings who neither had common symptoms nor the same disease severity. Consequent with inflammasome activation, abnormally elevated caspase-1 activity and IL-1β levels were seen. Patients in papers I and III highlight the risk of missing out such patients if attempting a very conventional diagnosis. Paper IV dissects the functional role of Q705K in NLRP3 using THP-1 cells in an in vitro model. Moderately elevated IL-1β and IL-18 levels could be observed in the THP-1 cells expressing Q705K, as compared to the wild type expressing cells, indicating a gain-of-function. Due to the presence of this alteration in healthy individuals it can be classified as a low-penetrance alteration. Additional studies are warranted to elucidate the mechanistic details of this polymorphism.