Mechanisms that Kindle Disease in Periodic Hereditary Diseases : How Isoprenoid Deficiency and SerpinB9 Mediate Autoinflammation

Abstract

Autoinflammatory disease (AID) is the uncontrolled and recurrent activation of the innate immune response, followed by complete resolution of the inflammation. Patients suffer from recurring fever episodes sometimes in combination with other inflammatory related symptoms. From a biochemical point of view, AID is characterized by the hyper-secretion of the pro-inflammatory cytokine interleukin-1β (IL-1β), originating from circulating monocytes. In AID the monocytes are primed to secrete more IL-1β than usual, or have a lower activation threshold. AID is diagnosed based on the clinical symptoms, but in most cases there is a genetic origin. AID has been classified in several syndromes, based on the identification of the causative mutations. The AID mevalonate kinase deficiency (MKD), the focus of this thesis, is caused by mutations the enzyme mevalonate kinase, reducing its activity to below 10% of the wild type enzyme. This deficiency leads to reduced prenylation, a posttranslational lipid modification, of proteins and results in AID. In this thesis we investigate the molecular mechanisms underlying reduced prenylation and how it leads to increased IL-1β secretion from monocytes. We determined that reduced prenylation leads to the accumulation of damaged mitochondria in the cell and increases mitochondrial potential. The defective protein prenylation results in altered autophagy, and failure to clear the damaged mitochondria from the cytosol. The mitochondrial content from these damaged mitochondria leaks into the cytosol. The released mitochondrial DNA and reactive oxygen species are then free to activate the inflammasome, a multi protein complex that triggers the immune response. The inflammasome contains the IL-1β converting enzyme, caspase-1. The leaked mitochondrial content primes the monocytes for the secretion of IL-1β. When the activation is triggered, activation of caspase-1 leads to the increased secretion of IL-1β. Furthermore we found that lack of prenylation alters the activity and localization of the small GTPases Rac1 and RhoA. In affected monocytes, Rac1 activity is increased, while RhoA activity decreases when the proteins are not prenylated. Blocking RhoA activity leads to increased Rac1 activity, suggesting a clear connection and possible crosstalk between Rac1 and RhoA. The increased Rac1 activation had already been linked to increased IL-1β, and could be a second route to the priming of monocytes in MKD. In an AID patient with no known AID causing mutation, we identified a mutant version of SerpinB9, an endogenous inhibitor of caspase-1.We assessed the biological function of this mutant and its possible contribution to the development of autoinflammatory disease. The mutant protein has limited potential to inhibit caspase-1 compared to the wild type protein, but retains equal inhibition on its other target protein, granzyme B. While the mutant is not the sole cause of AID, it does contribute to increased IL-1β secretion. The findings in this thesis shed light on the molecular basis of autoinflammatory disease and can be used to identify new future targets for therapeutic intervention in MKD and possible other AID.