MKD is an auto-inflammatory syndrome caused by a genetic defect in mevalonate pathway and isoprenoid biosynthesis. Recent studies have established that patients have hyperactive caspase-1 due to a shortage of geranylgeranylated proteins and an increased IL-1β
When PAMPs or DAMPs (ie, vaccine, microbial infection, stress and fatigue) stimulate innate immune system of MKD patients, it results in typical inflammatory attacks.
In this study, we considered the hypothesis that the activation of caspase-1 – due to the inhibition of mevalonate pathway – could be mediated by one of the three known inflammasomes. Indeed, the principal role showed for NALP1-, NALP3- and IPAF-inflammasome is the cleavage of pro-caspase-1 in its active form.
Because of the limited number of patients, we used a cellular model based on the treatment of human monocytes with the aminobisphosphonate alendronate (inhibitor of mevalonate pathway) and a bacterial compound, to substantiate the inflammasome involvement in the pathogenesis of MKD, through the study of the NALP1/NLRP1, NALP3/NLRP3 and IPAF/NLRC4 expression.
In this model, we have shown that alendronate alone, and even more together with LPS, induces a great IL-1β
secretion and also upregulates the transcription of CASP1
and of the three inflammasome' genes. In particular, NALP3/NLRP3
seems to be the most susceptible to inhibition of the mevalonate pathway. In physiologic condition NLRP3
expression could be induced by common pro-inflammatory compounds, such as LPS, TNF and IL-1β
this is the first time to our knowledge that the induction of NLRP3
through a metabolic impairment was reported.
When the same experiments were conducted in monocytes isolated from the only two MKD patients available at the moment of the study, we observed that NALP3/NLRP3 is already hyper-expressed in untreated cells and probably it was the reason why it could not be further inducible by LPS. This result suggests that MKD patients are characterized by basal high levels of NLRP3, independently of any kind of stimulation. The association between the genetic defect in mevalonate pathway and NLRP3 transcription needs to be replicated in a greater number of MKD patients.
Different to what was expected on the basis of the results obtained in our cellular model, the induction of NLRP1 and NLRC4 by LPS treatment was observed only in patients and not in the control samples, leading us to hypothesize that being NLRP3 transcription dysregulated, other inflammasomes (ie, NLRP1 and NLRC4) can have a role in the regulation of inflammation.
Of course, we cannot exclude that a more general consequence of the mevalonate pathway metabolic block involves other genes besides NLRP3 and inflammasome. How the metabolic dysregulation could affect the gene transcription is currently under investigation and we cannot exclude that an epigenetic effect occurs.
Further investigations are needed to confirm the involvement of NALP3-inflammasome deregulation in the pathogenesis of MKD. In chronic inflammatory disorders, such as rheumatoid arthritis, NLRP3
has been reported to be upregulated,11
but no role for NALP3-inflammasome has been shown so far in the pathogenesis of this disease. Considering that the constitutive activation of NALP3-inflammasome is associated with several auto-inflammatory syndromes, such as familial cold urticaria, Muckle–Wells syndrome, CINCA syndrome and familial Mediterranean fever, it was conceivable that, at least indirectly, its deregulation could also be involved in the pathogenesis of MKD. Furthermore, the constitutive activation of NALP3 inflammasome leads to decreased apoptosis,12
an event that was also observed in leukocytes from MKD subjects.13
It could be interesting to monitor NLRP3
expression in our previously reported animal model of MKD after treatment with alendronate.6
Moreover, the role of NALP3-inflammasome in the inflammation induced by the mevalonate pathway inhibition could be substantiated in mice KO for NLRP3
or other inflammasome genes.
A proteomic approach is difficult nowadays because of the lack of easy technical methods to discriminate the active form of NALP3 from its inactive one. All the research in this field has been made using recombinant mutated proteins.14
The results reported in this study, although preliminary, support our hypothesis about the involvement of inflammasomes, in particular NALP3-inflammasome, in the activation of caspase-1 within the pathogenetic events that, starting from the impairment of mevalonate-derived intermediates, lead to the inflammatory systemic effects observed in MKD.