Asma Smahi

The genetics of monogenic auto-inflammatory diseases

Asma Smahi
  • Christine Bodemer
  • Smail Hadj-Rabia
  • Asma Smahi
  • Pascale de Lonlay
  • Elodie Bal
  • Yamina Hamel
  • Laura Polivka
  • Rania Amar
  • Marine Madrange
  • Inderpreet Singh

The genetics of monogenic auto-inflammatory diseases

In recent years, we have characterized the molecular mechanisms underlying incontinentia pigmenti (IP, an inflammatory disease that mostly targets the skin and central nervous system) and demonstrated that it results from major inhibition of the NF-B signaling pathway related to genetic defects in its NEMO regulatory subunit. We have also characterized the molecular mechanisms of rhabdomyolysis related to fever.

1. We are continuing to identify novel NEMO mutants and characterize their respective impacts at different steps in the NF-B pathway. This process is important for better understanding severe forms of IP and thus defining the best drug targets in this orphan disorder. Linear ubiquitination has been recently emerged as a canonical pathway to activate NF-kB signaling via the linear ubiquitination of NEMO and RIP1. It is catalyzed by an enzymatic complex formed by HOIP, HOIL-1L and SHARPIN. We have identified a splice mutation in NEMO gene which arise in a central frame deletion resulting in a protein of 32kDa. The deleted domain is known in mice to be involved in linear ubiquitination.
We demonstrate a total impairing of NF-kB activation in fibroblasts derived from IP male aborted and carrying the splice mutation. We have demonstrated an impairing in linear ubiquitination due to the fact that the truncated form doesn’t interact specifically with SHARPIN (manuscript in revision JACI). With respect to the role of the NF-B pathway in inflammation and epidermal homeostasis, we hypothesized that elucidating the genetic and molecular abnormalities involved in generalized pustular psoriasis (GPP, an inflammatory skin disease displaying a Mendelian mode of inheritance) might unveil the key contribution of dysregulated inflammatory circuits. This latter strategy allowed us to characterize the molecular defects underlying GPP for the first time, which consist of deficiency of the IL-36 receptor antagonist activity and enhanced skin and systemic inflammatory responses. These genetically inherited abnormalities of the innate immune response have prompted reclassification of GPP as an auto-inflammatory disease. In another hand, In collaboration with the team of F. Capon, we have contributed to the identification of AP1S3, a gene which encodes a protein that promotes vesicular trafficking involved in endosomal translocation of the TLR-3 receptor involved in viral infection, consistent with the fact that viral infection is a major trigger of GPP flares. AP1S3 deficiency causes an abormal accumulation of p62 impairining keratinocytes autophagy which arise in final to enhanced NF-kB signalling. Finally, gain-of-function mutations in CARD14 gene encoding a positive regulator of NF-kB signaling have been identified in various forms of psoriasis. Consequently, the three genetic conditions described above display enhancing NF-kB signaling pathway as a common defect. In the same line, we have identified via linkage and exomes analyses in a multiplex family with a complex auto-inflammtory disease with predominant cutaneous manifestations homozygous mutations in a novel gene encoding for a negative regulator of both two canonical pathways, NF-kB and Beta-Catenin, both involved in epidermal homeostasis. We demonstrated an hyperactivation of the two signalling pathways in keratinocytes and monocytes upon Lipopolysaccharide stimulation. We have also been able to decipher a novel pathophysiological mechanism underlying a complexe genodermatosis with high inflammatory phenotype wich resulted from mutation in desmosomal protein and have linked the barrier function defect to inflammation via the NF-kB signalling pathway. The candidate protein is an unexpected inhibitor of epithelial inflammation via the inhibition of NF-B signaling pathway (manuscript in revision).

Our main objectives are thus to:
- identify new gene defects associated with GPP by using homozygosity mapping in several multiplex consanguineous families showing a Mendelian segregation of the GPP trait and by leveraging recent advances in the high-throughput sequencing of large genomic regions to screen for targeted loci.

- investigate the effector mechanisms responsible for the exacerbated inflammatory reaction, in view of our recent findings in support of a key role of significantly dysregulated skin and systemic innate immune responses depending on IL-1 family members. We shall focus on the range of inflammatory and regulatory cytokines released during the flares and will address the cellular response in vitro in keratinocytes, macrophages and dendritic cells (i.e. the main cell types putatively involved in systemic and skin inflammation in GPP).

- decipher the molecular mechanisms involved in the inflammatory cascade leading to cytokine release. We shall focus mainly on the NF-B signaling pathway which, in inflammatory macrophage populations, has been shown to be a key pathway downstream of the activation of many receptors by their respective ligands, including IL-1 and TNF.

- identify cellular and molecular interactions of the inflammatory cascade by using existing genetically engineered mouse models of the IL-36/IL-36Ra pathway. These studies should allow the identification of new targets for the design of innovative therapeutic strategies for not only GPP but also systemic diseases with excessive inflammatory responses that frequently involve the skin and locomotor system (bones and joints), as is usually observed in severe forms of pustular psoriasis.

- identify new genes responsible for uncharacterized auto-inflammatory diseases with skin involvement, in view of the recent classification of GPP as an autoinflammatory group disease, the known involvement of a misregulated immune innate response in GPP and our ability to recruit patients presenting with complex inflammatory syndromes. This applies to severe forms of febrile
neutrophilic dermatosis with major systemic inflammation, such as PASH syndrome (pyoderma gangrenosum, acne and suppurative hidradenitis - a new auto-inflammatory disease that is distinct from PAPA syndrome with pyogenic arthritis, pyoderma gangrenosum and acne) or other unclassified, systemic, pyoderma gangrenosum syndromes which might have an early onset in childhood or in young adults.

2. Concerning rhabdomyolysis, our working hypothesis is that fever-related rhabdomyolysis may be triggered and/ or worsened by a dysregulation in innate immunity and/ or inflammatory response, and by protein thermolability as showed in aldolase A deficiency. Moreover, irrespective of the cause of rhabdomyolysis, the pathophysiologic events follow a common pathway, the increased intracellular calcium concentration by either direct injury to the sarcolemma or failure of energy production. To test our hypotheses we will mainly focus on severe inherited rhabdomyolysis triggered by fever and due to i) a primary Fatty acid Beta-oxidation deficiency (FAO), as any pathogenic role of inflammation, thermolability and calcium release in FAO disorders has barely been studied, ii) and mutations in new genes discovered by exome sequencing. Five questions will be addressed:

• Is inflammation associated with rhabdomyolysis caused by FAO deficiencies and by mutations in new genes found by Exome sequencing? We will determine the spectrum of serum inflammatory and innate immune mediators released during flares for each causes of rhabdomyolysis described above, and identify immune and non-immune cellular actors responsible for hyper-inflammation in myoblasts cultured under innate immune stimuli.

• Is protein thermolability associated with rhabdomyolysis from all causes?

• Is there any toxicity for identified inflammatory mediators on skeletal muscle cells in vitro as assessed by calcium release and metabolic functions?

• Are there other consequences of gene defects at the cellular and the molecular levels in the context of severe rhabdomyolysis related to inflammation (cellular compartment composition and/or trafficking, biochemical evidences)?

• Are candidate drug therapies able to restore i) defective vesicular dynamic or inflammatory signaling pathways identified, ii) thermolability, iii) abnormal calcium flux?