Pos0378 Heterozygous Loss-of-Function Rela Mutations Lead to Systemic Lupus Erythematosus or Autoinflammatory Syndrome

Background: Systemic lupus erythematosus is a complex disease with highly variable clinical presentation and immune dysregulation. The study of rare inborn errors of immunity developing autoimmunity can help to better understand the pathophysiology of more common diseases. In human, haploinsufficiency of RELA/p65 has been described in patients with a dominant inheritance of mucocutaneous ulceration and systemic inflammation. RELA (p65) is the major transcription factor of the canonical NF-kB pathway implicated in the transcription of pro-inflammatory cytokines, type I interferon and cell survival genes, essential to the immune system. Objectives: To perform a multi-modal study of a cohort of patients with rare RELA variants associated with autoimmunity or autoinflammation. Methods: We performed the molecular characterization of the different variants by doing ectopic expression of the allele to assess their transcriptional capacity and dominant negative effect. We studied patients' cells to decipher the impact of the variant regarding NF-kB activity and type I interferon production. We compared patients with RELA variants and different clinical phenotype by performing single-cell RNA sequencing. Results: We described the clinical phenotype and molecular characterization of 15 new patients at the frontier of autoinflammation and autoimmunity. Five patients had a definite diagnosis of systemic lupus erythematosus (SLE), the other 10 patients had various penetrance of a Behçet-like condition associating recurrent bipolar aphtosis, systemic inflammation, pseudo-folliculitis, polyarthritis, and gastro-intestinal symptoms. Almost all patients exhibited an elevated type I interferon signature score. Molecular characterization showed that all private variants were loss-of-function for NF-kB activation in an ectopic expression model. We also demonstrated that none of the variants we tested had a dominant negative effect in vitro. We concluded that these variants were acting by haploinsufficiency. We showed that patients' fibroblasts were more sensitive to TNFα-induced cell death. This increased sensibility to TNFα-induced cell death could drive the pathophysiology of the disease by releasing inflammatory mediators in the environment. Strikingly, the patients' monocyte derived dendritic cells (MDDCs) were more prone to produce, or respond to, IFN after stimulation with TLR3-agonist. Of note, we identified the same RELA variant in a patient presenting with an early-onset SLE and in an unrelated patient affected by bipolar aphtosis. Single-cell RNA sequencing comparison of the different clinical phenotypes identified strong differences in interferon and NF-kB signatures. The highest interferon signature was found in the SLE group whereas the stronger NF-kB signature was found in the Behçet-like group. Lastly, in another family with a strong dominant inheritance of recurrent fever, inflammation, and gastro-intestinal symptoms, one member of the family was a healthy carrier of the RELA mutation. This variable penetrance and clinical expression of RELA associated disease strongly suggested that additional modifiers, such as epigenetic modifications, somatic events or environmental factors, were underlying the clinical phenotype. Conclusion: The description of 15 new cases of RELA associated inflammatory disease (RAID), and the literature review, shed light on the heterogeneity of the clinical presentations related to RELA insufficiency. Our data reinforce the notion that a monogenic disease could be associated with variable clinical presentations, likely mediated by additional factors. The use of multi-omics, the progress in their analysis and in new tools may help to understand this variable clinical presentation and incomplete penetrance in monogenic diseases. REFERENCES: NIL. Acknowledgements: NIL. Disclosure of Interests: None declared.