Somatic gain-of-function mutation in TLR7 causes early-onset systemic lupus erythematosus

We identified a case of early-onset systemic lupus erythematosus (SLE) characterised by acute immune thrombocytopenia, recurrent fever, pneumonia, myocardial damage, thyroid dysfunction, lymphadenopathy, hepatosplenomegaly, and intracranial calcification. Our objective was to investigate the genetic and molecular mechanisms underlying the disease. Whole exome sequencing and targeted sequencing were performed and a somatic mutation in TLR7 was identified. RNA sequencing, quantitative polymerase chain reaction (qPCR), intracellular cytokine staining, and phospho-flow cytometry were performed to characterise inflammatory signatures. In addition, nuclear factor κB dual-luciferase reporter assays, qPCR, and RNA pull-down assays were performed to assess the functional impact of the TLR7 mutation on immune signalling. We identified a novel somatic TLR7 mutation (p.Phe506Ser) that is likely to arise during early embryonic development. This mutation led to transcriptional upregulation of proinflammatory cytokines and interferon-stimulated genes, such as TNF and IFI27, with significant increases in intracellular cytokine expression, including TNF, following stimulation with the ligand single-stranded RNA (ssRNA) and the agonist R848 in the patient's peripheral blood mononuclear cells (PBMCs). In addition, functional analysis in HEK293T cells demonstrated that the mutant TLR7 exhibited increased binding affinity for ssRNA and enhanced responsiveness to agonists, resulting in hyperactivation of TLR7-mediated signalling. We report the first case of early-onset SLE caused by a somatic TLR7 gain-of-function mutation. Our findings demonstrate that the TLR7 F506S mutation drives excessive proinflammatory signalling in the patient's PBMCs, contributing to disease pathogenesis.