A Gain-of-function Variation in PLCG1 Causes a New Immune Dysregulation Disease.

Background: Phospholipase C (PLC) y1 is a critical enzyme regulating nuclear factor-kB (NF-kB), extracellular signal -related kinase, mitogen-activated protein kinase, and nuclear factor of activated T cells signaling pathways, yet germline PLCG1 mutation in human disease has not been reported. Objective: We aimed to investigate the molecular pathogenesis of a PLCG1 activating variant in a patient with immune dysregulation. Methods: Whole exome sequencing was used to identify the patient's pathogenic variants. Bulk RNA sequencing, single-cell RNA sequencing, quantitative PCR, cytometry by time of flight, immunoblotting, flow cytometry, luciferase assay, IP-One ELISA, calcium flux assay, and cytokine measurements in patient PBMCs and T cells and COS-7 and Jurkat cell lines were used to define inflammatory signatures and assess the impact of the PLCG1 variant on protein function and immune signaling. Results: We identified a novel and de novo heterozygous PLCG1 variant, p.S1021F, in a patient presenting with early-onset immune dysregulation disease. We demonstrated that the S1021F variant is a gain-of-function variant, leading to increased inositol-1,4,5-trisphosphate production, intracellular Ca21 release, and increased phosphorylation of extracellular signal-related kinase, p65, and p38. The transcriptome and protein expression at the single-cell level revealed exacerbated inflammatory responses in the patient's T cells and monocytes. The PLCG1 activating variant resulted in enhanced NF-kB and type II interferon pathways in T cells, and hyperactivated NF-kB and type I interferon pathways in monocytes. Treatment with either PLCy1 inhibitor or Janus kinase inhibitor reversed the upregulated gene expression profile in vitro. Conclusions: Our study highlights the critical role of PLCy1 in maintaining immune homeostasis. We illustrate immune dysregulation as a consequence of PLCy1 activation and provide insight into therapeutic targeting of PLCy1.