PLCG2 variants in cherubism

Abstract

Background: Cherubism is most commonly caused by rare heterozygous gain-of-function (GOF) missense variants in SH3BP2, which appear to signal through phospholipase C gamma 2 (PLCG2) to cause excessive osteoclast activity leading to expansile lesions in facial bones in childhood. GOF variants in PLCG2 lead to autoinflammatory PLCG2-associated antibody deficiency and immune dysregulation (autoinflammatory PLAID, or PLAID-GOF), characterized by variably penetrant autoinflammatory, autoimmune, infectious, and atopic manifestations. Cherubism has not been reported in PLAID to date.

Objective: We determined whether GOF PLCG2 variants may be associated with cherubism.

Methods: Clinical, laboratory, and genomic data from 2 patients with cherubism and other clinical symptoms observed in patients with PLCG2 variants were reviewed. Primary B-cell receptor-induced calcium flux was assessed by flow cytometry.

Results: Two patients with lesions consistent with cherubism but no SH3BP2 variants were found to have rare PLCG2 variants previously shown to be GOF in vitro, leading to increased primary B-cell receptor-induced calcium flux in one patient's B cells. Variable humoral defects, autoinflammatory rash, and other clinical and laboratory findings consistent with PLAID were observed as well.

Conclusion: GOF PLCG2 variants likely represent a novel genetic driver of cherubism and should be assessed in SH3BP2-negative cases. Expansile bony lesions expand the phenotypic landscape of autoinflammatory PLAID, and bone imaging should be considered in PLAID patients.

Keywords: PLAID; autoinflammation; cherubism; immune dysregulation; phospholipase C gamma 2.

Figure 1.

Computed tomography imaging of the paranasal sinuses of Patients 1 and 2. A and B represent respectively the coronal and axial images of Patient 1 demonstrating multiple bilateral multiloculated cystic bony lesions of the maxillae (orange arrows). C and D represent respectively the coronal and axial imaging of a normal age-matched control without cherubism. E and F represent respectively the sagittal and axial images of Patient 2 demonstrating an 18.2mm mandibular cystic bony lesion and bilateral maxillary multiloculated cystic bony lesions measuring 15.4mm and 14.4mm.

Figure 2:

Increased B-cell calcium flux in patient with Ser707Phe. Peripheral blood mononuclear cells isolated from healthy controls and Patient 1 were stained for surface expression of CD3+ for T cells or CD19+ for B cells and incubated with Indo-1, which is a colorimetric measure of cytoplasmic calcium. Baseline Indo-1 intensity amongst T cells and B cells was measured by flow cytometry for 30 seconds, followed by stimulation with a cocktail of anti-CD3 (for T-cell receptor) and anti-IgM (for B-cell receptor). Within gated T or B cells, fluorescence emitted corresponding to calcium uptake into the cytoplasm was measured over time (250 seconds total). In contrast to CD3+ cells or control CD19+ cells which show a normal peak and decrease in Indo-1 staining, the patient’s CD19+ Indo-1 staining remains elevated over the time course measured.

Figure 3.

Pathology images show the typical finding of multinucleated osteoclast-like giant cells (arrows) near bone (A) and within soft tissue stroma (B) seen in cherubism. The spindle cells were positive for Vimentin confirming a mesenchymal origin (C) and the multinucleated cells positive for CD68 as characteristically seen in cherubism (D).