A Germline Mutation in the C2 Domain of PLCγ2 Associated with Gain-of-Function Expands the Phenotype for PLCG2-Related Diseases

Case Report: de novo in-frame deletion in PLCG2 gene: a case report of B-cell lymphopenia, pulmonary bullae, and cutis laxa

Phospholipase C gamma 2 (PLCG2) gene mutations might cause PLCG2-associated antibody deficiency and immune dysregulation (PLAID)/autoinflammation and PLCG2-associated antibody deficiency and immune dysregulation (APLAID) syndrome. They are two forms of autosomal-dominant immune dysregulation (ID). APLAID patients are usually characterized by skin lesions, pulmonary involvement, and musculoskeletal, ophthalmic, and gastrointestinal tract symptoms, but unlike PLAID patients, these patients do not present with cold urticaria or autoimmunity. Here, we report a 25-year-old man with B-cell lymphopenia, pulmonary bullae, recurrent sinopulmonary infections, and cutis laxa but without cold-induced urticaria. Anti-nuclear antibodies were negative. Trio whole-genome sequencing revealed a de novo heterozygous PLCG2 gene (NM_002661.5) variant c.3417_3419del, p.E1139del, located on chromosome chr16-81973600-81973602. Our findings expand the variety of clinical and genetic phenotypes for APLAID and suggest that this variant would be meaningful.

Characterisation of molecular mechanisms for PLCγ2 disease-linked variants

The phospholipase C enzyme PLCγ2 is best characterised in the context of immune cell regulation. Furthermore, many mutations discovered in PLCγ2 have been linked to the development of complex immune disorders as well as resistance to ibrutinib treatment in chronic lymphocytic leukaemia. Importantly, it has also been found that a rare variant of PLCγ2 (P522R) has a protective role in Alzheimer's disease (AD). Despite initial characterisation of these disease-linked variants, a comprehensive understanding of their differences and underpinning molecular mechanisms, needed to facilitate therapeutic efforts, is lacking. Here, we used available structural insights for PLCγ enzymes to further analyse PLCγ2 M1141K mutation, representative for mutations in immune disorders and cancer resistance, and the AD-protective variant, PLCγ2 P522R. Together with several other mutations in the autoinhibitory interface, the PLCγ2 M1141K mutation was strongly activating in a cell-based assay, under basal and stimulated conditions. Measurements of PLC activity in various in vitro assays demonstrated enhanced activity of PLCγ2 M1141K while the activity of PLCγ2 P522R was not significantly different from the WT. Similar trends were observed in several other assays, including direct liposome binding. However, an enhanced rate of phosphorylation of a functionally important tyrosine by Btk in vitro was observed for PLCγ2 P522R variants. To further assess implications of these in vitro findings in a cellular context relevant for the PLCγ2 P522R variant, microglia (BV2) stable cell lines were generated and analysed under growth conditions. The PLC activity in cells expressing PLCγ2 P522R at physiologically relevant levels was clearly enhanced compared to the WT, and differences in cell morphology observed. These data, combined with the structural insights, suggest that the PLCγ2 P522R variant has subtle, localised structural changes that do not directly affect the PLC activity by compromising autoinhibition, as determined for PLCγ2 M1141K. It is also likely that in contrast to the PLCγ2 M1141K, the functional impact of the P522R substitution completely depends on further interactions with upstream kinases and other regulatory proteins in a relevant cellular context, where changes in the PLCγ2 P522R variant could facilitate processes such as phosphorylation and protein-protein interactions.

The crystal and cryo-EM structures of PLCγ2 reveal dynamic interdomain recognitions in autoinhibition

Phospholipase C gamma 2 (PLCγ2) plays important roles in cell signaling downstream of various membrane receptors. PLCγ2 contains a multidomain inhibitory region critical for its regulation, while it has remained unclear how these domains contribute to PLCγ2 activity modulation. Here we determined three structures of human PLCγ2 in autoinhibited states, which reveal dynamic interactions at the autoinhibition interface, involving the conformational flexibility of the Src homology 3 (SH3) domain in the inhibitory region, and its previously unknown interaction with a carboxyl-terminal helical domain in the core region. We also determined a structure of PLCγ2 bound to the kinase domain of fibroblast growth factor receptor 1 (FGFR1), which demonstrates the recognition of FGFR1 by the nSH2 domain in the inhibitory region of PLCγ2. Our results provide structural insights into PLCγ2 regulation that will facilitate future mechanistic studies to understand the entire activation process.

Characterization of a Novel Pathogenic PLCG2 Variant Leading to APLAID Syndrome Responsive to a TNF Inhibitor

OBJECTIVE

Autoinflammation and phospholipase C (PLC) γ2-associated antibody deficiency and immune dysregulation (APLAID) syndrome is an autoinflammatory disease caused by gain-of-function variants in PLCG2. This study investigates the pathogenic mechanism of a novel variant of PLCG2 in a patient with APLAID syndrome.

METHODS

Whole-exome sequencing and Sanger sequencing were used to identify the pathogenic variant in the patient. Single-cell RNA sequencing, immunoblotting, luciferase assay, inositol monophosphate enzyme-linked immunosorbent assay, calcium flux assay, quantitative PCR, and immunoprecipitation were used to define inflammatory signatures and evaluate the effects of the PLCG2 variant on protein functionality and immune signaling.

RESULTS

We identified a novel de novo variant, PLCG2 p.D993Y, in a patient with colitis, pansinusitis, skin rash, edema, recurrent respiratory infections, B-cell deficiencies, and hypogammaglobulinemia. The single-cell transcriptome revealed exacerbated inflammatory responses in the patient's peripheral blood mononuclear cells. Expression of the D993Y variant in HEK293T, COS-7, and PLCG2 knock-out THP-1 cell lines showed heightened PLCγ2 phosphorylation; elevated inositol-1,4,5-trisphosphate production and intracellular Ca2+ release; and activation of the MAPK, NF-κB, and NFAT signaling pathways compared with control-transfected cells. In vitro experiments indicated that the D993Y variant altered amino acid properties, disrupting the interaction between the catalytic and autoinhibitory domains of PLCγ2, resulting in PLCγ2 autoactivation.

CONCLUSION

Our findings demonstrated that the PLCG2 D993Y variant is a gain-of-function mutation via impairing its autoinhibition, activating multiple inflammatory signaling pathways, thus leading to APLAID syndrome. This study further broadens the molecular underpinnings and phenotypic spectrum of PLCγ2-related disorders.

The hypermorphic PLCγ2 S707Y variant dysregulates microglial cell function - Insight into PLCγ2 activation in brain health and disease, and opportunities for therapeutic modulation

Phospholipase C-gamma 2 (PLCγ2) is highly expressed in hematopoietic and immune cells, where it is a key signalling node enabling diverse cellular functions. Within the periphery, gain-of-function (GOF) PLCγ2 variants, such as the strongly hypermorphic S707Y, cause severe immune dysregulation. The milder hypermorphic mutation PLCγ2 P522R increases longevity and confers protection in central nervous system (CNS) neurodegenerative disorders, implicating PLCγ2 as a novel therapeutic target for treating these CNS indications. Currently, nothing is known about what consequences strong PLCγ2 GOF has on CNS functionality, and more precisely on the specific biological functions of microglia. Using the PLCγ2 S707Y variant as a model of chronic activation we investigated the functional consequences of strong PLCγ2 GOF on human microglia. PLCγ2 S707Y expressing human inducible pluripotent stem cells (hiPSC)-derived microglia exhibited hypermorphic enzymatic activity under both basal and stimulated conditions, compared to PLCγ2 wild type. Despite the increase in PLCγ2 enzymatic activity, the PLCγ2 S707Y hiPSC-derived microglia display diminished functionality for key microglial processes including phagocytosis and cytokine secretion upon inflammatory challenge. RNA sequencing revealed a downregulation of genes related to innate immunity and response, providing molecular support for the phenotype observed. Our data suggests that chronic activation of PLCγ2 elicits a detrimental phenotype that is contributing to unfavourable CNS functions, and informs on the therapeutic window for targeting PLCγ2 in the CNS. Drug candidates targeting PLCγ2 will need to precisely mimic the effects of the PLCγ2 P522R variant on microglial function, but not those of the PLCγ2 S707Y variant.

PLCG2-associated immune dysregulation (PLAID) comprises broad and distinct clinical presentations related to functional classes of genetic variants

BACKGROUND

Pathogenic variants of phospholipase C gamma 2 (PLCG2) cause 2 related forms of autosomal-dominant immune dysregulation (ID), PLCγ2-associated antibody deficiency and immune dysregulation (PLAID) and autoinflammatory PLAID (APLAID). Since describing these conditions, many PLCG2 variants of uncertain significance have been identified by clinical sequencing of patients with diverse features of ID.

OBJECTIVE

We sought to functionally classify PLCG2 variants and explore known and novel genotype-function-phenotype relationships.

METHODS

Clinical data from patients with PLCG2 variants were obtained via standardized questionnaire. PLCG2 variants were generated by mutagenesis of enhanced green fluorescent protein (EGFP)-PLCG2 plasmid, which was overexpressed in Plcg2-deficient DT-40 B cells. B-cell receptor-induced calcium flux and extracellular signal-regulated kinase phosphorylation were assayed by flow cytometry. In some cases, stimulation-induced calcium flux was also measured in primary patient cells.

RESULTS

Three-fourths of PLCG2 variants produced functional alteration of B-cell activation, in vitro. Thirteen variants led to gain of function (GOF); however, most functional variants defined a new class of PLCG2 mutation, monoallelic loss of function (LOF). Susceptibility to infection and autoinflammation were common with both GOF and LOF variants, whereas a new phenotypic cluster consisting of humoral immune deficiency, autoinflammation, susceptibility to herpesvirus infection, and natural killer cell dysfunction was observed in association with multiple heterozygous LOF variants detected in both familial and sporadic cases. In some cases, PLCG2 variants produced greater effects in natural killer cells than in B cells.

CONCLUSIONS

This work expands the genotypic and phenotypic associations with functional variation in PLCG2, including a novel form of ID in carriers of heterozygous loss of PLCG2 function. It also demonstrates the need for more diverse assays for assessing the impact of PLCG2 variants on human disease.

Activation Mechanisms and Diverse Functions of Mammalian Phospholipase C

Phospholipase C (PLC) plays pivotal roles in regulating various cellular functions by metabolizing phosphatidylinositol 4,5-bisphosphate in the plasma membrane. This process generates two second messengers, inositol 1,4,5-trisphosphate and diacylglycerol, which respectively regulate the intracellular Ca²⁺ levels and protein kinase C activation. In mammals, six classes of typical PLC have been identified and classified based on their structure and activation mechanisms. They all share X and Y domains, which are responsible for enzymatic activity, as well as subtype-specific domains. Furthermore, in addition to typical PLC, atypical PLC with unique structures solely harboring an X domain has been recently discovered. Collectively, seven classes and 16 isozymes of mammalian PLC are known to date. Dysregulation of PLC activity has been implicated in several pathophysiological conditions, including cancer, cardiovascular diseases, and neurological disorders. Therefore, identification of new drug targets that can selectively modulate PLC activity is important. The present review focuses on the structures, activation mechanisms, and physiological functions of mammalian PLC.

G-CSF drives autoinflammation in APLAID

Missense mutations in PLCG2 can cause autoinflammation with phospholipase C gamma 2-associated antibody deficiency and immune dysregulation (APLAID). Here, we generated a mouse model carrying an APLAID mutation (p.Ser707Tyr) and found that inflammatory infiltrates in the skin and lungs were only partially ameliorated by removing inflammasome function via the deletion of caspase-1. Also, deleting interleukin-6 or tumor necrosis factor did not fully prevent APLAID mutant mice from autoinflammation. Overall, these findings are in accordance with the poor response individuals with APLAID have to treatments that block interleukin-1, JAK1/2 or tumor necrosis factor. Cytokine analysis revealed increased granulocyte colony-stimulating factor (G-CSF) levels as the most distinct feature in mice and individuals with APLAID. Remarkably, treatment with a G-CSF antibody completely reversed established disease in APLAID mice. Furthermore, excessive myelopoiesis was normalized and lymphocyte numbers rebounded. APLAID mice were also fully rescued by bone marrow transplantation from healthy donors, associated with reduced G-CSF production, predominantly from non-hematopoietic cells. In summary, we identify APLAID as a G-CSF-driven autoinflammatory disease, for which targeted therapy is feasible.

A novel fluorogenic reporter substrate for 1-phosphatidylinositol 4,5-bisphosphate phosphodiesterase gamma-2 (PLCγ2): Application to high-throughput screening for activators to treat Alzheimer's disease

A rare coding variant in PLCγ2 (P522R) expressed in microglia induces a mild activation of enzymatic activity when compared to wild-type. This mutation is reported to be protective against the cognitive decline associated with late-onset Alzheimer's disease (LOAD) and therefore, activation of wild-type PLCγ2 has been suggested as a potential therapeutic target for the prevention and treatment of LOAD. Additionally, PLCγ2 has been associated with other diseases such as cancer and some autoimmune disorders where mutations with much greater increases in PLCγ2 activity have been identified. Here, pharmacological inhibition may provide a therapeutic effect. In order to facilitate our investigation of the activity of PLCγ2, we developed an optimized fluorogenic substrate to monitor enzymatic activity in aqueous solution. This was accomplished by first exploring the spectral properties of various "turn-on" fluorophores. The most promising turn-on fluorophore was incorporated into a water-soluble PLCγ2 reporter substrate, which we named C8CF3-coumarin. The ability of PLCγ2 to enzymatically process C8CF3-coumarin was confirmed, and the kinetics of the reaction were determined. Reaction conditions were optimized to identify small molecule activators, and a pilot screen of the Library of Pharmacologically Active Compounds 1280 (LOPAC₁₂₈₀) was performed with the goal of identifying small molecule activators of PLCγ2. The optimized screening conditions allowed identification of potential PLCγ2 activators and inhibitors, thus demonstrating the feasibility of this approach for high-throughput screening.

Identification of genes related to immune enhancement caused by heterologous ChAdOx1-BNT162b2 vaccines in lymphocytes at single-cell resolution with machine learning methods

The widely used ChAdOx1 nCoV-19 (ChAd) vector and BNT162b2 (BNT) mRNA vaccines have been shown to induce robust immune responses. Recent studies demonstrated that the immune responses of people who received one dose of ChAdOx1 and one dose of BNT were better than those of people who received vaccines with two homologous ChAdOx1 or two BNT doses. However, how heterologous vaccines function has not been extensively investigated. In this study, single-cell RNA sequencing data from three classes of samples: volunteers vaccinated with heterologous ChAdOx1-BNT and volunteers vaccinated with homologous ChAd-ChAd and BNT-BNT vaccinations after 7 days were divided into three types of immune cells (3654 B, 8212 CD4+ T, and 5608 CD8+ T cells). To identify differences in gene expression in various cell types induced by vaccines administered through different vaccination strategies, multiple advanced feature selection methods (max-relevance and min-redundancy, Monte Carlo feature selection, least absolute shrinkage and selection operator, light gradient boosting machine, and permutation feature importance) and classification algorithms (decision tree and random forest) were integrated into a computational framework. Feature selection methods were in charge of analyzing the importance of gene features, yielding multiple gene lists. These lists were fed into incremental feature selection, incorporating decision tree and random forest, to extract essential genes, classification rules and build efficient classifiers. Highly ranked genes include PLCG2, whose differential expression is important to the B cell immune pathway and is positively correlated with immune cells, such as CD8+ T cells, and B2M, which is associated with thymic T cell differentiation. This study gave an important contribution to the mechanistic explanation of results showing the stronger immune response of a heterologous ChAdOx1-BNT vaccination schedule than two doses of either BNT or ChAdOx1, offering a theoretical foundation for vaccine modification.

Clinical and immunological features of an APLAID patient caused by a novel mutation in PLCG2

Background

The APLAID syndrome is a rare primary immunodeficiency caused by gain-of-function mutations in the PLCG2 gene. We present a 7-year-old APLAID patient who has recurrent blistering skin lesions, skin infections in the perineum, a rectal perineal fistula, and inflammatory bowel disease.

Methods

To determine the genetic cause of our patient, WES and bioinformatics analysis were performed. Flow cytometry was used for phenotyping immune cell populations in peripheral blood. Cytokines released into plasma were analyzed using protein chip technology. The PBMCs of patient and a healthy child were subjected to single-cell RNA-sequencing analysis.

Results

The patient carried a novel de novo missense mutation c.2534T>C in exon 24 of the PLCG2 gene that causes a leucine to serine amino acid substitution (p.Leu845Ser). Bioinformatics analysis revealed that this mutation had a negative impact on the structure of the PLCγ2 protein, which is highly conserved in many other species. Immunophenotyping by flow cytometry revealed that in addition to the typical decrease in circulating memory B cells, the levels of myeloid dendritic cells (mDCs) in the children's peripheral blood were significantly lower, as were the CD4⁺ effector T cells induced by their activation. Single-cell sequencing revealed that the proportion of different types of cells in the peripheral blood of the APLAID patient changed.

Conclusions

We present the first case of APLAID with severely reduced myeloid dendritic cells carrying a novel PLCG2 mutation, and conducted a comprehensive analysis of immunological features in the ALPAID patient, which has not been mentioned in previous reports. This study expands the spectrum of APLAID-associated immunophenotype and genotype. The detailed immune analyses in this patient may provide a basis for the development of targeted therapies for this severe autoinflammatory disease.

Heterozygous expression of the Alzheimer's disease-protective PLCγ2 P522R variant enhances Aβ clearance while preserving synapses

BACKGROUND

A rare coding variant, P522R, in the phospholipase C gamma 2 (PLCG2) gene has been identified as protective against late-onset Alzheimer's disease (AD), but the mechanism is unknown. PLCG2 is exclusively expressed in microglia within the central nervous system, and altered microglial function has been implicated in the progression of AD.

METHODS

Healthy control hiPSCs were CRISPR edited to generate cells heterozygous and homozygous for the PLCG2P522R variant. Microglia derived from these hiPSC's were used to investigate the impact of PLCγ2P522R on disease relevant processes, specifically microglial capacity to take up amyloid beta (Aβ) and synapses. Targeted qPCR assessment was conducted to explore expression changes in core AD linked and microglial genes, and mitochondrial function was assessed using an Agilent Seahorse assay.

RESULTS

Heterozygous expression of the P522R variant resulted in increased microglial clearance of Aβ, while preserving synapses. This was associated with the upregulation of a number of genes, including the anti-inflammatory cytokine Il-10, and the synapse-linked CX3CR1, as well as alterations in mitochondrial function, and increased cellular motility. The protective capacity of PLCγ2P522R appeared crucially dependent on (gene) 'dose', as cells homozygous for the variant showed reduced synapse preservation, and a differential gene expression profile relative to heterozygous cells.

CONCLUSION

These findings suggest that PLCγ2P522R may result in increased surveillance by microglia, and prime them towards an anti-inflammatory state, with an increased capacity to respond to increasing energy demands, but highlights the delicate balance of this system, with increasing PLCγ2P522R 'dose' resulting in reduced beneficial impacts.

Variant in the PLCG2 Gene May Cause a Phenotypic Overlap of APLAID/PLAID: Case Series and Literature Review

Background: Variants in the phospholipase C gamma 2 (PLCG2) gene can cause PLCG2-associated antibody deficiency and immune dysregulation (PLAID)/autoinflammation and PLCG2-associated antibody deficiency and immune dysregulation (APLAID) syndrome. Linking the clinical phenotype with the genotype is relevant in making the final diagnosis. Methods: This is a single center case series of five related patients (4−44 years), with a history of autoinflammation and immune dysregulation. Clinical and laboratory characteristics were recorded and a literature review of APLAID/PLAID was performed. Results: All patients had recurrent fevers, conjunctivitis, lymphadenopathy, headaches, myalgia, abdominal pain, cold-induced urticaria and recurrent airway infections. Hearing loss was detected in two patients. Inflammatory parameters were slightly elevated during flares. Unswitched B-cells were decreased. Naïve IgD+CD27− B-cells and unswitched IgD+CD27+ B-cells were decreased; switched IgD-CD27+ B-cells were slightly increased. T-cell function was normal. Genetic testing revealed a heterozygous missense variant (c.77C>T, p.Thr26Met) in the PLCG2 gene in all patients. Genotype and phenotype characteristics were similar to previously published PLAID (cold-induced urticaria) and APLAID (eye inflammation, musculoskeletal complaints, no circulating antibodies) patients. Furthermore, they displayed characteristics for both PLAID and APLAID (recurrent infections, abdominal pain/diarrhea) with normal T-cell function. Conclusion: The heterozygous missense PLCG2 gene variant (c.77C>T, p.Thr26Met) might cause phenotypical overlap of PLAID and APLAID patterns.

Identification of ultra-rare genetic variants in pediatric acute onset neuropsychiatric syndrome (PANS) by exome and whole genome sequencing

Abrupt onset of severe neuropsychiatric symptoms including obsessive-compulsive disorder, tics, anxiety, mood swings, irritability, and restricted eating is described in children with Pediatric Acute-Onset Neuropsychiatric Syndrome (PANS). Symptom onset is often temporally associated with infections, suggesting an underlying autoimmune/autoinflammatory etiology, although direct evidence is often lacking. The pathological mechanisms are likely heterogeneous, but we hypothesize convergence on one or more biological pathways. Consequently, we conducted whole exome sequencing (WES) on a U.S. cohort of 386 cases, and whole genome sequencing (WGS) on ten cases from the European Union who were selected because of severe PANS. We focused on identifying potentially deleterious genetic variants that were de novo or ultra-rare (MAF) < 0.001. Candidate mutations were found in 11 genes (PPM1D, SGCE, PLCG2, NLRC4, CACNA1B, SHANK3, CHK2, GRIN2A, RAG1, GABRG2, and SYNGAP1) in 21 cases, which included two or more unrelated subjects with ultra-rare variants in four genes. These genes converge into two broad functional categories. One regulates peripheral immune responses and microglia (PPM1D, CHK2, NLRC4, RAG1, PLCG2). The other is expressed primarily at neuronal synapses (SHANK3, SYNGAP1, GRIN2A, GABRG2, CACNA1B, SGCE). Mutations in these neuronal genes are also described in autism spectrum disorder and myoclonus-dystonia. In fact, 12/21 cases developed PANS superimposed on a preexisting neurodevelopmental disorder. Genes in both categories are also highly expressed in the enteric nervous system and the choroid plexus. Thus, genetic variation in PANS candidate genes may function by disrupting peripheral and central immune functions, neurotransmission, and/or the blood-CSF/brain barriers following stressors such as infection.

The relationship of early- and late-onset Alzheimer’s disease genes with COVID-19

Individuals with Alzheimer’s disease and other neurodegenerative diseases have been exposed to excess risk by the COVID-19 pandemic. COVID-19’s main manifestations include high body temperature, dry cough, and exhaustion. Nevertheless, some affected individuals may have an atypical presentation at diagnosis but suffer neurological signs and symptoms as the first disease manifestation. These findings collectively show the neurotropic nature of SARS-CoV-2 virus and its ability to involve the central nervous system. In addition, Alzheimer’s disease and COVID-19 has a number of common risk factors and comorbid conditions including age, sex, hypertension, diabetes, and the expression of APOE ε4. Until now, a plethora of studies have examined the COVID-19 disease but only a few studies has yet examined the relationship of COVID-19 and Alzheimer’s disease as risk factors of each other. This review emphasizes the recently published evidence on the role of the genes of early- or late-onset Alzheimer’s disease in the susceptibility of individuals currently suffering or recovered from COVID-19 to Alzheimer’s disease or in the susceptibility of individuals at risk of or with Alzheimer’s disease to COVID-19 or increased COVID-19 severity and mortality. Furthermore, the present review also draws attention to other uninvestigated early- and late-onset Alzheimer’s disease genes to elucidate the relationship between this multifactorial disease and COVID-19.

Phospholipase Cγ2 is Essential for Experimental Models of Epidermolysis Bullosa Acquisita

Phospholipase Cγ2 (PLCγ2) mediates tyrosine kinase‒coupled receptor signaling in various hematopoietic lineages. Although PLCγ2 has been implicated in certain human and mouse inflammatory disorders, its contribution to autoimmune and inflammatory skin diseases is poorly understood. In this study, we tested the role of PLCγ2 in a mouse model of epidermolysis bullosa acquisita triggered by antibodies against type VII collagen (C7), a component of the dermo-epidermal junction. PLCγ2-deficient (Plcg2^-/-) mice and bone marrow chimeras with a Plcg2^-/- hematopoietic system were completely protected from signs of anti-C7-induced skin disease, including skin erosions, dermal‒epidermal separation, and inflammation, despite normal circulating levels and skin deposition of anti-C7 antibodies. PLCγ2 was required for the tissue infiltration of neutrophils, eosinophils, and monocytes/macrophages as well as for the accumulation of proinflammatory mediators (including IL-1β, MIP-2, and LTB₄) and reactive oxygen species. Mechanistic experiments revealed a role for PLCγ2 in the release of proinflammatory mediators and reactive oxygen species but not in the intrinsic migratory capacity of leukocytes. The phospholipase C inhibitor U73122 inhibited dermal-epidermal separation of human skin sections incubated with human neutrophils in the presence of anti-C7 antibodies. Taken together, our results suggest a critical role for PLCγ2 in the pathogenesis of the inflammatory form of epidermolysis bullosa acquisita.

There is no gene for CVID - novel monogenetic causes for primary antibody deficiency

'There is no gene for fate' (citation from the movie 'GATTACA') - and there is no gene for CVID. Common Variable ImmunoDeficiency (CVID) is the most prevalent primary immunodeficiency in humans. CVID is characterized by an increased susceptibility to infections, hypogammaglobulinemia, reduced switched memory B cell numbers in peripheral blood and a defective response to vaccination, often complicated by autoimmune and autoinflammatory conditions. However, as soon as a genetic diagnosis has been made in a patient with CVID, the diagnosis must be changed to the respective genetic cause (www.esid.org). Therefore, there are genetic causes for primary antibody deficiencies, but not for CVID. Primary antibody deficiencies (PADs) are a heterogeneous group of disorders. Several attempts have been made to gain further insights into the pathogenesis of PAD, using unbiased approaches such as whole exome or genome sequencing. Today, in just about 35% of cases with PAD, monogenic mutations (including those in the gene TNFRSF13B) can be identified in a set of 68 genes [1^•]. These mutations occur either sporadically or are inherited and do explain an often complex phenotype. In our review, we not only discuss gene defects identified in PAD patients previously diagnosed with CVID and/or CVID-like disorders such as IKZF1, CTNNBL1, TNFSF13 and BACH2, but also genetic defects which were initially described in non-CVID patients but have later also been observed in patients with PAD such as PLCG2, PIK3CG, PMS2, RNF31, KMT2D, STAT3. We also included interesting genetic defects in which the pathophysiology suggests a close relation to other known defects of the adaptive immune response, such as DEF6, SAMD9 and SAMD9L, and hence a CVID-like phenotype may be observed in the future. However, alternative mechanisms most likely add to the development of an antibody-deficient phenotype, such as polygenic origins, epigenetic changes, and/or environmental factors.

Case Report: A Rare Case of Autoinflammatory Phospholipase Cγ2 (PLCγ2)-Associated Antibody Deficiency and Immune Dysregulation Complicated With Gangrenous Pyoderma and Literature Review

Background

Autoinflammatory phospholipase Cγ2 (PLCγ2)-associated antibody deficiency and immune dysregulation (APLAID) is a rare autoinflammatory disease caused by gain-of-function mutations in the PLCG2 gene. Here we report a rare case of APLAID patient carrying a novel heterozygous missense PLCG2 I169V mutation with gangrenous pyoderma and concomitant high serum immunoglobulin (Ig) E level.

Methods

The patient was diagnosed as APLAID and has been treated in our department. His phenotype and genotype were carefully documented and studied. We also conducted a comprehensive literature review on APLAID.

Results

A 23-year-old Chinese Han man presented with recurrent fever for 18 years and vesiculopustular rashes for 9 years, along with chronic bronchitis, leukocytosis, increased C-reactive protein, immunodeficiency and high serum IgE. Skin biopsy showed chronic inflammatory cells infiltration. A paternal heterozygous missense variant in exon 6 of the PLCG2 gene p. I169V was identified. His vesiculopustular and IgE level responded to medium dose corticosteroids. After withdrawal of steroids, he developed severe arthritis and a large deteriorating ulceration resembling pyoderma gangrenosum on the left knee. Large dose corticosteroids were suboptimal. Then he received adalimumab with satisfactory response for arthritis and skin lesion. But he got an immunodeficiency-associated lymphoproliferative disorder 2 months later. Through literature review, there were a total of 10 APLAID patients reported by six English-language publications. Vesiculopustular rashes, sinopulmonary infection and immunodeficiency were the most frequent symptoms of APLAID patients. Glucocorticoids, intravenous immunoglobulin and biologics were clinically used to treat APLAID but none of these patients had a complete recovery.

Conclusions

The rarity and diversity of APLAID make it difficult to be diagnosed. Our study reported the first case of APLAID with gangrenous pyoderma and concomitant high IgE carrying a novel PLCG2 mutation, which may expand the clinical phenotype and genotype of APLAID.

The impact of rare and low-frequency genetic variants in common variable immunodeficiency (CVID)

Next Generation Sequencing (NGS) has uncovered hundreds of common and rare genetic variants involved in complex and rare diseases including immune deficiencies in both an autosomal recessive and autosomal dominant pattern. These rare variants however, cannot be classified clinically, and common variants only marginally contribute to disease susceptibility. In this study, we evaluated the multi-gene panel results of Common Variable Immunodeficiency (CVID) patients and argue that rare variants located in different genes play a more prominent role in disease susceptibility and/or etiology. We performed NGS on DNA extracted from the peripheral blood leukocytes from 103 patients using a panel of 19 CVID-related genes: CARD11, CD19, CD81, ICOS, CTLA4, CXCR4, GATA2, CR2, IRF2BP2, MOGS, MS4A1, NFKB1, NFKB2, PLCG2, TNFRSF13B, TNFRSF13C, TNFSF12, TRNT1 and TTC37. Detected variants were evaluated and classified based on their impact, pathogenicity classification and population frequency as well as the frequency within our study group. NGS revealed 112 different (a total of 227) variants with under 10% population frequency in 103 patients of which 22(19.6%) were classified as benign, 29(25.9%) were classified as likely benign, 4(3.6%) were classified as likely pathogenic and 2(1.8%) were classified as pathogenic. Moreover, 55(49.1%) of the variants were classified as variants of uncertain significance. We also observed different variant frequencies when compared to population frequency databases. Case-control data is not sufficient to unravel the genetic etiology of immune deficiencies. Thus, it is important to understand the incidence of co-occurrence of two or more rare variants to aid in illuminating their potential roles in the pathogenesis of immune deficiencies.

TREM2/PLCγ2 signalling in immune cells: function, structural insight, and potential therapeutic modulation

The central role of the resident innate immune cells of the brain (microglia) in neurodegeneration has become clear over the past few years largely through genome-wide association studies (GWAS), and has rapidly become an active area of research. However, a mechanistic understanding (gene to function) has lagged behind. That is now beginning to change, as exemplified by a number of recent exciting and important reports that provide insight into the function of two key gene products – TREM2 (Triggering Receptor Expressed On Myeloid Cells 2) and PLCγ2 (Phospholipase C gamma2) – in microglia, and their role in neurodegenerative disorders. In this review we explore and discuss these recent advances and the opportunities that they may provide for the development of new therapies.

Four diseases, PLAID, APLAID, FCAS3 and CVID and one gene (PHOSPHOLIPASE C, GAMMA-2; PLCG2): Striking clinical phenotypic overlap and difference

We suggest PLAID, APLAID, and FCAS3 have to be considered as different aspects of the same underlying condition, because of our long-term clinical and genetical experiences. Some CVID patients have the same disease-causing mutations in PLCG2 gene, so it may be better to define all of them as "PLCG2deficiency."

PLCG2 rs72824905 Variant Reduces the Risk of Alzheimer's Disease and Multiple Sclerosis

We aimed to evaluate the association of PLCG2 rs72824905 variant with Alzheimer's disease (AD) and multiple sclerosis (MS) using large-scale genetic association study datasets. We selected 50,024 AD cases and 467,330 controls, and 32,367 MS cases and 36,012 controls. We found moderate heterogeneity of rs72824905 in different studies. We found significant association between rs72824905 G allele and reduced AD risk (OR = 0.66, 95% CI 0.59-0.74, p = 5.91E-14). Importantly, rs72824905 G allele could also significantly reduce the risk of MS with OR = 0.94, p = 3.63E-05. Hence, the effects of rs72824905 on AD and MS are consistent.

A novel somatic PLCG2 variant associated with resistance to BTK and SYK inhibition in chronic lymphocytic leukemia

The treatment of chronic lymphocytic leukemia (CLL) has been transformed by the use of targeted small molecules inhibiting components of the B cell receptor (BCR) signaling pathway (Haematologica, 103, 2018 and e204; Curr Hematol Malig Rep, 14, 2019, 302). Chief among these is ibrutinib, an irreversible inhibitor of Bruton tyrosine kinase (BTK), which produces deep, durable responses in CLL with good tolerability (Haematologica, 103, 2018 and e204). Though prolonged exposure to the drug can exert selective pressure on CLL cells and allow for the emergence of drug-resistant clones, primary ibrutinib treatment failure is rare (Expert Rev Hematol, 11 and 2018, 185; N Engl J Med, 370, 2014 and 2352; N Engl J Med, 373, 2015 and 25, 2425; Blood, 128, 2016 and 2199). Activating mutations in the gene PLCG2, which encodes a downstream target of BTK, appear to enable constitutive BCR signaling and have been associated with ibrutinib resistance (Int J Cancer, 146 and 2020, 85; J Clin Oncol, 35, 2017 and 1437; Blood, 126, 2015 and 61). In recent years, novel investigational agents have targeted other components of the BCR pathway. Among these is entospletinib, an orally bioavailable, selective inhibitor of splenic tyrosine kinase (SYK) (Blood, 126, 2015 and 1744), which lies upstream of the enzyme phospholipase C-gamma-2 (PLCG2). Here, we describe a patient who was found to harbor a novel somatic variant of PLCG2 and experienced a lack of treatment response to both ibrutinib and entospletinib.

Phospholipase C families: Common themes and versatility in physiology and pathology

Phosphoinositide-specific phospholipase Cs (PLCs) are expressed in all mammalian cells and play critical roles in signal transduction. To obtain a comprehensive understanding of these enzymes in physiology and pathology, a detailed structural, biochemical, cell biological and genetic information is required. In this review, we cover all these aspects to summarize current knowledge of the entire superfamily. The families of PLCs have expanded from 13 enzymes to 16 with the identification of the atypical PLCs in the human genome. Recent structural insights highlight the common themes that cover not only the substrate catalysis but also the mechanisms of activation. This involves the release of autoinhibitory interactions that, in the absence of stimulation, maintain classical PLC enzymes in their inactive forms. Studies of individual PLCs provide a rich repertoire of PLC function in different physiologies. Furthermore, the genetic studies discovered numerous mutated and rare variants of PLC enzymes and their link to human disease development, greatly expanding our understanding of their roles in diverse pathologies. Notably, substantial evidence now supports involvement of different PLC isoforms in the development of specific cancer types, immune disorders and neurodegeneration. These advances will stimulate the generation of new drugs that target PLC enzymes, and will therefore open new possibilities for treatment of a number of diseases where current therapies remain ineffective.

Severe Autoinflammatory Manifestations and Antibody Deficiency Due to Novel Hypermorphic PLCG2 Mutations

Autoinflammatory diseases (AIDs) were first described as clinical disorders characterized by recurrent episodes of seemingly unprovoked sterile inflammation. In the past few years, the identification of novel AIDs expanded their phenotypes toward more complex clinical pictures associating vasculopathy, autoimmunity, or immunodeficiency. Herein, we describe two unrelated patients suffering since the neonatal period from a complex disease mainly characterized by severe sterile inflammation, recurrent bacterial infections, and marked humoral immunodeficiency. Whole-exome sequencing detected a novel, de novo heterozygous PLCG2 variant in each patient (p.Ala708Pro and p.Leu845_Leu848del). A clear enhanced PLCγ2 activity for both variants was demonstrated by both ex vivo calcium responses of the patient's B cells to IgM stimulation and in vitro assessment of PLC activity. These data supported the autoinflammation and PLCγ2-associated antibody deficiency and immune dysregulation (APLAID) diagnosis in both patients. Immunological evaluation revealed a severe decrease of immunoglobulins and B cells, especially class-switched memory B cells, with normal T and NK cell counts. Analysis of bone marrow of one patient revealed a reduced immature B cell fraction compared with controls. Additional investigations showed that both PLCG2 variants activate the NLRP3-inflammasome through the alternative pathway instead of the canonical pathway. Collectively, the evidences here shown expand APLAID diversity toward more severe phenotypes than previously reported including dominantly inherited agammaglobulinemia, add novel data about its genetic basis, and implicate the alternative NLRP3-inflammasome activation pathway in the basis of sterile inflammation.