Recurrent aseptic meningitis with PIGT mutations: a novel pathogenesis of recurrent meningitis successfully treated by eculizumab

Allogeneic stem cell transplantation-A curative treatment for paroxysmal nocturnal hemoglobinuria with PIGT mutation: A case report

BACKGROUND

Patients with paroxysmal nocturnal hemoglobinuria (PNH) have a clonal population of blood cells deficient in glycosylphosphatidylinositol-anchored (GPI-anchored) proteins, most of the time resulting from a mutation in the X-linked gene PIGA. We report a patient with PNH resulting from a rare biallelic PIGT mutation on chromosome 20.

CASE SUMMARY

A 47-year-old man was referred to our hospital for febrile pancytopenia. The patient reported a history of recurrent urticaria and arthralgia and he presented during 3 mo recurrent acute dermo-hypodermitis and aseptic meningitidis. Based on clinical cases published with PIGT-PNH, with clinically typical PNH and autoinflammatory symptoms, we treated our patients with repeated infusions of eculizumab to decrease autoinflammatory symptoms and then we performed an allogeneic stem cell transplantation (allo-SCT) with a mismatched unrelated donor. Our patient experienced no acute Graft vs Host disease (GvHD) and a moderate chronic GvHD and is now considered cured at 24 mo after allo-SCT.

CONCLUSION

This case report suggests that allo-SCT should be considered to cure PIGT-PNH patients.

Rare genetic causes of meningitis and encephalitis

Differential diagnosis of meningitis and encephalitis is often very challenging because it cannot be determined based on symptoms, and the diseases have various causes. This article explains rare genetic causes of meningitis and encephalitis. Autoinflammatory disorders include cryopyrin-associated periodic syndromes, familial Mediterranean fever, and tumor necrosis factor receptor-associated periodic syndrome. Furthermore, other genetic disorders, such as complement factor I deficiency, phosphatidylinositol glycan anchor biosynthesis class T mutation, and neuronal intranuclear inclusion disease, can present as meningitis and encephalitis.

Anti-complement Agents for Autoimmune Neurological Disease

In recent years, there has been increasing recognition of the diversity of autoimmune neurological diseases affecting all levels of the nervous system. A growing understanding of disease pathogenesis has enabled us to better target specific elements of the immune system responsible for the cell dysfunction and cell destruction seen in these diseases. This is no better demonstrated than in the development of complement directed therapies for the treatment of complement mediated autoimmune neurological conditions. Herein, we describe the basic elements of the complement cascade, provide an overview of select autoimmune neurological diseases whose pathogenesis is mediated by complement, the effector system of autoantigen bound autoantibodies, and discuss the complement directed therapies trialed in the treatment of these diseases. Several complement-directed therapies have demonstrated benefit in the treatment of autoimmune neurological diseases; we also review the trials resulting in the approval of these therapies for the treatment of AChR Ab-positive myasthenia gravis (MG) and neuromyelitis spectrum disorder. Finally, on the heels of the recent successes described, we discuss possibilities for the future, including additional targeted therapies with greater ease of administration, improved risk profiles, and other possible uses for therapeutics targeting elements of the complement cascade.

Complement in neurological disorders and emerging complement-targeted therapeutics

The complement system consists of a network of plasma and membrane proteins that modulate tissue homeostasis and contribute to immune surveillance by interacting with the innate and adaptive immune systems. Dysregulation, impairment or inadvertent activation of complement components contribute to the pathogenesis of some autoimmune neurological disorders and could even contribute to neurodegenerative diseases. In this Review, we summarize current knowledge about the main functions of the complement pathways and the involvement of complement in neurological disorders. We describe the complex network of complement proteins that target muscle, the neuromuscular junction, peripheral nerves, the spinal cord or the brain and discuss the autoimmune mechanisms of complement-mediated myopathies, myasthenia, peripheral neuropathies, neuromyelitis and other CNS disorders. We also consider the emerging role of complement in some neurodegenerative diseases, such as Alzheimer disease, amyotrophic lateral sclerosis and even schizophrenia. Finally, we provide an overview of the latest complement-targeted immunotherapies including monoclonal antibodies, fusion proteins and peptidomimetics that have been approved, that are undergoing phase I–III clinical trials or that show promise for the treatment of neurological conditions that respond poorly to existing immunotherapies.

In this Review, Dalakas et al. discuss the complement system, the role it plays in autoimmune neurological disease and neurodegenerative disease, and provide an overview of the latest therapeutics that target complement and that can be used for or have potential in neurological disorders.

Complement has an important physiological role in host immune defences and tissue remodelling.

The physiological role of complement extends to the regulation of synaptic development.

Complement has a key pathophysiological role in autoimmune neurological diseases and mediates the actions of pathogenic autoantibodies, such as acetylcholine receptor antibodies and aquaporin 4 antibodies.

For some autoimmune neurological diseases, such as myasthenia gravis and neuromyelitis optica spectrum disorders, approved complement-targeted treatments are now available.

Complement also seems to be of pathogenic relevance in neurodegenerative diseases such as Alzheimer disease, in which innate immune-driven inflammation is receiving increasing attention.

The field of complement-targeted therapeutics is rapidly expanding, with several FDA-approved agents and others currently in phase II and phase III clinical trials.

Complement and inflammasome overactivation mediates paroxysmal nocturnal hemoglobinuria with autoinflammation

Patients with paroxysmal nocturnal hemoglobinuria (PNH) have a clonal population of blood cells deficient in glycosylphosphatidylinositol-anchored (GPI-anchored) proteins, resulting from a mutation in the X-linked gene PIGA. Here we report on a set of patients in whom PNH results instead from biallelic mutation of PIGT on chromosome 20. These PIGT-PNH patients have clinically typical PNH, but they have in addition prominent autoinflammatory features, including recurrent attacks of aseptic meningitis. In all these patients we find a germ-line point mutation in one PIGT allele, whereas the other PIGT allele is removed by somatic deletion of a 20q region comprising maternally imprinted genes implicated in myeloproliferative syndromes. Unlike in PIGA-PNH cells, GPI is synthesized in PIGT-PNH cells and, since its attachment to proteins is blocked, free GPI is expressed on the cell surface. From studies of patients' leukocytes and of PIGT-KO THP-1 cells we show that, through increased IL-1β secretion, activation of the lectin pathway of complement and generation of C5b-9 complexes, free GPI is the agent of autoinflammation. Eculizumab treatment abrogates not only intravascular hemolysis, but also autoinflammation. Thus, PIGT-PNH differs from PIGA-PNH both in the mechanism of clonal expansion and in clinical manifestations.

Biosynthesis and biology of mammalian GPI-anchored proteins

At least 150 human proteins are glycosylphosphatidylinositol-anchored proteins (GPI-APs). The protein moiety of GPI-APs lacking transmembrane domains is anchored to the plasma membrane with GPI covalently attached to the C-terminus. The GPI consists of the conserved core glycan, phosphatidylinositol and glycan side chains. The entire GPI-AP is anchored to the outer leaflet of the lipid bilayer by insertion of fatty chains of phosphatidylinositol. Because of GPI-dependent membrane anchoring, GPI-APs have some unique characteristics. The most prominent feature of GPI-APs is their association with membrane microdomains or membrane rafts. In the polarized cells such as epithelial cells, many GPI-APs are exclusively expressed in the apical surfaces, whereas some GPI-APs are preferentially expressed in the basolateral surfaces. Several GPI-APs act as transcytotic transporters carrying their ligands from one compartment to another. Some GPI-APs are shed from the membrane after cleavage within the GPI by a GPI-specific phospholipase or a glycosidase. In this review, I will summarize the current understanding of GPI-AP biosynthesis in mammalian cells and discuss examples of GPI-dependent functions of mammalian GPI-APs.

Mutations in PIGU Impair the Function of the GPI Transamidase Complex, Causing Severe Intellectual Disability, Epilepsy, and Brain Anomalies

The glycosylphosphatidylinositol (GPI) anchor links over 150 proteins to the cell surface and is present on every cell type. Many of these proteins play crucial roles in neuronal development and function. Mutations in 18 of the 29 genes implicated in the biosynthesis of the GPI anchor have been identified as the cause of GPI biosynthesis deficiencies (GPIBDs) in humans. GPIBDs are associated with intellectual disability and seizures as their cardinal features. An essential component of the GPI transamidase complex is PIGU, along with PIGK, PIGS, PIGT, and GPAA1, all of which link GPI-anchored proteins (GPI-APs) onto the GPI anchor in the endoplasmic reticulum (ER). Here, we report two homozygous missense mutations (c.209T>A [p.Ile70Lys] and c.1149C>A [p.Asn383Lys]) in five individuals from three unrelated families. All individuals presented with global developmental delay, severe-to-profound intellectual disability, muscular hypotonia, seizures, brain anomalies, scoliosis, and mild facial dysmorphism. Using multicolor flow cytometry, we determined a characteristic profile for GPI transamidase deficiency. On granulocytes this profile consisted of reduced cell-surface expression of fluorescein-labeled proaerolysin (FLAER), CD16, and CD24, but not of CD55 and CD59; additionally, B cells showed an increased expression of free GPI anchors determined by T5 antibody. Moreover, computer-assisted facial analysis of different GPIBDs revealed a characteristic facial gestalt shared among individuals with mutations in PIGU and GPAA1. Our findings improve our understanding of the role of the GPI transamidase complex in the development of nervous and skeletal systems and expand the clinical spectrum of disorders belonging to the group of inherited GPI-anchor deficiencies.

Free, unlinked glycosylphosphatidylinositols on mammalian cell surfaces revisited

Glycosylphosphatidylinositols (GPIs) are linked to many cell-surface proteins, anchor these proteins in the membrane, and are well characterized. However, GPIs that exist in the free form on the mammalian cell surface remain largely unexplored. To investigate free GPIs in cultured cell lines and mouse tissues, here we used the T5-4E10 mAb (T5 mAb), which recognizes unlinked GPIs having an N-acetylgalactosamine (GalNAc) side chain linked to the first mannose at the nonreducing terminus. We detected free GPIs bearing the GalNAc side chain on the surface of Neuro2a and CHO, but not of HEK293, K562, and C2C12 cells. Furthermore, free GPIs were present in mouse pons, medulla oblongata, spinal cord, testis, epididymis, and kidney. Using a panel of Chinese hamster ovary cells defective in both GPI-transamidase and GPI remodeling pathway, we demonstrate that free GPIs follow the same structural remodeling pathway during passage from the endoplasmic reticulum to the plasma membrane as do protein-linked GPI. Specifically, free GPIs underwent post-GPI attachment to protein 1 (PGAP1)-mediated inositol deacylation, PGAP5-mediated removal of the ethanolamine phosphate from the second mannose, and PGAP3- and PGAP2-mediated fatty acid remodeling. Moreover, T5 mAb recognized free GPIs even if the inositol-linked acyl chain or ethanolamine-phosphate side chain linked to the second mannose is not removed. In contrast, addition of a fourth mannose by phosphatidylinositol glycan anchor biosynthesis class Z (PIGZ) inhibited T5 mAb-mediated detection of free GPIs. Our results indicate that free GPIs are normal components of the plasma membrane in some tissues and further characterize free GPIs in mammalian cells.