Phenotypic variation in Aicardi-Goutières syndrome explained by cell-specific IFN-stimulated gene response and cytokine release

The brain microvasculature is a primary mediator of interferon-α neurotoxicity in human cerebral interferonopathies

Aicardi-Goutières syndrome (AGS) is an autoinflammatory disease characterized by aberrant interferon (IFN)-α production. The major cause of morbidity in AGS is brain disease, yet the primary source and target of neurotoxic IFN-α remain unclear. Here, we demonstrated that the brain was the primary source of neurotoxic IFN-α in AGS and confirmed the neurotoxicity of intracerebral IFN-α using astrocyte-driven Ifna1 misexpression in mice. Using single-cell RNA sequencing, we demonstrated that intracerebral IFN-α-activated receptor (IFNAR) signaling within cerebral endothelial cells caused a distinctive cerebral small vessel disease similar to that observed in individuals with AGS. Magnetic resonance imaging (MRI) and single-molecule ELISA revealed that central and not peripheral IFN-α was the primary determinant of microvascular disease in humans. Ablation of endothelial Ifnar1 in mice rescued microvascular disease, stopped the development of diffuse brain disease, and prolonged lifespan. These results identify the cerebral microvasculature as a primary mediator of IFN-α neurotoxicity in AGS, representing an accessible target for therapeutic intervention.

STING signaling in the brain: Molecular threats, signaling activities, and therapeutic challenges

Stimulator of interferon genes (STING) is an innate immune signaling protein critical to infections, autoimmunity, and cancer. STING signaling is also emerging as an exciting and integral part of many neurological diseases. Here, we discuss recent advances in STING signaling in the brain. We summarize how molecular threats activate STING signaling in the diseased brain and how STING signaling activities in glial and neuronal cells cause neuropathology. We also review human studies of STING neurobiology and consider therapeutic challenges in targeting STING to treat neurological diseases.

Emerging concepts of type I interferons in SLE pathogenesis and therapy

Type I interferons have been suspected for decades to have a crucial role in the pathogenesis of systemic lupus erythematosus (SLE). Evidence has now overturned several long-held assumptions about how type I interferons are regulated and cause pathological conditions, providing a new view of SLE pathogenesis that resolves longstanding clinical dilemmas. This evidence includes data on interferons in relation to genetic predisposition and epigenetic regulation. Importantly, data are now available on the role of interferons in the early phases of the disease and the importance of non-haematopoietic cellular sources of type I interferons, such as keratinocytes, renal tubular cells, glial cells and synovial stromal cells, as well as local responses to type I interferons within these tissues. These local effects are found not only in inflamed target organs in established SLE, but also in histologically normal skin during asymptomatic preclinical phases, suggesting a role in disease initiation. In terms of clinical application, evidence relating to biomarkers to characterize the type I interferon system is complex, and, notably, interferon-blocking therapies are now licensed for the treatment of SLE. Collectively, the available data enable us to propose a model of disease pathogenesis that invokes the unique value of interferon-targeted therapies. Accordingly, future approaches in SLE involving disease reclassification and preventative strategies in preclinical phases should be investigated.

Regulation of cGAS Activity and Downstream Signaling

Cyclic GMP-AMP synthase (cGAS) is a predominant and ubiquitously expressed cytosolic onfirmedDNA sensor that activates innate immune responses by producing a second messenger, cyclic GMP-AMP (cGAMP), and the stimulator of interferon genes (STING). cGAS contains a highly disordered N-terminus, which can sense genomic/chromatin DNA, while the C terminal of cGAS binds dsDNA liberated from various sources, including mitochondria, pathogens, and dead cells. Furthermore, cGAS cellular localization dictates its response to foreign versus self-DNA. Recent evidence has also highlighted the importance of dsDNA-induced post-translational modifications of cGAS in modulating inflammatory responses. This review summarizes and analyzes cGAS activity regulation based on structure, sub-cellular localization, post-translational mechanisms, and Ca²⁺ signaling. We also discussed the role of cGAS activation in different diseases and clinical outcomes.

Keratinocytes: From passive targets to active mediators of systemic autoimmunity

Keratinocyte-mediated interferon production in nonlesional skin drives activation of CD16⁺ dendritic cells and is associated with cutaneous inflammation in lupus (Billi et al.).

DNA damage contributes to neurotoxic inflammation in Aicardi-Goutières syndrome astrocytes

Aberrant induction of type I IFN is a hallmark of the inherited encephalopathy Aicardi-Goutières syndrome (AGS), but the mechanisms triggering disease in the human central nervous system (CNS) remain elusive. Here, we generated human models of AGS using genetically modified and patient-derived pluripotent stem cells harboring TREX1 or RNASEH2B loss-of-function alleles. Genome-wide transcriptomic analysis reveals that spontaneous proinflammatory activation in AGS astrocytes initiates signaling cascades impacting multiple CNS cell subsets analyzed at the single-cell level. We identify accumulating DNA damage, with elevated R-loop and micronuclei formation, as a driver of STING- and NLRP3-related inflammatory responses leading to the secretion of neurotoxic mediators. Importantly, pharmacological inhibition of proapoptotic or inflammatory cascades in AGS astrocytes prevents neurotoxicity without apparent impact on their increased type I IFN responses. Together, our work identifies DNA damage as a major driver of neurotoxic inflammation in AGS astrocytes, suggests a role for AGS gene products in R-loop homeostasis, and identifies common denominators of disease that can be targeted to prevent astrocyte-mediated neurotoxicity in AGS.

Intracellular virus sensor MDA5 mutation develops autoimmune myocarditis and nephritis

Mutations in IFIH1 gene encoding viral RNA sensor MDA5 have been reported responsible for many interferonopathies, including Aicardi-Goutières syndrome (AGS) and monogenic lupus, however, the pathological link between IFIH1 mutations and various autoimmune symptoms remains unclear. Here, we generated transgenic mice expressing human MDA5 R779H mutant (R779H Tg), reported in AGS and monogenic lupus patient. Mice spontaneously developed myocarditis and nephritis with upregulation of type I IFNs in the major organs. R779H Tg Mavs-/- and R779H Tg Ifnar-/- showed no phenotypes, indicating direct MDA5-signaling pathway involvement. Rag-2 deficiency and bone marrow cells transfer from wild type to adult mice did not prevent myocarditis development, while mice with cardiomyocyte-specific expression of hMDA5 R779H showed cardiomegaly and high expression of inflammatory cytokines. Taken together, our study clarifies that type I IFNs production and chemokines from cardiomyocytes starts in neonatal period and is critical for the development of myocarditis. Activated lymphocytes and auto-antibodies exacerbate the pathogenesis but are dispensable for the onset.

Mutations in the adenosine deaminase ADAR1 that prevent endogenous Z-RNA binding induce Aicardi-Goutières-syndrome-like encephalopathy

Mutations in the adenosine-to-inosine RNA-editing enzyme ADAR1 p150, including point mutations in the Z-RNA recognition domain Zα, are associated with Aicardi-Goutières syndrome (AGS). Here, we examined the in vivo relevance of ADAR1 binding of Z-RNA. Mutation of W197 in Zα, which abolished Z-RNA binding, reduced RNA editing. Adar1^W197A/W197A mice displayed severe growth retardation after birth, broad expression of interferon-stimulated genes (ISGs), and abnormal development of multiple organs. Notably, malformation of the brain was accompanied by white matter vacuolation and gliosis, reminiscent of AGS-associated encephalopathy. Concurrent deletion of the double-stranded RNA sensor MDA5 ameliorated these abnormalities. ADAR1 (W197A) expression increased in a feedback manner downstream of type I interferons, resulting in increased RNA editing at a subset of, but not all, ADAR1 target sites. This increased expression did not ameliorate inflammation in Adar1^W197A/W197A mice. Thus, editing of select endogenous RNAs by ADAR1 is essential for preventing inappropriate MDA5-mediated inflammation, with relevance to the pathogenesis of AGS.

Aicardi-Goutières syndrome-associated mutation at ADAR1 gene locus activates innate immune response in mouse brain

Background

Aicardi-Goutières syndrome (AGS) is a severe infant or juvenile-onset autoimmune disease characterized by inflammatory encephalopathy with an elevated type 1 interferon-stimulated gene (ISG) expression signature in the brain. Mutations in seven different protein-coding genes, all linked to DNA/RNA metabolism or sensing, have been identified in AGS patients, but none of them has been demonstrated to activate the IFN pathway in the brain of an animal. The molecular mechanism of inflammatory encephalopathy in AGS has not been well defined. Adenosine Deaminase Acting on RNA 1 (ADAR1) is one of the AGS-associated genes. It carries out A-to-I RNA editing that converts adenosine to inosine at double-stranded RNA regions. Whether an AGS-associated mutation in ADAR1 activates the IFN pathway and causes autoimmune pathogenesis in the brain is yet to be determined.

Methods

Mutations in the ADAR1 gene found in AGS patients were introduced into the mouse genome via CRISPR/Cas9 technology. Molecular activities of the specific p.K999N mutation were investigated by measuring the RNA editing levels in brain mRNA substrates of ADAR1 through RNA sequencing analysis. IFN pathway activation in the brain was assessed by measuring ISG expression at the mRNA and protein level through real-time RT-PCR and Luminex assays, respectively. The locations in the brain and neural cell types that express ISGs were determined by RNA in situ hybridization (ISH). Potential AGS-related brain morphologic changes were assessed with immunohistological analysis. Von Kossa and Luxol Fast Blue staining was performed on brain tissue to assess calcification and myelin, respectively.

Results

Mice bearing the ADAR1 p.K999N were viable though smaller than wild type sibs. RNA sequencing analysis of neuron-specific RNA substrates revealed altered RNA editing activities of the mutant ADAR1 protein. Mutant mice exhibited dramatically elevated levels of multiple ISGs within the brain. RNA ISH of brain sections showed selective activation of ISG expression in neurons and microglia in a patchy pattern. ISG-15 mRNA was upregulated in ADAR1 mutant brain neurons whereas CXCL10 mRNA was elevated in adjacent astroglia. No calcification or gliosis was detected in the mutant brain.

Conclusions

We demonstrated that an AGS-associated mutation in ADAR1, specifically the p.K999N mutation, activates the IFN pathway in the mouse brain. The ADAR1 p.K999N mutant mouse replicates aspects of the brain interferonopathy of AGS. Neurons and microglia express different ISGs. Basal ganglia calcification and leukodystrophy seen in AGS patients were not observed in K999N mutant mice, indicating that development of the full clinical phenotype may need an additional stimulus besides AGS mutations. This mutant mouse presents a robust tool for the investigation of AGS and neuroinflammatory diseases including the modeling of potential “second hits” that enable severe phenotypes of clinically variable diseases.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02217-9.

Hypoxia-induced RNASEH2A limits activation of cGAS-STING signaling in HCC and predicts poor prognosis

BACKGROUND

Hypoxia is a hallmark of solid cancers, including hepatocellular carcinoma (HCC). There is scarce information about how hypoxia avoids immunologic stress and maintains a cancer-promoting microenvironment.

METHODS

The Cancer Genome Atlas, RNA-seq data, and Oncomine database were used to discover the correlation of RNASEH2A with tumor progression; then expression of RNASEH2A mRNA and protein were detected in HCC tissues and cells subjected to hypoxia or with the treatment of CoCl₂ via real-time quantitative polymerase chain reaction and immunochemistry assays. Finally, the effect of RNASEH2A on cell proliferation and the involved signaling pathway was explored further.

RESULTS

RNASEH2A was positively correlated with tumor grade, size, vascular invasion, and poor prognosis. The expression of RNASEH2A mRNA and protein were increased and dependent on hypoxia-inducible factor 2α in HCC tissues and cell lines. Knockout of RNASEH2A in HCC cells greatly reduced cell proliferation and induced the transcription of multiple cGAS-STING (cyclic GMP-AMP synthase-stimulator of interferon genes) targeted type 1 interferon-related genes, including IFIT1, USP18, and CXCL10, which suggests knockout of RNASEH2A may produce immunologic stress and tumor suppressive effects.

CONCLUSIONS

RNASEH2A plays a critical role and potentially predicts patient outcomes in HCC, which uncovers a new mechanism that RNASEH2A contributes to limit immunologic stress of cancer cells in the context of hypoxia.

Monogenic Autoinflammatory Diseases: State of the Art and Future Perspectives

Systemic autoinflammatory diseases are a heterogeneous family of disorders characterized by a dysregulation of the innate immune system, in which sterile inflammation primarily develops through antigen-independent hyperactivation of immune pathways. In most cases, they have a strong genetic background, with mutations in single genes involved in inflammation. Therefore, they can derive from different pathogenic mechanisms at any level, such as dysregulated inflammasome-mediated production of cytokines, intracellular stress, defective regulatory pathways, altered protein folding, enhanced NF-kappaB signalling, ubiquitination disorders, interferon pathway upregulation and complement activation. Since the discover of pathogenic mutations of the pyrin-encoding gene MEFV in Familial Mediterranean Fever, more than 50 monogenic autoinflammatory diseases have been discovered thanks to the advances in genetic sequencing: the advent of new genetic analysis techniques and the discovery of genes involved in autoinflammatory diseases have allowed a better understanding of the underlying innate immunologic pathways and pathogenetic mechanisms, thus opening new perspectives in targeted therapies. Moreover, this field of research has become of great interest, since more than a hundred clinical trials for autoinflammatory diseases are currently active or recently concluded, allowing us to hope for considerable acquisitions for the next few years. General paediatricians need to be aware of the importance of this group of diseases and they should consider autoinflammatory diseases in patients with clinical hallmarks, in order to guide further examinations and refer the patient to a specialist rheumatologist. Here we resume the pathogenesis, clinical aspects and diagnosis of the most important autoinflammatory diseases in children.

Investigation of type I interferon responses in ANCA-associated vasculitis

Type I interferon (IFN) dysregulation is a major contributory factor in the development of several autoimmune diseases, termed type I interferonopathies, and is thought to be the pathogenic link with chronic inflammation in these conditions. Anti-neutrophil cytoplasmic antibody (ANCA)-Associated Vasculitis (AAV) is an autoimmune disease characterised by necrotising inflammation of small blood vessels. The underlying biology of AAV is not well understood, however several studies have noted abnormalities in type I IFN responses. We hypothesised that type I IFN responses are systemically dysregulated in AAV, consistent with features of a type I interferonopathy. To investigate this, we measured the expression of seven interferon regulated genes (IRGs) (ISG15, SIGLEC1, STAT1, RSAD2, IFI27, IFI44L and IFIT1) in peripheral blood samples, as well as three type I IFN regulated proteins (CXCL10, MCP-1 and CCL19) in serum samples from AAV patients, healthy controls and disease controls. We found no difference in type I IFN regulated gene or protein expression between AAV patients and healthy controls. Furthermore, IRG and IFN regulated protein expression did not correlate with clinical measurements of disease activity in AAV patients. Thus, we conclude that systemic type I IFN responses are not key drivers of AAV pathogenesis and AAV should not be considered a type I interferonopathy.

Type I Interferonopathies in Children: An Overview

Notable advances in gene sequencing methods in recent years have permitted enormous progress in the phenotypic and genotypic characterization of autoinflammatory syndromes. Interferonopathies are a recent group of inherited autoinflammatory diseases, characterized by a dysregulation of the interferon pathway, leading to constitutive upregulation of its activation mechanisms or downregulation of negative regulatory systems. They are clinically heterogeneous, but some peculiar clinical features may lead to suspicion: a familial "idiopathic" juvenile arthritis resistant to conventional treatments, an early necrotizing vasculitis, a non-infectious interstitial lung disease, and a panniculitis associated or not with a lipodystrophy may represent the "interferon alarm bells." The awareness of this group of diseases represents a challenge for pediatricians because, despite being rare, a differential diagnosis with the most common childhood rheumatological and immunological disorders is mandatory. Furthermore, the characterization of interferonopathy molecular pathogenetic mechanisms is allowing important steps forward in other immune dysregulation diseases, such as systemic lupus erythematosus and inflammatory myositis, implementing the opportunity of a more effective target therapy.

Systemic inflammation and chronic kidney disease in a patient due to the RNASEH2B defect

INTRODUCTION

Aicardi-Goutières (AGS) is a rare immune dysregulated disease due to mutations in TREX1, RNASEH2A, RNASEH2B, RNASEH2C, SAMHD1, ADAR1, or IFIH1. Clinical features include basal ganglia calcifications, white matter abnormalities, and cerebral atrophy. Severe systemic inflammation and chronic kidney disease (CKD) are extremely rare in AGS. Herein, we report a patient presenting with systemic inflammation and CKD to broaden the clinical phenotype spectrum of the RNASEH2B defect.

METHODS

All testing and molecular genetic analysis were performed after obtaining the informed consent of the parents. Demographic, clinical, and laboratory findings were abstracted from outpatient and inpatient encounters. Cerebral magnetic resonance imaging (MRI), computed tomography (CT) scans, and renal biopsy histopathology reports were reviewed and summarized. Whole exome sequencing (WES) was performed on peripheral blood cells. After exposure to cGAMP in vitro for 24 h, mRNA expression of 12 IFN-stimulated cytokine genes in PBMCs was assessed. Serum cytokine levels were detected by Milliplex.

RESULTS

A 11-year-old girl presented with recurrent aseptic fever, arthritis, chilblains, failure to thrive, mild hearing loss, and neurological manifestations. Laboratory and immunologic findings demonstrated lymphopenia, low complement levels, positive autoantibodies, elevated levels of acute-phase reactants and inflammatory cytokines. Cerebral imaging showed cerebral atrophy, white matter abnormalities, and intracranial calcification. Renal biopsy showed glomerular sclerosis in 3 of 14 glomeruli, infiltration of lymphocytes and other mononuclear cells. WES revealed a homozygous and heterozygous mutations in RNASEH2B. Over-expression of IFN-stimulated cytokine genes was observed, including IFI44, IFI27, IFIT1, IFIT2, IFIT3, ISG15, OAS1, and SIGLEC1.

CONCLUSIONS

To date, only two cases with AGS have been reported to have renal disease. Here, we describe a patient with both homozygous and heterozygous variants in RNASEH2B, presenting with neurological manifestations, persistently systemic autoinflammation, and CKD. CKD has never been reported in patients with AGS due to the RNASEH2B defect.

TRIAL REGISTRATION

Not applicable; this was a retrospective study.

Retroelement-derived RNA and its role in the brain

Comprising ~40% of the human genome, retroelements are mobile genetic elements which are transcribed into RNA, then reverse-transcribed into DNA and inserted into a new site in the genome. Retroelements are referred to as "genetic parasites", residing among host genes and relying on host machinery for transcription and evolutionary propagation. The healthy brain has the highest expression of retroelement-derived sequences compared to other somatic tissue, which leads to the question: how does retroelement-derived RNA influence human traits and cellular states? While the functional importance of upregulating retroelement expression in the brain is an active area of research, RNA species derived from retroelements influence both self- and host gene expression by contributing to chromatin remodeling, alternative splicing, somatic mosaicism and translational repression. Here, we review the emerging evidence that the functional importance of RNA derived from retroelements is multifaceted. Retroelements can influence organismal states through the seeding of epigenetic states in chromatin, the production of structured RNA and even catalytically active ribozymes, the generation of cytoplasmic ssDNA and RNA/DNA hybrids, the production of viral-like proteins, and the generation of somatic mutations. Comparative sequencing suggests that retroelements can contribute to intraspecies variation through these mechanisms to alter transcript identity and abundance. In humans, an increasing number of neurodevelopmental and neurodegenerative conditions are associated with dysregulated retroelements, including Aicardi-Goutieres syndrome (AGS), Rett syndrome (RTT), Amyotrophic Lateral Sclerosis (ALS), Alzheimer's disease (AD), multiple sclerosis (MS), schizophrenia (SZ), and aging. Taken together, these concepts suggest a larger functional role for RNA derived from retroelements. This review aims to define retroelement-derived RNA, discuss how it impacts the mammalian genome, as well as summarize data supporting phenotypic consequences of this unique RNA subset in the brain.

Relapsing-remitting clinical course expands the phenotype of Aicardi-Goutières syndrome

Aicardi-Goutières syndrome (AGS) is a rare and likely underdiagnosed genetic leukoencephalopathy, typically presenting in infancy with encephalopathy and characteristic neuroimaging features, with residual static neurological deficits. We describe a patient who, following an initial presentation at the age of 12 months in keeping with AGS, exhibited a highly atypical relapsing course of neurological symptoms in adulthood with essentially normal neuroimaging. Whole-exome sequencing confirmed a pathogenic RNASEH2B gene variant consistent with AGS. This case highlights the expanding phenotypes associated with AGS and the potential role of whole-exome sequencing in facilitating an increase in the rate of diagnosis.

Astroglia in Leukodystrophies

Leukodystrophies are genetically determined disorders affecting the white matter of the central nervous system. The combination of MRI pattern recognition and next-generation sequencing for the definition of novel disease entities has recently demonstrated that many leukodystrophies are due to the primary involvement and/or mutations in genes selectively expressed by cell types other than the oligodendrocytes, the myelin-forming cells in the brain. This has led to a new definition of leukodystrophies as genetic white matter disorders resulting from the involvement of any white matter structural component. As a result, the research has shifted its main focus from oligodendrocytes to other types of neuroglia. Astrocytes are the housekeeping cells of the nervous system, responsible for maintaining homeostasis and normal brain physiology and to orchestrate repair upon injury. Several lines of evidence show that astrocytic interactions with the other white matter cellular constituents play a primary pathophysiologic role in many leukodystrophies. These are thus now classified as astrocytopathies. This chapter addresses how the crosstalk between astrocytes, other glial cells, axons and non-neural cells are essential for the integrity and maintenance of the white matter in health. It also addresses the current knowledge of the cellular pathomechanisms of astrocytic leukodystrophies, and specifically Alexander disease, vanishing white matter, megalencephalic leukoencephalopathy with subcortical cysts and Aicardi-Goutière Syndrome.

Genetic contributors and soluble mediators in prediction of autoimmune comorbidity

Comorbidities including subclinical atherosclerosis, neuropsychological aberrations and lymphoproliferation represent a major burden among patients with systemic autoimmune diseases; they occur either as a result of intrinsic disease related characteristics including therapeutic interventions or traditional risk factors similar to those observed in general population. Soluble molecules recently shown to contribute to subclinical atherosclerosis in the context of systemic lupus erythematosus (SLE) include among others B-cell activating factor (BAFF), hyperhomocysteinemia, parathormone (PTH) levels and autoantibodies against oxidized lipids. Variations of the 5, 10- methylenetetrahydrofolate reductase (MTHFR) gene -the main genetic determinant of hyperhomocystenemia in humans-as well the interferon regulatory factor-8 (IRF8), FcγRIIA and BAFF genes have been all linked to subclinical atherosclerosis in SLE. BAFF variants have been also found to confer increased risk for subclinical atherosclerosis and lymphoma development in Sjogren's syndrome (SS) patients. Other genes shown to be implicated in SS lymphoproliferation include genes involved a. in inflammatory responses such as the NFκB regulator Tumor necrosis factor alpha-induced protein 3 (TNFAIP3) and the Leukocyte immunoglobulin-like receptor A3 (LILRA3) immunoreceptor, b. B cell activation and signaling (BAFF/BAFF-receptor), c. type I IFN pathway such as three-prime repair exonuclease 1 (TREX1), d. epigenetic processes including DNA methylation (MTHFR rs1801133, 677T allele) and e. genomic instability (MTHFR rs1801131, 1298C allele). Emerging soluble biomarkers for SS related lymphoma include mediators of B cell growth and germinal center formation such as BAFF, FMS-like tyrosine kinase 3 ligand (Flt-3L) and CXCL13 as well as inflammatory contributors such as inteleukin (IL)-17, IL-18, ASC, LILRA3 and the extracellular lipoprotein-associated phospholipase A2 (Lp-PLA2). In regard to fatigue and neuropsychologic features in the setting of SS, contributing factors such as BAFF variants, antibodies against neuropeptides, proteins involved in nervous system function as well as inflammatory cytokines have been reported.

TREX1 variants in Sjogren's syndrome related lymphomagenesis

Genetic variants of the three-prime repair exonuclease 1 (TREX1) -an exonuclease involved in DNA repair and degradation- have been previously found to increase susceptibility to Aicardi Goutieres syndrome, familial chilblain lupus and systemic lupus erythematosus. We aimed to explore whether TREX1 common variants could influence the risk of primary Sjogren's syndrome (SS) and SS-related lymphoma. Three single nucleotide polymorphisms (SNPs) of the TREX1 gene (rs11797, rs3135941 and rs3135945) were evaluated in 229 SS, 89 SS-lymphoma (70 SS-MALT and 19 SS non-MALT) and 240 healthy controls by PCR-based assays. In available 52 peripheral blood and 26 minor salivary gland tissues from our SS cohort, mRNA expression of type I interferon (IFN) related genes and TREX1 was determined by real-time PCR. Significantly decreased prevalence of rs11797 A minor allele was detected in SS patients complicated by non-MALT lymphoma compared to controls (ΟR [95% CI]: 0.4 [0.2-0.9], p-value: 0.02). SS patients carrying the rs11797 AA genotype had increased type I IFN related gene mRNA expression in minor salivary gland tissues. These data support genetically related dampened type I IFN production as an additional mechanism for SS-related lymphomagenesis.

The role of protein complexes in human genetic disease

Many human genetic disorders are caused by mutations in protein-coding regions of DNA. Taking protein structure into account has therefore provided key insight into the molecular mechanisms underlying human genetic disease. Although most studies have focused on the intramolecular effects of mutations, the critical role of the assembly of proteins into complexes is being increasingly recognized. Here, we review multiple ways in which consideration of protein complexes can help us to understand and explain the effects of pathogenic mutations. First, we discuss disorders caused by mutations that perturb intersubunit interactions in homomeric and heteromeric complexes. Second, we address how protein complex assembly can facilitate a dominant-negative mechanism, whereby mutated subunits can disrupt the activity of wild-type protein. Third, we show how mutations that change protein expression levels can lead to damaging stoichiometric imbalances. Finally, we review how mutations affecting different subunits of the same heteromeric complex often cause similar diseases, whereas mutations in different interfaces of the same subunit can cause distinct phenotypes.

Astrocytes, an active player in Aicardi-Goutières syndrome

Aicardi-Goutières syndrome (AGS) is an early-onset, autoimmune and genetically heterogeneous disorder with severe neurologic injury. Molecular studies have established that autosomal recessive mutations in one of the following genes are causative: TREX1, RNASEH2A, RNASEH2B, RNASEH2C, SAMHD1, ADAR1 and IFIH1/MDA5. The phenotypic presentation and pathophysiology of AGS is associated with over-production of the cytokine Interferon-alpha (IFN-α) and its downstream signaling, characterized as type I interferonopathy. Astrocytes are one of the major source of IFN in the central nervous system (CNS) and it is proposed that they could be key players in AGS pathology. Astrocytes are the most ubiquitous glial cell in the CNS and perform a number of crucial and complex functions ranging from formation of blood-brain barrier, maintaining ionic homeostasis, metabolic support to synapse formation and elimination in healthy CNS. Involvement of astrocytic dysfunction in neurological diseases-Alexander's disease, Epilepsy, Alzheimer's and amyotrophic lateral sclerosis (ALS)-has been well-established. It is now known that compromised astrocytic function can contribute to CNS abnormalities and severe neurodegeneration, nevertheless, its contribution in AGS is unclear. The current review discusses known molecular and cellular pathways for AGS mutations and how it stimulates IFN-α signaling. We shed light on how astrocytes might be key players in the phenotypic presentations of AGS and emphasize the cell-autonomous and non-cell-autonomous role of astrocytes. Understanding the contribution of astrocytes will help reveal mechanisms underlying interferonopathy and develop targeted astrocyte specific therapeutic treatments in AGS.

TREX1 Mutation Causing Autosomal Dominant Thrombotic Microangiopathy and CKD-A Novel Presentation

Renal thrombotic microangiopathy (TMA) involves diverse causes and clinical presentations. Genetic determinants causing alternate pathway complement dysregulation underlie a substantial proportion of cases. In a significant proportion of TMAs, no defect in complement regulation is identified. Mutations in the major mammalian 3' DNA repair exonuclease 1 (TREX1) have been associated with autoimmune and cerebroretinal vasculopathy syndromes. Carboxy-terminal TREX1 mutations that result in only altered localization of the exonuclease protein with preserved catalytic function cause microangiopathy of the brain and retina, termed retinal vasculopathy and cerebral leukodystrophy (RVCL). Kidney involvement reported with RVCL usually accompanies significant brain and retinal microangiopathy. We present a pedigree with autosomal dominant renal TMA and chronic kidney disease found to have a carboxy-terminal frameshift TREX1 variant. Although symptomatic brain and retinal microangiopathy is known to associate with carboxy-terminal TREX1 mutations, this report describes a carboxy-terminal TREX1 frameshift variant causing predominant renal TMA. These findings underscore the clinical importance of recognizing TREX1 mutations as a cause of renal TMA. This case demonstrates the value of whole-exome sequencing in unsolved TMA.

The classification, genetic diagnosis and modelling of monogenic autoinflammatory disorders

Monogenic autoinflammatory disorders are an increasingly heterogeneous group of conditions characterised by innate immune dysregulation. Improved genetic sequencing in recent years has led not only to the discovery of a plethora of conditions considered to be 'autoinflammatory', but also the broadening of the clinical and immunological phenotypic spectra seen in these disorders. This review outlines the classification strategies that have been employed for monogenic autoinflammatory disorders to date, including the primary innate immune pathway or the dominant cytokine implicated in disease pathogenesis, and highlights some of the advantages of these models. Furthermore, the use of the term 'autoinflammatory' is discussed in relation to disorders that cross the innate and adaptive immune divide. The utilisation of next-generation sequencing (NGS) in this population is examined, as are potential in vivo and in vitro methods of modelling to determine pathogenicity of novel genetic findings. Finally, areas where our understanding can be improved are highlighted, such as phenotypic variability and genotype-phenotype correlations, with the aim of identifying areas of future research.

Late diagnosis and atypical brain imaging of Aicardi-Goutières syndrome: are we failing to diagnose Aicardi-Goutières syndrome-2?

Aicardi-Goutières syndrome (AGS) is a rare disorder with in utero or postnatal onset of encephalopathy and progressive neurological deterioration. The seven genetic subtypes of AGS are associated with abnormal type I interferon-mediated innate immune response. Most patients with AGS present with progressive microcephaly, spasticity, and cognitive impairment. Some, especially those with type 2 (AGS2), manifest milder phenotypes, reduced childhood mortality, and relative preservation of physical and cognitive abilities. In this report, we describe two siblings (sister and brother) diagnosed with AGS2 in their second decade, who exhibited static encephalopathy since 1 year of age with spastic quadriplegia and anarthria but preserved intellect. Both were homozygous for the common pathogenic RNASEH2B allele (c.529G>A, p.Ala177Thr). Rather than manifesting calcifications and leukoencephalopathy, both had increased iron signal in the basal ganglia. Our report broadens the clinical and imaging spectrum of AGS2 and emphasizes the importance of including AGS2 in the differential diagnosis of idiopathic spastic cerebral palsy.

WHAT THIS PAPER ADDS

We identified two siblings (sister and brother) with atypical Aicardi-Goutières syndrome type 2 due to RNASEH2B mutation. Manifestations included spastic quadriplegia and anarthria but preserved intellect and increased iron signal in the basal ganglia. RNASEH2B-related Aicardi-Goutières syndrome type 2 can have present with a variable phenotype, including idiopathic spastic cerebral palsy.

RNA editing by ADAR1 regulates innate and antiviral immune functions in primary macrophages

ADAR1-dependent A-to-I editing has recently been recognized as a key process for marking dsRNA as self, therefore, preventing innate immune activation and affecting the development and resolution of immune-mediated diseases and infections. Here, we have determined the role of ADAR1 as a regulator of innate immune activation and modifier of viral susceptibility in primary myeloid and lymphoid cells. We show that ADAR1 knockdown significantly enhanced interferon, cytokine and chemokine production in primary macrophages that function as antiviral paracrine factors, rendering them resistant to HIV-1 infection. ADAR1 knockdown induced deregulation of the RLRs-MAVS signaling pathway, by increasing MDA5, RIG-I, IRF7 and phospho-STAT1 expression, an effect that was partially rescued by pharmacological blockade of the pathway. In summary, our results demonstrate a role of ADAR1 in regulating innate immune function in primary macrophages, suggesting that macrophages may play an essential role in disease associated to ADAR1 dysfunction. We also show that viral inhibition is exclusively dependent on innate immune activation consequence of ADAR1 knockdown, pointing towards ADAR1 as a potential target to boost antiviral immune response.

Roles of SAMHD1 in antiviral defense, autoimmunity and cancer

The enzyme, sterile α motif and histidine-aspartic acid domain-containing protein 1 (SAMHD1) diminishes infection of human immunodeficiency virus type 1 (HIV-1) by hydrolyzing intracellular deoxynucleotide triphosphates (dNTPs) in myeloid cells and resting CD4+ T cells. This dNTP degradation reduces the dNTP concentration to a level insufficient for viral cDNA synthesis, thereby inhibiting retroviral replication. This antiviral enzymatic activity can be inhibited by viral protein X (Vpx). The HIV-2/SIV Vpx causes degradation of SAMHD1, thus interfering with the SAMHD1-mediated restriction of retroviral replication. Recently, SAMHD1 has been suggested to restrict HIV-1 infection by directly digesting genomic HIV-1 RNA through a still controversial RNase activity. Here, we summarize the current knowledge about structure, antiviral mechanisms, intracellular localization, interferon-regulated expression of SAMHD1. We also describe SAMHD1-deficient animal models and an antiviral drug on the basis of disrupting proteasomal degradation of SAMHD1. In addition, the possible roles of SAMHD1 in regulating innate immune sensing, Aicardi-Goutières syndrome and cancer are discussed in this review.

Type I interferon-mediated monogenic autoinflammation: The type I interferonopathies, a conceptual overview

Type I interferon is a potent substance. As such, the induction, transmission, and resolution of the type I interferon-mediated immune response are tightly regulated. As defined, the type I interferonopathies represent discrete examples of a disturbance of the homeostatic control of this system caused by Mendelian mutations. Considering the complexity of the interferon response, the identification of further monogenic diseases belonging to this disease grouping seems likely, with the recognition of type I interferonopathies becoming of increasing clinical importance as treatment options are developed based on an understanding of disease pathology and innate immune signaling. Definition of the type I interferonopathies indicates that autoinflammation can be both interferon and noninterferon related, and that a primary disturbance of the innate immune system can "spill over" into autoimmunity in some cases. Indeed, that several non-Mendelian disorders, most particularly systemic lupus erythematosus and dermatomyositis, are also characterized by an up-regulation of type I interferon signaling suggests the possibility that insights derived from this work will have relevance to a broader field of clinical medicine.

Interferon-induced sterile alpha motif and histidine/aspartic acid domain-containing protein 1 expression in astrocytes and microglia is mediated by microRNA-181a

OBJECTIVE

Sterile alpha motif and histidine/aspartic acid domain-containing protein 1 (SAMHD1), a newly discovered HIV-1 host restriction factor, has been found to be induced by interferons and to be regulated by microRNA-181a (miR-181a). However, the mechanism of interferons-induced SAMHD1 expression is unclear.

DESIGN

We hypothesized that interferons induce SAMHD1 expression through Janus kinase-signal transducer and activator of transcription (JAK-STAT) signaling pathways, which is mediated by miR-181a.

METHODS

We examined the effect of IFN-α and IFN-γ on SAMHD1 mRNA and protein expression, as well as the levels of phosphorylated SAMHD1 and miR-181a in astrocytes and microglia. To determine whether interferons-induced SAMHD1 expression was mediated by miR-181a, we overexpressed or inhibited miR-181a in these cells and exposed them to interferons. We also detected the effect of SAMHD1 and miR-181a on HIV-1 infection in astrocytes and microglia.

RESULTS

Both IFN-α and IFN-γ increased SAMHD1 mRNA and protein expression, and reduced miR-181a levels, particularly in microglia. Phosphorylated SAMHD1was not induced by interferons. Overexpression of miR-181a counteracted induction of SAMHD1 expression by interferons, and inhibition of miR-181a mimicked interferons treatment. Inhibition of JAK-STAT signaling pathways resulted in increased miR-181a levels and decreased SAMHD1 mRNA expression. Knock-down of SAMHD1 or overexpression of miR-181a enhanced HIV-1 infection, whereas inhibition of miR-181a reduced HIV-1 infection. However, inhibition of HIV-1 infection induced by IFN-α was not significantly affected by miR-181a and SAMHD1.

CONCLUSION

MiR-181a is an important mediator for interferons-induced SAMHD1 expression in astrocytes and microglia, but not for inhibition of HIV-1 infection induced by IFN-α.

Ribonuclease H2 mutations induce a cGAS/STING-dependent innate immune response

Aicardi-Goutières syndrome (AGS) provides a monogenic model of nucleic acid-mediated inflammation relevant to the pathogenesis of systemic autoimmunity. Mutations that impair ribonuclease (RNase) H2 enzyme function are the most frequent cause of this autoinflammatory disorder of childhood and are also associated with systemic lupus erythematosus. Reduced processing of eitherRNA:DNAhybrid or genome-embedded ribonucleotide substrates is thought to lead to activation of a yet undefined nucleic acid-sensing pathway. Here, we establishRnaseh2b(A174T/A174T)knock-in mice as a subclinical model of disease, identifying significant interferon-stimulated gene (ISG) transcript upregulation that recapitulates theISGsignature seen inAGSpatients. The inflammatory response is dependent on the nucleic acid sensor cyclicGMP-AMPsynthase (cGAS) and its adaptorSTINGand is associated with reduced cellular ribonucleotide excision repair activity and increasedDNAdamage. This suggests thatcGAS/STINGis a key nucleic acid-sensing pathway relevant toAGS, providing additional insight into disease pathogenesis relevant to the development of therapeutics for this childhood-onset interferonopathy and adult systemic autoimmune disorders.

Devastating recurrent brain ischemic infarctions and retinal disease in pediatric patients with CD59 deficiency

Identification of CD59 p.Cys89Tyr mutation in 5 patients from North-African Jewish origin presenting with chronic inflammatory demyelinating polyradiculoneuropathy like disease and chronic hemolysis, led us to reinvestigate an unsolved disease in 2 siblings from the same origin who died 17 years ago. The two patients carried the same CD59 gene mutation previously described by our group. These children had quiet similar disease course but in addition developed devastating recurrent brain infarctions, retinal and optic nerve involvement. Revising the brain autopsy of one of these patients confirmed the finding of multiple brain infarctions of different ages. CD59 protein expression was missing on brain endothelial cells by immunohistochemical staining. This new data expands the clinical spectrum of CD59 mutations and further emphasizes the need for its early detection and treatment.

NBS1 is required for macrophage homeostasis and functional activity in mice

Nijmegen breakage syndrome 1 (NBS1) is a component of the MRE11 complex, which is a sensor of DNA double-strand breaks and plays a crucial role in the DNA damage response. Because activated macrophages produce large amounts of reactive oxygen species (ROS) that can cause DNA lesions, we examined the role of NBS1 in macrophage functional activity. Proliferative and proinflammatory (interferon gamma [IFN-γ] and lipopolysaccharide [LPS]) stimuli led to increased NBS1 levels in macrophages. In mice expressing a hypomorphic allele of Nbs1, Nbs1(∆B/∆B), macrophage activation-induced ROS caused increased levels of DNA damage that were associated with defects in proliferation, delayed differentiation, and increased senescence. Furthermore, upon stimulation, Nbs1(∆B/∆B) macrophages exhibited increased expression of proinflammatory cytokines. In the in vivo 2,4-dinitrofluorobenzene model of inflammation, Nbs1(∆B/∆B) animals showed increased weight and ear thickness. By using the sterile inflammation by zymosan injection, we found that macrophage proliferation was drastically decreased in the peritoneal cavity of Nbs1(∆B/∆B) mice. Our findings show that NBS1 is crucial for macrophage function during normal aging. These results have implications for understanding the immune defects observed in patients with NBS and related disorders.

Type I interferon dysregulation and neurological disease

Type I interferon is an essential component of the brain's innate immune defence, conferring protection against viral infection. Recently, dysregulation of the type I interferon pathway has been implicated in the pathogenesis of a spectrum of neuroinfectious and neuroinflammatory disorders. Underactivity of the type I interferon response is associated with a predisposition to herpes simplex encephalitis. Conversely, a group of 'interferonopathic' disorders, characterized by severe neuroinflammation and overactivity of type I interferon, has been described. Elucidation of the genetic basis of these Mendelian neuroinflammatory diseases has uncovered important links between nucleic acid sensors, innate immune activation and neuroinflammatory disease. These mechanisms have an important role in the pathogenesis of more common polygenic diseases that can affect the brain, such as lupus and cerebral small vessel disease. In this article, we review the spectrum of neurological disease associated with type I interferon dysregulation, as well as advances in our understanding of the molecular and cellular pathogenesis of these conditions. We highlight the potential utility of type I interferon as both a biomarker and a therapeutic target in neuroinflammatory disease.