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Purpose: STAT2 is both an effector and negative regulator of type I interferon (IFN-I) signalling. We describe the characterization of a novel homozygous missense STAT2 substitution in a patient with a type I interferonopathy. Methods: Whole-genome sequencing (WGS) was used to identify the genetic basis of disease in a patient with features of enhanced IFN-I signalling. After stable lentiviral reconstitution of STAT2-null human fibrosarcoma U6A cells with STAT2 wild type or p.(A219V), we performed quantitative polymerase chain reaction, western blotting, immunofluorescence, and co-immunoprecipitation to functionally characterize the p.(A219V) variant. Results: WGS identified a rare homozygous single nucleotide transition in STAT2 (c.656C > T), resulting in a p.(A219V) substitution, in a patient displaying developmental delay, intracranial calcification, and up-regulation of interferon-stimulated gene (ISG) expression in blood. In vitro studies revealed that the STAT2 p.(A219V) variant retained the ability to transduce an IFN-I stimulus. Notably, STAT2 p.(A219V) failed to support receptor desensitization, resulting in sustained STAT2 phosphorylation and ISG up-regulation. Mechanistically, STAT2 p.(A219V) showed defective binding to ubiquitin specific protease 18 (USP18), providing a possible explanation for the chronic IFN-I pathway activation seen in the patient. Conclusion: Our data indicate an impaired negative regulatory role of STAT2 p.(A219V) in IFN-I signalling and that mutations in STAT2 resulting in a type I interferonopathy state are not limited to the previously reported R148 residue. Indeed, structural modelling highlights at least 3 further residues critical to mediating a STAT2-USP18 interaction, in which mutations might be expected to result in defective negative feedback regulation of IFN-I signalling. [ABSTRACT FROM AUTHOR]

Aicardi-Goutières syndrome (AGS1-9) is a genetically determined encephalopathy that falls under the type I interferonopathy disease class, characterized by excessive type I interferon (IFN-I) activity, coupled with upregulation of IFN-stimulated genes (ISGs), which can be explained by the vital role these proteins play in self-non-self-discrimination. To date, few mouse models fully replicate the vast clinical phenotypes observed in AGS patients. Therefore, we investigated the use of zebrafish as an alternative species for generating a clinically relevant model of AGS. Using CRISPR-cas9 technology, we generated a stable mutant zebrafish line recapitulating AGS5, which arises from recessive mutations in SAMHD1. The resulting homozygous mutant zebrafish larvae possess a number of neurological phenotypes, exemplified by variable, but increased expression of several ISGs in the head region, a significant increase in brain cell death, microcephaly and locomotion deficits. A link between IFN-I signaling and cholesterol biosynthesis has been highlighted by others, but not previously implicated in the type I interferonopathies. Through assessment of neurovascular integrity and qPCR analysis we identified a significant dysregulation of cholesterol biosynthesis in the zebrafish model. Furthermore, dysregulation of cholesterol biosynthesis gene expression was also observed through RNA sequencing analysis of AGS patient whole blood. From this novel finding, we hypothesize that cholesterol dysregulation may play a role in AGS disease pathophysiology. Further experimentation will lend critical insight into the molecular pathophysiology of AGS and the potential links involving aberrant type I IFN signaling and cholesterol dysregulation. [ABSTRACT FROM AUTHOR]

Objective: Juvenile idiopathic inflammatory/immune myopathies (IIMs) constitute a highly heterogeneous group of disorders with diagnostic difficulties and prognostic uncertainties. Circulating myositis‐specific autoantibodies (MSAs) have been recognized as reliable tools for patient substratification. Considering the key role of type I interferon (IFN) up‐regulation in juvenile IIM, we undertook the present study to investigate whether IFN‐induced 15‐kd protein (ISG‐15) could be a reliable biomarker for stratification and diagnosis and to better elucidate its role in juvenile IIM pathophysiology. Methods: The study included 56 patients: 24 with juvenile dermatomyositis (DM), 12 with juvenile overlap myositis (OM), 10 with Duchenne muscular dystrophy, and 10 with congenital myopathies. Muscle biopsy samples were assessed by immunohistochemistry, immunoblotting, and real‐time quantitative polymerase chain reaction. Negative regulators of type I IFN (ISG15 and USP18) and positive regulators of type I IFN (DDX58 and IFIH1) were analyzed. Results: ISG15 expression discriminated patients with juvenile IIM from those with nonimmune myopathies and, among patients with juvenile IIM, discriminated those with DM from those with OM. Among patients with juvenile DM, up‐regulation of the type I IFN positive regulators DDX58 and IFIH1 was similar regardless of MSA status. In contrast, the highest levels of the type I IFN negative regulator ISG15 were observed in patients who were positive for melanoma differentiation–associated gene 5 (MDA‐5). Finally, ISG15 levels were inversely correlated with the severity of muscle histologic abnormalities and positively correlated with motor performance as evaluated by the Childhood Myositis Assessment Scale and by manual muscle strength testing. Conclusion: Muscle ISG15 expression is strongly associated with juvenile DM, with patients exhibiting a different ISG‐15 muscle signature according to their MSA class. Patients with juvenile DM who are positive for MDA‐5 have higher expression of ISG15 in both gene form and protein form compared to the other subgroups. Moreover, our data show that negative regulation of type I IFN correlates with milder muscle involvement. [ABSTRACT FROM AUTHOR]

Objective To define the molecular basis of a multisystem phenotype with progressive musculoskeletal disease of the hands and feet, including camptodactyly, subluxation, and tendon rupture, reminiscent of Jaccoud's arthropathy. Methods We identified 2 families segregating an autosomal-dominant phenotype encompassing musculoskeletal disease and variable additional features, including psoriasis, dental abnormalities, cardiac valve involvement, glaucoma, and basal ganglia calcification. We measured the expression of interferon (IFN)-stimulated genes in the peripheral blood and skin, and undertook targeted Sanger sequencing of the IFIH1 gene encoding the cytosolic double-stranded RNA (dsRNA) sensor melanoma differentiation-associated protein 5 (MDA-5). We also assessed the functional consequences of IFIH1 gene variants using an in vitro IFNβ reporter assay in HEK 293T cells. Results We recorded an up-regulation of type I IFN-induced gene transcripts in all 5 patients tested and identified a heterozygous gain-of-function mutation in IFIH1 in each family, resulting in different substitutions of the threonine residue at position 331 of MDA-5. Both of these variants were associated with increased IFNβ expression in the absence of exogenous dsRNA ligand, consistent with constitutive activation of MDA-5. Conclusion These cases highlight the significant musculoskeletal involvement that can be associated with mutations in MDA-5, and emphasize the value of testing for up-regulation of IFN signaling as a marker of the underlying molecular lesion. Our data indicate that both Singleton-Merten syndrome and neuroinflammation described in the context of MDA-5 gain-of-function constitute part of the same type I interferonopathy disease spectrum, and provide possible novel insight into the pathology of Jaccoud's arthropathy. [ABSTRACT FROM AUTHOR]

Objective Patients with systemic lupus erythematosus (SLE) have an increased risk of cardiovascular disease (CVD) and impaired endothelial repair. Although vitamin D deficiency is associated with increased CVD risk in the general population, a causal relationship has not been demonstrated. We aimed to determine whether vitamin D deficiency directly modulates endothelial dysfunction and immune responses in a murine model of SLE. Methods Vitamin D deficiency was induced in lupus-prone MRL/ lpr mice by dietary restriction for 6 weeks. Endothelium-dependent vasorelaxation was quantified using aortic ring myography, and endothelial repair mechanisms were assessed by evaluating the phenotype and function of bone marrow endothelial progenitor cells (EPCs) and with the use of an in vivo Matrigel plug model. Lupus disease activity was determined by evaluating expression of interferon-stimulated genes (ISGs) in splenic tissue, positivity for serum autoantibodies, and renal histology. To validate the findings, expression of ISGs was also measured in whole blood from vitamin D-deficient and vitamin D-sufficient patients with SLE. Results Vitamin D deficiency resulted in impaired endothelium-dependent vasorelaxation and decreases in neoangiogenesis without a change in the total number of EPCs. There were no differences in anti-double-stranded DNA titers, proteinuria, or glomerulonephritis (activity or chronicity) between vitamin D-deficient or sufficient mice. Vitamin D deficiency was associated with a trend toward increased ISG expression both in mice and in patients with SLE. Conclusion These findings indicate that vitamin D deficiency is associated with hampered vascular repair and reduced endothelial function, and may modulate type I interferon responses. [ABSTRACT FROM AUTHOR]

Background Hereditary spastic paraplegia is a neurodegenerative phenotype characterized by a progressive loss of corticospinal motor tract function. In a majority of affected individuals the pathogenesis remains undetermined. Methods We identified a series of patients with a phenotype of nonsyndromic spastic paraplegia in whom no diagnosis had been reached before exome sequencing. We measured the expression of interferon stimulated genes (ISGs) in peripheral blood from these patients. Results Five patients from four families with previously unexplained spastic paraplegia were identified with mutations in either ADAR1 (one patient), IFIH1 (one patient), or RNASEH2B (three patients from two families). All patients were developmentally normal before the onset of features beginning in the second year of life. All patients remain of normal intellect. Four patients demonstrated normal neuroimaging, while a single patient had features of nonspecific dysmyelination. The patients with ADAR1 and IFIH1-related disease showed a robust interferon signature. The patients with mutations in RNASEH2B demonstrated no (two patients) or a minimal (one patient) upregulation of ISGs compared with controls. Conclusions Mutations in ADAR1, IFIH1, and RNASEH2B can cause a phenotype of spastic paraplegia with normal neuroimaging, or in association with nonspecific dysmyelination. Although the presence of an interferon signature can be helpful in interpreting the significance of gene variants in this context, patients with pathogenic mutations in RNASEH2B may demonstrate no upregulation of ISGs in peripheral blood. However, it remains possible that type I interferons act as a neurotoxin in the context of all genotypes. [ABSTRACT FROM AUTHOR]

We studied ten individuals from eight families showing features consistent with the immuno-osseous dysplasia spondyloenchondrodysplasia. Of particular note was the diverse spectrum of autoimmune phenotypes observed in these individuals (cases), including systemic lupus erythematosus, Sjögren's syndrome, hemolytic anemia, thrombocytopenia, hypothyroidism, inflammatory myositis, Raynaud's disease and vitiligo. Haplotype data indicated the disease gene to be on chromosome 19p13, and linkage analysis yielded a combined multipoint log10 odds (LOD) score of 3.6. Sequencing of ACP5, encoding tartrate-resistant acid phosphatase, identified biallelic mutations in each of the cases studied, and in vivo testing confirmed a loss of expressed protein. All eight cases assayed showed elevated serum interferon alpha activity, and gene expression profiling in whole blood defined a type I interferon signature. Our findings reveal a previously unrecognized link between tartrate-resistant acid phosphatase activity and interferon metabolism and highlight the importance of type I interferon in the genesis of autoimmunity. [ABSTRACT FROM AUTHOR]