Nadja König

SAMHD1 prevents autoimmunity by maintaining genome stability

Objectives The HIV restriction factor, SAMHD1 (SAM domain and HD domain-containing protein 1), is a triphosphohydrolase that degrades deoxyribonucleoside triphosphates (dNTPs). Mutations in SAMHD1 cause Aicardi–Goutières syndrome (AGS), an inflammatory disorder that shares phenotypic similarity with systemic lupus erythematosus, including activation of antiviral type 1 interferon (IFN). To further define the pathomechanisms underlying autoimmunity in AGS due to SAMHD1 mutations, we investigated the physiological properties of SAMHD1. Methods Primary patient fibroblasts were examined for dNTP levels, proliferation, senescence, cell cycle progression and DNA damage. Genome-wide transcriptional profiles were generated by RNA sequencing. Interaction of SAMHD1 with cyclin A was assessed by coimmunoprecipitation and fluorescence cross-correlation spectroscopy. Cell cycle-dependent phosphorylation of SAMHD1 was examined in synchronised HeLa cells and using recombinant SAMHD1. SAMHD1 was knocked down by RNA interference. Results We show that increased dNTP pools due to SAMHD1 deficiency cause genome instability in fibroblasts of patients with AGS. Constitutive DNA damage signalling is associated with cell cycle delay, cellular senescence, and upregulation of IFN-stimulated genes. SAMHD1 is phosphorylated by cyclin A/cyclin-dependent kinase 1 in a cell cycle-dependent manner, and its level fluctuates during the cell cycle, with the lowest levels observed in G1/S phase. Knockdown of SAMHD1 by RNA interference recapitulates activation of DNA damage signalling and type 1 IFN activation. Conclusions SAMHD1 is required for genome integrity by maintaining balanced dNTP pools. dNTP imbalances due to SAMHD1 deficiency cause DNA damage, leading to intrinsic activation of IFN signalling. These findings establish a novel link between DNA damage signalling and innate immune activation in the pathogenesis of autoimmunity.

Familial chilblain lupus due to a gain-of-function mutation in STING

Objectives Familial chilblain lupus is a monogenic form of cutaneous lupus erythematosus caused by loss-of-function mutations in the nucleases TREX1 or SAMHD1. In a family without TREX1 or SAMHD1 mutation, we sought to determine the causative gene and the underlying disease pathology. Methods Exome sequencing was used for disease gene identification. Structural analysis was performed by homology modelling and docking simulations. Type I interferon (IFN) activation was assessed in cells transfected with STING cDNA using an IFN-β reporter and Western blotting. IFN signatures in patient blood in response to tofacitinib treatment were measured by RT-PCR of IFN-stimulated genes. Results In a multigenerational family with five members affected with chilblain lupus, we identified a heterozygous mutation of STING, a signalling molecule in the cytosolic DNA sensing pathway. Structural and functional analyses indicate that mutant STING enhances homodimerisation in the absence of its ligand cGAMP resulting in constitutive type I IFN activation. Treatment of two affected family members with the Janus kinase (JAK) inhibitor tofacitinib led to a marked suppression of the IFN signature. Conclusions A heterozygous gain-of-function mutation in STING can cause familial chilblain lupus. These findings expand the genetic spectrum of type I IFN-dependent disorders and suggest that JAK inhibition may be of therapeutic value.

RPA and Rad51 constitute a cell intrinsic mechanism to protect the cytosol from self DNA

Immune recognition of cytosolic DNA represents a central antiviral defence mechanism. Within the host, short single-stranded DNA (ssDNA) continuously arises during the repair of DNA damage induced by endogenous and environmental genotoxic stress. Here we show that short ssDNA traverses the nuclear membrane, but is drawn into the nucleus by binding to the DNA replication and repair factors RPA and Rad51. Knockdown of RPA and Rad51 enhances cytosolic leakage of ssDNA resulting in cGAS-dependent type I IFN activation. Mutations in the exonuclease TREX1 cause type I IFN-dependent autoinflammation and autoimmunity. We demonstrate that TREX1 is anchored within the outer nuclear membrane to ensure immediate degradation of ssDNA leaking into the cytosol. In TREX1-deficient fibroblasts, accumulating ssDNA causes exhaustion of RPA and Rad51 resulting in replication stress and activation of p53 and type I IFN. Thus, the ssDNA-binding capacity of RPA and Rad51 constitutes a cell intrinsic mechanism to protect the cytosol from self DNA.

Autoimmune phenotype with type I interferon signature in two brothers with ADA2 deficiency carrying a novel CECR1 mutation

BackgroundLoss-of-function CECR1 mutations cause polyarteritis nodosa (PAN) with childhood onset, an autoinflammatory disorder without significant signs of autoimmunity. Herein we describe the unusual presentation of an autoimmune phenotype with constitutive type I interferon activation in siblings with adenosine deaminase 2 (ADA2) deficiency.Case presentationWe describe two siblings with early-onset recurrent strokes, arthritis, oral ulcers, discoid rash, peripheral vascular occlusive disease and high antinuclear antibody titers. Assessment of interferon signatures in blood revealed constitutive type I interferon activation. Aicardi-Goutières syndrome (AGS) was suspected, but no mutation in the known AGS genes were detected. Whole exome sequencing identified compound heterozygosity for a known and a novel mutation in the CECR1 gene. Functional consequences of the mutations were demonstrated by marked reduction in ADA2 catalytic activity.ConclusionsOur findings demonstrate that ADA2 deficiency can cause an unusual autoimmune phenotype extending the phenotypic spectrum of PAN. Constitutive interferon I activation in patient blood suggests a possible role of type I interferon in disease pathogenesis which may have therapeutic implications.

Response to: ‘JAK inhibition in STING-associated interferonopathy’ by Crow et al

We appreciate the comments by Crow et al on our report of a gain-of-function mutation in STING in familial chilblain lupus and the effect of the JAK inhibitor tofacitinib.1 In a concurrent study,2 Crow et al reported a marked clinical improvement in three patients with stimulator of interferon genes (STING)-associated vasculopathy3 using the JAK inhibitor ruxolitinib. This was accompanied by an incomplete reduction in the expression of interferon (IFN)-stimulated genes (ISGs) in blood raising the question as to the IFN signature as a read-out of clinical efficacy. This is a very important question that we cannot answer with regard to tofacitinib as we have no long-term experience with this JAK inhibitor in patients with type I interferonopathies. However, we have used ruxolitinib in two patients with Aicardi-Goutieres syndrome (AGS), an inflammatory encephalopathy characterised by chronic type I IFN activation.4 Both patients carried biallelic RNASEH2B mutations and suffered from severe developmental delay. They received no other medications, when treatment with ruxolitinib …

Typ-I-Interferonopathien

Type I interferons mediate immune defense against viral infections. The induction of type I interferons has stimulating and modulating effects on the innate and adaptive immune systems thereby reducing tolerance against self-antigens. Genetic defects that result in an inadequate activation of the type I interferon system can cause a group of inflammatory disorders, which are collectively referred to as type I interferonopathies. While the clinical spectrum of type I interferonopathies is broad and heterogeneous, neurological and cutaneous symptoms are the most frequent manifestations. Some clinical and genetic features of type I interferonopathies are shared by multifactorial diseases, such as systemic lupus erythematosus and systemic vasculitis. Advances in understanding the disease mechanisms underlying type I interferonopathies have pinpointed novel targets for therapeutic interventions.

Type I interferonopathies. Systemic inflammatory diseases triggered by type I interferons

Type I interferons mediate immune defense against viral infections. The induction of type I interferons has stimulating and modulating effects on the innate and adaptive immune systems thereby reducing tolerance against self-antigens. Genetic defects that result in an inadequate activation of the type I interferon system can cause a group of inflammatory disorders, which are collectively referred to as type I interferonopathies. While the clinical spectrum of type I interferonopathies is broad and heterogeneous, neurological and cutaneous symptoms are the most frequent manifestations. Some clinical and genetic features of type I interferonopathies are shared by multifactorial diseases, such as systemic lupus erythematosus and systemic vasculitis. Advances in understanding the disease mechanisms underlying type I interferonopathies have pinpointed novel targets for therapeutic interventions.

Typ-I-Interferonopathien@@@Type I interferonopathies: Durch Typ-1-Interferone bedingte entzündliche Systemerkrankungen@@@Systemic inflammatory diseases triggered by type I interferons

Type I interferons mediate immune defense against viral infections. The induction of type I interferons has stimulating and modulating effects on the innate and adaptive immune systems thereby reducing tolerance against self-antigens. Genetic defects that result in an inadequate activation of the type I interferon system can cause a group of inflammatory disorders, which are collectively referred to as type I interferonopathies. While the clinical spectrum of type I interferonopathies is broad and heterogeneous, neurological and cutaneous symptoms are the most frequent manifestations. Some clinical and genetic features of type I interferonopathies are shared by multifactorial diseases, such as systemic lupus erythematosus and systemic vasculitis. Advances in understanding the disease mechanisms underlying type I interferonopathies have pinpointed novel targets for therapeutic interventions.

Familial chilblain lupus caused by an activating mutation in STING.

Familial chilblain lupus is a monogenic form of cutaneous lupus erythematosus characterized by cold-induced cutaneous lesions at acral location. It is caused by loss-of-function mutations in the nucleic acid metabolizing enzymes TREX1 or SAMHD1. Gain-of-function mutations in STING (stimulator of Interferon genes) have been described in an infancy-onset autoinflammatory syndrome with fever, inflammatory cutaneous lesions and interstitial lung disease. Here we report on a family with dominant chilblain lupus over 4 generations. Affected family members presented with acral inflammatory and partially necrotizing lesions beginning in early childhood. In some cases, low-titered ANAs and immune complexes were detectable. The family tested negative for TREX1 or SAMHD1 mutations. Exome sequencing revealed a heterozygous STING mutation segregating with chilblain lupus in the family. The mutation affects a highly conserved residue within the STING dimer interface and is predicted to be pathogenic. Quantitative RT-PCR analysis showed an increased expression of IFN-stimulated genes in blood cells of affected family members suggesting that the identified mutation has an activating effect on type I IFN signaling. Taken together, our findings demonstrate that gain-of-function mutations in STING can cause familial chilblain lupus and expand the spectrum of type I interferonopathies.