Photosensitivity and cGAS-dependent type I IFN activation in lupus patients with TREX1 deficiency

Chronic endoplasmic reticulum stress in myotonic dystrophy type 2 promotes autoimmunity via mitochondrial DNA release

Myotonic dystrophy type 2 (DM2) is a tetranucleotide CCTG repeat expansion disease associated with an increased prevalence of autoimmunity. Here, we identified an elevated type I interferon (IFN) signature in peripheral blood mononuclear cells and primary fibroblasts of DM2 patients as a trigger of chronic immune stimulation. Although RNA-repeat accumulation was prevalent in the cytosol of DM2-patient fibroblasts, type-I IFN release did not depend on innate RNA immune sensors but rather the DNA sensor cGAS and the prevalence of mitochondrial DNA (mtDNA) in the cytoplasm. Sublethal mtDNA release was promoted by a chronic activation of the ATF6 branch of the unfolded protein response (UPR) in reaction to RNA-repeat accumulation and non-AUG translated tetrapeptide expansion proteins. ATF6-dependent mtDNA release and resulting cGAS/STING activation could also be recapitulated in human THP-1 monocytes exposed to chronic endoplasmic reticulum (ER) stress. Altogether, our study demonstrates a novel mechanism by which large repeat expansions cause chronic endoplasmic reticulum stress and associated mtDNA leakage. This mtDNA is, in turn, sensed by the cGAS/STING pathway and induces a type-I IFN response predisposing to autoimmunity. Elucidating this pathway reveals new potential therapeutic targets for autoimmune disorders associated with repeat expansion diseases.

Epidermal ZBP1 stabilizes mitochondrial Z-DNA to drive UV-induced IFN signaling in autoimmune photosensitivity

Photosensitivity is observed in numerous autoimmune diseases and drives poor quality of life and disease flares. Elevated epidermal type I interferon (IFN) production primes for photosensitivity and enhanced inflammation, but the substrates that sustain and amplify this cycle remain undefined. Here, we show that IFN-induced Z-DNA binding protein 1 (ZBP1) stabilizes ultraviolet (UV)B-induced cytosolic Z-DNA derived from oxidized mitochondrial DNA. ZBP1 is significantly upregulated in the epidermis of adult and pediatric patients with autoimmune photosensitivity. Strikingly, lupus keratinocytes accumulate extensive cytosolic Z-DNA after UVB, and transfection of keratinocytes with Z-DNA results in stronger IFN production through cGAS-STING activation compared to B-DNA. ZBP1 knockdown abrogates UV-induced IFN responses, whereas overexpression results in a lupus-like phenotype with spontaneous Z-DNA accumulation and IFN production. Our results highlight Z-DNA and ZBP1 as critical mediators for UVB-induced inflammation and uncover how type I IFNs prime for cutaneous inflammation in photosensitivity.

Pathogenesis of systemic lupus erythematosus: risks, mechanisms and therapeutic targets

Research elucidating the pathogenesis of systemic lupus erythematosus (SLE) has defined two critical families of mediators, type I interferon (IFN-I) and autoantibodies targeting nucleic acids and nucleic acid-binding proteins, as fundamental contributors to the disease. On the fertile background of significant genetic risk, a triggering stimulus, perhaps microbial, induces IFN-I, autoantibody production or most likely both. When innate and adaptive immune system cells are engaged and collaborate in the autoimmune response, clinical SLE can develop. This review describes recent data from genetic analyses of patients with SLE, along with current studies of innate and adaptive immune function that contribute to sustained IFN-I pathway activation, immune activation and autoantibody production, generation of inflammatory mediators and tissue damage. The goal of these studies is to understand disease mechanisms, identify therapeutic targets and stimulate development of therapeutics that can achieve improved outcomes for patients.

Suppression of TREX1 deficiency-induced cellular senescence and interferonopathies by inhibition of DNA damage response

TREX1 encodes a major DNA exonuclease and mutations of this gene are associated with type I interferonopathies in human. Mice with Trex1 deletion or mutation have shortened life spans accompanied by a senescence-associated secretory phenotype. However, the contribution of cellular senescence in TREX1 deficiency-induced type I interferonopathies remains unknown. We found that features of cellular senescence present in Trex1^-/- mice are induced by multiple factors, particularly DNA damage. The cGAS-STING and DNA damage response pathways are required for maintaining TREX1 deletion-induced cellular senescence. Inhibition of the DNA damage response, such as with Checkpoint kinase 2 (CHK2) inhibitor, partially alleviated progression of type I interferonopathies and lupus-like features in the mice. These data provide insights into the initiation and development of type I interferonopathies and lupus-like diseases, and may help inform the development of targeted therapeutics.

DNA damage and repair in age-related inflammation

Genomic instability is an important driver of ageing. The accumulation of DNA damage is believed to contribute to ageing by inducing cell death, senescence and tissue dysfunction. However, emerging evidence shows that inflammation is another major consequence of DNA damage. Inflammation is a hallmark of ageing and the driver of multiple age-related diseases. Here, we review the evidence linking DNA damage, inflammation and ageing, highlighting how premature ageing syndromes are associated with inflammation. We discuss the mechanisms by which DNA damage induces inflammation, such as through activation of the cGAS-STING axis and NF-κB activation by ATM. The triggers for activation of these signalling cascades are the age-related accumulation of DNA damage, activation of transposons, cellular senescence and the accumulation of persistent R-loops. We also discuss how epigenetic changes triggered by DNA damage can lead to inflammation and ageing via redistribution of heterochromatin factors. Finally, we discuss potential interventions against age-related inflammation.

DNA Containing Cyclobutane Pyrimidine Dimers Is Released from UVB-Irradiated Keratinocytes in a Caspase-Dependent Manner

Solar radiation induces the formation of cyclobutane pyrimidine dimers (CPDs) and other UV photoproducts in the genomic DNA of epidermal keratinocytes. Although CPDs have been detected in urine from UV- and sun-exposed individuals, the pathway by which they arrive there and the mechanisms by which UV-induced DNA damage in the skin has systemic effects throughout the body are not clear. Consistent with previous reports that DNA associates with small extracellular vesicles that are released from a variety of cell types, we observed that a small fraction of CPDs formed in genomic DNA after UVB exposure can later be detected in the culture medium. These extracellular CPDs are found within large fragments of histone-associated DNA and are released in a time- and UVB dose‒dependent manner. Moreover, studies with both cultured cells and human skin explants revealed that CPD release into the extracellular environment is blocked by caspase inhibition, which indicates a role for apoptotic signaling in CPD release from UVB-irradiated keratinocytes. Finally, we show that this released CPD-containing DNA can be taken up by other keratinocytes. These results therefore provide possible mechanisms for the export of damaged DNA from UVB-irradiated cells and for systemic effects of UVB exposure throughout the body.

Current concepts of photosensitivity in cutaneous lupus erythematosus

Cutaneous lupus erythematosus (CLE) represents a complex autoimmune disease with a broad phenotypic spectrum ranging from acute to chronic destructive cutaneous lesions. Patients with CLE exhibit high photosensitivity and ultraviolet (UV) irradiation can lead to systemic flares in systemic lupus erythematosus. However, the exact mechanisms how UV irradiation enhances cutaneous inflammation in lupus are not fully understood. Recently, new molecular mechanisms of UV-driven immune responses in CLE were identified, offering potential therapeutic approaches. Especially the induction of type I interferons, central cytokines in lupus pathogenesis which are released by various skin cells, have become the focus of current research. In this review, we describe current pathogenic concepts of photosensitivity in lupus erythematosus, including UV-driven activation of intracellular nucleic acid sensors, cellular cytokine production and immune cell activation. Furthermore, we discuss activated pathways contributing to enhanced apoptosis as well as intracellular translocation of autoantigens thereby promoting CLE upon UV light exposure.

A Novel Type I Interferon Primed Dendritic Cell Subpopulation in TREX1 Mutant Chilblain Lupus Patients

Heterozygous TREX1 mutations are associated with monogenic familial chilblain lupus and represent a risk factor for developing systemic lupus erythematosus. These interferonopathies originate from chronic type I interferon stimulation due to sensing of inadequately accumulating nucleic acids. We here analysed the composition of dendritic cell (DC) subsets, central stimulators of immune responses, in patients with TREX1 deficiency. We performed single-cell RNA-sequencing of peripheral blood DCs and monocytes from two patients with familial chilblain lupus and heterozygous mutations in TREX1 and from controls. Type I interferon pathway genes were strongly upregulated in patients. Cell frequencies of the myeloid and plasmacytoid DC and of monocyte populations in patients and controls were similar, but we describe a novel DC subpopulation highly enriched in patients: a myeloid DC CD1C⁺ subpopulation characterized by the expression of LMNA, EMP1 and a type I interferon- stimulated gene profile. The presence of this defined subpopulation was confirmed in a second cohort of patients and controls by flow cytometry, also revealing that an increased percentage of patient's cells in the subcluster express costimulatory molecules. We identified a novel type I interferon responsive myeloid DC subpopulation, that might be important for the perpetuation of TREX1-induced chilblain lupus and other type I interferonopathies.

TREX1 degrades the 3' end of the small DNA oligonucleotide products of nucleotide excision repair in human cells

The nucleotide excision repair (NER) machinery removes UV photoproducts from DNA in the form of small, excised damage-containing DNA oligonucleotides (sedDNAs) ∼30 nt in length. How cells process and degrade these byproducts of DNA repair is not known. Using a small scale RNA interference screen in UV-irradiated human cells, we identified TREX1 as a major regulator of sedDNA abundance. Knockdown of TREX1 increased the level of sedDNAs containing the two major UV photoproducts and their association with the NER proteins TFIIH and RPA. Overexpression of wild-type but not nuclease-inactive TREX1 significantly diminished sedDNA levels, and studies with purified recombinant TREX1 showed that the enzyme efficiently degrades DNA located 3′ of the UV photoproduct in the sedDNA. Knockdown or overexpression of TREX1 did not impact the overall rate of UV photoproduct removal from genomic DNA or cell survival, which indicates that TREX1 function in sedDNA degradation does not impact NER efficiency. Taken together, these results indicate a previously unknown role for TREX1 in promoting the degradation of the sedDNA products of the repair reaction. Because TREX1 mutations and inefficient DNA degradation impact inflammatory and immune signaling pathways, the regulation of sedDNA degradation by TREX1 may contribute to photosensitive skin disorders.