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Coronavirus disease 2019 (COVID-19) mainly affects the respiratory system, but the involvement of other organ systems has also been commonly reported. Acute acro-ischemia or chilblain like lesions were among the first recognized dermatological presentations of COVID-19. Though the occurrence of such lesions has been attributed to the similar interferon-1 mediated immune response in both COVID-19 and systemic lupus erythematosus, we propose another possible explanation based on a common genetic background. In a recent genome-wide association study, the 3p21.31 region was found to be associated with COVID-19 severity. This region also contains the TREX1 gene. Missense mutations of the TREX1 gene are responsible for familial chilblain lupus and its genetic polymorphisms have been implicated in the pathogenesis of systemic lupus erythematosus. Based on this observation, herein we have reviewed other COVID-19 risk loci for potential overlap with dermatological conditions.

Background: Familial chilblain lupus (FCL) is a rare, chronic form of cutaneous lupus erythematosus, which is characterized by painful bluish-red inflammatory cutaneous lesions in acral locations. Mutations in TREX1, SAMHD1 and STING have been described in FCL patients. Less than 10 TREX1 mutation positive FCL families have been described in the literature., Case Presentation: Genetic study was performed in a large, nonconsanguineous Chinese family with 13 members over 4 generations affected by chilblain lupus. Whole exome sequencing was performed for the index patient. Significant variant detection was subsequently validated by resequencing using Sanger sequencing in the index patient and other family members. A novel pathogenic mutation TREX1 p.Asp18His was iditified in the index patient. The mutation was present in affected individuals and was absent in non-affected individuals in the familiy., Conclusions: We present a four-generation Chinese family with FCL caused by a novel heterozygous mutation TREX1 p.Asp18His, which had been reported in a patient with Aicardi-Goutie'res syndrome. This is the first reported Chinese family with FCL based on mutation in TREX1.

Importance: Familial chilblain lupus is a monogenic autosomal dominant form of cutaneous lupus erythematosus that in most cases is caused by mutations in the 3 prime repair exonuclease 1 (TREX1). Familial chilblain lupus presents in early childhood with cold-induced painful erythematous infiltrates leading to mutilation and is associated with systemic involvement illustrated by an elevated type I interferon (IFN) signature in the skin and blood. Effective treatment is currently not available., Objectives: To evaluate the clinical response to the Janus kinase inhibitor baricitinib in familial chilblain lupus and assess the effect of cold on patient fibroblasts., Design, Setting, and Participants: In this case series, 3 patients with familial chilblain lupus due to TREX1 mutation underwent treatment with baricitinib for 3 months., Interventions: Doses of baricitinib, 4 mg, were administered daily for 3 months., Main Outcomes and Measures: Reduction of cutaneous lupus lesions was measured by the revised cutaneous lupus area and severity index, pain due to skin and joint involvement was assessed by visual analog scale, type I IFN signature in blood was determined by polymerase chain reaction, and the in vitro response of fibroblasts to cold exposure was analyzed., Results: All 3 patients (2 women and 1 man; mean [SD] age, 51 [24] years) showed a significant improvement of cutaneous lupus lesions with suppression of systemic type I IFN activation. One patient had a complete remission regarding pain and, in 2 patients, pain associated with joint inflammation was partially reduced. No severe adverse reactions were reported. Exposure of patient fibroblasts to cold induced a stress response and enhanced senescence along with induction of IFN-stimulated gene in vitro., Conclusions and Relevance: These findings demonstrate the therapeutic efficacy of Janus kinase inhibition in a monogenic form of lupus among 3 patients and provide mechanistic insight into the process of disease exacerbation by cold in TREX1-deficient cells. This finding may be relevant to other type I IFN-mediated disorders and implicates Janus kinase inhibition as a potential therapeutic option also for multifactorial cutaneous lupus erythematosus.

Purpose: Cryofibrinogenemia is a rare cryopathy presenting as acrocyanosis following exposure to cold. Familial presentation has been described but the underlying molecular cause remained undetermined., Methods: Forty (40) members from a large family with an initial diagnosis of familial cryofibrinogenemia were interviewed and examined to determine affected status and collect DNA. Exome sequencing was performed on three affected individuals from distinct branches of the pedigree., Results: Seventeen (17) family members reported a history of acrocyanosis with cold exposure. None reported symptoms were suggestive of lupus. Exome sequencing of three subjects identified the heterozygous mutation D18N in the TREX1 gene which was then confirmed by Sanger sequencing in all affected as well as 2 unaffected family members. The mutation is already being associated with familial chilblain lupus erythematosus (CHLE), and a systematic review of literature was undertaken to compare reports of familial CHLE and cryofibrinogenemia. Both entities were found to share highly similar clinical presentations suggesting they are part of a same syndrome in which cryofibrinogenemia and lupus manifestations have variable penetrance., Conclusions: Familial cryofibrinogenemia without lupus should be added to the spectrum of TREX1-related disease.

Three prime repair exonuclease 1 degrades single and double stranded DNA with 3'-5' nuclease activity and its mutations are related to type 1 IFN mediated autoinflammation due to accumulated intracellular nucleic acids. To date, several cases of systemic lupus erythematosus, Aicardi-Goutieres syndrome, familial chilblain lupus, retinal vasculopathy-cerebral leukodystrophy have been reported with TREX1 mutations. Chilblain lupus is a skin disease characterized by blue-reddish coloring, swelling or ulcers on acral regions of body such as fingertips, heels, nose and auricles. Central nervous system vasculitis is a prominent cause of childhood strokes. 10 families with familial chilblain lupus related to TREX1 mutations were reported previously in the literature, in which homozygote D18N variant in TREX1 gene was related to chilblains with cerebral vasculitis. In this report, whole-exome-sequencing revealed a homozygote R114C mutation in TREX1 gene was shown in two siblings with recurrent chilblains whom one of them was the second case accompanied by cerebral vasculitis in the literature. As a result, the approach of WES in clinical use revealed a novel mutation in clinically heterogenous patients to provide genetic counseling., (Copyright © 2017 Elsevier Masson SAS. All rights reserved.)

The TREX1 gene encodes a potent DNA exonuclease, and mutations in TREX1 cause a spectrum of lupus-like autoimmune diseases. Most lupus patients develop autoantibodies to double-stranded DNA (dsDNA), but the source of DNA antigen is unknown. The TREX1 D18N mutation causes a monogenic, cutaneous form of lupus called familial chilblain lupus, and the TREX1 D18N enzyme exhibits dysfunctional dsDNA-degrading activity, providing a link between dsDNA degradation and nucleic acid-mediated autoimmune disease. We determined the structure of the TREX1 D18N protein in complex with dsDNA, revealing how this exonuclease uses a novel DNA-unwinding mechanism to separate the polynucleotide strands for single-stranded DNA (ssDNA) loading into the active site. The TREX1 D18N dsDNA interactions coupled with catalytic deficiency explain how this mutant nuclease prevents dsDNA degradation. We tested the effects of TREX1 D18N in vivo by replacing the TREX1 WT gene in mice with the TREX1 D18N allele. The TREX1 D18N mice exhibit systemic inflammation, lymphoid hyperplasia, vasculitis, and kidney disease. The observed lupus-like inflammatory disease is associated with immune activation, production of autoantibodies to dsDNA, and deposition of immune complexes in the kidney. Thus, dysfunctional dsDNA degradation by TREX1 D18N induces disease in mice that recapitulates many characteristics of human lupus. Failure to clear DNA has long been linked to lupus in humans, and these data point to dsDNA as a key substrate for TREX1 and a major antigen source in mice with dysfunctional TREX1 enzyme.

Importance: Familial chilblain lupus is a rare, autosomal dominant form of lupus erythematosus characterized by cold-induced inflammatory lesions at acral locations presenting in early childhood. Familial chilblain lupus is usually caused by a mutation in TREX1 (3' repair exonuclease 1)., Observations: We report on a family with dominant chilblain lupus segregating a novel TREX1 mutation (c.585C>G; H195Q) within the highly conserved exonuclease (Exo) III domain. Affected family members experienced cold-induced chilblain lesions of varying degrees, ranging from bluish-red infiltrations to mutilating necrotic ulcerations. In addition, all patients showed signs of systemic disease, such as arthritis, lymphopenia, or antinuclear antibodies. An increased expression of myxovirus resistance protein A in the skin and induction of interferon-stimulated genes in peripheral blood cells demonstrated activation of type I interferon., Conclusions and Relevance: This case further implicates type I interferon-dependent innate immune activation in the pathogenesis of TREX1-associated familial chilblain lupus. Unlike previously reported TREX1 mutations, which affect the Exo I or Exo II domains, the mutation presented here alters the Exo III domain, suggesting a particular role of mutations within the catalytic Exo domains in the pathogenesis of familial chilblain lupus. The high prevalence of extracutaneous manifestations, along with activation of type I interferon, underlines the systemic nature of familial chilblain lupus.

Considering that it is a single exon gene encoding a 314 amino acid protein, the genotype-phenotype landscape of TREX1 is remarkably complex. Here we briefly describe the human diseases so-far associated with mutations in TREX1, which include Aicardi-Goutières syndrome, familial chilblain lupus, systemic lupus erythematosus and retinal vasculopathy with cerebral leukodystrophy.

TREX1 is a potent 3' → 5' exonuclease that degrades single- and double-stranded DNA (ssDNA and dsDNA). TREX1 mutations at amino acid positions Asp-18 and Asp-200 in familial chilblain lupus and Aicardi-Goutières syndrome elicit dominant immune dysfunction phenotypes. Failure to appropriately disassemble genomic DNA during normal cell death processes could lead to persistent DNA signals that trigger the innate immune response and autoimmunity. We tested this concept using dsDNA plasmid and chromatin and show that the TREX1 exonuclease locates 3' termini generated by endonucleases and degrades the nicked DNA polynucleotide. A competition assay was designed using TREX1 dominant mutants and variants to demonstrate that an intact DNA binding process, coupled with dysfunctional chemistry in the active sites, explains the dominant phenotypes in TREX1 D18N, D200N, and D200H alleles. The TREX1 residues Arg-174 and Lys-175 positioned adjacent to the active sites act with the Arg-128 residues positioned in the catalytic cores to facilitate melting of dsDNA and generate ssDNA for entry into the active sites. Metal-dependent ssDNA binding in the active sites of the catalytically inactive dominant TREX1 mutants contributes to DNA retention and precludes access to DNA 3' termini by active TREX1 enzyme. Thus, the dominant disease genetics exhibited by the TREX1 D18N, D200N, and D200H alleles parallel precisely the biochemical properties of these TREX1 dimers during dsDNA degradation of plasmid and chromatin DNA in vitro. These results support the concept that failure to degrade genomic dsDNA is a principal pathway of immune activation in TREX1-mediated autoimmune disease.

Chilblain lupus erythematosus is a rare form of cutaneous lupus erythematosus characterized by bluish red infiltrates in acral locations of the body mostly affecting middle-aged women. We recently described a familial form of chilblain lupus manifesting in early childhood caused by a heterozygous mutation in the TREX1 gene, which encodes a 3'-5' DNA exonuclease. Thus, familial chilblain lupus represents the first monogenic form of cutaneous lupus erythematosus. Here we describe the unusual clinical course of this newly defined genodermatosis in an 18-year-old female member of the family in which familial chilblain lupus was originally described., (Copyright 2009 S. Karger AG, Basel.)

TREX1 acts in concert with the SET complex in granzyme A-mediated apoptosis, and mutations in TREX1 cause Aicardi-Goutières syndrome and familial chilblain lupus. Here, we report monoallelic frameshift or missense mutations and one 3' UTR variant of TREX1 present in 9/417 individuals with systemic lupus erythematosus but absent in 1,712 controls (P = 4.1 x 10(-7)). We demonstrate that two mutant TREX1 alleles alter subcellular targeting. Our findings implicate TREX1 in the pathogenesis of SLE.

We recently described a novel autosomal-dominant genodermatosis, termed familial chilblain lupus, and mapped its genetic locus to chromosome 3p21. Familial chilblain lupus manifests in early childhood with ulcerating acral skin lesions and is associated with arthralgias and circulating antinuclear antibodies. In this study, we report the identification of a heterozygous missense mutation (D18N) in TREX1 encoding the 3'-5'repair exonuclease 1 in affected individuals of the family with chilblain lupus. The homodimeric TREX1 is the most abundant intracellular DNase in mammalian cells. We have recently shown that TREX1 plays a role in apoptotic single-stranded DNA damage induced by the killer lymphocyte protease granzyme A. D18N affects a highly conserved amino acid residue critical for catalytic activity. Recombinant mutant TREX1 homodimers are enzymatically inactive, while wild type/mutant heterodimers show residual exonucleolytic activity, suggesting a heterozygous loss of function. Lymphoblastoid cells carrying the D18N mutation are significantly less sensitive to granzyme A-mediated cell death, suggesting a novel role for this caspase-independent form of apoptosis in the pathogenesis of familial chilblain lupus. Our findings also warrant further investigation of TREX1 in common forms of lupus erythematosus.