Barbara Kind

Defective removal of ribonucleotides from DNA promotes systemic autoimmunity

Genome integrity is continuously challenged by the DNA damage that arises during normal cell metabolism. Biallelic mutations in the genes encoding the genome surveillance enzyme ribonuclease H2 (RNase H2) cause Aicardi-Goutières syndrome (AGS), a pediatric disorder that shares features with the autoimmune disease systemic lupus erythematosus (SLE). Here we determined that heterozygous parents of AGS patients exhibit an intermediate autoimmune phenotype and demonstrated a genetic association between rare RNASEH2 sequence variants and SLE. Evaluation of patient cells revealed that SLE- and AGS-associated mutations impair RNase H2 function and result in accumulation of ribonucleotides in genomic DNA. The ensuing chronic low level of DNA damage triggered a DNA damage response characterized by constitutive p53 phosphorylation and senescence. Patient fibroblasts exhibited constitutive upregulation of IFN-stimulated genes and an enhanced type I IFN response to the immunostimulatory nucleic acid polyinosinic:polycytidylic acid and UV light irradiation, linking RNase H2 deficiency to potentiation of innate immune signaling. Moreover, UV-induced cyclobutane pyrimidine dimer formation was markedly enhanced in ribonucleotide-containing DNA, providing a mechanism for photosensitivity in RNase H2-associated SLE. Collectively, our findings implicate RNase H2 in the pathogenesis of SLE and suggest a role of DNA damage-associated pathways in the initiation of autoimmunity.

Single-stranded nucleic acids promote SAMHD1 complex formation

SAM domain and HD domain-containing protein 1 (SAMHD1) is a dGTP-dependent triphosphohydrolase that degrades deoxyribonucleoside triphosphates (dNTPs) thereby limiting the intracellular dNTP pool. Mutations in SAMHD1 cause Aicardi–Goutières syndrome (AGS), an inflammatory encephalopathy that mimics congenital viral infection and that phenotypically overlaps with the autoimmune disease systemic lupus erythematosus. Both disorders are characterized by activation of the antiviral cytokine interferon-α initiated by immune recognition of self nucleic acids. Here we provide first direct evidence that SAMHD1 associates with endogenous nucleic acids in situ. Using fluorescence cross-correlation spectroscopy, we demonstrate that SAMHD1 specifically interacts with ssRNA and ssDNA and establish that nucleic acid-binding and formation of SAMHD1 complexes are mutually dependent. Interaction with nucleic acids and complex formation do not require the SAM domain, but are dependent on the HD domain and the C-terminal region of SAMHD1. We finally demonstrate that mutations associated with AGS exhibit both impaired nucleic acid-binding and complex formation implicating that interaction with nucleic acids is an integral aspect of SAMHD1 function.

The nuclear pore complex protein ALADIN is anchored via NDC1 but not via POM121 and GP210 in the nuclear envelope

The nuclear pore complex (NPC) consists of approximately 30 different proteins and provides the only sites for macromolecular transport between cytoplasm and nucleus. ALADIN was discovered as a new member of the NPC. Mutations in ALADIN are known to cause triple A syndrome, a rare autosomal recessive disorder characterized by adrenal insufficiency, alacrima, and achalasia. The function and exact location of the nucleoporin ALADIN within the NPC multiprotein complex is still unclear. Using a siRNA-based approach we downregulated the three known membrane integrated nucleoporins NDC1, GP210, and POM121 in stably expressing GFP-ALADIN HeLa cells. We identified NDC1 but not GP210 and POM121 as the main anchor of ALADIN within the NPC. Solely the depletion of NDC1 caused mislocalization of ALADIN. Vice versa, the depletion of ALADIN led also to disappearance of NDC1 at the NPC. However, the downregulation of two further membrane-integral nucleoporins GP210 and POM121 had no effect on ALADIN localization. Furthermore, we could show a direct association of NDC1 and ALADIN in NPCs by fluorescence resonance energy transfer (FRET) measurements. Based on our findings we conclude that ALADIN is anchored in the nuclear envelope via NDC1 and that this interaction gets lost, if ALADIN is mutated. The loss of integration of ALADIN in the NPC is a main pathogenetic aspect for the development of the triple A syndrome and suggests that the interaction between ALADIN and NDC1 may be involved in the pathogenesis of the disease.

Intracellular ROS level is increased in fibroblasts of triple A syndrome patients

Triple A syndrome is named after the main symptoms of alacrima, achalasia, and adrenal insufficiency but also presents with a variety of neurological impairments. To investigate the causes of progressive neurodegeneration, we examined the oxidative status of fibroblast cultures derived from triple A syndrome patients in comparison to control cells. Patient cells showed a 2.1-fold increased basal level of reactive oxygen species (ROS) and a massive boost after induction of artificial oxidative stress by paraquat. We examined the expression of the ROS-detoxifying enzymes superoxide dismutase 1 and 2 (SOD1, SOD2), catalase, and glutathione reductase. The basal expression of SOD1 was significantly (1.3-fold) increased, and the expression of catalase was 0.7-fold decreased in patient cells after induction of artificial oxidative stress. We show that the mitochondrial network is 1.8-fold more extensive in patient cells compared to control fibroblasts although the maximal ATP synthesis was unchanged. Despite having the same energy potential as the controls, the patient cells showed a 1.4-fold increase in doubling time. We conclude that fibroblasts of triple A patients have a higher basal ROS level and an increased response to artificially induced oxidative stress and undergo “stress-induced premature senescence”. The increased sensitivity to oxidative stress may be a major mechanism for the neurodegeneration in triple A syndrome.

Role of ALADIN in human adrenocortical cells for oxidative stress response and steroidogenesis.

Triple A syndrome is caused by mutations in AAAS encoding the protein ALADIN. We investigated the role of ALADIN in the human adrenocortical cell line NCI-H295R1 by either over-expression or down-regulation of ALADIN. Our findings indicate that AAAS knock-down induces a down-regulation of genes coding for type II microsomal cytochrome P450 hydroxylases CYP17A1 and CYP21A2 and their electron donor enzyme cytochrome P450 oxidoreductase, thereby decreasing biosynthesis of precursor metabolites required for glucocorticoid and androgen production. Furthermore we demonstrate that ALADIN deficiency leads to increased susceptibility to oxidative stress and alteration in redox homeostasis after paraquat treatment. Finally, we show significantly impaired nuclear import of DNA ligase 1, aprataxin and ferritin heavy chain 1 in ALADIN knock-down cells. We conclude that down-regulating ALADIN results in decreased oxidative stress response leading to alteration in steroidogenesis, highlighting our knock-down cell model as an important in-vitro tool for studying the adrenal phenotype in triple A syndrome.

Altered spatio-temporal dynamics of RNase H2 complex assembly at replication and repair sites in Aicardi–Goutières syndrome

Ribonuclease H2 plays an essential role for genome stability as it removes ribonucleotides misincorporated into genomic DNA by replicative polymerases and resolves RNA/DNA hybrids. Biallelic mutations in the genes encoding the three RNase H2 subunits cause Aicardi-Goutières syndrome (AGS), an early-onset inflammatory encephalopathy that phenotypically overlaps with the autoimmune disorder systemic lupus erythematosus. Here we studied the intracellular dynamics of RNase H2 in living cells during DNA replication and in response to DNA damage using confocal time-lapse imaging and fluorescence cross-correlation spectroscopy. We demonstrate that the RNase H2 complex is assembled in the cytosol and imported into the nucleus in an RNase H2B-dependent manner. RNase H2 is not only recruited to DNA replication foci, but also to sites of PCNA-dependent DNA repair. By fluorescence recovery after photobleaching, we demonstrate a high mobility and fast exchange of RNase H2 at sites of DNA repair and replication. We provide evidence that recruitment of RNase H2 is not only PCNA-dependent, mediated by an interaction of the B subunit with PCNA, but also PCNA-independent mediated via the catalytic domain of the A subunit. We found that AGS-associated mutations alter complex formation, recruitment efficiency and exchange kinetics at sites of DNA replication and repair suggesting that impaired ribonucleotide removal contributes to AGS pathogenesis.

Tert expression and telomerase activity in gonads and somatic cells of the Japanese medaka (Oryzias latipes)

A telomerase reverse transcriptase (Tert) encoding gene was cloned from the testis of the teleost fish Oryzias latipes. The expression pattern of Japanese medaka tert (Ola_tert) was analyzed by reverse transcription–polymerase chain reaction and in situ hybridization. Ola_tert was expressed in embryonic stages as well as in differentiated adult tissues. In tissues of adult medakas the highest tert expression was found in gonads and brain. Furthermore, two different splice variants were described and an Ola_tert antisense transcript was identified. The enzyme activity of Tert was determined using a non‐radioactive telomeric amplification protocol and the telomerase activity in various tissues was shown to correlate with the tert expression. The telomerase activity was found to be high in contrast to the generally low activity in differentiated human tissues.

Axonal neuropathy with unusual pattern of amyotrophy and alacrima associated with a novel AAAS mutation p.Leu430Phe

The triple A syndrome is caused by autosomal recessively inherited mutations in the AAAS gene and is characterized by achalasia, alacrima and adrenal insufficiency as well as progressive neurological impairment. We report on a 14-year-old girl with slowly progressive axonal motor neuropathy with conspicuous muscle wasting of hypothenars and calves as well as alacrima. The mutation analysis of the AAAS gene revealed a compound heterozygous mutation: a c.251G>A mutation in exon 2 that had been reported previously, and a novel c.1288C>T mutation in exon 14. At the transcriptional level, the c.251G>A transition results in an aberrant splicing and decay of this RNA strand so that the particular clinical picture results from the novel c.1288C>T, (p.Leu430Phe, L430F) mutation in a hemizygous form. With transfection experiments, we demonstrate that GFP–ALADINL430F correctly localizes to nuclear pore complexes. Therefore, we conclude that this point mutation impairs ALADIN function at the nuclear pore.

Changes in differential gene expression in fibroblast cells from patients with triple A syndrome under oxidative stress.

The triple A syndrome is a rare autosomal recessive disease caused by mutations in the AAAS gene, which encodes the nucleoporin ALADIN. Recently it was shown that ALADIN plays a role in the import of different factors into the nucleus, which prevent the cell from DNA damage and consecutive cell death under oxidative stress. In order to investigate the changes in differential gene expression in ALADIN-deficient or mutated cells under oxidative stress we used fibroblast cell cultures of triple A syndrome patients and compared these to controls. Analysis of 84 genes, which are associated with oxidative stress and antioxidant defense, showed that 7 genes were significantly and differentially regulated, namely BCL2/adenovirus E1B 19kD-interacting protein 3 (BNIP3), 24-dehydrocholesterol reduc-tase (DHCR24), dual specificity phosphatase 1 (DUSP1), forkhead box M1 (FOXM1), nudix-type motif 1 (NUDT1), prostaglandin-endoperoxide synthase 2 (PTGS2), and scavenger receptor class A, member 3 (SCARA3). Whereas in control cells the expression of DHCR24, FOXM1, NUDT1, and SCARA3 was decreased after paraquat treatment, the expression did not change significantly in patient cells. However, the basal expression of SCARA3 and BNIP3 was significantly higher in patient cells than in controls whereas PTGS2 was less expressed. Furthermore, after paraquat treatment the expression of BNIP3, DUSP1, and PTGS2 was significantly increased in control cells while in patient cells the increase of DUSP1 and PTGS2 expression was significantly reduced. With this work we confirm that cells of triple A patients show an altered induction or downregulation of genes associated with oxidative stress and antioxidant defense.

Therapeutic Approaches to Type I Interferonopathies

Purpose of ReviewTo review recent scientific advances and therapeutic approaches in the expanding field of type I interferonopathies.SummaryType I interferonopathies represent a genetically and phenotypically heterogenous group of disorders of the innate immune system caused by constitutive activation of antiviral type I interferon (IFN). Clinically, type I interferonopathies are characterized by autoinflammation and varying degrees of autoimmunity or immunodeficiency. The elucidation of the underlying genetic causes has revealed novel cell-intrinsic mechanisms that protect the organism against inappropriate immune recognition of self nucleic acids by cytosolic nucleic acid sensors. The type I IFN system is subject to a tight and complex regulation. Disturbances of its checks and balances can spark an unwanted immune response causing uncontrolled type I IFN signaling. Novel mechanistic insight into pathways that control the type I IFN system is providing opportunities for targeted therapeutic approaches by repurposing drugs such as Janus kinase inhibitors or reverse transcriptase inhibitors.