SAMHD1 prevents autoimmunity by maintaining genome stability.
OBJECTIVE
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.
Department of Pediatrics, Medizinische Fakultät Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany.
Stefanie
S
Kretschmer
Department of Pediatrics, Medizinische Fakultät Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany.
Christine
C
Wolf
Department of Pediatrics, Medizinische Fakultät Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany.
Nadja
N
König
Biotechnology Center, Technische Universität Dresden, Dresden, Germany.
Wolfgang
W
Staroske
Biotechnology Center, Technische Universität Dresden, Dresden, Germany Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge, UK.
Jochen
J
Guck
Department of Pediatrics, University Hospital, University of Aachen, Aachen, Germany.
Martin
M
Häusler
Department of Pediatrics, Medizinische Fakultät Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany.
Hella
H
Luksch
Department of Pediatrics, Center for Drug Discovery, Emory University, Atlanta, Georgia, USA.
Laura A
LA
Nguyen
Department of Pediatrics, Center for Drug Discovery, Emory University, Atlanta, Georgia, USA College of Pharmacy, Kyung-Hee University, Seoul, South Korea.
Baek
B
Kim
Biotechnology Center, Technische Universität Dresden, Dresden, Germany Center for Regenerative Therapies, Technische Universität Dresden, Dresden, Germany.
Dimitra
D
Alexopoulou
Biotechnology Center, Technische Universität Dresden, Dresden, Germany Center for Regenerative Therapies, Technische Universität Dresden, Dresden, Germany.
Andreas
A
Dahl
Department of Biology, Technische Universität Darmstadt, Germany.
Alexander
A
Rapp
Department of Biology, Technische Universität Darmstadt, Germany.
M Cristina
MC
Cardoso
Max Planck Institute of Molecular Cell Biology and Genetics, Dresden, Germany.
Anna
A
Shevchenko
Department of Pediatrics, Medizinische Fakultät Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany.
Min Ae
MA
Lee-Kirsch
D020274:Autoimmune Diseases of the Nervous System / Q000235:genetics* / Q000378:metabolism; D015551:Autoimmunity / Q000235:genetics*; D016203:CDC2 Protein Kinase; D002478:Cells, Cultured; D019925:Cyclin A / Q000378:metabolism*; D018844:Cyclin-Dependent Kinases / Q000378:metabolism; D004249:DNA Damage / Q000235:genetics / Q000276:immunology; D005347:Fibroblasts / Q000378:metabolism*; D020869:Gene Expression Profiling; D042822:Genomic Instability / Q000235:genetics*; D057809:HEK293 Cells; D006367:HeLa Cells; D006801:Humans; D007370:Interferon Type I / Q000276:immunology; D020559:Monomeric GTP-Binding Proteins / Q000235:genetics* / Q000378:metabolism; D009421:Nervous System Malformations / Q000235:genetics* / Q000378:metabolism; D010766:Phosphorylation; D034622:RNA Interference; D012333:RNA, Messenger / Q000235:genetics*; D000076106:SAM Domain and HD Domain-Containing Protein 1; D015398:Signal Transduction
D019925:Cyclin A; D007370:Interferon Type I; D012333:RNA, Messenger; D016203:CDC2 Protein Kinase; C528899:CDK1 protein, human; D018844:Cyclin-Dependent Kinases; D000076106:SAM Domain and HD Domain-Containing Protein 1; C484280:SAMHD1 protein, human; D020559:Monomeric GTP-Binding Proteins
Autoimmune Diseases; Autoimmunity; Fibroblasts; Systemic Lupus Erythematosus