SAMHD1 prevents autoimmunity by maintaining genome stability

Abstract

ObjectivesThe 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.MethodsPrimary 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.ResultsWe 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.ConclusionsSAMHD1 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.

Phenotypic Variation in Aicardi–Goutières Syndrome Explained by Cell-Specific IFN-Stimulated Gene Response and Cytokine Release

“…SAMHD1-null mutant fibroblasts have been shown to exhibit a cell cycle arrest (41). Another investigation also revealed a delayed cell cycle progression after transient knockdown of SAMHD1 in HeLa cells using small interfering RNA (42). Alternatively, transient knockdown of ADAR1 in HeLa cells did not alter the cell growth (43).…”

HIV-1–Mediated BAFF Secretion in Macrophages Does Not Require Endosomal TLRs, Type-I IFN, and Nef, but Depends on the Cellular Phenotype Status

“…n.s., not significant. stability (86). SAMHD1 is required for genome integrity by maintaining balanced dNTP pools.…”

Structural basis of cellular dNTP regulation by SAMHD1

“…SAMHD1 was also found to inhibit infection by certain DNA viruses including herpes simplex virus type 1 (HSV-1) and vaccinia virus in nondividing macrophages (11,12). Apart from viral restriction, SAMHD1 is ubiquitously expressed in both differentiated and undifferentiated cells of various human organs (2,13), where it functions in the regulation of DNA damage signaling and proper activation of the innate immune response (14,15). Mutations in SAMHD1, many of which result in deficiency in the dNTPase activity, are associated with chronic lymphocytic leukemia (CLL) (15,16) and the autoimmune condition AicardiGoutieres syndrome (AGS) (17,18).…”