SAMHD1-dependent retroviral control and escape in mice

Abstract

SAMHD1 is a host restriction factor for human immunodeficiency virus 1 (HIV‐1) in cultured human cells. SAMHD1 mutations cause autoimmune Aicardi‐Goutières syndrome and are found in cancers including chronic lymphocytic leukaemia. SAMHD1 is a triphosphohydrolase that depletes the cellular pool of deoxynucleoside triphosphates, thereby preventing reverse transcription of retroviral genomes. However, in vivo evidence for SAMHD1's antiviral activity has been lacking. We generated Samhd1 null mice that do not develop autoimmune disease despite displaying a type I interferon signature in spleen, macrophages and fibroblasts. Samhd1−/− cells have elevated deoxynucleoside triphosphate (dNTP) levels but, surprisingly, SAMHD1 deficiency did not lead to increased infection with VSV‐G‐pseudotyped HIV‐1 vectors. The lack of restriction is likely attributable to the fact that dNTP concentrations in SAMHD1‐sufficient mouse cells are higher than the KM of HIV‐1 reverse transcriptase (RT). Consistent with this notion, an HIV‐1 vector mutant bearing an RT with lower affinity for dNTPs was sensitive to SAMHD1‐dependent restriction in cultured cells and in mice. This shows that SAMHD1 can restrict lentiviruses in vivo and that nucleotide starvation is an evolutionarily conserved antiviral mechanism.

Heterozygous colon cancer-associated mutations of SAMHD1 have functional significance

“…We base our conclusion on the following observations: (i) These cancers have mutations in SAMHD1, an enzyme responsible for the degradation of dNTPs; (ii) the identified SAMHD1 mutations negatively affect its dNTPase activity in vitro; and (iii) actively dividing hemizygous SAMHD1 +/− mouse embryos have increased dNTP pools compared with congenic WT embryos of the same age. Previous dNTP pool measurements were performed in various types of WT and SAMHD1 −/− homozygous cells, including E14.5 mouse embryonic fibroblasts (56,57), but, to our knowledge, ours is the first study to measure dNTP pools in hemizygous SAMHD1 mouse embryos. This observation is important because it demonstrates that cancer cells do not need to inactivate both SAMHD1 alleles to increase the dNTP pools.…”

Ribonuclease H2 mutations induce a cGAS/STING‐dependent innate immune response

“…In particular, there was no evidence of intracranial calcification, leukodystrophy, chilblain vasculitis or cardiomyopathy, clinical features associated with AGS in humans (Crow et al , 2015). Notably, histopathological signs of inflammation are also not evident in Samhd1 −/− mice, although activation of innate immune signalling does occur, with ISG upregulation evident (Behrendt et al , 2013; Rehwinkel et al , 2013). We therefore investigated whether this was also the case in Rnaseh2b A174T/A174T mice.…”

“…It will therefore be important to confirm that the innate immune pathways implicated in Trex1, Adar1 and now RNase H2 deficiency, through the use of mouse models, are also relevant to the human autoinflammatory phenotype in AGS patients in whom these genes are affected. Samhd1 −/− mice, like Rnaseh2b A174T/A174T mice, display an ISG response in the absence of detectable pathology (Behrendt et al , 2013; Rehwinkel et al , 2013). In contrast a strong ISG response in Adar1 null or editing‐deficient mice is associated with embryonic lethality (Mannion et al , 2014; Liddicoat et al , 2015; Pestal et al , 2015), and with autoinflammatory cardiomyopathy and multi‐tissue involvement in Trex1 −/− mice (Morita et al , 2004; Stetson et al , 2008; Gall et al , 2012).…”

Deoxynucleoside triphosphate (dNTP) synthesis and destruction regulate the replication of both cell and virus genomes

“…Some lentiviruses, such as HIV2 and the Simian Immunodeficiency Virus, carry in a protein called Vpx that causes the degradation of SAMHD1, thereby allowing an increase in dNTPs and copying of the RNA genome into DNA (6,8,15). The K m for different reverse transcriptases vary and contribute to the host cell-specificity for virus replication, a process influenced by the presence or absence of SAMHD1 (16). The phenotype of AGS is consistent with SAMHD1 mutations causing higher dNTP levels that, in turn, could lead to more robust virus infection for viruses that have a DNA polymerase with a K m that requires increased dNTP levels.…”