Retroviral DNA Methylation and Epigenetic Repression Are Mediated by the Antiviral Host Protein Daxx

Abstract

Integrated retroviral DNA is subject to epigenetic transcriptional silencing at different frequencies. This process is mediated by repressive DNA methylation and histone modifications on viral chromatin. However, the detailed mechanisms by which retroviral silencing is initiated and maintained are not well understood. Using a model system in which avian sarcoma virus (ASV) DNA is epigenetically repressed in mammalian cells, we previously found that a cellular scaffolding protein, Daxx, acts as an antiretroviral factor that promotes epigenetic repression through recruitment of histone deacetylases (HDACs). Here we show that human Daxx protein levels are increased in response to retroviral infection and that Daxx acts at the time of infection to initiate epigenetic repression. Consistent with a rapid and active antiviral epigenetic response, we found that repressive histone marks and long terminal repeat (LTR) DNA methylation could be detected within 12 h to 3 days postinfection, respectively. Daxx was also found to be required for long-term ASV silencing maintenance and full viral DNA methylation, and it was physically associated with both viral DNA and DNA methyltransferases (DNMTs). These findings support a model in which incoming retroviral protein-DNA complexes are detected by Daxx, and the integrated provirus is rapidly chromatinized and repressed by DNA methylation and histone modification as part of an antiviral response. These results uncover a possible direct and active antiviral mechanism by which DNMTs can be recruited to retroviral DNA. Retroviruses are important agents of disease and serve as valuable vectors for gene delivery, and their study has provided seminal insights into cellular functions. A defining feature of retroviral replication is the integration of a DNA copy of the retroviral RNA genome into host chromatin, a process that establishes the DNA provirus. Integration provides a permanent association of viral DNA with the host cell and all of its progeny, and it also allows the provirus to efficiently mobilize the cellular transcriptional machinery for synthesis of viral mRNAs and viral RNA genomes.DNA integration is an essential step in retroviral replication, and it is catalyzed by the virus-encoded integrase (IN) protein. However, establishment of the provirus does not guarantee its expression; transcriptional repression by epigenetic mechanisms (epigenetic silencing) is often observed in both natural and interspecies retroviral infections. Examples include the silencing of retroviruses in embryonic stem cells (1, 2), the progressive silencing of expression of genes transduced by retroviral vectors during long-term cell propagation (3), and HIV latency (4). Epigenetic mechanisms also repress the expression of endogenous retroviruses (5-9).Retroviral epigenetic silencing is mediated by the enzymatic placement, and subsequent reading, of DNA methylation marks (addition of a methyl group to position 5 of the cytosine pyrimidine [5MeCpG]) and repressive nucleosomal histone modifications. Th...

Virion Factors That Target Daxx To Overcome Intrinsic Immunity

“…2A) (87). In addition, Daxx also recruits Dmnt1 to the viral DNA, resulting in long terminal repeat (LTR) methylation and transcriptional repression (88). These findings suggest that Daxx/HDAC/Dmnt1 complexes play a role in initiating epigenetic silencing, possibly as part of an antiviral response against retroviruses (87,89).…”

The DAXX co-repressor is directly recruited to active regulatory elements genome-wide to regulate autophagy programs in a model of human prostate cancer

“…Many studies have shown that cell death-modulating protein DAXX functions as a transcriptional repressor [1-4]. DAXX associates with RelB, a transcription factor of the NF-κB family that directly controls the expression of numerous apoptosis- and autophagy-relevant genes, including tumor suppressor protein kinases DAPK1/3 [1;2].…”

“…In mouse embryo fibroblasts (MEFs) [2], as well as in the PCa cell lines [unpublished data], DAXX represses expression of these genes via mechanisms that include its binding to DNA methyl transferases (DNMTases) and hypermethylation of the promoter regions of the corresponding genes [2]. DAXX also recruits DNMTases to viral DNA so as to initiate its epigenetic repression [4]. A number of publications have reproducibly shown that DAXX interacts directly with DNMTs and employs DNA methylation as a major mechanism of repression [2-4].…”

“…DAXX also recruits DNMTases to viral DNA so as to initiate its epigenetic repression [4]. A number of publications have reproducibly shown that DAXX interacts directly with DNMTs and employs DNA methylation as a major mechanism of repression [2-4]. DAXX is also known to repress through additional mechanisms, including recruitment of histone deacetylases (HDACs) to target promoters [2], and histone H3.3 association/regulation [9].…”

Emerging Role of PML Nuclear Bodies in Innate Immune Signaling

“…Nevertheless, both studies indicate an unconventional mobility of PML-NBs, which appear to sense viral nucleoprotein complexes or replication intermediates, resulting in movement of the respective proteins even to the cytoplasm. Additionally, PML and hDaxx were implicated as contributing to the control of retroviral latency via epigenetic repression (12,13).…”