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...