UHRF1 suppresses viral mimicry through both DNA methylation-dependent and -independent mechanisms
Some chemotherapeutic agents which cause loss of DNA methylation have been recently shown to induce a state of "viral mimicry" involving upregulation of endogenous retroviruses (ERV) and a subsequent innate immune response. This approach may be useful in combination with immune checkpoint cancer therapies, but relatively little is known about normal cellular control of ERV suppression. The UHRF1 protein can interact with the maintenance methylation protein DNMT1 and is known to play an important role in epigenetic control in the cell. To examine potential roles of this protein in differentiated cells, we first established stable knockdowns in normal human lung fibroblasts. While these knockdown cells showed the expected loss of DNA methylation genome-wide, transcriptional changes were instead dominated by a single response, namely activation of innate immune signalling, consistent with viral mimicry. We confirmed using mechanistic approaches that activation of interferons and interferon-stimulated genes involved in double-stranded RNA detection was crucial to the response. ERVs were demethylated and transcriptionally activated in UHRF1 knockdown cells. As in these normal cell lines, ERV activation and interferon response also occurred following the transient loss of UHRF1 in both melanoma and colon cancer cell lines. Restoring UHRF1 in either transient- or stable knockdown systems abrogated ERV reactivation and interferon response, but without substantial restoration of DNA methylation. Rescued cell lines were hypersensitive to depletion of SETDB1, implicating H3K9me3 as crucial to UHRF1-mediated repression in the absence of DNA methylation. Confirming this, cells rescued with UHRF1 containing point mutations affecting H3K9me3 binding could not mediate silencing of ERV transcription or the innate immune response. Finally, by introducing similar point mutations in the mouse homologue, we could show that this pathway is conserved in mice. Our results therefore implicate UHRF1 as a key regulator of ERV suppression and strengthen the basis for cancer cell hypomethylation therapy.