Human Papillomavirus E7 Oncoprotein Subverts Host Innate Immunity via SUV39H1-Mediated Epigenetic Silencing of Immune Sensor Genes

Abstract

Subversion of innate immunity by oncoviruses, such as human papillomavirus (HPV), favors carcinogenesis because the mechanism(s) of viral immune evasion can also hamper cancer immunosurveillance. Previously, we demonstrated that high-risk (hr) HPVs trigger simultaneous epigenetic silencing of multiple effectors of innate immunity to promote viral persistence. Here, we expand on those observations and show that the HPV E7 oncoprotein upregulates the H3K9-specific methyltransferase, whose action shuts down the host innate immune response. Specifically, we demonstrate that SUV39H1 contributes to chromatin repression at the promoter regions of the viral nucleic acid sensors RIG-I and cGAS and the adaptor molecule STING in HPV-transformed cells. Inhibition of SUV39H1 leads to transcriptional activation of these genes, especially RIG-I, followed by increased beta interferon (IFN-β) and IFN-λ1 production after poly(dA·dT) or RIG-I agonist M8 transfection. Collectively, our findings provide new evidence that the E7 oncoprotein plays a central role in dampening host innate immunity and raise the possibility that targeting the downstream effector SUV39H1 or the RIG-I pathway is a viable strategy to treat viral and neoplastic disease. IMPORTANCE High-risk HPVs are major viral human carcinogens responsible for approximately 5% of all human cancers. The growth of HPV-transformed cells depends on the ability of viral oncoproteins to manipulate a variety of cellular circuits, including those involved in innate immunity. Here, we show that one of these strategies relies on E7-mediated transcriptional activation of the chromatin repressor SUV39H1, which then promotes epigenetic silencing of RIG-I, cGAS, and STING genes, thereby shutting down interferon secretion in HPV-transformed cells. Pharmacological or genetic inhibition of SUV39H1 restored the innate response in HPV-transformed cells, mostly through activation of RIG-I signaling. We also show that IFN production upon transfection of poly(dA·dT) or the RIG-I agonist M8 predominantly occurs through RIG-I signaling. Altogether, the reversible nature of the modifications associated with E7-mediated SUV39H1 upregulation provides a rationale for the design of novel anticancer and antiviral therapies targeting these molecules.

Vesicular trafficking permits evasion of cGAS/STING surveillance during initial human papillomavirus infection

“…The HPV early proteins have been shown to block cGAS/STING and downstream IRF3-dependent IFN responses, as well as NFκB-dependent cytokine responses through a variety of mechanisms [70,71,[79][80][81]97,98]. The early viral proteins that counteract the cellular antiviral responses are not packaged within incoming virions.…”

“…Indeed, IFN responses are detrimental to persistent HPV infections, reducing cellular proliferation and causing apoptosis, episome loss, mutation, and/or integration [73][74][75][76][77]. Likewise, it is well known that some HPV early genes-E5, E6, and E7 -counteract these detrimental antiviral IFN and IFN-stimulated gene (ISG) responses through a variety of mechanisms [78][79][80][81].…”

Abrogation of Constitutive and Induced Type I and Type III Interferons and Interferon-Stimulated Genes in Keratinocytes by Canine Papillomavirus 2 E6 and E7

“…Both high-risk mucosal HPV16 and HPV18 antagonize the adaptor protein within the dsDNA signaling pathway Stimulator of IFN genes (STING) to repress signaling through the cytosolic DNA receptor pathway, although they do this through different mechanisms [20,38]. Further, HPV18 E7 can epigenetically silence RIGI, the DNA sensor cyclic GMP-AMP synthase (cGAS), and STING preventing upregulation of IFN and IFN-stimulated genes activated by dsRNA and dsDNA [39]. We found in this current study that the main impact of CPV2 E7 was on the dsDNA-induced expression of IFN and IFN-stimulated genes.…”

The Interplay between Antiviral Signalling and Carcinogenesis in Human Papillomavirus Infections

“…binds to the NF-κB kinase (IKK) complex, impairing NF-κB signalling [75] HPV16 impairs DNA binding activity of NF-κB, through impairment of p65 subunit functions [82,85] HPV16 binds to p48, inhibiting the IFN-stimulated gene factor 3 (ISGF3)-mediated gene expression [86] HPV16 binds to the DNA methyltransferase (DNMT1), impairing antiviral gene transcription through epigenetic modification [87] HPV18 binds to STING, impairing IFNs and pro-inflammatory cytokines expression [88] HPV18 induces of the H3K9 methyltransferase (SUV39H1) transcription, which promotes epigenetic silencing of PRRs [89] HPV16 HPV38…”

“…In the last few years, new data on HPV epigenetic control has been emerging. Lo Cigno et al showed that HPV E7 upregulates the H3K9 methyltransferase SUV39H1, which, through alterations in the chromatin structure, promotes epigenetic inhibition of nucleic acid sensors, such as RIG-I, cyclic GMP-AMP synthase (cGAS) or even STING [89].…”