Epigenetic Enzyme Inhibitors Are Broad-Spectrum Antivirals

Ricki Lewis, PhD

September 26, 2017

Inhibitors of a key epigenetic enzyme blocks infection by herpes simplex virus 1 (HSV1) with a coordinated innate immune response also seen in other viruses, according to findings published August 15 in mBio.

Researchers at the National Institute of Allergy and Infectious Diseases discovered that small molecule inhibitors of the histone H3K27 methyltransferases EZH2 and EZH1 (EZH2/1), which were expected to activate transcription of HSV1 genes, instead repress it. The effect is also seen for the DNA viruses human cytomegalovirus and adenovirus 5 and the RNA virus Zika.

"The discovery was counterintuitive, but it fits into the complex and exploding field of epigenetic regulation of host cells and viruses," coauthor Thomas M. Kristie, PhD, chief of the Molecular Genetics Section of the Laboratory of Viral Diseases at National Institute of Allergy and Infectious Diseases, Bethesda, Maryland, told Medscape Medical News.

Genes are accessed for transcription as repressive methyl groups are removed and activating methyl groups are added to specific sites on the histone proteins around which DNA winds, forming nucleosome subunits. Methyltransferases add the methyl groups that can selectively either quell or enhance expression of an organism's genes. The enzymes also regulate transcription of viral DNA in a cell.

Because histone H3K27 is known as a site where EZH2/1 binds and represses viral transcription, an inhibitor of EZH2/1 should activate transcription of viral genes. Coauthor Jesse H. Arbuckle, PhD, research associate, National Institute of Allergy and Infectious Diseases, applied the inhibitors, and was stunned to see just the opposite effect in cell culture and in mice: a dampening of HSV1 infection. "We looked at gene expression profiles induced by the inhibitors, and that made it clear what was going on. From there, the experiments just took off," Dr Arbuckle told Medscape Medical News.

For several years, the investigators had been trying to identify and understand the epigenetic components that regulate herpes viruses through the lytic, latent, and reactivated stages, focusing on demethylases and methyltransferases. "We had thought that if we inhibited this enzyme, EZH2/1, we'd induce viral immediate early genes and viral lytic genes and be able to induce viruses from latency in ganglia. That was Jesse's initial experiment," said Dr Kristie.

The researchers tested three inhibitors — GSK126, GSK343, and UNC1999 — in three sets of experiments that revealed an innate immune response that seems to override the expected activation of viral genes resulting from the lifting of the repressive methylation. The experiments were on human fetal fibroblasts, in mice, and in explants of mouse sensory ganglia latently infected with HSV1.

Treating infected fibroblasts with the inhibitors decreased the number of viral genomes and transcription of viral genes. The eyes of infected mice revealed a rapid neutrophil response and falling viral titers and DNA loads in response to the inhibitors, effects not seen in animals given a control intervention. Finally, the inhibitors suppressed spread from initial viral reactivation in ganglia explants.

All three inhibitors were active against HSV1 infection. GSK126 and NC1999 also had anti-cytomegalovirus and anti-AV effects, and collaboration with other National Institute of Allergy and Infectious Diseases laboratories showed that GSK126 had anti-Zika effects.

The researchers zeroed in on the immune response. "These compounds were inducing a broad spectrum of interferons and interferon-stimulated genes and antiviral pathways in these cells," Dr Kristie said, not a general inflammatory response. "A number of studies suggested EZH2/1 regulates interferons and cytokines. Our study is striking in that it's been assumed this protein complex is an important repressor with respect to the establishment and maintenance of viral latency. It's surprising that induction of these multiple intersecting antiviral pathways is dominant over suppression of the virus by the EZH2/1 complex," he added.

The response includes interferons alpha, alpha 1, and alpha 2; interleukins 8 and 6 and the interleukin 6 receptor; and transcription factors, regulatory proteins involved in immune signaling pathways, and stress response factors that function in the endoplasmic reticulum. "It seems this enzyme complex plays an important role in regulating the cellular antiviral response at a number of different levels, providing a broad effect. These compounds amplify the natural antiviral response of a cell," Dr Kristie said.

Some EZH2/1 inhibitors are already in clinical trials, as "epipharmaceuticals," to treat lymphoma and multiple myeloma with mutations in EZH2/1. "A new class of antivirals based on this study might be useful for patients who are resistant to existing antivirals like acyclovir and ganciclovir, which is important in herpes keratitis, or other viral infections for which there aren't pharmaceuticals to boost an individual's immune response," Dr Kristie said.

"The importance of the study is that they clearly demonstrated, both in vivo and in vitro, that the benefit of using epigenetic drugs against viral infection is not only that they can directly affect the expression of viral genes, but can also induce immunomodulatory genes that can block viral infection. Their results support the notion that epigenetic drugs could be successfully employed in treating diverse viral infections," said Zsolt Toth, PhD, an assistant professor in the Department of Oral Biology at the University of Florida College of Dentistry, Gainesville, told Medscape Medical News.

But more work is needed to move this "exciting finding" toward the clinic, Christine Johnston, MD, MPH, an associate professor from the Division of Allergy and Infectious Diseases at the University of Washington, Seattle, told Medscape Medical News. "The currently available antivirals are very safe, and suppressive therapy needs to be taken on a daily basis. Therefore, new anti-HSV drugs will need to clear a high bar for safety, which may be more difficult when targeting a cellular rather than a viral protein. But the concept that cellular proteins can be targeted to increase the ability of the cell to broadly resist viral replication may eventually lead to new ways to fight viral infections."

Dr Kristie and Dr Arbuckle have filed patents related to this work. Dr Toth and Dr Johnston have disclosed no relevant financial relationships.

mBio. Published online August 15, 2017. Abstract

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