The tumor suppressor p16 INK4A inhibits formation of enzymatically active complexes of cyclin-dependent kinases 4 and 6 (CDK4/6) with D-type cyclins. Oncogenic stress induces p16 INK4A expression, which in turn triggers cellular senescence through activation of the retinoblastoma tumor suppressor. Subversion of oncogene-induced senescence is a key step during cancer development, and many tumors have lost p16 INK4A activity by mutation or epigenetic silencing. Human papillomavirus (HPV)-associated tumors express high levels of p16 INK4A in response to E7 oncoprotein expression. Induction of p16 INK4A expression is not a consequence of retinoblastoma tumor suppressor inactivation but is triggered by a cellular senescence response and is mediated by epigenetic derepression through the H3K27-specific demethylase (KDM)6B. HPV E7 expression causes an acute dependence on KDM6B expression for cell survival. The p16 INK4A tumor suppressor is a critical KDM6B downstream transcriptional target and its expression is critical for cell survival. This oncogenic p16 INK4A activity depends on inhibition of CDK4/CDK6, suggesting that in cervical cancer cells where retinoblastoma tumor suppressor is inactivated, CDK4/CDK6 activity needs to be inhibited in order for cells to survive. Finally, we note that HPV E7 expression creates a unique cellular vulnerability to small-molecule KDM6A/B inhibitors.synthetic lethality | apoptosis | biomarker | cancer therapy P osttranslational histone modifications, including phosphorylation, ubiquitination, acetylation, and methylation, impact both the physical state and the transcriptional competence of chromatin. These modifications are dynamic and regulate a variety of cellular processes, such as stem cell maintenance, cell fate determination and maintenance, cell cycle control, and epigenetic heritability of transcriptional programs (reviewed in refs. 1 and 2). Methylation of lysine residues on histones is modulated by histone lysine methyltransferases (KMTs) and histone lysine demethylases (KDMs). Although histone lysine methylation is involved in both transcriptional activation and repression, histone H3 trimethylation of lysine 27 (H3K27me3) on gene promoters is crucial for epigenetic silencing mediated by polycomb group proteins (3-5). The JmjC-domain containing histone demethylases, KDM6A (UTX) and KDM6B (JMJD3) remove this repressive mark, allowing for transcriptional activation (6-10). Although KDM6A and KDM6B appear identical with regards to catalytic activities and histone substrate specificities, they have a number of nonoverlapping and nonredundant biological activities. KDM6B-but not KDM6A-regulates RAS/RAF-mediated oncogene-induced senescence (OIS) (11, 12). OIS was originally described as a cell-abortive response to ectopic RAS oncogene expression in normal human cells. It is now recognized that OIS represents one of several cell-intrinsic tumor-suppressor responses that function to eliminate aberrantly proliferating, potentially premalignant cells. Evidence for OIS has been dete...

Numerous studies have focused on the regulatory functions of ICP27, an immediate-early (IE) protein of herpes simplex virus 1 (HSV-1). However, its homolog in HSV-2, termed ICP27t2, has been little studied. Here, we used two different approaches to functionally compare ICP27t2 and ICP27. In transfection-based assays, ICP27t2 closely resembled ICP27 in its capacity to enhance HSV-1 late gene expression, suppress the splicing of a viral intron, and complement the growth of an HSV-1 ICP27 null mutant. To study ICP27t2 in the context of viral infection, we engineered K2F1, an HSV-1 mutant that encodes ICP27t2 in place of ICP27. In Vero cells, K2F1 replicated with wild-type (WT) kinetics and yields, expressed delayed-early and late proteins normally, and was fully capable of activating several cellular signal transduction pathways that are ICP27 dependent. Thus, we conclude that ICP27t2 and ICP27 are functionally very similar and that ICP27t2 can mediate all ICP27 activities that are required for HSV-1 replication in cell culture. Surprisingly, however, we found that K2F1 forms plaques that are morphologically different from those of WT HSV-1. Investigation of this trait demonstrated that it results from the decreased release of progeny virions into the culture medium. This appears to be due to a reduction in the detachment of K2F1 progeny from the extracellular surface of the infected cell. We identified two HSV-1 ICP27 amino-terminal deletion mutants with a similar release defect. Together, these results demonstrate that ICP27 plays a heretofore-unappreciated role in modulating the efficiency of progeny virion release. IMPORTANCEICP27 is an essential, multifunctional regulatory protein that has a number of critical roles in the HSV-1 life cycle. Although ICP27 homologs are encoded by all known members of the Herpesviridae, previous work with several of these homologs has shown that they cannot substitute for ICP27 in the context of HSV-1-infected cells. Here, we identify ICP27t2 as the first homolog that can efficiently replace ICP27 in HSV-1 infection. Unexpectedly, our results also reveal that the sequence of the ICP27 gene can affect the release of HSV-1 progeny virions from the infected cell. Thus, our comparative study has revealed a novel function for ICP27 in the regulation of virus release. Herpes simplex virus 1 (HSV-1), a human alphaherpesvirus, is widespread in the human population, with greater than 57% of U.S. adults seropositive (1). The interaction of HSV-1 with the human host has been well studied (reviewed in reference 2). The virus is transmitted to new hosts via body secretions such as saliva, with the primary site of infection often being the oral mucosa. Following replication in epithelial cells, HSV-1 infects sensory neurons, where it establishes a lifelong latent infection in which the viral genome persists in the trigeminal ganglion as a semiquiescent episome. Periodically, the virus reactivates and returns to the oral cavity, where it can cause lesions (cold sores) and be transmitte...