Genome-wide lentiviral shRNA screen identifies serine/arginine-rich splicing factor 2 as a determinant of oncolytic virus activity in breast cancer cells.

Oncolytic human herpes simplex virus type 1 (HSV-1) shows promising treatment efficacy in late-stage clinical trials. The anticancer activity of oncolytic viruses relies on deregulated pathways in cancer cells, which make them permissive to oncolysis. To identify pathways that restrict HSV-1 KM100-mediated oncolysis, this study used a pooled genome-wide short hairpin RNA library and found that depletion of the splicing factor arginine-rich splicing factor 2 (SRSF2) leads to enhanced cytotoxicity of breast cancer cells by KM100. Serine/arginine-rich (SR) proteins are a family of RNA-binding phosphoproteins that control both constitutive and alternative pre-mRNA splicing. Further characterization showed that KM100 infection of HS578T cells under conditions of low SRSF2 leads to pronounced apoptosis without a corresponding increase in virus replication. As DNA topoisomerase I inhibitors can limit the phosphorylation of SRSF2, we combined a topoisomerase I inhibitor chemotherapeutic with KM100 and observed synergistic anticancer effect in vitro and prolonged survival of tumor-bearing mice in vivo.

Oncolytic bovine herpesvirus type 1 infects and kills breast tumor cells and breast cancer-initiating cells irrespective of tumor subtype.

Oncolytic viruses are attractive cancer therapeutics because of their unique mechanisms of tumor cell targeting and the absence of toxic side effects associated with current treatments. Bovine herpesvirus type 1 (BHV-1) is a species-specific herpesvirus that fails to induce cytopathic effects in normal human cells, but is capable of infecting and killing a variety of immortalized and transformed human cell types, including human breast tumor cell lines from luminal, basal A and basal B subtypes, representing a variety of receptor expression profiles. BHV-1 is capable of initiating replication in and killing both bulk and side population cells, the latter of which have enhanced tumor-initiating capacity. Despite the lack of a productive infection or secretion of cytotoxic factors, BHV-1 infection decreases cellular viability in long-term culture following low multiplicity of infection. Moreover, BHV-1-infected MCF7 cells are significantly diminished in their capacity to form tumors in vivo. Overall, these studies suggest that oncolytic BHV-1 targets bulk breast cancer cells and cancer-initiating cells from luminal and basal subtypes by a novel mechanism that is not contingent upon cellular receptor expression status.

Adenovirus evasion of interferon-mediated innate immunity by direct antagonism of a cellular histone posttranslational modification.

Overcoming the cellular type I interferon (IFN) host defense response is critical for a virus to ensure successful infection. Investigating the effects of human adenovirus (HAdV) infection on global cellular histone posttranslational modification (hPTM), we discovered that virus infection-induced activation of IFN signaling triggers a global increase in the monoubiquitination of histone 2B (H2B) at lysine 120, which is a mark for transcriptionally active chromatin. This hPTM, catalyzed by the hBre1/RNF20 complex, is necessary for activation of the cellular IFN-stimulated gene (ISG) expression program in response to viruses. To establish effective infection, the HAdV E1A protein binds to and dissociates the hBre1 complex to block IFN-induced H2B monoubiquitination and associated ISG expression. Together, these data uncover a key role for H2B monoubiquitination in the type I IFN response and a viral mechanism of antagonizing this hPTM to evade the IFN response.

PML has a predictive role in tumor cell permissiveness to interferon-sensitive oncolytic viruses.

The oncotropic phenotypes of several viruses correlate with tumor-associated deficiencies within interferon (IFN) signaling pathways. This observation formed the conceptual basis for developing oncolytic viruses deleted for viral proteins that inhibit the host IFN-dependent antiviral response, such as herpes simplex virus type-1 infected cell protein-0 (ICP0) and vesicular stomatitis virus matrix protein. Many viruses have evolved means to disrupt promyelocytic leukemia protein (PML) nuclear bodies. For example, ICP0 promotes PML degradation to inhibit the antiviral activities of this IFN-stimulated gene. As PML is downregulated in a variety of tumors, we hypothesized ICP0-null herpes simplex type-1 viruses are selectively oncolytic in tumors with impaired PML expression. We illustrate that ICP0-null herpes simplex type-1 viruses target tumor cells that either possess impaired PML signaling or cannot upregulate PML because of impaired IFN responsiveness. Disruption of PML signaling through overexpression of the dominant-negative protein PML-retinoic acid receptor alpha in PML-positive cells renders them sensitive to oncolysis by ICP0-null herpes simplex virus type-1 and vesicular stomatitis virus M protein mutant viruses, whereas PML overexpression reverses this phenomenon. Together, these data illustrate that PML mediates an antiviral mechanism that predicts the tropism of IFN-sensitive oncolytic viruses. To our knowledge, these viruses are the first examples of anti-cancer therapeutics capable of targeting deficiencies in PML expression.

Deconstructing radiation hormesis.

This paper explores some factors that may explain why the possibility of hormesis has not been embraced by the radiation protection community. If shown to be sustainable, hormesis might ameliorate several serious issues plaguing radiation protection including the high economic cost of environmental regulatory compliance and public fear of radiation exposure. Some but not all analyses of data from various sources, including the Japanese survivors of the atomic bombs and residential radon studies, suggest that low levels of ionizing radiation may be beneficial to human health. The evidence, however, has not been viewed as compelling for the following reasons: (1) Data in support of radiation hormesis in human populations is limited and much of it is based on re-evaluation of selected epidemiological data that has been used to test a different hypothesis; (2) Hormetic effects are weak and inconsistent, and are subject to large statistical uncertainties as is the case for carcinogenic effects at small doses; (3) A consensus is lacking on how hormesis should be defined and quantified; and (4) It is unclear how hormesis can be incorporated into the regulatory framework when beneficial health effects exceed the requirement for protection of health.

Herpes simplex virus triggers and then disarms a host antiviral response.

Virus infection induces an antiviral response that is predominantly associated with the synthesis and secretion of soluble interferon. Here, we report that herpes simplex virus type 1 virions induce an interferon-independent antiviral state in human embryonic lung cells that prevents plaquing of a variety of viruses. Microarray analysis of 19,000 human expressed sequence tags revealed induction of a limited set of host genes, the majority of which are also induced by interferon. Genes implicated in controlling the intracellular spread of virus and eliminating virally infected cells were among those induced. Induction of the cellular response occurred in the absence of de novo cellular protein synthesis and required viral penetration. In addition, this response was only seen when viral gene expression was inhibited, suggesting that a newly synthesized viral protein(s) may function as an inhibitor of this response.

Evidence that herpes simplex virus VP16 is required for viral egress downstream of the initial envelopment event.

During infection with herpes simplex virus type 1 (HSV-1), VP16 serves multiple functions, including transcriptional activation of viral immediate early genes and downregulation of the virion host shutoff protein vhs. Furthermore, VP16 has been shown to be involved in some aspect of virus assembly and/or maturation. Experiments with a VP16 null virus, 8MA, suggested that VP16 plays a direct role in virion assembly, since removal of VP16 from the HSV-1 genome results in reduced levels of encapsidated DNA and a failure to produce extracellular enveloped particles. However, VP16 null mutants display a severe translational arrest due to unrestrained vhs activity, thus complicating interpretation of these data. We examine here the role of VP16 in virion assembly and egress in the context of a vhs null background, using the virus 8MA/DeltaSma (VP16(-) vhs(-)). Comparison of 8MA and 8MA/DeltaSma with respect to viral DNA accumulation and encapsidation and accumulation of the major capsid protein, VP5, revealed that the 8MA lethal phenotype is only partially due to uncontrolled vhs activity, indicating that VP16 is required in HSV-1 virion formation. Electron microscopy confirmed these results and further showed that VP16 is required for HSV-1 egress beyond the perinuclear space. In addition, we describe the isolation and characterization of an 8MA derivative capable of propagation on Vero cells, due to second site mutations in the vhs and UL53 (gK) genes. Taken together, these results show that VP16 is required for viral egress downstream of the initial envelopment step and further underscore the importance of VP16 in controlling vhs activity within an infected cell.

Herpes simplex virus ICP0 mutants are hypersensitive to interferon.

Interferon (IFN) is an important immune system molecule capable of inducing an antiviral state within cells. Herpes simplex virus type 1 (HSV-1) replication is somewhat reduced in tissue culture in the presence of IFN, presumably due to decreased viral transcription. Here, we show mutations that inactivate immediate-early (IE) gene product ICP0 render HSV-1 exquisitely sensitive to IFN inhibition, resulting in greatly decreased levels of viral mRNA transcripts and the resulting polypeptides and a severe reduction in plaque formation ability. Mutations in other HSV-1 genes, including the genes coding for virion transactivator VP16 and the virion host shutoff protein vhs, IE gene ICP22, and the protein kinase UL13 gene, do not increase the IFN sensitivity of HSV-1. Interestingly, ICP0 mutants demonstrate the same level of sensitivity to IFN as wild-type virus on U2OS cells, an osteosarcoma cell line that is known to complement mutations in ICP0 and VP16. Thus, in some cell types, functional ICP0 is required for HSV-1 to efficiently bypass the inhibitory effects of IFN in order to ensure its replication. The significance of this link between ICP0 and IFN resistance is discussed.

Truncation of the C-terminal acidic transcriptional activation domain of herpes simplex virus VP16 renders expression of the immediate-early genes almost entirely dependent on ICP0.

The herpes simplex virus (HSV) proteins VP16 and ICP0 play key roles in stimulating the onset of the viral lytic cycle. We sought to explore the regulatory links between these proteins by studying the phenotypes of viral mutants in which the activation functions of both were simultaneously inactivated. This analysis unexpectedly revealed that truncation of the C-terminal transcriptional activation domain of VP16 (allele V422) in an ICP0-deficient background almost completely eliminated immediate-early gene expression and virus replication in Vero and HEL cells. The doubly mutant viral genome persisted in a quiescent state for at least 10 days in HEL cells infected at high multiplicity and could be reactivated by superinfection with wild-type HSV. In contrast, the in1814 VP16 mutation produced a markedly less severe phenotype in the same ICP0-deficient background. These data demonstrate that expression of the immediate-early genes requires ICP0 when the C-terminal activation domain of VP16 is deleted and raise the possibility that the in1814 form of VP16 retains a residual ability to stimulate gene expression during virus infection.

The linear no-threshold debate: where do we go from here?

For the past several years, the LNT (linear no-threshold) theory has come under attack within the scientific community. Analysis of a number of epidemiological studies of the Japanese survivors of the atomic bombings and workers exposed to low level radiation suggest that the LNT philosophy is overly conservative, and low-level radiation may be less dangerous than commonly believed. Proponents of current standards argue that risk conservatism is justified because low level risks remain uncertain and it is prudent public health policy; LNT opponents maintain that regulatory compliance costs are excessive, and there is now substantial scientific information arguing against the LNT model. Regulators use the LNT theory in the standards setting process to predict numbers of cancers due to exposure to low level radiation because direct observations of radiation-induced cancers in populations exposed to low level radiation are difficult. The LNT model is simplistic and provides a conservative estimate of risk. Abandoning the LNT philosophy and relaxing regulations would have enormous economic implications. However, alternative models to predict risk at low dose are as difficult to justify as the LNT model. Perhaps exposure limits should be based on model-independent approaches. There is no requirement that exposure limits be based on any predictive model. It is prudent to base exposure limits on what is known directly about health effects of radiation exposure of human populations.

Radiation protection of radiosensitive populations.

Radiosensitive populations (defined as subpopulations with increased radiogenic risk for cancer) may include individuals with known genetic predispositions for cancer (e.g., breast cancer), and persons with certain diseases, such as ataxia telangiectasia, which are characterized by enhanced radiation sensitivity and increased risk for cancer. Although approximately 10% of the population may be radiosensitive, current radiological protection limits, based on the "average" individual, should be retained. Radiation is a weak environmental carcinogen. Diet and cigarette smoking each account for 15-18 times more cancer deaths. Radiosensitivity is a much less important host factor than age and genetic predisposition. Workers should be allowed to "declare" a predisposition for cancer. Workers who have declared radiosensitivities should be provided with additional information regarding job-related risks and strategies to reduce dose. "Declared" employees should not be subjected to different terms and conditions of employment.