Viral Link to Chronic Fatigue Chronic fatigue syndrome (CFS) is a complex and debilitating disorder that is often linked to immune system dysfunction but whose cause(s) remain mysterious. Lombardi et al. (p. 585 , published online 8 October; see the Perspective by Coffin and Stoye ) now present a tantalizing new lead. In blood samples from 101 patients with well-documented CFS, over two-thirds (68) contained DNA from a recently described human gammaretrovirus, xenotropic murine leukemia virus–related virus (XMRV), which possesses sequence similarity to a murine leukemia virus. Cell culture assays confirmed that XMRV derived from CFS patient plasma and from T and B lymphocytes was infectious. Although the correlation with CFS is striking, whether the virus plays a causal role in the disorder remains to be determined. Interestingly, nearly 4% of the 218 healthy donors tested were positive for XMRV, which suggests that this virus—whose pathogenic potential is unknown—may be present in a significant proportion of the general population.

We recently reported identification of a previously undescribed gammaretrovirus genome, xenotropic murine leukemia virusrelated virus (XMRV), in prostate cancer tissue from patients homozygous for a reduced activity variant of the antiviral enzyme RNase L. Here we constructed a full-length XMRV genome from prostate tissue RNA and showed that the molecular viral clone is replication-competent. XMRV replication in the prostate cancer cell line DU145 was sensitive to inhibition by IFN-␤. However, LNCaP prostate cancer cells, which are deficient in JAK1 and RNase L, were resistant to the effects of IFN-␤ against XMRV. Furthermore, DU145 cells rendered deficient in RNase L with siRNA were partially resistant to IFN inhibition of XMRV. Expression in hamster cells of the xenotropic and polytropic retrovirus receptor 1 allowed these cells to be infected by XMRV. XMRV provirus integration sites were mapped in DNA isolated from human prostate tumor tissue to genes for two transcription factors (NFATc3 and CREB5) and to a gene encoding a suppressor of androgen receptor transactivation (APPBP2/PAT1/ARA67). Our studies demonstrate that XMRV is a virus that has infected humans and is susceptible to inhibition by IFN and its downstream effector, RNase L.cancer ͉ RNase L ͉ xenotropic murine leukemia virus-related virus A diverse range of mammalian species are susceptible to infections by viruses from the gammaretrovirus genus of Retroviridae (1). Examples of these simple viruses whose genomes include gag, pro, pol, and env genes only are murine leukemia virus (MLV), feline leukemia virus, koala retrovirus, and gibbon ape leukemia virus. These viruses are responsible for leukemogenesis and other diseases in their respective host species (1-3). However, until recently evidence of authentic infections of humans by gammaretroviruses was lacking. We reported in 2006 identification of viral genomes for a previously undescribed gammaretrovirus, termed xenotropic MLV-related virus (XMRV), in a subset of men with prostate cancer (4). The discovery of XMRV followed investigations of the role of the antiviral enzyme RNase L in hereditary prostate cancer, a disease in which tumors arise in three or more first-degree relatives (5). The human RNase L gene (RNASEL) was initially proposed as a candidate for the hereditary prostate cancer 1 (HPC1) gene based on a positional cloning/candidate gene method (6).RNase L is a regulated endoribonuclease for single-stranded RNA that functions in the IFN antiviral response (7,8). IFN treatment of cells induces a family of 2Ј-5Ј oligoadenylate synthetases that produce 5Ј-phosphorylated, 2Ј-5Ј-linked oligoadenylates (2-5A) from ATP in response to stimulation by viral dsRNA. 2-5A activates the preexisting, latent, and ubiquitous RNase L, resulting in degradation of viral and cellular RNA. Sustained activation of RNase L leads to apoptosis, a function consistent with a role in the suppression of tumor growth (9). Although mice lacking RNase L do not spontaneously develop tumors at higher rates than wild-typ...

Xenotropic murine leukemia virus-related virus (XMRV) is a new human gammaretrovirus identified inprostate cancer tissue from patients homozygous for a reduced-activity variant of the antiviral enzyme RNase L. Neither a casual relationship between XMRV infection and prostate cancer nor a mechanism of tumorigenesis has been established. To determine the integration site preferences of XMRV and the potential risk of proviral insertional mutagenesis, we carried out a genome-wide analysis of viral integration sites in the prostate cell line DU145 after an acute XMRV infection and compared the integration site pattern of XMRV with those found for murine leukemia virus and two human retroviruses, human immunodeficiency virus type 1 and human T-cell leukemia virus type 1. Among all retroviruses analyzed, XMRV has the strongest preference for transcription start sites, CpG islands, DNase-hypersensitive sites, and gene-dense regions; all are features frequently associated with structurally open transcription regulatory regions of a chromosome. Analyses of XMRV integration sites in tissues from prostate cancer patients found a similar preference for the aforementioned chromosomal features. Additionally, XMRV integration sites in cancer tissues were associated with cancer breakpoints, common fragile sites, microRNA, and cancer-related genes, suggesting a selection process that favors certain chromosomal integration sites. In both acutely infected cells and cancer tissues, no common integration site was detected within or near proto-oncogenes or tumor suppressor genes. These results are consistent with a model in which XMRV may contribute to tumorigenicity via a paracrine mechanism.Prostate cancer is the most common noncutaneous cancer diagnosed in men in developed countries and is responsible for the deaths of approximately 30,000 men per year in the United States (43). Despite its impact on male health, the molecular mechanisms involved in the pathogenesis of prostate cancer, particularly the events contributing to initiation and progression, remain relatively unknown in comparison with those for other common cancers. Epidemiological studies of kindreds with hereditary prostate cancer, who often display early-onset disease and account for 9% of all cases (16), identified HPC1 as a susceptibility locus for prostate cancer (94). HPC1 is linked to RNASEL, which encodes a regulated endoribonuclease for single-stranded RNA and functions in the antiviral action of interferon (IFN) (15, 17). In response to stimulation by viral double-stranded RNA, IFN treatment of cells induces a family of 2Ј-5Ј oligoadenylate synthetases that produce 2Ј-5Ј-linked oligoadenylates, which then activate the latent and ubiquitous protein RNase L, resulting in degradation of viral and cellular RNA and apoptosis induction (112). Several germ line variants of HPC1 and RNASEL have been observed in hereditary prostate cancer (91), including a common (35% allelic frequency) missense variant of RNase L in which a G-to-A transition at nucleotide position 13...

RNase L, a principal mediator of innate immunity to viral infections in higher vertebrates, is required for a complete IFN antiviral response against certain RNA stranded viruses. dsRNA produced during viral infections activates IFN-inducible synthetases that produce 5 -phosphorylated, 2 ,5 -oligoadenylates (2-5A) from ATP. 2-5A activates RNase L in a wide range of different mammalian cell types, thus blocking viral replication. However, 2-5A has unfavorable pharmacologic properties; it is rapidly degraded, does not transit cell membranes, and leads to apoptosis. To obtain activators of RNase L with improved drug-like properties, high-throughput screening was performed on chemical libraries by using fluorescence resonance energy transfer. Seven compounds were obtained that activated RNase L at micromolar concentrations, and structureactivity relationship studies resulted in identification of an additional four active compounds. Two lead compounds were shown to have a similar mechanistic path toward RNase L activation as the natural activator 2-5A. The compounds bound to the 2-5A-binding domain of RNase L (as determined by surface plasmon resonance and confirmed by computational docking), and the compounds induced RNase L dimerization and activation. Interestingly, the low-molecular-weight activators of RNase L had broad-spectrum antiviral activity against diverse types of RNA viruses, including the human pathogen human parainfluenza virus type 3, yet these compounds by themselves were not cytotoxic at the effective concentrations. Therefore, these RNase L activators are prototypes for a previously uncharacterized class of broad-spectrum antiviral agents.high-throughput screening ͉ interferon ͉ 2-5A ͉ virus

Xenotropic murine leukemia-related virus (XMRV) was identified in association with human prostate cancer and chronic fatigue syndrome. To examine the infection potential, kinetics, and tissue distribution of XMRV in an animal model, we inoculated five macaques with XMRV intravenously. XMRV established a persistent, chronic disseminated infection, with low transient viremia and provirus in blood lymphocytes during acute infection. Although undetectable in blood after about a month, XMRV viremia was reactivated at 9 months, confirming the chronicity of the infection. Furthermore, XMRV Gag was detected in tissues throughout, with wide dissemination throughout the period of monitoring. Surprisingly, XMRV infection showed organ-specific cell tropism, infecting CD4 T cells in lymphoid organs including the gastrointestinal lamina propria, alveolar macrophages in lung, and epithelial/interstitial cells in other organs, including the reproductive tract. Of note, in spite of the intravenous inoculation, extensive XMRV replication was noted in prostate during acute but not chronic infection even though infected cells were still detectable by fluorescence in situ hybridization (FISH) in prostate at 5 and 9 months postinfection. Marked lymphocyte activation occurred immediately postinfection, but antigen-specific cellular responses were undetectable. Antibody responses were elicited and boosted upon reexposure, but titers decreased rapidly, suggesting low antigen stimulation over time. Our findings establish a nonhuman primate model to study XMRV replication/dissemination, transmission, pathogenesis, immune responses, and potential future therapies.Xenotropic murine leukemia-related virus (XMRV) is a novel gammaretrovirus, initially identified in human prostate cancer using a Virochip DNA microarray (43) in men with a low-activity variant of RNASEL, an enzyme involved in innate immunity via type I interferons (14). Although related to murine leukemia virus (MLV) and probably acquired by zoonotic infection, human tissue-derived XMRV clearly segregates from other gammaretroviruses, genotypically arguing against the hypothesis that such human infection is acquired via repeated zoonotic transmission (43). The association of XMRV with prostate cancer has since been confirmed by other laboratories, albeit with a potentially different cellular tropism (34). In addition, association with RNASEL deficiency has been variable (1, 7, 15, 34), suggesting that low levels of RNASEL may not be a requirement for productive infection or viral propagation in humans. Nevertheless, the association of RNASEL mutations and prostate cancer has been reinforced by the recent discovery that a prostate cell line, 22Rv1, was derived from a patient with a low-activity RNASEL genotype (15). RNASEL dysfunction has also been associated with another disease, chronic fatigue syndrome (CFS) (8,20,21,38,42), which prompted an investigation into a potential association of XMRV with CFS. In a geographically restricted cohort, up to 67% of CFS patients were found ...