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The avian virus-derived protein apoptin induces p53-independent apoptosis in a tumor-specific way. Apoptin acts as a multimeric complex and forms superstructures upon binding to DNA. In tumor cells, apoptin is phosphorylated and mainly nuclear, whereas in normal cells it is unphosphorylated, cytoplasmic, and becomes readily neutralized. Interestingly, apoptin phosphorylation, nuclear translocation, and apoptosis can transiently be induced in normal cells by cotransfecting SV40 large T oncogene, indicating that apoptin recognizes early stages of oncogenic transformation. In cancer cells, apoptin appears to recognize survival signals, which it is able to redirect into cell death impulses. Apoptin targets include DEDAF, Nur77, Nmi, Hippi, and the potential drug target APC1. Apoptin-transgenic mice and animal tumor models have revealed apoptin as a safe and efficient antitumor agent, resulting in significant tumor regression. Future antitumor therapies could use apoptin either as a therapeutic bullet or as an early sensor of druggable tumor-specific processes.

The major immediate-early (MIE) genes of cytomegaloviruses (CMV) are broadly thought to be decisive regulators of lytic replication and reactivation from latency. To directly assess the role of the MIE protein IE1 during the infection of murine CMV (MCMV), we constructed an MCMV with exon 4 of the ie1 gene deleted. We found that, independent of the multiplicity of infection, the resulting recombinant virus, MCMVdie1, which fails to express the IE1 protein, was fully competent for early gene expression and replicated in different cultured cell types with identical kinetics to those of parental or revertant virus. Immunofluorescence microscopy studies revealed that MCMVdie1 was greatly impaired in its capacity to disrupt promyelocytic leukemia bodies in NIH 3T3 cells early after infection, a process that has been proposed to increase viral transcription efficiency. We examined MCMVdie1 in the murine model using both immunocompetent BALB/c and severe combined immunodeficient (SCID) mice. When MCMVdie1 was inoculated into these two types of mice, significantly lower viral titers were detected in infected organs than in those of the wild-type virus-infected animals. Moreover, the ie1-deficient MCMV exhibited a markedly reduced virulence. While all animals infected with 5 ؋ 10 4 PFU of parental virus died by 30 days postinfection, SCID mice infected with a similar dose of MCMVdie1 did not succumb before 60 days postinfection. The in vivo defective growth phenotype of MCMVdie1 was abrogated upon rescue of ie1. These results demonstrate the significance of the ie1 gene for promoting an acute MCMV infection and virulence yet indicate that MCMV is able to grow in vivo, although impaired, in the absence of the ie1 gene.Similar to other herpesviruses, the transcription of the cytomegalovirus (CMV) genome during the lytic infection is temporarily regulated (for a review, see reference 47). The immediate-early (IE or ␣) genes are the first ones to be expressed in the replicative cycle, and their expression does not depend on prior viral protein synthesis. Together with some virion proteins, the IE products activate viral genes and alter the infected cell to generate an appropriate milieu that favors viral replication. Transcription of early (E or ␤) genes requires the expression of at least one of the IE proteins, and only after viral replication has started, the transcription of late (L or ␥) genes proceeds. The majority of the CMV IE transcripts originate from the major IE (MIE) locus. This locus is structurally similar between human CMV (HCMV) and the closely related mouse CMV (MCMV) (14,53,59). The primary transcript from the MIE region is under the control of the strong MIE enhancer-promoter and is differentially spliced to generate two predominant transcripts, the ie1 transcript that consists of exons 1 to 4, and the ie2 transcript that is composed of exons 1 to 3 and 5. In HCMV, the ie1 and ie2 transcripts are translated into the acidic 72-kDa IE1 and the 86-kDa IE2 nuclear phosphoproteins, respectively (for a review, ...

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