The herpesvirus saimiri ORF 73 regulatory region provides long-term transgene expression in human carcinoma cell lines.

Herpesvirus saimiri (HVS) is capable of establishing a persistent infection in a variety of human carcinoma cell lines, by virtue of episomal maintenance. Moreover, the viral episome provides expression of a transgene in both in vitro and in vivo environments. At present, HVS vectors utilize heterologous promoters such as the IE hCMV promoter. However, this promoter maybe unsuitable for long-term expression in vivo, as promoter silencing has been observed in this and other herpesvirus-based vector systems. Ideal regulatory regions would be functional when the herpesvirus genome is maintained as a latent episome. We have previously shown that gene expression in an HVS-persistently-infected human carcinoma cell line is limited to an adjacent set of genes encoding ORFs 71-73. These genes are transcribed as a polycistronic mRNA species from a common regulatory region upstream of the ORF 73 gene. In this report, we assess the potential of the ORF 73 regulatory region to provide heterologous gene expression in a wide variety of human cancer cell lines. We demonstrate, utilizing transient transfection assays, that the ORF 73 regulatory region can provide transgene expression in a variety of human carcinoma cell lines, although levels of transgene expression are not as high as achieved under the control of heterologous promoters such as the IE hCMV promoter. Furthermore, incorporation of the minimal ORF 73 regulatory region in a recombinant HVS-based vector provides sustained expression of the green fluorescent protein in both in vitro and in vivo environments. These results suggest that the ORF 73 regulatory region may be suitable for use in HVS-based cancer gene therapy applications.