Molecular genetic analysis of herpesviruses and their potential use as vectors for gene therapy applications.

Molecular genetic experiments with large human herpesviruses have provided a means whereby large heterologous DNA fragments can be cloned, propagated and established in cells permissive for infection with herpesviruses. Some of these cells include neuronal cells and B-lymphocytes infected with herpes simplex virus (HSV) and Epstein-Barr virus (EBV), respectively as well as human T-lymphocytes with herpesvirus saimiri. These large DNA viruses have the potential to deliver fragments of human heterologous DNA > 150 kbp to specific cells. Additionally, EBV recombinants can maintain these large pieces of DNA in the infected B-cells as episomal DNA. The maintenance of these episomes requires a specific EBV nuclear protein, EBNA1, constitutively expressed during infection with EBV. Additionally, these vectors can be used for transfection, where large amounts of protein can be generated transiently in vitro. Herpesvirus amplicon systems are also being used to package pieces of DNA > 150 kbp and to infect cells that can stably maintain DNA as episomes. Moreover, other herpesvirus vector systems can be utilized as a source for the propagation of specific DNA fragments of interest, including wild-type genes to correct genetic defects in human cells. Other herpesvirus systems include defective infectious single cycle (DISC-HSV) mutants of HSV, which are now undergoing trials for the treatment of genital herpesvirus infections and certain types of cancers. The use of herpesviruses as an agent to deliver heterologous gene products has the potential to make a significant difference to the development of therapeutic approaches to human diseases.