Herpes simplex virus type 1 amplicons and their hybrid virus partners, EBV, AAV, and retrovirus.

HSV-1 amplicons can accommodate foreign DNA of any size up to 150 kbp. Genomic sequences as well as cDNA, large transcriptional regulatory sequences for cell type-specific expression, or multiple transgenes can be inserted in a modular fashion. HSV-1 amplicon vectors deliver DNA efficiently into the cell nucleus as an extrachromosomal, non-replicating circular concatenate, which is rapidly diluted, at least in dividing cells. Consequently, transgene expression is lost within days to weeks in dividing cells, but may be retained for months in non-dividing cells. In contrast, vectors based on Epstein-Barr virus, adeno-associated virus, or retroviruses can mediate long-term transgene expression, as vector DNA is retained by episomal replication or chromosomal integration. Hybrid amplicons use genetic elements from HSV-1 that allow replication and packaging of the vector DNA into HSV-1 virions, thereby conserving the large transgene capacity of HSV-1, and genetic elements from other viruses that confer genetic stability to the vector DNA within transduced cells. Additional strategies to sustain genetic material in infected cells include the incorporation of recombinases from different bacteriophages or transposable elements of the Tc1/mariner family in the amplicon vector. Moreover, modification of the HSV-1 virion itself offers a myriad of possibilities to improve gene delivery by targeting specific cell populations or transporting foreign proteins, such as Cre recombinase or the adeno-associated virus Rep protein, which can control the fate and expression of the therapeutic transgene.