Herpes Simplex Virus Type 1 Amplicons and their Hybrid Virus Partners, EBV, AAV, and Retrovirus

Abstract

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.

Advances in Gene Delivery Systems

“…[75] Other hybrid viral vectors include HSV/AAV and HSV/Epstein-Barr virus. [76] Interestingly, several groups have attempted to coat the viral vectors with polymers or lipids. [77] The primary objective of the coating is to hide the virus from the host immune system upon in vivo administration, which in turn prevents degradation and increases vector circulation time and, subsequently, tissue transduction.…”

Herpes Simplex Virus Type 1/Adeno-Associated Virus Hybrid Vectors

“…Hybrid gene transfer vectors are designed to combine advantageous properties of different viruses to enhance efficiency of transgene delivery, vector stability and long-term transgene expression, while maintaining high safety standards [133, 202]. For example, the instability of HSV-1 amplicon vector delivered transgene DNA and transient transgene expression can be overcome by introducing genetic elements that allow the amplicon DNA to be maintained as an episome or to integrate into the host cell genome [203].…”

Viral vectors for in vivo gene transfer in Parkinson's disease: Properties and clinical grade production

“…A significant safety concern, however, is insertional mutagenesis leading to de novo oncogenesis as has been observed in ex vivo gene therapy using retroviruses for the treatment of severe combined immunodeficiency (adenosine deaminase deficiency) and chronic granulomatous disease (Nienhuis, 2006). In general, rAd and HSV do not integrate into the genome and form episomes (Harui et al, 1999;Hillgenberg et al, 2001;Oehmig et al, 2004b). Both rAd and HSV can be engineered to integrate into the genome, thus, addition of a transposon to a helper-dependent (high capacity) rAd, increases the frequency of integration and enhances long-term gene expression (Yant et al, 2002).…”

“…Both rAd and HSV can be engineered to integrate into the genome, thus, addition of a transposon to a helper-dependent (high capacity) rAd, increases the frequency of integration and enhances long-term gene expression (Yant et al, 2002). Likewise, HSV amplicon vectors can be induced to integrate by including sequences from viruses that normally integrate into the genome (Oehmig et al, 2004b).…”

“…In addition, the factors controlling longterm in vivo expression are also being elucidated (Burton et al, 2005;Tyler et al, 2006). For example, hybrid HSV amplicon vectors that combine features of other viral vectors that allow long-term transgene expression but retain the large cDNA packaging capacity of HSV are being investigated (Oehmig et al, 2004a;Oehmig et al, 2004b). HSV's inherent neural tropism, retrograde transport, and large transgene capacity are attractive features to be exploited in the future.…”