Gene Delivery Using Viral Vectors - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 245

by RNA polymerase II of the infected host and immediate early genes, which encode
proteins that regulate the expression of Early and Late viral genes. Immediate early
genes are the first to be expressed following infection. The early gene expression
allows the synthesis of enzymes involved in DNA replication and the production of
certain envelope glycoproteins. Expression of late genes occurs last and these groups
of genes predominantly encode proteins that form the virion particle [181-184].
Different aspects of HSV biology render this virus attractive for the designing of
gene therapy vectors:
1. HSV can display a broad host cell range, and its cellular receptors, heparan sulfate (HS),
herpesvirus entry mediator (HVEM), and nectin-1 and -2, are widely expressed on the cell
surface of numerous cell types.
2. HSV is highly infectious, and it is possible to transduce 70% cells in vitro at a low multi-
plicity of infection (1.0), with a replication-defective vector.
3. Nondividing cells may be efficiently infected and transduced by HSV.
4. Almost half of the 84 known viral genes are nonessential for growth in tissue culture and
may be deleted to create genomic space for exogenous transgenes and to delete functions
essential for virulence and toxicity in vivo .
5. Recombinant HSV vectors can be easily produced to high titer and purity without wild-
type (wt) contaminants.
6. The latent behavior of the virus can be exploited for stable long-term expression of thera-
peutic transgenes in neurons.
Herpes viruses have several advantages. A general information exists by which
genes and DNA sequences may be deleted, and at those sites foreign DNA may be
inserted into the DNA genome [185] with minimal requirements for viral replication
and packaging [186] . HSV-based vector approaches rely on the ability of HSV to infect
neuronal cells and to establish a latent infection. Generally, latency is defined as a state
in which viral DNA is maintained within the cell nucleus in the absence of any viral
replication. During this latency period, viral gene expression is largely absent with the
exception of the latency-associated transcripts (LATs), which may remain transcrip-
tionally active [187] .
5.5.1.3 Cellular Entry
Generally, entry of HSV into the host cell involves two steps:
1. Several glycoproteins on the surface of the enveloped virus interact with receptors on the
surface of the host cell.
2. The envelope covering the virus particle, when bound to specific receptors on the cell sur-
face, will fuse with the host cell membrane and create an opening, or pore , through which
the virus enters the host cell.
The sequential stages of HSV entry are as follows:
At first, complementary receptors on the virus and the cell surface bring the viral
and cell membranes into proximity. In an intermediate state, the two membranes
begin to merge, forming a hemifusion state , and finally a stable entry pore is formed
through which the viral envelope contents are introduced to the host cell [187] .