Herpes Simplex Virus 1 (HSV-1)-Based Vectors - Viral Vector Approaches in Neurobiology and Brain Diseases - page 55

a

Transgene 1 Reporter gene

Transgene 3

U U

a OriL a' a

Transgene 2 Transgene 4

HSV-1 recombinant vector Engineered HSV-1 DNA

b

Transgene

Ori S GFP

“a”

HSV-1 amplicon vector

HSV-1 amplicon plasmid

Fig. 4 HSV-1-derived vectors. ( a ) Recombinant vectors are HSV-1 particles carrying an engineered HSV-1
genome. They usually carry one (or several) transgene(s) of interest, and a reporter gene (for example lucifer-
ase or GFP) to facilitate the identifi cation of infected cells. ( b ) HSV-1-derived amplicon vectors are HSV-1
particles carrying a head-to-tail concatemer of a DNA derived from the amplicon plasmid, instead of the virus
genome. The amplicon plasmid is a standard Escherichia coli plasmid carrying one HSV-1 origin of DNA
replication (oriS) and one cleavage/packaging a sequence, to allow amplifi cation and packaging of the plasmid
into HSV-1 particles. In addition, it carries the transgenic and reporter (here GFP) sequences of interest (repre-
sented by arrows )

recombinant vectors are replication-competent HSV-1 generally
carrying attenuating mutations that restrict spread and lytic viral
replication to actively dividing cells, such as cancer cells, while
causing no toxicity to normal quiescent tissues [ 15 , 16 ]. A differ-
ent type of attenuating mutations constrains the virus genome to
establish and remain in latency in neurons, thus inhibiting virus
reactivation, but without affecting its capability to grow and spread
in epithelial cells before infecting neurons. Lastly, amplicons
(Fig. 4b ) are defective, helper-dependent vectors that take advan-
tage essentially of the large transgenic capacity of the virus particle.
Actually, there is no other available mammalian vector that could
equal the ability of herpesvirus-based amplicons to deliver up to
150 kbp of foreign DNA with no simultaneous delivering of viral
genes [ 17 - 19 ].
In all HSV-1 vector types, the virus particles are basically
identical to that of wild-type HSV-1 (Fig. 1 ). The structural virus
proteins and the DNA that are delivered into the cell during vector
infection can trigger cell signaling and cellular responses and, con-
sequently, may have a transient impact on the cell homeostasis and
gene expression. However, at least when using fully defective
vectors not expressing virus genes, such as some defective