Herpes Simplex Virus 1 (HSV-1)-Based Vectors - Viral Vector Approaches in Neurobiology and Brain Diseases - page 58

cells, due to deletion of the “fl oxed” packaging signal. Nevertheless,
some helper genomes can escape action of the Cre recombinase,
allowing the production of some contaminant helper particles. For
this reason, the two genes surrounding the cleavage/packaging sig-
nal, respectively encoding a virulence factor known as ICP34.5 and
the essential protein ICP4, were further deleted from the helper virus
genome. Although the amplicon stocks prepared with this helper
virus (in a complementing cell line expressing both Cre and ICP4
proteins) still can contain a small amount of contaminating helper
particles, this helper is replication incompetent and cannot spread
upon infection of target cells or tissues. Use of the HSV-1 LaL J

helper virus generally results in the production of large stocks of
amplicon vectors only barely contaminated (0.05-0.5 %) with
defective, nonpathogenic helper particles.

Δ

11 Applications of HSV-1-Based Vectors

In the last 20 years, the three types of HSV-1-based vectors have
been used in a large set of cell and animal models, both for funda-
mental studies and for experimental gene therapy protocols of neu-
rological disorders, and some of these vectors have already reached
the clinic. This chapter does not intend to fully recapitulate these
applications but to illustrate how these vectors can be rationally
designed and used in selected disease models.

12 Defective Recombinant Vectors

Different types of defective recombinant vectors have been devel-
oped. The problems related to HSV-1 vector design fall into the
following general categories (1) elimination of the lytic viral gene
expression and of the innate and immune responses; (2) engineer-
ing of promoter systems to achieve appropriate, lasting transgene
expression; (3) identifi cation of strategies to target heterologous
gene expression to specifi c neurons; and (4) simultaneous
expression of multiple genes. In recent years, novel technologies
have allowed researchers to get deeper into these problems. To
date, several replication-defective vectors have been constructed in
which the immediate-early (IE) genes, expressing ICP0, 4, 22, 27,
and 47, have been deleted in various combinations [ 30 - 32 ].
Nonreplicative HSV-1 vectors have been tested in different gene
therapy animal models of various neuropathies [ 33 - 35 ], including
epilepsy [ 36 ], demyelinating diseases such as experimental autoim-
mune encephalomyelitis (EAE) [ 37 ], Parkinson's disease [ 38 ],
chronic pain [ 39 - 41 ], or lysosomal storage disorders with neuro-
logical involvement [ 42 , 43 ]. In this chapter, we illustrate only
some of these applications (see Fig. 5 ).