Herpes Simplex Viral Vector

The wild-type herpes simplex virus type-1 (HSV-1) is an enveloped, double-stranded DNA virus. Nearly half of its 152-kb viral genome sequence is nonessential for viral replication. Theoretically, a HSV-1 vector can be built to carry a 76-kb foreign DNA sequence without compromising viral replication. In the laboratory, recombinant HSV-1 vectors with a 38-kb packaging capacity have been generated (reviewed in (Burton et al. 2002; Glorioso et al. 1997; Goins et al. 2004; Huard et al. 1997b)). Early experimentation with HSV-1 vectors demonstrated quite efficient myoblast transduction in vitro. However, direct muscle injection only yielded transient, restricted expression (Akkaraju et al. 1999; Huard et al., 1995, 1996, 1997a).

A variant of the HSV-1 vector called an amplicon has also been investigated for gene transfer. HSV-1 amplicons only contain ~1% of the wild-type viral sequences encoding the HSV replication origin and the packaging signal (reviewed in (Hibbitt and Wade-Martins 2006; Link et al. 2003; Wang 2006)). HSV-1 amplicons are generated by supplying the essential viral replication machinery and structural proteins in trans. Although in vivo muscle transduction efficiency is quite disappointing, HSV-1 amplicons offer a unique opportunity to deliver some of the largest therapeutic genes (such as titin, see Table 1) for ex vivo muscle gene therapy application (Bujold et al. 2002).

Fig. 12.3 Expanding AAV packaging capacity with the hybrid dual vector strategy. (a) Schematic outline of transgene reconstitution in the hybrid AAV vectors. (b) LacZ dual AAV vector transduction in muscle. The tibialis anterior muscles of 6-week-old BL10 mice were either infected with an intact LacZ vector (1010 vg particles/muscle) or co-infected with dual AAV vectors. In co-infection, a total of 2 x 1010 vg particles were delivered to each muscle (1010 vg particles/vector/muscle). Transgene expression was determined 6 weeks after infection by histochemical staining in muscle section and b-galactosidase activity assay in muscle lysate. Photomicrographs are representative images from dual vector co-infected muscles.

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