Hsv1 Strains

Ground breaking studies by Martuza and colleagues were the first to demonstrate the therapeutic promise of an engineered oncolytic HSV-1 strain, the thymidine kinase (tk) negative HSV-1 mutant, c/Zsptk (3). This mutant derivative was chosen because tk mutants replicate effectively in actively dividing cells such as those found in tumors, but are relatively impaired for replication in nondividing cells, such as neurons and therefore display reduced neurovirulence compared with wild-type strains upon introduction into the CNS of adult mice (4-8). The tumor selected for oncolytic therapy was malignant glioma, as the outcome for patients with this devastating brain tumor is grim, remaining essentially unchanged over the past 50 yr despite advances in surgery, radiation, and chemotherapy. Direct injection of d/sptk into established tumors inhibited the growth of human glioma implants (subcutanteous or subrenal) in athymic mice and prolonged survival of mice with intracranial gliomas as well. However, fatal encephalitis was still observed in 70 to 100% of the treated mice despite the fact that the tk mutant was significantly less neurovirulent than wild-type HSV-1 (3). In subsequent years, the oncolytic potential of HSV-1 derivatives with mutations in the viral ribonucleotide reductase large subunit (hr3) or DNA polymerase genes (AraA) was evaluated. Unlike the tk mutant d/sptk, these strains were sensitive to acyclovir, an extremely effective, safe antiviral drug adding a component of safety to control the infection if necessary. However, despite their ability to inhibit tumor growth and responsiveness to antiviral chemotherapy, these viral strains were still not adequately attenuated to merit further investigation. Thus, whereas it proved possible to use HSV-1 as an oncolytic virus to destroy cancer cells, the understanding of virulence was not sufficiently advanced to render the virus safe.

Fig. 2. Genetic structure of oncolytic HSV-1 y34.5 mutant derivatives. The HSV-1 genome is 152-Kb in length and is composed of a unique long segment (UL), a unique short segment (Us), and several reiterated components (rectangles). The y34.5 gene is contained within these repetitive components that flank the UL segment and is therefore diploid. (A) A34.5 null mutant where both copies of the y34.5 gene have been deleted. The recombinants R3616 (strain F) and 1716 (strain 17) represent this class of simple deletion mutations. (B) A multimutated strain that contains a bacterial P-galactosi-dase (lacZ) expression cassette disrupting the viral UL39 gene in addition to a deletion affecting both copies of the y34.5 gene. This A34.5 null mutant is also unable to produce the large subunit of the viral ribonucleotide reductase, the UL39 gene product. The recombinants G207 and MGH1 (both strain F) are of this genotype.

Fig. 2. Genetic structure of oncolytic HSV-1 y34.5 mutant derivatives. The HSV-1 genome is 152-Kb in length and is composed of a unique long segment (UL), a unique short segment (Us), and several reiterated components (rectangles). The y34.5 gene is contained within these repetitive components that flank the UL segment and is therefore diploid. (A) A34.5 null mutant where both copies of the y34.5 gene have been deleted. The recombinants R3616 (strain F) and 1716 (strain 17) represent this class of simple deletion mutations. (B) A multimutated strain that contains a bacterial P-galactosi-dase (lacZ) expression cassette disrupting the viral UL39 gene in addition to a deletion affecting both copies of the y34.5 gene. This A34.5 null mutant is also unable to produce the large subunit of the viral ribonucleotide reductase, the UL39 gene product. The recombinants G207 and MGH1 (both strain F) are of this genotype.

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