Potential and limitations of a γ34.5 mutant of herpes simplex 1 as a gene therapy vector in the CNS

Direct injection of viral vectors into the central nervous system has become a valuable technique for exploring the function of neurological systems and is a potential therapy for neural disease. To this end a number of herpes simplex virus (HSV)-derived vectors are currently being developed for the introduction of foreign DNA into the brain. In this study a non-neurovirulent HSV 17(+) mutant, variant 1716, deleted in the gamma 34.5 gene and expressing the marker gene lacZ under the control of the latency-associated transcripts promoter was injected stereotactically into the central nervous system of two strains of rat (AO and PVG). We show (1) that transgene expression was low at the site of injection, in the striatum, at all times studied (12 h to 30 days after injection); (2) dramatically more transgene expression was observed at distant sites which contain neurons projecting directly to the site of injection, with maximal expression at these sites being at 1-2 days; (3) immunostaining with a polyclonal anti-HSV antibody and with an antibody which detects a 65 kDa HSV DNA binding protein (the product of the UL42 gene of the virus) demonstrated that viral gene products could be detected at the injection site as early as 12 h and up to I week after injection. Moreover these could also be detected at several secondary sites not all of which have direct connections with the injection site. These findings suggest that gamma 34.5 negative vectors have potential for gene transfer but may require further attenuation to limit viral antigen expression before they can be used successfully for gene therapy in the brain.