Towards global and long-term neurological gene therapy: unexpected transgene dependent, high-level, and widespread distribution of HSV-1 thymidine kinase throughout the CNS

One of the challenges of neurological gene therapy for the treatment of chronic neurodegenerative disorders, such as Parkinson's and Alzheimer's diseases, is achieving high levels, widespread distribution, and long-lived transgene expression in the brain. Here, following the intracerebral injection of a recombinant adenovirus (RAd) encoding herpes simplex virus type 1 thymidine kinase (HSV1-TK), we detect very high levels of HSV1-TK immunoreactivity throughout the brain both ipsilaterally and contralaterally to the injection site, for up to 12 months following vector administration. This study concludes that long-term, high-level, and anatomically distributed HSV1-TK immunoreactivity can be obtained, and that this is most likely due to transgene-specific properties, because neither the distribution nor the longevity were observed for the transgene beta-galactosidase encoded by a co-injected vector. Thus, we demonstrate that transgene expression can be achieved over widespread areas of the rodent brain, even 12 months after a single injection of first-generation adenovirus vector.

Chronic brain inflammation and persistent herpes simplex virus 1 thymidine kinase expression in survivors of syngeneic glioma treated by adenovirus-mediated gene therapy: implications for clinical trials

The long-term consequences of adenovirus-mediated conditional cytotoxic gene therapy for gliomas remain uncharacterized. We report here detection of active brain inflammation 3 months after successful inhibition of syngeneic glioma growth. The inflammatory infiltrate consisted of activated macrophages/microglia and astrocytes, and T lymphocytes positive for leucosyalin, CD3 and CD8, and included secondary demyelination. We detected strong widespread herpes simplex virus 1 thymidine kinase immunoreactivity and vector genomes throughout large areas of the brain. Thus, patient evaluation and the design of clinical trials in ongoing and future gene therapy for brain glioblastoma must address not only tumor-killing efficiency, but also long-term active brain inflammation, loss of myelin fibers and persistent transgene expression.