Parkinson’s disease (PD) is the second most common neurodegenerative disease with no disease-modifying therapy currently available. Viral vector-mediated gene therapy, however, is being tested in clinical trials as a method to slow disease progression. Although multiple clinical trials have tested the efficacy of this therapy for PD, all strategies have failed to provide adequate relief. Although possibly due to a variety of reasons, failing to consider aging as a covariate in preclinical trials could have contributed to these results. The importance of accounting for age when designing viral vectors for PD stems from the fact that age is the greatest risk factor for developing PD and age-related impairments in cellular processes overlap with steps of viral vector transduction. As a result, I sought to test whether advanced age impacts viral vector transduction efficiency. In the present dissertation, I tested the efficiency of four viral vector constructs in two systems of the brain involved in PD. Three pseudotypes of recombinant adeno-associated virus (rAAV2/2, rAAV2/5, rAAV2/9) and a lentiviral vector (LV) were chosen due to their use in clinical trials for PD, preclinical trials in PD animal models, or proposed use in clinical trials for PD. These viral vectors were injected into the substantia nigra pars compacta (SNpc) to transduce the nigrostriatal system—the system that degenerates in PD—or the striatum to transduce the striatonigral system—the system most often targeted in viral vector-mediated gene therapy clinical trials for PD. The first evidence for an age-related transduction deficiency came from observations that rAAV2/5 expressing green fluorescent protein (rAAV2/5 GFP) was deficient in transducing the aged as compared to young adult rat midbrain and nigrostriatal system two weeks post-injection. I continued this investigation by systematically characterizing the ability of rAAV2/2, rAAV2/5, rAAV2/9, and LV, all expressing GFP, to transduce the aged as compared to young adult rat nigrostriatal and striatonigral systems at one month post-injection when expression levels asymptote. I observed robust age-related deficiencies in the ability of rAAV2/2 and rAAV2/5 to transduce the nigrostriatal and striatonigral systems. Age-related deficiencies were also observed in rAAV2/9 and LV transduction; however, deficiencies were observed in the nigrostriatal but not striatonigral system with LV and vice versa with rAAV2/9. Taken together, these results indicate robust age-related transduction deficiencies that are structure- and vector-specific. The clinical relevance of this finding was investigated to determine whether the viral vector construct used in an upcoming clinical trial is similarly deficient in the aged striatonigral system. I found robust deficiencies in rAAV2/2 overexpressing glial cell line-derived neurotrophic factor (rAAV2/2 GDNF) in the injected striatum as well as in the ipsilateral anterograde structures, suggesting age may negatively impact an upcoming PD clinical trial. Finally, I sought to determine the mechanism of the age-related transduction deficiency of rAAV2/2 in the nigrostriatal system and uncovered a role for cell surface receptors in the deficiency. Taken together, the results of my experiments highlight the need to account for age when developing therapies for age-related neurodegenerative diseases. Specifically, age can greatly impact the ability of viral vectors to transduce brain systems involved in PD, with robust age-related deficiencies observed across multiple viral vectors, target structures, transgenes expressed, rat strains, and durations of expression. Furthermore, the mechanism of this deficiency may involve virion interactions with cell surface receptors. With this information, future clinical and preclinical studies using viral vectors in the aged brain may be able to circumvent age-related transduction deficiencies and develop a disease-modifying treatment for neurodegenerative diseases like PD.
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