715. Impact of Aging on Recombinant Adeno-Associated and Lentiviral Vector Transduction Efficiency in the Rat Nigrostriatal and Striatonigral Systems

Abstract

Clinical trials currently examine the efficacy of viral vector-mediated gene delivery for treating age-related neurodegenerative diseases such as Parkinson's disease (PD). While viral vector strategies have been successful in preclinical studies, to date, human clinical trials have been unsuccessful. This may be partially due to the fact that preclinical studies fail to account for advanced age as an important covariate despite aging being the primary risk factor for PD. Previously, we found that gene transfer using recombinant adeno-associated virus pseudotype 2/5 (rAAV2/5) results in decreased transduction efficiency in the aged (20 month) rat midbrain as compared to young adult (3 month) rat at 12 days post-injection (Polinski et al., 2014, PMID: 25457558). Specifically, injection of rAAV2/5 expressing green fluorescent protein (rAAV2/5 GFP) to the substantia nigra (SN) of aged rats resulted in: 1) ~60% fewer cells expressing detectable GFP, 2) ~50% less striatal GFP protein, and 3) 4-fold lower GFP mRNA expression than identical injections into the young adult rat SN. These results were generalizable over rat strain, duration of expression, and location sampled in the nigrostriatal system. In the present series of experiments, we investigated whether the phenomenon of deficient transduction in the aged brain is generalizable to other vector constructs. Further, since the striatum is the structure most often targeted in PD gene therapy clinical trials, we examined the impact of aging on transduction efficiency in striatum. Aged (20 month) and young adult (3 month) male Fischer 344 rats were stereotaxically injected into the SN or striatum with rAAV2/5, rAAV2/2, rAAV2/9, or lentivirus (LV) expressing GFP. Four weeks after injection in the SN, three of the four vector constructs (rAAV2/5, rAAV2/2, and LV) were less efficient in transduction of the aged nigrostriatal system as assessed through levels of GFP protein in the striatum. In contrast, rAAV2/9 was equally as efficient in aged as compared to young adult rats. These results implicate vector capsid-related steps of transduction in the deficiencies observed in the aged brain as this is the only difference between the rAAV pseudotypes used. Results examining whether similar deficiencies are observed following intrastriatal injection of the four vector constructs are pending. Outcome measures will include stereological quantification of NeuN+ or GFP+ cells in the striatum and measurement of GFP protein levels in striatonigral terminals. Aging-related deficits in transduction have the potential to negatively impact current and future gene therapy trials. Our results will identify which vector constructs are better suited for transduction of the aged nigrostriatal and striatonigral systems to optimize future gene therapy clinical trials for age-related neurodegenerative diseases.