Recombinant adenoassociated virus 2/5-mediated gene transfer is reduced in the aged rat midbrain

Abstract

Clinical trials are examining the efficacy of viral vector-mediated gene delivery for treating Parkinson’s disease (PD). While viral vector strategies have been successful in preclinical studies, to date clinical trials have disappointed. This may be due to the fact that preclinical studies fail to account for aging. Aging is the single greatest risk factor for developing PD and age alters cellular processes utilized by viral vectors. We hypothesized that the aged brain would be relatively resistant to transduction when compared to the young adult. We examined recombinant adeno-associated virus 2/5 mediated green fluorescent protein (rAAV2/5 GFP) expression in the young adult and aged rat nigrostriatal system. GFP overexpression was produced in both age groups. However, following rAAV2/5 GFP injection to the substantia nigra (SN) aged rats displayed 40-60% less GFP protein in the striatum, regardless of rat strain or duration of expression. Further, aged rats exhibited 40% fewer cells expressing GFP and 4-fold less GFP mRNA. rAAV2/5-mediated gene transfer is compromised in the aged rat midbrain, with deficiencies in early steps of transduction leading to significantly less mRNA and protein expression.

Rationally Engineered AAV Capsids Improve Transduction and Volumetric Spread in the CNS

“…Numerous considerations, in addition to interspecies scaling, have to be made when translating rodent studies into the NHP and, ultimately, humans. For instance, we have established that age is an important covariate in transduction efficiency, where certain serotypes exhibit deficiencies in distinct anatomical locations 5, 49. Thus, age-matching experimental subjects in preclinical testing is increasingly important.…”

“…Additional considerations that may warrant the use of one serotype over another is whether a patient has pre-existing immunity to a specific serotype. Moreover, various serotypes are differentially impaired in the aged brain 5, 49. Finally, the increased efficacy of these vectors would also allow the use of lower titers, thereby lowering the risk of an immune response.…”

“…Nevertheless, clinical efficacy is largely absent, and concerns related to the immunogenicity of the viral capsids still remain. The limited clinical success has been attributed to the inability to efficiently scale transduction between species (i.e., from mice to non-human primates [NHPs] to humans), 4 differential properties of the healthy and the diseased brain, 4 and the differential ability of rAAV to infect the aged versus the young brain, 5 among others. To address such shortcomings, researchers have engaged in a veritable arms race to enhance the efficacy of rAAV using both rational design methods based on the known structure of rAAV 6 and molecular evolution techniques to select for capsids with various desirable properties 7, 8, 9…”

Gene Therapy Models of Alzheimer’s Disease and Other Dementias

“…1). Our group recently found that rAAV2/5-GFP transduction in the SN is reduced in aged animals compared to young animals [82], but other studies have shown that rAAV2/9-tau and -GFP transduction is unaffected in the SN [83, 84]. The differences in transduction efficiency with age may reflect the use of different rAAV serotypes.…”

Immunomodulators as Therapeutic Agents in Mitigating the Progression of Parkinson’s Disease

“…Many of these studies were done using either glial cell lines or in young animals, neglecting the impact of the pro-inflammatory actions of α-syn within the primed glial environment of an aging brain, thus causing even more damage. In fact, when α-syn is introduced into an aged animal it is more cytotoxic compared to the young controls [34]. …”