58. Pushing the Limits for Canavan Gene Therapy into Adulthood: Is There an Age Limit for Gene Therapy in CNS Disorders?

Canavan Disease (CD) is a rare and lethal inherited leukodystrophy caused by autosomal recessive mutations in the aspartoacylase (Aspa) gene. To date, no effective treatment is available. Thus, gene therapy is an attractive approach to treat this devastating disease. Commonly associated with early death in childhood, an increasing group of patients reach teen age or even adulthood and in the process pose new challenges for families and the scientific community. The availability of a new mouse model (Nur7) with barely reduced life expectancy but full blown Canavan-like clinical manifestation prompted us to determine if there is a point-of-no-return in the gene therapeutic treatment of Canavan disease and to identify potentially limiting factors in the development of the central nervous system (CNS) and severity of the neuropathology. We previously reported that early postnatal systemic delivery of the human aspartoacylase (hASPA) gene by recombinant adeno-associated virus (rAAV) to the CNS of a CD mouse model with neonatal death rescued lethality and partially restored motor function. Now in its 3rd generation, our Canavan gene therapy completely reverses the disease phenotype in the CD KO mouse. For clinical translation to treat juvenile and adult patients, we sought to study and understand the age-limitations for Canavan disease gene therapy. We hypothesized that there is a point-of-no-return, when gene replacement therapy alone is no longer sufficient to successfully alter the disease outcome. First, Nur7 mice were treated at post-natal (p) day 1 as the gold standard. In the next step, experimental groups were dosed at 6, 12, and 24 weeks of age with a dose 10-fold higher than that for neonates. Motor function was tested for all mice 4 weeks after treatment and subsequent intervals up to one year of age for direct comparison. As expected, the earlier mice were treated, the better the therapeutic outcome. To our surprise, juvenile mice at 6 weeks of age recovered completely within 4 weeks post-injection. Although mice treated at 3 months of age and older did not respond immediately within the first 4 weeks post-treatment, they eventually showed significant improvements over Nur7 mutant control mice. Of note, cognitive function testing revealed that treated mice recover cognitively before motor function improves; this was even true for late treatment time points. Furthermore, response to rAAVhASPA gene therapy was confirmed via MRS for N-acetyaspartate, MRI, and neuropathology. Finally, our preliminary data characterizing oligodendrocytes and myelin in the Nur7 mouse model shows rapid reconstitution of myelin in mice treated at 6 weeks, underlining the potential of our therapy for later treatment. Currently, we are working on identifying molecular mechanisms limiting complete disease rescue in adult Nur7 mice to further explore options in order to successfully treat older patients. Overall, our data demonstrate that rAAV mediated hASPA expression of our 3rd generation gene therapy vector not only prevents but also rescues the clinical manifestation and pathology of the juvenile and adult model of Canavan disease at an unprecedented level, which might also have implications for other CNS disorders that require treatment in later stages of life.