Adeno-associated virus (AAV)-mediated gene therapies for central nervous system (CNS) disorders are experiencing a period of rapid development. A string of recent clinical trial outcomes for rare and complex CNS disorders complemented by preclinical studies in animal models continue to highlight both the prospects and challenges facing therapeutic gene transfer to the brain. 1–3 Notably, regional disparities in AAV vector spread within the brain parenchyma and transgene expression have spurred efforts in two key areas: optimizing physical methods of CNS dosing and better characterizing AAV biology in the brain. 4 In this issue of Molecular Therapy, Heller et al. 5 highlight a third temporal component that shines a spotlight on the interface of CNS disease and AAV biology.

Krabbe disease (KD), also known as globoid cell leukodystrophy, affects newborn children, often leading to death within the first few years of life. 6 As KD is caused by loss-of-function mutations in the lysosomal enzyme galactosylceramidase (GALC), several groups have published promising results showing that AAV-delivered GALC significantly improves behavior and pathology in Twitcher mice, a murine model for KD. 7–9 These results have sparked optimism for treating KD via AAV gene therapy. In a recent long-term study, the Heller group found that AAV9-GALC-treated Twitcher mice showed little to no signs of disease for most of their significantly lengthened lifespans. 7 However, a wide range of lifespans were found to result from the treatment, suggesting differences in underlying factors affecting long-term survival. Upon deeper investigation, Heller et al. 5 found that late-onset focal lesions of demyelination develop in white matter following AAV9-GALC treatment. Interestingly, the lesions developed in areas where, at earlier time points, the investigators observed healthy tissue with robust GALC expression, suggesting waning efficacy.