Early AAV9-mediated SLC9A6 gene replacement preserves cerebellar structure and motor function in the shaker rat model of Christianson syndrome

Christianson syndrome (CS) is an X-linked neurodevelopmental and neurodegenerative disorder caused by SLC9A6 loss of function, resulting in NHE6 deficiency and epilepsy, intellectual disability, cerebellar atrophy, and ataxia. Following on our prior gene replacement therapeutic development, we preclinically evaluated AAV9-mediated SLC9A6 gene replacement in the context of promoter selection, dosing, and treatment timing. Intracerebroventricular delivery of neuronal-targeted gene replacement at 1 x 10^11 vector genomes (vg) produced substantial improvements in motor coordination and preserved cerebellar volume, quantified through structural MRI. Early postnatal treatment prevented the development of gait ataxia throughout the longitudinal assessment period, statistically indistinguishable from wildtype controls, with molecular analyses demonstrating restored NHE6 expression and preservation of cerebellar markers. Interestingly, later treatments at timepoints corresponding to early neurodegenerative stages yielded strong initial effects, but these effects were largely lost over time, with half-life modelling demonstrating the need for early treatment. This conflicted with more persistent motor benefit shown in prior work with an engineered capsid, suggesting the need to consider the interaction between treatment timing and capsid-specific tropism and/or persistence. Finally, we built models capable of accurately predicting motor coordination from several protein markers. These findings will impact therapeutic development for CS, with relevance across neurodevelopmental genetic disorders.