Introduction: Christianson syndrome (CS) is a rare x-linked neurodevelopmental and neurodegenerative disorder characterized by severe symptoms, including intellectual disability, cerebellar degeneration, ataxia, seizures, and autistic behaviour. CS is caused by loss-of-function mutations in the SLC9A6 gene, which encodes NHE6, a sodium/hydrogen exchanger that is highly expressed in endosomes. We sought to preclinically evaluate a viral gene therapeutic strategy aimed at replacing missing SLC9A6 in a rodent model of Christianson syndrome.

Methods: We developed AAV9-CAG-SLC9A6, making use of the AAV9 serotype, a ubiquitous promoter, and the human SLC9A6 gene. Using the Wistar Furth shaker rat, a spontaneous rat model of CS, we administered AAVs via intracerebroventricular injection and quantified effects on molecular, cellular, and motor dysfunction.

Results: AAV9-CAG-SLC9A6 yielded substantial improvements in cerebellar health markers and motor function at doses as low as 8E9 viral genome copies. We identified strong, significant relationships between NHE6 expression and motor function, NHE6 expression and CALB1 expression, and CALB1 expression and motor function. Interestingly, AAV gene replacement differentially modulated tremor and ataxia and substantially modified their temporal relationship.

Conclusions: Gene therapy is a promising, potential treatment strategy for Christianson syndrome. NHE6 loss of function leads to an overly acidified endosome, which may assist with viral transduction of target neurons, yielding strong expression and uptake at low doses. Underlying cerebellar health markers are highly predictive of motor coordination. Further, tremor and ataxia may arise from dissociable mechanisms in Christianson syndrome. Additional work is required, particularly in terms of non-motor symptoms and safety / toxicology.