Exploration of a DDX3X Gene Supplementation Therapy Including Expanded Characterization and Novel Findings of Sleep Disturbances in Ddx3x Haploinsufficient Mice.

Background: DDX3X syndrome is a neurodevelopmental disorder caused by mutations in the DDX3X gene. It is one of many rare diseases for which there is no adequate treatment, but it has characteristics that make it potentially amenable to gene supplementation therapy. Methods: An AAV9 (adeno-associated virus serotype 9)-mediated gene supplementation therapy was designed and tested for efficacy and safety in a mouse model of DDX3X syndrome. A total of 182 female Ddx3x +/- mutant and Ddx3x flox/+ control littermates were dosed at postnatal day 1 with AAV9/DDX3X or vehicle and assessed throughout development for phenotypic benefit. To expand the readouts, sleep-a behavior disrupted in patients-was assessed by wireless telemetry. Results: There was no effect of treatment on early postnatal developmental milestones. At 3 months, a dose-dependent amelioration in anxiety-like behavior was observed in treated Ddx3x +/- mice, and a dose-dependent worsening in anxiety-like behavior was observed in treated Ddx3x flox/+ control mice. These treatment effects were not sustained at 12 months and were absent in mice that had not been subjected to prior developmental milestone testing. Significant sleep disturbances were observed in the Ddx3x +/- mice at 4 months, but treatment had no effect on sleep. Conclusions: While the gene therapy design used in this study was not effective, the data suggest that behavior can be modified by altering Ddx3x expression, warranting further exploration of gene therapy as a potential treatment approach. Additionally, the sleep disturbances present in Ddx3x +/- mutant mice phenocopy the reduced sleep quality seen in patients, further validating this model for research.