DDX3X overexpression in mice can cause rapid tissue-specific toxicity and mortality.

DEAD-Box Helicase 3 X-Linked (DDX3X) is a ubiquitously expressed RNA helicase with diverse cellular roles implicated in a neurodevelopmental disorder called DDX3X syndrome. Although DDX3X is a leading genetic cause of intellectual disability in females, there is no treatment. While gene supplementation is a plausible therapeutic strategy, previous studies suggest DDX3X is carefully regulated and dose sensitive. To understand the consequences of overexpressing DDX3X with unregulated adeno-associated virus-mediated gene supplementation, we generated a vector driving strong ubiquitous DDX3X expression and administered it through a direct cerebrospinal fluid injection in newborn mice. Mice injected with a high dose died within 1 week from myocardial degeneration. Increased expression of stress response markers together with elevated apoptotic signaling in the heart suggested activation of stress-induced apoptotic pathways. Incidental findings included excess lipid accumulation, most prominent in the liver, and other liver injury. The innate immune system was also highly activated in the heart and liver. Interestingly, the brain was overall unaffected. The results suggest that DDX3X overexpression can cause rapid transgene-driven, tissue-specific toxicity, underscoring the need for tight DDX3X gene dosage control. These findings illustrate the possibility for improper transgene expression to drive severe toxicity including death within days following administration.