Abstracts

SYNGAP1-related disorders (SRDs) are rare neurodevelopmental conditions characterized by severe neurological symptoms, including epileptiform activity, motor impairments, and cognitive dysfunction. Existing treatment options are limited, focusing primarily on symptom management rather than addressing the underlying genetic cause. SRDs are primarily caused by haploinsufficiency of the SYNGAP1 gene, which encodes SynGAP, a synaptic scaffolding and signaling protein. Gene supplementation using adeno-associated virus (AAV) to deliver SYNGAP1 offers a promising disease-modifying approach. This study evaluated the therapeutic potential of AAV-mediated delivery of SYNGAP1 in a mouse model of SRDs.

We engineered AAV vectors expressing full-length human SYNGAP1-Aα1 under the SYNAPSIN I promoter for broad neuronal expression. AAV was administered to Syngap1 heterozygous mice either neonatally (postnatal day 2, intracerebroventricular) or during the juvenile period (postnatal day 21, retro-orbital). Transgene expression, localization, and function were confirmed using molecular, immunohistochemical, and Western blot analyses. In vivo efficacy was assessed using chronic video EEG/EMG recordings to quantify interictal spikes, generalized tonic-clonic (GTC) seizures, and FFT-based frequency band alterations. Behavioral outcomes were evaluated using open field and elevated zero maze tests to assess locomotor activity, anxiety-like/exploratory behavior, and risk-taking behavior.

The transgene-expressed SynGAP protein localized to dendrites and synapses and retained interactions with PSD-95 and its GTPase-activating protein (GAP) function. Neonatal AAV-SYNGAP1 delivery led to forebrain- and cortex-biased expression, reducing interictal spikes but failing to rescue GTC seizures or behavioral phenotypes in Syngap1 heterozygous mice. In contrast, juvenile delivery achieved brain-wide expression and produced robust, dose-dependent rescue across epileptic and behavioral domains. Treated Syngap1 heterozygous mice showed reduced interictal spikes and restored oscillatory activity across slow-wave, theta, alpha, beta, and gamma frequencies. Behavioral improvements included reduced hyperactivity and normalized anxiety-like/exploratory and risk-taking behaviors in elevated zero maze tests.

This study provides the first preclinical evidence that AAV-mediated delivery of SYNGAP1 can restore function and reverse key disease phenotypes in a mouse model of SRDs. Juvenile AAV-SYNGAP1 administration, corresponding to the typical age of diagnosis in humans, rescued both epileptiform activity and behavioral deficits, underscoring its clinical relevance. These results highlight AAV-mediated SYNGAP1 gene therapy as a transformative strategy for SRDs and offer critical insights for translational development.

This project was supported through the Paul G. Allen family foundation’s continued investment in the Allen Institute and through a sponsored research agreement with BioMarin Pharmaceutical for basic scientific discovery research.