Decline of olfactory function in Parkinson's disease: A ten-year longitudinal study.

(also known as ) is an essential component of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex, which mediates synaptic vesicle exocytosis and synaptic transmission. Heterozygous (HET) pathogenic variants in the gene lead to a spectrum of early-onset neurodevelopmental and epileptic disorders, which can result in severe developmental delay, treatment-resistant seizures, and sudden unexpected death in epilepsy. While a diverse range of mutations cause STXBP1-related disorders (STXBP1-RD), haploinsufficiency leading to reduced synaptic vesicle release is generally recognized as the dominant underlying disease mechanism. No disease-modifying therapies exist for STXBP1-RD, and current treatment is focused on symptom management with anti-seizure medications and supportive care. Gene therapy offers the potential for a transformative, disease-modifying approach for STXBP1-RD. Adeno-associated virus (AAV)-based vectors are currently the most widely used and well-established technology for safe and effective delivery of genetic therapies. In particular, AAV9 has emerged as the leading viral vector for central nervous system (CNS)-targeted gene therapies, gaining widespread clinical approval; thousands of patients to date have received AAV9-based therapies. However, CNS disorders are a challenging target for gene therapy treatment due to the rich cell type diversity and systems properties of the brain, and the need for efficient and potent transgene delivery. In addition, off-target overexpression in the liver and dorsal root ganglion (DRG) are known potential toxicities associated with AAV vector administration. While influenced by serotype, dose, and route of administration, restricting transgene expression to target cells helps reduce this risk. is expressed pan-neuronally, with subclasses of excitatory and inhibitory neurons contributing to distinct phenotypic components of disease pathogenesis and excitatory/inhibitory (E/I) balance. Mouse models with targeted loss-of-function in either gamma-aminobutyric acid (GABA)-ergic/glycinergic or glutamatergic neurons recapitulate distinct phenotypes, suggesting that restoring expression in both cell types may be required for complete rescue. Achieving balanced rescue across excitatory and inhibitory populations may be similarly important for maintaining network properties, which are disrupted in STXBP1-RD. On-target expression in these neuronal subtypes may be key to therapeutic efficacy; however, off-target AAV class effects in the DRG and liver have led to dose-limiting toxicities, presenting a key limitation for clinical development of CNS gene therapies. While early genetic therapies have relied on ubiquitous viral promoter elements such as cytomegalovirus early enhancer/chicken beta-actin (CAG) or cytomegalovirus immediate-early enhancer (CB) to drive high levels of transgene overexpression, expression cassettes tailored to the desired therapeutic target are now considered an important component in the design of AAV vectors. Therefore, we aimed to develop a gene replacement AAV9 candidate for STXBP1-RD, building on clinically established delivery modalities, while incorporating both potent and specific cell coverage across excitatory and inhibitory neurons with reduced expression in DRG and non-neuronal tissues.