Impressions of the 2024 STXBP1 Researcher Roundtable Meeting

The 2024 STXBP1 Researcher Roundtable meeting was held this past July 18th and 19th just outside Philadelphia in the quaint suburb of Drexel Hill. This year’s meeting brought together the insights and expertise of over 100 researchers, clinicians, and pharma folks from around the world to share and discuss the newest advances in our understanding of STXBP1 and STXBP1-related disorders.

Here are my 3 main takeaways from the meeting:

1. We are still learning new things about STXBP1.

The STXBP1 protein and its primary function in synaptic release have been studied for decades; however, as the years have gone on, more and more interesting findings are uncovered concerning this fascinating protein. This was evident during the meeting, which began with three talks focusing on functions of the STXBP1 protein beyond neurotransmitter release. Elina Kostyanovskaya (UCSF) revealed that STXBP1 is associated with primary cilia—a cellular structure on neurons that acts like an antenna, receiving environmental signals and transmitting them into the neuron. Defective cilia, as might occur in STXBP1-RD, can lead to human development disorders called ciliopathies. Elina’s colleague tested this by measuring nitric oxide gas release from STXBP1 individuals’ noses during the Family Summit; we await the results. Tristan Wallis (Queensland University, Australia) described STXBP1’s interaction with DDHD2, a molecule linked to free fatty acid (FFA) generation. Certain FFAs have been revealed to be involved in learning and memory and Tristan showed that generation of FFAs are disrupted in Stxbp1+/- mice. These findings may help to explain some of the cognitive impairments observed in STXBP1-RD. Lastly, Praveen Papareddy (Lund University, Sweden) discussed how STXBP1 might impact our immune response by controlling the release of extracellular vesicles (EVs) from cells. EVs facilitate communication between cells, including those involved in generating an immune response to something like a bacterial infection. 

Additional new insights into how mutations in the STXBP1 gene lead to the constellation of symptoms observed in STXBP1-RD were also presented during the meeting. Jacquelin Burre (Weill Cornell Medicine) discussed how mutations in the gene trigger dysfunction in not only syntaxin-1, its well-known binding partner, but also three other proteins, syntaxin-3, syntaxin-5, and Doc2, which could contribute to the various pathologies we observe in patients. Amparo Roig Adam (VU Amsterdam) explained that haploinsufficiency in Stxbp1+/- mice leads to changes in the genes that are expressed in certain types of neurons. Furthermore, she found that the exact type of neuron and the exact genes affected were unique to STXBP1 compared to other neurodevelopmental disorders. The STXBP1 gene harbors numerous mutations, some causing STXBP1-RD, others having no disease association, and many classified as variants of uncertain significance (VUSs), or variants where the potential to cause STXBP1-RD is unknown. Caroline Pearson (Weill Cornell Medicine) and Jeffrey Calhoun (Northwestern University) presented their work as part of the EpiMVP project aimed at predicting whether specific VUSs are likely to lead to disease. Their approach combines testing known patient mutations in cultured neurons (functional modeling) with machine learning. 

2. The development of new therapies is progressing at a rapid pace.

Just a few years ago the only potential new therapy for STXBP1-RD was the repurposed drug, 4-phenylbutyrate (more on this later), but currently there are several precision therapeutic approaches making their way through preclinical drug development, as summarized nicely during the meeting by Mike Boland (UPenn/ENDD), and hopefully within a couple of years several will be starting clinical trials. 

One precision therapy approach is to add a normal, working, copy of the STXBP1 gene into neurons using an adeno-associated virus (AAV) based delivery system. Suresh Poda from Encoded Therapeutics described the development of a gene therapy vector that uses an AAV virus already approved for human use. Initial testing in mice and nonhuman primates has been promising. Charles Chen (Baylor College of Medicine) shared data on how different doses of this AAV gene therapy can improve behavioral deficits in Stxbp1+/- mice. This study, in collaboration with Capsida Biotherapeutics, aims at identifying optimal doses for a clinical trial. In a separate talk, Swati Tole and Alberto Lopez (from Capsida) introduced a new AAV vector designed to deliver genes into the brain following an intravenous injection. Similarly, Beverly Davidson (CHOP/ENDD) described how her lab is developing new AAV vectors that can be directed directly into the nervous system to more easily target the brain, or specific brain structures, minimizing potential liver toxicity—a significant concern with current AAV-based approaches.

Ben Prosser (UPenn/ENDD) and Xuebing Wu (Columbia University) presented talks on using antisense oligonucleotides (ASOs) to target specific regions in the STXBP1 RNA. Their goal was to enhance protein production. Notably, therapies based on this approach don’t rely on a virus-based delivery system and have already received FDA approval for treating other rare diseases, though they require repeated administration over time. Luis Williams, from Quiver Bioscience, discussed a novel method for assessing synaptic function in thousands of cultured STXBP1+/- neurons simultaneously. This technique allows their team to evaluate the therapeutic effects of ASOs and other drugs. Lara Janssen (VU Amsterdam) explored the role of ‘poison exons’ in generating disabled STXBP1 RNA transcripts within neurons. Disrupting these exons could potentially increase functional STXBP1 RNA and, consequently, protein levels. Additionally, Molly Reilly (UPenn/ENDD) described a CRISPR activation (CRISPRa) approach aimed at boosting RNA production from the STXBP1 gene. 

In addition to the development of precision therapeutic approaches, drug repurposing continues to be an important avenue of research. Zach Grinspan gave an update for the ongoing clinical trial using phenylbutyrate to treat STXBP1-RD. After approximately 2-3 years, about 50% of the enrolled STXers are showing a response to the drug as defined by a reduction in seizures. Chris Hopkins (Devine Bio) and Trisha Brock (InVivo Biosystems) discussed the use of worms (C. elegans) and zebrafish, genetically modified with patient-specific STXBP1 mutations, to evaluate the potential of repurposed drugs to treat STXBP1-RD. Sagi Gidali, whose son Rafael has STXBP1-RD, shared how he and his wife, Ella Gordon, established Rafa’s Moonshot and have already identified one repurposed drug candidate and continue to identify others while also supporting research into precision therapies. 

3. We are rocking our clinical trial readiness efforts.

With the remarkable advances in therapeutic development, we must prepare for the clinical trial stage of these drugs. The identification of biomarkers and the collection of natural history data are vital towards this goal. During the meeting three categories of biomarkers under development for STXBP1-RD were discussed: biochemical, electrophysiological, and functional. Wendy Gold (University of Sydney) is using an integrative -omics approach, combining transcriptomics, proteomics, and metabolomics to study biomolecule differences between individuals with STXBP1-RD and related controls. Ganna Balagura (University of Genoa) is extending this approach by assessing gut bacteria (microbiota). Evidence suggests that communication between gut microbiota and the brain could influence disease progression or drug response. Electrophysiological biomarkers were also discussed. Jay Pathmanathan (Beacon Biosignals) described specific EEG patterns in individuals with developmental epileptic encephalopathies (DEEs) and introduced a wearable device for home EEG data collection. Peter Galer (CHOP) analyzed EEG data from STXBP1-RD patients, revealing a specific decrease in the alpha/delta ratio compared to neurotypical controls and other DEEs. Additionally, Hannah Stamberger (University of Antwerp) explored gait analysis as a tool to assess drug effectiveness on sensory, motor, and cerebellar systems in STXBP1-RD patients.

The collection of natural history data through the STARR Study was the subject of talks from Ingo Helbig, Sam Pierce, and Kristin Cunningham from CHOP and Andrea Miele from Children’s Colorado. With 100 participants now enrolled in the study they presented data on seizures, motor, behavioral, and cognitive assessments in STXers of various ages. So far, the data is very encouraging that multiple clinical outcome measures will be amenable for use in assessing therapeutic efficacy in future interventional drug trials. Collection of data will continue as the STARR Study moves into a new longitudinal phase. Juliet Knowles also gave a talk on setting up a new STARR Study clinic site at Stanford University and how the site will also be used to test repurposed drugs being investigated by her lab. Longitudinal data from 25 STXBP1-RD patients in Canada was described by Alex Freibauer from the University of British Columbia. This 3-year study found a slight decline in adaptive functioning and is ongoing. Antonio Gil-Nagel from Hospital Ruber Internacional in Madrid presented data on a cross-sectional study of Spanish patients and provided some initial findings suggesting that the location of  missense variants in the 3-dimensional structure of the STXBP1 protein could have an effect on the severity of symptoms. Constance Smith-Hicks (Kennedy Krieger Institute/Johns Hopkins) discussed sleep disruption in STXers, how to measure sleep, and how sleep disturbances in the population affect behavior and quality of life. Lastly, Francesca Furia from the Danish Epilepsy Center presented study data on 40 cases of early mortality in STXBP1-RD. The study found that STXBP1-RD has a similar mortality rate to other DEEs. SUDEP (sudden unexpected death in epilepsy) accounted for about 1/3 of cases examined and, as expected, tended to occur in mid-childhood and adolescence. There appeared to be no effect of genotype on early mortality.

This year’s Researcher Roundtable continued to showcase the hard work and dedication of our researchers and clinicians whose efforts expand our basic, translational, and clinical knowledge of STXBP1. Recordings of the presentations have been posted on our website.

Next year’s meeting will take place in Heidelberg, Germany on October 8th – 10th.

James Goss