Alan Beggs, PhDDirector of the Manton Center for Orphan Disease ResearchSir Edwin and Lady Manton Professor of Pediatrics, Boston Children's HospitalHarvard Medical School, Boston, MA, USA Julie A. Parsons, MDHaberfield Endowed Chair in Pediatric Neuromuscular DisordersProfessor of Clinical Pediatrics and NeurologyUniversity of Colorado School of Medicine, Children's Hospital ColoradoAurora, CO, USADoctors Beggs and Parsons discuss the current status of gene therapies in rare neuromuscular disorders in this eight part podcast series. This is derived from the symposium that was presented at the MDA 2025 conference in Dallas, Texas, in March 2025 and is intended for healthcare professionals only. This podcast includes information about investigational compounds that do not yet have a regulatory approval or authorization for a specific indication. The safety and efficacy of the agents under investigation have not been established. In contents of this podcast, shall not be used in any manner to directly or indirectly promote or sell the product for unapproved uses. The ASPIRO clinical trial is on clinical hold since September 2021.In this part, Doctor Beggs will provide an explanation of AAV-mediated gene therapies.Alan Beggs, PhDAAV vectors, which I'm going to be talking about more today, or Adeno associated viral vectors are small viruses. Their DNA gets delivered into the cell and remains extrachromosomal. There are very rare occasional integrations, but the risk of oncogenesis as a result is significantly lower as a consequence of remaining extrachromosomal, though, we do have to think about what happens as the cells divide and potentially the durability of treatment is more limited.There have been a lot of movement and development over the years, starting back in the 1980s when the first AAV genomes were isolated and sequenced. This led to a development of methods to produce recombinant AAVs that would lack the genes necessary for viral replication, but contain a therapeutic gene you wish to deliver. Through this, the structure of AAVs have been developed. There have been isolation of a number of naturally occurring variants. You've heard of AAV8, AAV9, also RH 74, derived from a rhesus monkey for the RH. These have all been used in clinical trials. Then at the end I'll talk a little bit about directed evolution methods to actually engineer capsids with particular properties that are beneficial.Throughout this we've identified some of the issues that arise in this. It was initially thought that AAV vectors were non-immunogenic, but in fact there are immune responses not just to the viral payload to the therapeutic protein, but also to the viral vectors, and you're going to hear about that from Doctor Parsons. Over time, as we've come to understand these challenges, we've also been developing approaches to mitigate them. In terms of clinical trials and treatments, the very first studies were done back in the 1970s.By the early 2000, the very first clinical therapeutic was approved in China. It was actually an oncolytic virus carrying a p53 gene to treat head and neck cancers. By now there are over 40 approved treatments for various types of AAV delivered gene therapies. Of course, the ones we know a lot about are Zolgensma, which was approved in 2019, and Elevidys, which was approved last year. A number of challenges and then also a number of approaches to overcome those challenges. First of all, the preclinical data are not always sufficient to predict the response of a human patient.For example, in X-linked myotubular myopathy we had mouse and dog models that exhibited a myopathy but nothing else, and yet when we treated human patients, we discovered that patients with X-linked myotubular myopathy actually had a previously only poorly recognized hepatopathology that led to potential liver consequences following gene therapy. The animal models don't always predict the clinical outcome in humans.Also, we have small disease populations. These are rare diseases. It's important to understand the natural history of these diseases, understand the heterogeneity among the clinical population. It's very important to engage with families and with patients and communities, understand who might be at increased risk to treatment with one of these. This feeds into safety considerations. We need to think also about some of the immune responses. I think we're starting to learn, for example, with the gene therapies for Duchenne, and we know this from SMA that some patients get into trouble and others don't. We need to understand why that may be, and we don't know about the long term effects. This has been very recent.