In the latest episode of The Genetics Podcast, Patrick spoke with Dr. Jeffrey Chamberlain, Professor at the University of Washington School of Medicine and Director of the Wellstone Muscular Dystrophy Center. Chamberlain has spent more than 35 years studying the dystrophin gene and advancing gene therapy for Duchenne muscular dystrophy (DMD). The conversation traces the early discovery of the gene, milestones in vector development, and the challenges the field must solve to reach consistent clinical benefit.
Jeffrey entered the field shortly after the dystrophin gene was first cloned in the late 1980s. At the time, few human disease genes had been identified and the size of the dystrophin locus made it difficult to study. His early work confirmed that the mdx mouse was a valid Duchenne model and helped define deletion hotspots that frequently appeared in patients.
This led to the development of multiplex PCR, one of his most influential contributions. Before this innovation, diagnosis relied on Southern blotting that was slow, labor intensive, and difficult to apply to a gene spanning more than 2 million base pairs. Multiplex PCR allowed nine regions to be tested at once and became the global standard for DMD diagnosis and carrier testing for almost two decades.
When Jeffrey established his own lab, he focused first on basic biology. By assembling full length cDNAs and creating transgenic mouse models, his team studied which regions of dystrophin were essential and which tissues required expression.
One key finding was that expressing dystrophin only in striated muscle was sufficient to eliminate detectable disease features in the mouse model. Another was that many patients with in-frame deletions had mild disease, which showed that large portions of the protein could be removed without complete loss of function. These insights guided his team’s effort to design shorter therapeutic versions of dystrophin. The result was micro-dystrophin, a construct under 4 kilobases that preserved essential domains and remained functional in animal models.
Throughout the 1990s, researchers explored many delivery approaches including plasmid injections, electroporation, retroviral vectors, and adenoviral vectors. Most produced only local expression or caused inflammation that limited their usefulness.
AAV vectors changed the direction of the field. Working with colleagues at the University of Washington, including David Russell, Jeffrey helped identify AAV6 as a serotype with strong muscle tropism. Improvements in vector production increased titers to levels that made new types of experiments possible.
A pivotal moment came when the group showed that once a critical concentration of AAV6 was reached in the bloodstream, the vector could exit capillaries and reach muscles throughout the body. Their publication in 2004 demonstrated the first systemic delivery of micro-dystrophin using AAV. This work forms the basis of modern gene therapy strategies for neuromuscular disease, including approved treatments for spinal muscular atrophy.
More than 1,500 boys have now received systemic AAV micro-dystrophin from several companies testing different serotypes, promoters, and cassette designs. Some children show stabilization of motor function and limited gains, typically accompanied by measurable micro-dystrophin expression.
The effect sizes remain smaller than those seen in mice and dogs. Jeffrey noted that the amount of micro-dystrophin produced in human muscle is generally lower than expected, even at high doses. Trial participants are usually at least four years old and already have inflammation and fibrosis that make delivery and expression more difficult.
Safety limits are another constraint. Systemic AAV requires high doses, and adverse events such as liver injury, complement activation, and rare fatalities have occurred across neuromuscular programs. These risks limit dose escalation as a strategy for improving efficacy.
Jeffrey believes that the field now needs vectors that reach muscle at lower doses, therapeutic constructs with greater potency, and approaches that allow treatment earlier in life.
Jeffrey is optimistic about the long-term promise of gene editing but realistic about its challenges for Duchenne today. The dystrophin gene is extremely large and mutations vary widely. Early editing methods that target individual exons would require many separate products to reach most patients.
Classical Cas9 creates double strand breaks, which can lead to unwanted deletions or rearrangements. Systemic delivery of Cas enzymes still depends on high dose AAV and introduces immune risks because the enzymes originate from bacteria.
Newer tools such as base editors and prime editors may reduce these problems. Next generation myotropic AAV capsids may also lower the required dose. Jeffrey is especially interested in strategies that insert larger cDNA segments to correct broad categories of mutations. Although promising, these approaches are still at an early stage.
The team is working to close the gap between animal studies and human outcomes by improving delivery and increasing the potency of therapeutic constructs. Current efforts include:
Jeffrey emphasized that gene therapy for Duchenne is not yet a cure. However, the field now has a clear view of the main barriers and a growing set of technologies that can address them.
Listen to the full episode below.