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#apaperaday: Assessment of systemic AAV-microdystrophin gene therapy in the GRMD model of Duchenne muscular dystrophy

In today’s #apaperaday, Prof. Aartsma-Rus reads and comments on the paper titled: Assessment of systemic AAV-microdystrophin gene therapy in the GRMD model of Duchenne muscular dystrophy

Today’s pick is from Science Translational Medicine by Birch et al on an AAV microdystrophin study in golden retriever muscular dystrophy dogs DOI: 10.1126/scitranslmed.abo1815

AAV microdystrophin gene therapy is currently evaluated in Duchenne patients in clinical trials. Studies in mdx mice have revealed body wide distribution of microdystrophin – however, mice are very small. Authors argue that dogs models are better for gene therapy delivery.

I agree for 2 reasons:

  1. Dogs are larger so more translational to humans than a 20-30 gram mouse.
  2. Dogs live longer – impossible to study long term microdystrophin expression in a mouse with a life expectancy of 2-3 years.

An advantage of studying in animal models is that it allows studying all muscles and also other tissues – in human trials often 2-3 biopsies can be obtained. The results from animal studies can provide insight in expected variability between muscles and e.g. heart.

Here authors studied three doses of AAV microdystrophin (low: 1.10(13), medium: 1.10(14) and high 2.10(14) viral particles per kg) and no treatment in a blinded fashion in 3 dogs per group. The microdystrophin was the canine version of the Solid BioSciences trial.

Dogs were pretreated with steroids to suppress an immune response. Viral vector shedding was seen in nose, urine and stool until 90 days for high dose. For the lower doses shedding disappeared earlier in urine and stools. Dogs were sacrificed 90 days (3 months) after treatment. Virus was detected in liver, spleen and kidney and muscles. Dystrophin was restored for the medium and high doses for most skeletal muscles analyzed and in the heart (albeit at lower levels). There was variability across muscles and between dogs.

Western blot analysis and mass spec analysis confirmed dystrophin restoration. Associated proteins were restored as well on cross sections. As expected an immune response was seen against AAV after treatment. Also, antidystrophin immunity was observed.

This is something that is also seen in Duchenne patients with mutations in the beginning of the gene (NB this dog model has a mutation in the N-terminal domain so part of the micro-dystrophin is indeed a neoepitope). On histology pathology was improved for medium and high doses.

There was also less fibrosis, although fibrosis levels were still rather low at this stage in untreated and low dose treated dogs. MRI did not reveal differences between treated and untreated dogs. For muscle function authors had the challenges of small numbers per group. They therefore pooled groups: low and untreated vs medium and high dose. This showed improved force (torque) for medium and high dose and a trend for the 6 minute walk test. No significant differences were seen due to variability between dogs.

Respiratory function was more normal for medium and high doses. Heart function did not differ between any of the group but was also not impaired at this stage. Muscle damage markers (CK, ALT, AST) were reduced in high and medium dose treated dogs.

They were not fully normalized – as expected as micro-dystrophin is only partially functional (as also stressed by the authors). Authors further discuss the differences between the dog study (treatment well tolerated) and human trials (severe side effects seen in some patients) In humans especially the complement activation by AAV treatment appears to be involved in side effects. Authors also indicate that data on efficacy and safety are currently incomplete but 4 micro-dystrophin AAVs are in clinical trials.

They outline the limitations of their study: small groups of dogs were used so powering is challenging. Still this is one of the largest systematic dog studies with micro-dystrophin and authors present a wealth of data. I hope in the future longer term studies will also be published with effective doses to assess long term effects on function and heart pathology. It will also allow studying longevity of the micro-dystrophin: we know with time the amount of micro-dystrophin will reduce.

This is because AAV does not target stem cells, so with muscle turnover, micro-dystrophin transgenes will slowly decrease. As micro-dystrophin is partially functional, muscle turnover will happen. The question is how long this will take: years or decades?

I appreciate the authors performing the dog studies as they provide data that clinical trials cannot provide and look forward to additional data from this group (and others).