The Progress of Micro-Dystrophin Gene Therapy

October 21, 2018

Since the inception of the idea more than 3 decades ago and its initial development 20 years later, Sarepta Therapeutics’ micro-dystrophin gene therapy has now made its way to human trials.

Louise Rodino-Klapac, PhD

Although it is still in early phase testing, Sarepta Therapeutics’ micro-dystrophin gene therapy, being developed for the treatment of Duchenne Muscular Dystrophy (DMD), has come a long way already.

Just after reiterating recently announced positive results from an early stage, open-label trial of 4 patients, a presentation by Louise Rodino-Klapac, PhD, the president of gene therapy at Sarepta, given at the 143rd Annual Meeting of the American Neurological Association in Atlanta, Georgia, detailed its journey to this point.1

The original challenge faced in the therapy’s progress was the inability to use full-length dystrophin with adeno-associated virus (AAV) viral vectors, as the gene is 14 kb in length. Ultimately, Rodino-Klapac noted that in order to fit the transgene into the viral vector along with its promoter, it would need to be about 3.5 kb.

“Really the idea behind many micro-dystrophins came about almost 30 years ago,” Rodino-Klapac said. “It was really identified based on a natural experiment of Kay Davies’s lab, where she identified a patient, a 61-year-old ambulatory patient with [Becker muscular dystrophy] that had 46% of the dystrophin gene missing.”

At the time, in their work,2 Davies and colleagues noted that their results were “particularly significant in the context of gene therapy which, if it is ever envisaged, would be facilitated by the replacement of the very large dystrophin gene with a more manipulatable mini-gene construct.” They had developed the theory that that patients with DMD possess deletions that disrupt the reading frame of the protein, whereas BMD patients have deletions that keep the translational reading framework, allowing the muscle cells to produce altered versions of dystrophin products.

The deletion in that patient ranged from exon 17 to 48, a large portion of the center of the gene. Rodino-Klapac noted that this work led to the development of a micro-dystrophin, though it was about 6.5 kb in length—still too large for use with AAV viral vectors. “But it really led to the idea of how we could further modify these dystrophins to be able to fit,” she said.

The construct used by Sarepta Therapeutics began over a decade ago, with the small micro-dystrophin the investigators kept coming back to carrying similarities to what Jeff Chamberlin and Scott Harper identified in 2002.3 One of the attractive points of this version, Rodino-Klapac explained, was that the hinge regions included in the micro-dystrophin were all located in their natural orientation within the gene.

“One of the things we spent a lot of time on was the potential immunogenicity of micro-dystrophin,” she said. “We’re injecting this micro-dystrophin into patients with very large deletions, and the potential for an immune response to the transgene is a possibility, so trying to make sure that we don’t illicit novel epitopes within the gene was important. There’s really only 1 novel epitope within this gene.”

Rodino-Klapac noted that the AAVrh74 vector was selected due to its robust affinity for muscle, which allowed for wide distribution, as well as its relatively low pre-existing immunity characteristics. The promoter chosen is the MHCK7 promoter developed by Stephen Hauschka, PhD, of the University of Washington.

“This promotor is specific to skeletal muscle, but then has this myosin heavy gene enhancer that allows it to express in cardiomyocytes,” she explained. “This is important for this disease because we want to make sure we’re protecting the heart in these patients.”

This model was then tested in preclinical mouse models with success and has since been explored in the aforementioned small cohort of 4 patients. The mean gene expression for the study, as measured by the percentage of micro-dystrophin positive fibers, was 81.2%, and the mean intensity of the fibers was 96.0% compared to normal control.

The therapy was also shown to be safe, with some transient nausea observed, though it was believed to be associated with increased steroid dosing. To adjust for this in the larger planned trial, Rodino-Klapac explained that the control group will also have additional steroid dosing added on to their qualifying stable regimen.

REFERENCES

1. Rodino-Klapac L. AAVrh74,MHCK7.micro-dystrophin gene therapy for DMD. Presented at: 143rd Annual Meeting of the American Neurological Association; October 20, 2018; Atlanta, Georgia.

2. England SB, Nicholson LV, Johnson MA. Very mild muscular dystrophy associated with the deletion of 46% of dystrophin. Nature. 1990;343(6254):180-182. doi: 10.1038/343180a0.

3. Harper SQ, Crawford RW, DelloRusso C, Chamberlain JS. Hum Mol Genetics. 2002;11(16):1807-1815. doi: 10.1093/hmg/11.16.1807.