Rare diseases and the power of precision medicine

When molecular biologists Steve Wilton and Sue Fletcher first started exploring dystrophin exon skipping for Duchenne muscular dystrophy (DMD), they faced a lot of scepticism.

Fast forward 20 years, and the drug developed by the researchers has altered disease progression — dramatically improving the quality of life of DMD sufferers. Strong support from the local Western Australian and US-based Muscular Dystrophy Association was a powerful motivator.

Caused by mutations in the DMD gene, the disorder affects 1 in 5000 males around the world, according to Muscular Dystrophy Western Australia. The disorder typically affects boys but girls can be carriers and in very rare instances present with the disease. Symptoms include delayed walking, inability of children to run and jump, followed by progressive muscle weakness and breathing difficulties.

The dystrophin gene has 79 exons spanning 2.3 megabases. In the severe forms of DMD, there is a protein truncating mutation so the end of gene message is missing — these kids have the most severe phenotype and typically become wheelchair bound before the age of 12, explained Wilton. One of the most common types of mutation in this gene is a deletion of one or more exons. If a deletion disrupts the reading frame, the result is Duchenne but if the deletion maintains the reading frame, the consequence is Becker muscular dystrophy, a milder form of the disease.

The drug developed by Wilton and Fletcher, now at Murdoch University, is designed to skip over the disease causing part of the gene message and reframe the message. For example, if a patient is missing exon 50, the reading frame is lost and the message is terminated in exon 51. Hence, by skipping exon 51, the early stop signal is ‘skipped over’ during splicing and the reading frame is restored.

This is the first splice switching drug to address the underlying genetic defect of Duchenne, and the first of its type to be approved by the US Food and Drug Administration (FDA), according to Wilton. The rights to develop the drug have been licensed through the University of Western Australia to Sarepta Therapeutics, a Massachusetts-based commercial-stage biopharmaceutical company.

In September 2016, Sarepta received accelerated approval for Exondys 51 from the FDA. The drug has drastically improved the health and wellbeing of DMD sufferers during clinical trials, according to Wilton. A number of young boys suffering from DMD who would have been in a wheelchair are still walking, thanks to the treatment. The longer these boys stay out of a wheelchair, the progression of their disease has been delayed.

Exondys 51 is the first dystrophin-restoring drug that has shown a modest but unequivocal increase in the missing protein dystrophin, after treatment, according to Wilton. Exondys 51 will be relevant to about 1 in every 10 DMD patients, as DMD is caused by numerous different mutations in the same gene, but Professors Wilton and Fletcher have developed a panel of ‘genetic patches’ for other dystrophin spelling errors causing DMD. Over time, they hope that treatments should become available for the vast majority of DMD sufferers.

Sarepta also has second- and third-generation DMD drugs in the pipeline that are “going to be even more potent”, according to Wilton. The researchers now have a proof-of-concept that they can change disease progression. Now, they are working on improving the treatment further.

When asked if CRISPR could provide a permanent solution, Wilton said that the gene editing has to be absolutely specific and as soon as there is a guarantee that there are no off-target effects, CRISPR could be one way of doing it. Our exon skipping approach cannot be regarded as a cure. We aim to reduce disease severity and progression by redirecting a DMD gene to make a semi-functional Becker-like protein, said Wilton. “What we are hoping for is either gene or cell replacement therapies become more efficient or CRISPR/Cas comes along and that the technology improves. I’m not going to say any of that cannot work, but right now we are able to buy more time for some DMD boys/young men with exon skipping.”

Professor Wilton and his team are also applying these drugs to look into treating other conditions, including cystic fibrosis, multiple sclerosis and spinal muscular atrophy, the most common genetic cause of death under the age of two.

In November, Murdoch University researchers Wilton and Fletcher received significant funding from the National Health and Medical Research Council (NHMRC) to develop genetic therapies for rare diseases. Professor Wilton, the Foundation Chair in Molecular Therapies at Murdoch University and Director of the Perron Institute for Neurological and Translational Science and co-head of Molecular Therapy Laboratory with Professor Fletcher in the Centre for Comparative Genomics at Murdoch University, will lead the project, which will receive $800,000 over the next three years.

This project will build on two decades of research by Professor Wilton and Professor Sue Fletcher that has resulted in the first-ever treatment to have altered the progression of the fatal disease Duchenne muscular dystrophy, according to Murdoch University.

“We have exploited the fact that some genes linked to inherited diseases have sections that are potentially dispensable,” Professor Fletcher said. Our technology — exon skipping — acts as a genetic whiteout that tricks cells into skipping over a genetic mutation, said Fletcher.

“This has resulted in a far better functionality of the dystrophin protein and improved function in DMD, and we believe these strategies can be applied to some other genetic diseases.” Rare diseases affect around one in 12 Australians, but there are over 6000 known rare diseases, which means often there are only a handful of men and women with a disorder. Rare diseases pose challenges to researchers — they can’t be studied through conventional trials, patient numbers are limited and getting an accurate diagnosis is difficult and costly. However, through research and personal advocacy, Professor Wilton has increased the profile of rare diseases.

The Murdoch research team will focus initially on eight genes that contribute to 46 serious inherited disorders. “This technology provides an exciting new platform to develop targeted therapies for a host of rare diseases that currently do not have successful treatments,” Professor Fletcher said.

“We anticipate that several potentially therapeutic compounds will be developed during the course of this project.” The Murdoch University team will work with long-term collaborators at UWA, Orthocell and Sarepta to develop the new therapies.

Murdoch University is also involved in six successful projects totalling over $5.5m led by UWA, TKI, Griffith University and Curtin University.

Wilton and Fletcher’s research has received numerous awards, including the 2012 WA Innovator of the Year Award, 2013 Australian Museum Eureka Prize for Medical Research Translation, the 2014 LabGear Australia Discovery Science Award, and selection of Professor Wilton as a finalist in the 2016 Western Australian of the Year (Professions category).

Image caption: Professor Steve Wilton. Image courtesy of Murdoch University.