Patients with Duchenne muscular dystrophy have a mutated gene for dystrophin

An international team of scientists headed by groups at the U.K. Universities of Exeter, and Nottingham, has identified a way to rescue muscle cells that have genetically mutated, paving the way to a possible new treatment for Duchenne muscular dystrophy (DMD). The researchers’ approach used novel drugs that are in development at the University of Exeter, to “metabolically reprogram” mitochondria muscle cells by providing them with a fuel source to generate metabolic energy. Results from studies in the model organism Caenorhabditis elegans, and investigations in mouse models of DMD, indicate that rectifying hydrogen sulfide (H2S) deficiency in muscle could offer a therapeutic strategy to improving muscle function. The University of Exeter is spinning out a new company, MitoRx Therapeutics, focused on developing small molecule mitochondrial sulfide donor drugs, for treating rare neuromuscular disorders and rare metabolic diseases.

University of Exeter School of Medicine professor Matt Whiteman, PhD, said, “We’re really excited that our findings show that a deficit in muscle sulfide may contribute to the development of Duchenne muscular dystrophy. Rectifying this deficit may lead to new treatment approaches for this and other currently incurable diseases, without relying on potentially harmful steroids.” Whiteman and collaborators reported on their work in the Proceedings of the National Academy of Sciences (PNAS) in a paper titled, “Mitochondrial hydrogen sulfide supplementation improves health in the C. elegans Duchenne muscular dystrophy model.”

DMD is an X-linked neuromuscular disorder caused by a mutation in the dystrophin gene. The condition, which is characterized by progressive muscle degeneration and weakness, affects more than 1 in 3,500 live male births, the authors explained. Symptoms include muscle atrophy, leading to loss of the ability to walk in children. The prognosis is very poor, the researchers continued, and there is currently no cure for DMD. Existing steroid-based therapy can have side effects. “Treatments are largely targeted at controlling the symptoms and focus on maximizing quality of life,” the team noted. “Although the current standard pharmaceutical treatment, the corticosteroid prednisone, extends the ambulatory period by a couple of years, prednisone treatment is associated with several undesirable side effects, such as weight gain and behavioral difficulties, and may lead to osteoporosis.”

The newly reported work, spearheaded by Whiteman at Exeter, and Nate Szewczyk, PhD, a University of Nottingham professor, focused on alternative strategies for improving muscle performance when the dystrophin gene is missing or is defective. Working with collaborating scientists in the U.K., Australia, the United States, the Netherlands, and Germany, the team studied the C. elegans DMD model, and also mice with specific genetic mutations affecting muscle strength, which match mutations that cause DMD in humans. The researchers found that these animals had defects in gait, movement, and muscle strength. The animals also demonstrated marked defects in the structure of their muscle mitochondria, lower levels of metabolic enzymes used for the generation of hydrogen sulfide in their muscles, and lower levels of the gas itself. “H2S is a signaling ‘gasotransmitter’ that is produced endogenously in mammals and in C. elegans,” the scientists explained.

Interestingly, loss of mitochondrial structure is an early feature of both aging and DMD phenotype in C. elegans, the investigators wrote, and alterations in mitochondrial gene expression are known to occur prior to onset of symptoms in DMD C. elegans. “It has recently been shown that hydrogen sulfide (H2S), improved both survival and health of aging C. elegans by attenuating intracellular reactive oxygen species (ROS) generation and protecting against various stressors.”

Metabolic H2S deficiencies have also been implicated in conditions such as phenylketonuria, which also affects muscle mass, highlighting the potential use of H2S supplementation in muscle-related disorders, the authors commented. “In recent years, the use of H2S supplementation has been gaining attention due to its potential use in aging and age-associated diseases.”

Their newly reported studies have now shown that treating the C. elegans model roundworms with a hydrogen sulfide-releasing compound called sodium GYY4137 (NaGYY) effectively replaced lost hydrogen sulfide, and partially reversed some of the muscle and mitochondrial defects in a way that was equivalent to that of the standard of care drug prednisone. “… we found that NaGYY treatment (100 μM) improved movement, strength, gait, and muscle mitochondrial structure, similar to the gold-standard therapeutic treatment, prednisone …” the scientists commented.

Encouragingly, specifically targeting mitochondria with hydrogen sulfide using another compound, AP39, resulted in the same effects, but at a 3.7 million-fold lower dose. “Using C. elegans, we have identified pharmacologic treatments that supplement hydrogen sulfide (H2S),” the authors noted. “One, sodium GYY4137, largely acts like prednisone to improve neuromuscular health; the other, AP39, targets H2S delivery to mitochondria … AP39 treatment also improved movement and strength, suggesting that mitochondrial H2S has therapeutic value due to action in the mitochondria, such as improved cellular bioenergetics and adenosine triphosphate (ATP) synthesis, prevention of oxidant formation, preservation of mitochondrial DNA integrity, and delaying onset of cell death and inflammation.” The investigators pointed out that as the two test molecules are not steroids, they are also unlikely to produce steroid-induced side effects.”

Szewczyk, who is also principal investigator of the Ohio Musculoskeletal & Neurological Institute, commented, “Steroids are very effective and safe drugs but their use over a long period of time causes effects to wear off and they can have some very unpleasant and life-changing side effects. The compounds we’ve used in our study are not steroids and they work in a very similar way to these drugs give the same improvement in muscle function, but at a much, much lower dose and because they are not steroids, they are unlikely to produce steroid-induced side effects such as weaker muscle and decreased ability to fight infection.”

The team concluded, “The finding that NaGYY treatment recapitulated the beneficial effects of prednisone for DMD phenotypes in worms suggests that not only could H2S be examined as a potential treatment for DMD, but also for greater understanding of why H2S has therapeutic value and what the role of H2S is in the DMD pathology … Overall, we provide evidence for the use of H2S compounds, including those which target H2S delivery to mitochondria, in the treatment of DMD and raise important questions of the role of H2S in the onset and progression of DMD pathology.”

The paper’s first author, Rebecca Ellwood, a PhD student, at the Medical Research Council (MRC) Versus Arthritis Centre for Musculoskeletal Ageing Research, Royal Derby Hospital, University of Nottingham, added, “Life first emerged on earth in a sulfide-rich environment and thrived for billions of years before it was replaced by the oxygen we have today. Our cells and our mitochondria have maintained the ability to both make and use very small amounts of sulfide to keep healthy. Our study now shows that in DMD models, this metabolic pathway is defective, offering a potential for therapeutic intervention to correct this defect.”

Whiteman developed the tool compounds used in the reported study, and is also developing next-generation molecules for commercialization. He said, “At Exeter we are developing more advanced approaches to target muscle mitochondria, and we aim to spin-out a new biotech company called MitoRx Therapeutics to develop these newer approaches for clinical use during 2021.”

Kate Adcock, PhD, director of research and innovation at the charity Muscular Dystrophy UK, said: “We welcome research that increases our understanding of molecular pathways that could both contribute to the symptoms of Duchenne muscular dystrophy and offer potential new therapeutic targets. Although a long way from patient studies, this research has shown interesting results in animal models of Duchenne muscular dystrophy and it is encouraging to see these early-stage studies for such a complex, rare condition.”

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