Read-through of nonsense mutations in Duchenne muscolar dystrophy.
Duchenne muscular dystrophy (DMD) is a devastating disease caused by mutations in the gene called dystrophin. DMD-causing mutations vary in nature and account for deletion, replications, duplications or single point mutations which disrupt the coding reading frame of the dystrophin gene. The result is absence of dystrophin expression into skeletal and cardiac muscles. Among the single point mutations, 10 to 15% are, so called, nonsense mutations. They are caused by a change of a single base in the DNA that prematurely ends the translation of a gene into a protein. This occurs after the gene is translated into mRNA and during the translation of the mRNA into protein.

The result is the expression of a truncated (non-functional) protein. Certain compounds like gentamicin or more recently Ataluren (PTC124) have shown to suppress the mutation by interfering with the premature termination of protein synthesis during mRNA processing.

However, clinical trials using both drugs have produced only limited effects. During the past years, there has been increasing interest in identifying new drugs that could read through the premature stop codon and counteract the genetic defect.

In 2009, new read-through compounds (RTCs) with promising activity were identified at the University of California, Los Angeles (UCLA) using high throughput screening. This labor-intensive project sought the involvement of different groups and campus resources including the California NanoSystems Institute and the UCLA Molecular Shared Screening Resource center and was led by the laboratory of Dr. Richard Gatti, professor of pathology and laboratory medicine and of human genetics at the David Geffen School of Medicine. The screening of thousands of small molecules identified two promising RTCs. The results were published in the Journal of Experimental Medicine. In that article, my laboratory demonstrated the ability of RTC13 and RTC14 to suppress the nonsense mutation in muscle cells in culture isolated from the mdx, a mouse model for DMD.

These encouraging results have prompted my research group to further test the RTCs in vivo and to determine their therapeutic relevance for the treatment of DMD.

In the new study that we published in the journal of Human Molecular Genetics we demonstrate that RTC13 restores significant amounts of dystrophin into muscle and that the effects are superior to those seen with gentamicin or PTC124.

Importantly, our results demonstrate that even a limited number of treatments can achieve significant beneficial effects in the mouse model. The compound was well tolerated in animals suggesting that it could be safe to use in humans. These results are noteworthy because they open new hope for the treatment of qualifying DMD patients that could benefit from a drug that suppresses nonsense mutations.

The next step is to test whether an orally viable formulation of the drug can achieved sustained levels of dystrophin expression and its overall effects on muscle pathology. The work that lies ahead will determine whether the drug can be moved into clinical trials and has reasonable chances of entering the clinic.

The cost of developing a drug, from its early stages of drug discovery to its first testing in humans and its commercialization has been estimated to be in the range of 10 to 14 million dollars and in difficult economical times such the current ones, it is not easy for many laboratories to find this type of resources.

We owe much of the success achieved with RTC13 to the Muscular Dystrophy Association of the US, the world’s largest non-profit organization sponsoring research for neuromuscular disorders.

The MDA has supported the preclinical studies in the mdx mouse and has allowed us to gather the preliminary data necessary to obtain additional funding from federal agencies like the National Institute of Health (NIH).

In my opinion, this is a perfect example of how everybody can make an impact in advancing basic and clinical research through donations. We hope our studies will ultimately lead to the development of an effective therapy that could benefit thousands of patients diagnosed worldwide.

Carmen Bertoni