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A paper just out in PLOS Biology is unable to confirm that the drug Ataluren is able to read through premature stop codons (nonsense mutations, for example 255X, 168X). The drug is already in clinical trials for Cystic Fibrosis and Duchenne’s muscular dystrophy. The new data highlights not only the difficulties in drug development and the importance of rigorous clinical trials but also the need for independent confirmation of data. This drug is currently being tested in a mouse model of Rett Syndrome with a 168X mutation.
Drug Mechanism Questioned
A study fails to confirm that the small molecule PTC124, in development for multiple genetic disorders, aids in read-through of premature stop codons.
By Kate Yandell
An in vitro study of PTC124, a drug in clinical trials for the treatment of cystic fibrosis and several other genetic disorders, has some scientists wondering whether the molecule works the way its developers say it does.
PTC124 is intended to treat disorders, including some cases of cystic fibrosis and Duchenne muscular dystrophy, that stem from mutations that cause stop codons to erroneously appear in critical gene transcripts, thereby inhibiting translation of vital proteins. The new study, published today (June 25) in PLOS Biology, demonstrates that PTC124 does not promote read-through of such premature stop codons, or “nonsense” mutations, in several reporter assays done using a human cell line.
“The paper is well designed and carefully executed, and the dataset is unambiguous,” said Bryan Roth, a professor of pharmacology at the University of North Carolina at Chapel Hill who was not involved in the work, in an email to The Scientist. “I think these are important findings which cast doubt on the role of nonsense suppression on the actions of PTC124.”
“From a basic mechanistic standpoint, it doesn’t look like [PTC124 is causing] translational read-through, certainly in our assays,” said Stuart McElroy, a molecular pharmacologist at the University of Dundee in Scotland and an author of the paper.
But representatives from PTC Therapeutics, the New Jersey biotech company that is developing the drug, said that it has produced strong evidence that PTC124, also called ataluren, does encourage the translation of complete proteins despite the nonsense mutations. “Numerous independent laboratories have provided confirmation of our results, demonstrating ataluren’s read-through activity in studies using reporters as well as multiple animal and cell-based nonsense mutation disease models,” the company said in an emailed response to questions sent by The Scientist.
A year and a half ago my wife, Rachel, and I received the worst phone call of our lives—it was the Children’s Hospital of Pennsylvania informing us that our beautiful, bright-eyed, giggling two-and-a-half year old daughter, Eleanor, had tested positive for the MECP2 mutation that causes Rett Syndrome. We were simply devastated and didn’t know what to do or where to turn. The ensuing months were the hardest of our lives. Our dreams and hopes for our only child had been crushed. We watched helplessly as Eleanor stopped scooting on her rear end, as she had been doing, and developed a repetitive hand motion and other symptoms.
Several months later, Rachel found the RSRT website. We made a call and RSRT’s co-founder and executive director, Monica Coenraads, answered the phone. We soon learned that we were not alone in our sadness and that there was a community of wonderful, supportive parents, grandparents, family members, and friends of families who have been impacted by Rett Syndrome. And even more exciting, we learned that there was hope for a cure for Rett. In fact, it was clear that there was a good deal more than hope—Rett symptoms had been reversed in an animal model, and very promising scientific progress was being made, much of it encouraged and supported by RSRT.
Monica and I continued to correspond. I was seeking her advice and thoughts about Eleanor’s diagnosis, and I was trying to understand the research and science. Monica began seeking my advice about fundraising and public relations when she learned that I headed the development office of the Woodrow Wilson Foundation in Princeton, New Jersey, and before that had served as director of development at Columbia University’s Teachers College. It may have dawned on Monica and me at the same time that there was a fit here—that I cared deeply and personally about the work that RSRT was doing and that I had nearly 20 years of experience that could help grow RSRT and its impact on the lives of girls and women with Rett.
The rest, to use the cliché, is history. My first official day as a Program Director at RSRT was June 17. Under Monica’s direction, I’m responsible for fundraising, public relations, and strategic thinking about the organization. I couldn’t be more excited. So many people, Monica foremost among them, have worked so hard and contributed so much to making RSRT the respected force that it is and to building the cumulative scientific knowledge that will lead to a cure. I’m honored and humbled to join this team. Frankly, I never imagined that I would be able to put my knowledge and skills to use for something so important to me.
I’ve gone on longer than I intended, but I have one further thought. I’ve been thinking lately about President Kennedy’s 1961 speech to Congress in which he announce the dramatic and ambitious goal of sending an American to the Moon before the end of the decade. It took a huge team effort, conviction and confidence in a very clear mission, ample resources, and leadership, for the goal to be met. I think RSRT’s goal is not unlike President Kennedy’s in its ambition, its clarity, its importance, and its attainability. I also believe that all of us working together are the team that will get us to the moon. And, like building a rocket, a cure depends on cumulative knowledge that is the sum of its parts. The rocket needs its nose cone, its fuel tank, its electronics, its landing pads, and other components to meet the goal. Research is like this too. All the parts have to work together. It is cumulative knowledge that will get us there.
I am tremendously grateful to Monica, to the RSRT Board, and to all of you who contribute your time, energy, and resources to RSRT for your confidence in me. I promise Eleanor and all of our daughters that I will do my best in everything I do for RSRT. I will need your help, advice, and counsel—most of you know RSRT and all of its accomplishments and the Rett community at large far better than I do—so I hope I can call on you. Please don’t hesitate to contact me any time. My office line is 609-309-5676; my cell is 609-815-5102; and my email is email@example.com. I look forward to meeting you.
It stands to reason that in our battle to cure Rett Syndrome it would be of great benefit to understand the function of the “Rett protein”, MeCP2. Towards this end RSRT launched the MECP2 Consortium in 2011, a unique $1.8 MM collaboration between three distinguished scientists, Adrian Bird, Michael Greenberg, Gail Mandel. On June 16th the first two publications from this collaborative effort are published in Nature Neuroscience and Nature. Together these papers provide further clarification of the elusive function of the MeCP2 protein and how mutations within it contribute to Rett.
We thank Kathy and Tony Schoener whose visionary $1 MM gift made the Consortium possible. We thank all of our donors and parent organizations worldwide who support us, in particular our funding partners Rett Syndrome Research Trust UK and the Rett Syndrome Research & Treatment Foundation.
We are providing a variety of resources to help you understand the progress being reported today.
Animation of Nature Neuroscience Paper (courtesy of Jeff Canavan)
Interview with Matt Lyst, post-doc in Bird lab
Interview with Michael Greenberg and Dan Ebert,
post-doc in Greenberg lab
We break down a few interesting results recently published.
Multiple mutations on MeCP2
In most cases of Rett syndrome, one mutation occurs on (a single copy of) the MeCP2 gene. But, rarely, multiple mutations occur on the same copy. Researchers from the University of Alabama at Birmingham characterized the largest group of individuals to date, 15 of them, who have more than one mutation. (The girls were participating in the Rett Syndrome Natural History Study, which aims to gather detailed historical and physical examination data on a large cohort of females with Rett — of which there are now more than 800.)
In contrast to two previous case studies suggesting that girls with more than one MeCP2 tend to have more severe disease, the new study found that participants with multiple mutations assessed using two quantitative measures — the Clinical Severity Scale and the motor behavioral analysis — showed no difference from those individuals with single, similar mutations. The findings appear in the American Journal of Medical Genetics Part A.
Other factors, including the individual mutations themselves or how much of the mutated gene is inactivated as part of normal development, can worsen symptoms, the authors suggest. Future drug therapies that account for a person’s genetics should consider this small group of individuals, the authors add.
Rett Syndrome and epilepsy
Seizures affect 50% to 90% of individuals with Rett syndrome. According to a recent review in Pediatric Neurology by Alison Dolce and her colleagues at Johns Hopkins Hospital in Baltimore, Maryland:
- It’s unclear whether earlier onset of seizures signals a poorer overall prognosis.
- EEG, or electroencephalography, an indirect measure of brain activity, is an important diagnostic tool in children with Rett syndrome because it allows clinicians to identify true seizures — as opposed to symptoms, ranging from hyperventilation or motor abnormalities, that are often mistaken for seizures. From 2 to 4 years of age and beyond, all girls with Rett syndrome develop abnormal EEGs, the authors write.
- There are few studies assessing anticonvulsant therapies in Rett, and many of them are short-term and involve only a small number of participants. “At our centers, we have circumstantial evidence of good responses to levetiracetam, lamotrigine, and topiramate,” the authors write. Although the clinicians also use valproate, the side effects can be problematic. In general, many children with Rett require more than one medicine.
- It’s still difficult to predict which patients will tend to develop seizures early and also whose seizures will be easier to treat.
- According to the authors, physicians should begin to consider the ketogenic diet and vagal nerve stimulation earlier in the clinical course of Rett, because these treatments “might be helpful.”
More on MeCP2 mechanisms
MeCP2 protein has several defined parts that researchers are studying. The methyl-CpG-binding domain or MBD, for example, binds to DNA in studies conducted in vitro and is thought of as crucial for the protein’s function of turning genes on or off, or altering the way DNA is stored in a cell. Given that the interactions between MeCP2 and DNA are difficult to mimic in a dish, a team of Canadian scientists wanted to see what would happen to the interaction in a mouse whose front end of the protein was deleted. The Mecp2tm1.1Jae model, commonly used in studies of Rett syndrome, is missing its first 116 amino acids—which include the 48 amino acids that make up the MBD. (What’s more, MeCP2 mutations in people with Rett occur in this region, such as R106W, R1333C, P152R, F155S and T158M.)
In the study, mutant mice still had MeCP2 in their tissues but only about half as much as healthy mice did. Although the mutated MeCP2 still bound to DNA and chromatin, the interactions were weaker and less specific for methylated DNA. In addition, although the ‘nuclear localization signal’ region of the mutated protein was still intact, protein was less able to move from the cytoplasm to the nucleus, where it can influence the production of other proteins. The paper is published in the May issue of Nucleic Acids Research.
News from the Fragile X community highlights the challenges of clinical trials.
Below is an article from the New York Times written by Andrew Pollack.
Holly Usrey-Roos will never forget when her son, Parker, then 10, accidentally broke a drinking glass and said, “I’m sorry, Mom. I love you.”
It was the first time she had ever heard her son say he loved her — or say much of anything for that matter. Parker, now 14, has fragile X syndrome, which causes intellectual disability and autistic behavior.
Ms. Usrey-Roos is certain that Parker’s new verbal ability resulted from an experimental drug he was taking in a clinical trial, and has continued to take for three years since then. She said she no longer had to wear sweaters to cover up the bruises on her arms she used to get from Parker hitting or biting her.
Now, however, the drug is being taken away. It has not met the goals set for it in clinical trials testing it as a treatment for either autism or fragile X syndrome. And Seaside Therapeutics, the company developing it, is running out of money and says it can no longer afford to supply the drug to former participants in its trials.
The setback is a blow in the effort to treat autism since the drug, arbaclofen, was one of the furthest along in clinical trials. And the company’s decision has caused both heartbreak and outrage among some parents.
“I waited 10 and a half years for him to tell me he loved me,” said Ms. Usrey-Roos, who lives in Canton, Ill. “With fragile X, you’re like living in a box and someone is holding the lid down. The medication opened the lid and let Parker out.”
“I don’t want to go back to the way life was,” she added.
The situation raises questions about what, if anything, drug companies owe to patients participating in their clinical trials. It also points out the difficulties in developing drugs to treat autism and fragile X syndrome. If the drug worked so well in some patients, why has it not succeeded so far in clinical trials?