Fragile X syndrome (FXS) is a leading genetic cause of intellectual disability, especially in males. Intellectual disability (formerly called “mental retardation”) is characterized by significant limitations both in intelligence and in adaptive behavior affecting many everyday social and practical skills. Many people with FXS have attention deficit hyperactivity disorder (ADHD) and autism. FXS is also associated with physical abnormalities such as long faces, protruding ears, and flat feet. The condition affects 1 in 4,000 males and 1 in 8,000 females. (See box, “Fragile X Syndrome: The Basics.”)
Recently, researchers uncovered the mechanism that causes FXS. Published in the medical journal Science in February 2014, the study is a major breakthrough—but experts say treatments may still be a long way off.
A FRAGILE SECTION OF DNA
As early as the 1940s, scientists knew that FXS was inherited through the X-chromosome. Every person normally has one pair of sex chromosomes in each cell. Females have two X chromosomes; males have one X and one Y chromosome.
In the 1960s, researchers discovered that people with FXS have an unusual “fragile” section on their X-chromosome. Normally, the X-chromosome is made up of two strands of the same thickness. But the X-chromosome of people with FXS contains a thin section on both strands. Viewed through a microscope, the bottom of the chromosome looks like it's attached by a thread.
It wasn't until the 1990s that scientists discovered that the gene next to this fragile section was turned off, meaning it would not produce its protein. The gene is called fragile X mental retardation 1 (FMR1). Normally, it produces fragile X mental retardation 1 protein (FMRP), which has important functions in brain development. FMRP helps communication between nerve cells (neurons) in the brain. In FXS, the gene stops making the protein in the womb. The gene remains turned off for the rest of the person's life, which leads to the symptoms of FXS.
Scientists didn't see any major differences between the FMR1 gene in people with or without FXS. The only difference was the long fragile thread next to the gene. What we think of as a gene—sections of genetic material that contain the code for proteins—makes up only about 2 percent of our DNA. The rest of the material includes sections that help turn genes on and off, as well as many sections without any known function.
THE IMPORTANCE OF STEM CELLS
Unfortunately, researchers could not recreate—in cells or animals—the exact process of turning off the FMR1 gene that occurs in human FXS in order to study it. That is, until a team led by Samie Jaffrey, M.D., professor of pharmacology at Weill Cornell Medical College in New York, NY, studied embryonic stem cells with the fragile section next to the FMR1 gene.
Stem cells have the capacity to turn into any type of cell in the body. Approved by the National Institutes of Health (NIH) for research, the stem cells used by Dr. Jaffrey and his team were derived from embryos donated by women who had previously given birth to children with FXS. Embryonic stem cells are derived from embryos that develop from eggs that have been fertilized in vitro—that is, in an artificial environment as opposed to inside the human body—and then donated for research with informed consent of the donors.
“There is something specific about the development of the human embryo that starts this process of turning off the gene. I don't see any other way we could have studied this except for using embryonic stem cells,” Dr. Jaffrey says.
Dr. Jaffrey and his team treated their stem cell lines with chemical stimulants so they would develop into neurons. Just as happens in FXS in the womb, these stem cells stopped producing FMRP at around the 50th day.
“We could see the process, in the Petri dish, where the gene turns off. And we asked whether we could stop the process from occurring in the first place,” Dr. Jaffrey says.
What they uncovered was a mechanism that may open up new pathways for research into not just FXS but other similar disorders.
A person's genes are made up of paired strands of DNA, which contain the code for how to make proteins needed by the body. This code is made up of four building blocks, called nucleotides, in various combinations. In the first stage of making a protein (called transcription), the two DNA strands separate and a copy of the code is made, called messenger RNA (mRNA). The DNA strands then reconnect. In the second stage (called translation), the mRNA code is read by the cell and a protein is made.
The four building blocks of the genetic code naturally form pairs. However, it's not uncommon to find repeating segments of three, called triplet repeats. In most cases, these abnormalities are harmless. But when these abnormalities happen too often, as in FXS, they can cause problems.
People without FXS have between 5 and 50 of these repeats. People with between 50 and 199 repeats may show no signs of the condition, but they can pass the condition on to their children. People with FXS have more than 200 of these repeats. This extended section of repeats is what makes the chromosome look fragile and thin.
What Dr. Jaffrey and his team found was that, in FXS, mRNA remains stuck to DNA instead of breaking away once it has copied the information. The reason is the fragile section next to the gene. Messenger RNA typically copies not just the gene but also some of the material around it. In the case of FXS, a large section of these abnormal repeats is copied. As a result, the DNA won't close back up after separating, and the cell is unable to make the protein.
This is the first time that such a mechanism—RNA remaining stuck to DNA, called RNA-DNA duplexes—has been identified as the possible cause of a disease in humans, according to Dr. Jaffrey. If this mechanism can cause FXS, he notes, it may also play a role in the inherited form of amyotrophic lateral sclerosis (ALS, also known as Lou Gehrig's disease), Huntington's disease, and several other diseases that involve triplet repeats.
“The next step is to see if what we found in FXS occurs in these other disorders. Inherited ALS is the most likely because the repeat sequences are so similar,” Dr. Jaffrey explains.
Dr. Jaffrey and his team also found that treating the stem cells with a small molecule, 1a, prevents RNA from sticking to DNA. Cells treated with 1a continue to produce FMRP even after 50 days. “Our research suggests that these small molecules, which can stop RNA from combining with the DNA, should be explored as potential ways to treat these diseases,” Dr. Jaffrey says.
SCIENCE MOVES SLOWLY
“The discovery of the mechanism is a big advance,” says Elizabeth Berry-Kravis, M.D., Ph.D., professor of pediatrics, neurological sciences, and biochemistry at Rush University Medical Center, member of the American Academy of Neurology, and expert on FXS, who was not involved in the study. “It opens up avenues for new research, particularly into how genes are inactivated. But right now, the study does not have implications for treatment,” she notes.
Dr. Berry-Kravis, who consults directly with patients and the families of patients with FXS, has been dismayed by media coverage claiming Dr. Jaffrey's discovery may lead to a cure in the near future. And she points out that Dr. Jaffrey and his team do not claim to have discovered a cure.
Although identifying 1a as a way to prevent the gene from turning off is crucial, it would have to be given to an embryo before the 50th day after conception to have an effect, Dr. Berry-Kravis says. “This would be enormously difficult. You would have to know that an embryo was carrying the mutation earlier than we normally test for other genetic disorders. You would have to find a means of delivering the molecule and regulating the amount given to the embryo. And you would have to determine if it is safe for both mother and child,” she says.
Other promising approaches have proven difficult to translate into treatment, Dr. Berry-Kravis points out.
“In 2002, there was a lot of excitement around replacing FMRP in people without it. Unfortunately, it proved more difficult than anticipated,” she recalls. “You can't take FMRP orally and expect it to get to the right places in the right cells in the right quantities. It's a large protein with different functions in different parts of the body at different points in development. Even if it were possible to deliver FMRP at the correct time and place, we might do more harm than good if we can't regulate the amount of FMRP in the body. At the end of the day, FMRP replacement has been too challenging thus far to test in clinical trials.”
But Dr. Berry-Kravis also emphasizes the importance of Dr. Jaffrey's research. Science advances through the often slow accumulation of knowledge. Research that sheds light on a fundamental mechanism causing a disease is necessary for someday treating that disease.
HOPE FOR TARGETED TREATMENTS
While researchers have not found a cure for FXS, targeted treatments that alleviate some of the symptoms are being tested in clinical trials.
“From studying mice, we know what happens when FMRP is not produced in the body. We may be able to use that information as a basis for treatment in humans,” Dr. Berry-Kravis says.
For example, FMRP regulates the production of many other proteins. In the absence of FMRP, some of these proteins are produced in an unregulated way. “If we can develop a drug to dampen this overactivity, then we might alleviate a symptom or even reverse a particular aspect of the disorder,” Dr. Berry-Kravis says. The same is true of proteins FMRP might stimulate.
In fact, drugs being developed to change or reverse these effects in mice are showing some promise, Dr. Berry-Kravis points out. “These new drugs are a targeted treatment. In other words, they correct the abnormalities seen in the mouse model of the disease. Of course, not everything that works in a mouse will work in humans. But some of them will.”
“FMRP is a complex molecule with many different functions, so it's unlikely that we will have one drug that will do everything that FMRP does,” Dr. Berry Kravis says. “But if you get enough targeted treatments together, you may be able to reverse many of the symptoms of FXS.”
Fragile X Syndrome: The Basics
What is fragile X syndrome? Fragile X syndrome (FXS) is a leading genetic cause of intellectual disability, affecting 1 in 4,000 males and 1 in 8,000 females.
What causes fragile X syndrome? FXS is caused by a thin, “fragile” section of genetic material next to a gene called fragile X mental retardation 1 (FMR1). This fragile section of genetic material contains an abnormally high number of repetitions of genetic code. Normally, FMR1 produces fragile X mental retardation 1 protein (FMRP), which has important functions in brain development. FMRP helps communication between nerve cells (neurons) in the brain. In FXS, however, the gene stops making the protein in the womb and then remains turned off for the rest of the person's life, which leads to the symptoms of FXS.
What are the symptoms of fragile X syndrome? The symptoms of FXS include intellectual disability (formerly called “mental retardation”), characterized by significant limitations both in intelligence and in adaptive behavior affecting many everyday social and practical skills; delayed speech, language, and motor skills; attention deficit hyperactivity disorder (ADHD); and autism. The physical symptoms of FXS include large, protruding ears; low muscle tone; flat feet; a long face; frequent ear infections; mitral valve prolapse, in which the valve between the left upper and lower chambers of the heart doesn't close properly; strabismus, or crossed eyes; and presbyopia, in which the eyes don't focus properly.
How is fragile X syndrome treated? Currently, FXS is treated with a combination of medications to address symptoms, including antipsychotics, stimulants, and antidepressants. In addition, a number of therapeutic interventions are used, including speech and language therapy, therapy for behavioral disorders, sensory integration therapy, physical therapy, and occupational therapy.
FOR MORE INFORMATION
- ▸ For more Neurology Now coverage of fragile X syndrome (FXS), go to bit.ly/1jb4OOE
- ▸ For coverage of FXS in the Neurology Today, the American Academy of Neurology's news publication for neurologists, go to bit.ly/1nG4fvK
- ▸ For a Patient Page on FXS from the American Academy of Neurology's journal Neurology, go to bit.ly/1hI25Xz