Thanks to the latest in a series of findings dating to 1999, researchers may be closing in on treatments for Rett syndrome, a neurological disorder that primarily affects girls and leaves them physically and mentally disabled.
Children with Rett syndrome develop normally for six to eighteen months, but then they start to regress, losing language skills and physical coordination. The children lose the ability to make purposeful movements and many habitually wring their hands. The syndrome causes breathing problems and seizures in some children, and many withdraw socially around the time other symptoms arise.
Despite these profound problems, people with Rett syndrome can live into adulthood with proper care. Researchers find no evidence of neurodegeneration in affected patients. Rather, they hypothesize that the neurons fail to develop completely but remain intact.
In the latest findings, announced in February, Adrian Bird, a professor at the Wellcome Trust Centre for Cell Biology at the University of Edinburgh, was able to reverse Rett symptoms in a mouse model of the disease.
“We have not devised a therapy, but what we have done is say there is an enormous amount of hope,” Bird says. “This is a potentially reversible disorder.”
In 1999, Huda Zoghbi, a molecular geneticist at Baylor College of Medicine in Houston, reported that mutations in a gene called MECP2 cause Rett syndrome. MECP2 resides on the X chromosome, so girls have two copies of the gene and boys have just one.
Girls who have one normal copy and one mutant copy of MECP2 develop Rett syndrome. Boys with the gene mutation usually die of respiratory failure by age 2, which is why few boys are diagnosed with the disorder.
Recently, Zoghbi’s team has found that not all children with MECP2 mutations develop classic Rett syndrome. When the researchers looked at the DNA of children diagnosed with other developmental disorders, including autism and mild mental retardation, they found that several had MECP2 mutations.
For example, of 300 children with diagnosed with autism and being treated at Baylor, 11 had alterations in the MECP2 gene. That suggests that any treatments developed for Rett syndrome patients might also benefit these other children. These data also mean that Rett syndrome may be more common than the current estimate of 1 in 10,000 female births.
Why would mutations in a single gene lead to such a variety of developmental disorders? The answer, Zoghbi says, lies in the way cells use their X chromosomes.
Because the two sexes do not have the same “dose” of genes on the X chromosome, each cell in a female permanently turns off one copy of the X chromosomes. However, which copy the cells turn off is not consistent, so that some turn off the paternally derived copy and others the maternally derived one.
Most of the time roughly the same number of cells turn off the mutant X chromosome as turn off the normal one, which results in classic severe Rett syndrome.
But the random assortment of inactivation can also work in a child’s favor. Some children get lucky, Zoghbi says, with the vast majority of their cells turning off the chromosome that includes the mutant MECP2 gene. These children develop a milder disorder. If they do not show the typical Rett symptoms, such as hand wringing, they may be misdiagnosed as having autism or some type of mental retardation.
Now that the genetic mutation that causes Rett syndrome is known, physicians can look for it in testing children who have some of the symptoms of the disorder. This approach will lead to a more accurate diagnosis and an appreciation for just how variable the syndrome is.
The identification of the mutation is also helping researchers to understand what specifically has gone awry in these children’s nervous system. MECP2 is a regulatory gene and therefore influences the activity of numerous genes, but pinning down exactly which of these target genes are critical to Rett syndrome biology has not been easy.
To get a better idea of how MECP2 mutations might cause neurological dysfunction and what genes it might regulate, Zoghbi’s team engineered a strain of mice that carry a mutant copy of MECP2. Like humans who have Rett syndrome, the mice appear normal early in their lives but then start to deteriorate. They develop tremors in their forelimbs, suffer from seizures, and have breathing problems.
Researchers were surprised that the mutant animals also showed psychological disturbances that resembled those seen in patients. The animals spent less time with other mice than did animals with normal MECP2. Moreover, they seemed overly anxious and spent less time in the open portions of the cage than control animals.
Given the anxiety in the mice, Zoghbi’s group hypothesized that the mutation was somehow triggering the secretion of excess stress hormones. When they measured the level of corticosterone in the animals’ blood after a stressful event, they found that it was significantly higher in the mutant animals than in healthy controls. The team found a similar result when they measured the amount of cortisol, the human stress hormone, in the urine of patients in their Rett syndrome study.
The investigators subsequently found that a neuropeptide that controls stress hormone production, called corticotropin releasing hormone (CRH), is overly abundant in the brains of the MECP2-mutant animals and that MECP2 normally controls the activity of the CRH-encoding gene. Previous work by other investigators demonstrated that animals lacking the cell surface receptor for CRH had reduced anxiety, so Zoghbi hypothesizes that using a drug to block the activity of these receptors could reduce patients’ anxiety.
The best part, Zoghbi says, is that such a drug—as yet unnamed—already exists and is being tested in patients with depression and anxiety. It is too early to know whether the drug will help Rett syndrome patients, but the Baylor team is testing the approach in the MECP2-mutant mice. If the drug works in the animals, the investigators will test it in patients.
The drug will not relieve all symptoms, or even perhaps some of the most severe ones, but Zoghbi thinks that reducing stress could be significant.
“If we can find a way to at least modulate the anxiety, even if it is one symptom of the many that Rett patients go through—if we can do that early before all of their other symptoms develop, we might prevent other symptoms,” she says.
Several studies have shown that stress can lead to neuronal damage. High levels of stress hormones have also been associated with decreased bone density and diabetes, which occur at an abnormally high rate in Rett patients.
Can Neural Function Be Revived?
With all of these new data in hand, Zoghbi and others have been working to answer a key question: Could restoring or repairing the damaged MECP2 pathway reverse or limit the symptoms?
Shortly after identifying the gene culprit, Zoghbi’s team found that simply adding a healthy copy of MECP2 does not solve the problem. In fact, the extra copy seemed to cause its own problems.
“To our surprise, mice that had one extra copy of the gene were quite abnormal,” Zoghbi says. They appeared normal for a couple of months but then developed spasticity, very abnormal movements with their forepaws, and recurrent seizures. “The levels of this protein are tightly regulated, and having just the right amount of this protein is very important for neuronal function.”
Several research groups have subsequently found that humans who have an extra copy of the MECP2 gene, due to a random duplication of that portion of the X chromosome, have neurodevelopmental problems that partially overlap with Rett syndrome. These patients have moderate to severe mental retardation, seizures, spasticity, and recurrent infections.
Those observations rule out the possibility of using gene therapy for patients, but they do not rule out gene therapy for the mutant mice. If such experiments work, the scientists may be able to identify a point in the pathway the gene regulates that can be safely turned back on with a drug in humans, returning the pathway to what it would look like if MECP2 were functioning normally. Two different approaches to gene therapy demonstrate that many symptoms associated with Rett syndrome may be reversible.
Rudolf Jaenisch, a molecular biologist at the Whitehead Institute for Biomedical Research in Cambridge, Mass., found that the MECP2 protein controls expression of brain-derived neurotrophic factor (BDNF), which is critical for neuronal survival and function. When his group generated mice that lack functional BDNF in their brains, the animals developed some symptoms that resemble Rett syndrome.
More remarkable, however, is that when Jaenisch’s team introduced excess BDNF into MECP2-mutant mice, the animals either had a delayed onset of Rett syndrome symptoms or the symptoms were milder than in untreated mutant animals. Additionally, his group found that if they reactivated the BDNF gene just as the first symptoms were appearing, they could relieve some of the problems.
Bird’s group went one step further: they engineered the mouse mutation so that they could willfully reactivate the gene at different points in development. Doing so reversed the symptoms.
Bird agrees that gene therapy in humans is not an option, but his new results, published in Science, and those of Jaenisch suggest the possibility that a drug therapy could restore neural function. One possibility might be to find a way to turn back on the normal copy of the MECP2 gene that lies on the silenced copy of the X chromosome.
“It worked like a dream experiment,” Jaenisch says of Bird’s work. “It is totally consistent with what we’ve done. You can’t reactivate the gene in humans [at this time], but it shows that the neurons are not irreversibly damaged.
“It is an incredible advance.”