SEATTLE—Study of fragile X syndrome has “come full circle,” according to Stephen Warren, PhD, from discovery of the candidate gene to understanding the basic biology of the normal gene, and the rational design of therapies to compensate for deleterious effects of the mutation. Trials of those therapies are now underway, the Emory University professor of genetics told the audience in a Frontiers of Clinical Neuroscience plenary session here at the annual meeting.
Dr. Warren described fragile X syndrome — the most common inherited form of cognitive deficiency, with a prevalence of 1 in 5,000 newborn boys, and affecting about 30,000 boys in the US — as having a “subtle but distinctive” phenotype. There is no frank dysmorphia, so that identifying patients as infants is difficult. But even patients with different ethnic backgrounds share a physical resemblance, including an enlarged head and a long face.
Macroorchidism is common, and epilepsy occurs in about 30 percent of patients. “But the biggest impact is on cognition,” he said. The average IQ is 40, and about 30 percent of patients meet DSM-IV criteria for autism.
The gene responsible for fragile X syndrome is called FMR1. The mutation is in the “upstream” untranslated, or noncoding, region. The region normally contains a CGG repeat about 30 units long. Fragile X patients, on the other hand, have repeat lengths of over 200 units, and often as many as 900 units. This extra DNA gives the X chromosome a distinctive constriction, leading to the “fragile” appearance that gives the syndrome its name.
The expansion triggers methylation of the gene, the way the genome prevents expression of the gene. Thus, in boys, with only one X chromosome, little or no FMR protein is made. Expressivity in females is quite variable, Dr. Warren said, accounting for a corresponding variability in the clinical phenotype among girls.
Research by Dr. Warren and others has revealed that the FMR protein is an RNA binding protein. It binds to 3 percent of all messenger RNAs in the brain, and inhibits their translation by about half. In the absence of the protein, all these messages are overtranslated.
“What was unexpected,” Dr. Warren said, “was that nearly all the messages regulated by FMR protein are synaptic activity-dependent, and the majority are localized to the dendrite. This was interesting because we think of fragile X as a dendritic disorder,” based on morphological changes seen in the syndrome, with a diminished number of mature boutons on dendrites. “They are failing to mature as they should.”
EXCESS PROTEIN SYNTHESIS
The FMR protein helps regulate synaptic plasticity, a form of learning that depends on protein synthesis. In its active state, the protein suppresses synthesis of its target proteins. When neuronal activity stimulates the synaptic metabotropic glutamate receptor, the FMR protein becomes inactivated, allowing translation of messenger RNAs and synthesis of multiple proteins at the dendritic spine, promoting plasticity.
In fragile X syndrome, on the other hand, there is no FMR protein. “The neuron is acting as if it is being stimulated by glutamate, despite that there is no external stimulation,” he said, leading to “runaway synaptic protein synthesis, independent of synaptic activity,” and a deficiency of learning-dependent synaptic changes.
If the mutation mimics excess glutamate stimulation, then one strategy to compensate for it would be to inhibit the glutamate receptor. Experiments in mice, reported in a 2007 report in Neuron, showed the potential of this approach, with a complete rescue of many of the phenotypic consequences of mutation.
At the same time, a drug screen in flies revealed another class of drugs, gamma-aminobutyric acid (GABA) agonists, might also be beneficial, and animal experiments have confirmed the potential of this strategy as well.
Drug trials using one or the other of these two approaches are underway in 15 clinics across the country.
“We have now gone around the circle, from human disease to finding the gene to making models to understanding the pathophysiology to developing drugs,” he said. “This resulted largely from doing basic science on the clinical phenotype,” and represents one of the most striking examples of translational research in neurology.
Commenting on Dr. Warren's lecture, Paul Hagerman, PhD, professor in the department of biochemistry and molecular medicine at the University of California–Davis, said the understanding of the role of metabotropic glutamate receptors “valuable and productive” for the development of new therapies for fragile X syndrome.
Nonetheless, he said, “there are other things this is not addressing.” There is evidence there is a primary deficit in the GABAergic system, for which the mechanism is not clear. “It is still a black box as to what's going on at the synapse. In the longer term, the story is still really out on this.”
“The other thing that is really important in thinking about treatment is that we many want to treat chronically, and as with many drugs, I'd worry about attenuation of effect. It is clear this takes place in other therapeutics. It is a big unknown here,” he said.