Cellular Mechanisms of Fragile X Syndrome Examined

Molecular and cellular studies of fragile-X syndrome may lead to insights on the biological mechanisms underlying this common form of developmental delay. Fragile X results from a mutation in a gene called FMR1. This gene, discovered in 1991, is found on the long arm of the X chromosome. The X chromosome helps determine gender: girls have two X chromosomes, and boys have an X and a Y chromosome.

Boys with the disorder are usually more affected by it than are girls. Because females have two copies of FMR1, girls with the fragile-X mutation usually have a normally functioning version of the gene. The normal version can partially compensate for the mutant one. Males have only one X chromosome. As a consequence, boys with the mutation often have more severe symptoms. Physical features can include large ears, a long face, connective tissue problems, and skeletal problems. Some people with the syndrome have speech disturbances and autistic behaviors, and may flap or bite their hands. Because symptoms vary, the diagnosis is often delayed.

Fragile X syndrome affects an estimated one in 4,000 to 6,000 males and about half as many females in all ethnic groups, and is second only to Down syndrome as a cause of cognitive disability. About one in 100 to 600 females is an unaffected carrier of the mutated gene.

The UW has a five-year grant of $5.86 million from the National Institute of Child Health and Human Development at the National Institutes of Health to study fragile X syndrome. The UW Center on Human Development and Disability (CHDD) administers the grant. The grant supports fragile X research at several labs and institutions.

The lead researchers are Dr. Charles Laird, UW professor of biology and director of the Fragile X Research Center; Dr. R. Scott Hansen, UW research assistant professor of medicine in the Division of Medical Genetics; Dr. Stephen Tapscott, member of the Fred Hutchinson Cancer Research Center and UW associate professor of neurology; and Dr. Randi Hagerman, director of the MIND Institute at the University of California at Davis. She co-directs the grant and oversees patient recruitment and evaluation.

Laird emphasized the interdisciplinary nature of the project: "CHDD has an outstanding record of bringing together faculty from different units, in this case the UW College of Arts and Sciences and the UW School of Medicine, the Fred Hutchinson, and the University of California. This coalescence of interested groups and approaches is crucial to understanding the biology of this astonishingly complex genetic disease. We also expect that fragile-X research will provide insights into normal cellular processes. For example, our work on fragile X has made us re-think our understanding of cell division."

The grant has many aspects, among them:

• Hansen's lab will study the timing of DNA replication during the cell cycle, including the disruption of replication and its consequences in fragile-X syndrome.
  
• Tapscott's group will look at the mutation's effects on the chromosome's organization and its chromatin structure.
  
• Laird's lab will examine mosaicism, in which different cells of a person with fragile-X syndrome contain various forms of the mutation. Mosaicism is observed in 15 percent to 20 percent of fragile-X patients.
  
• Hagerman's team will manage patient recruitment and evaluation. Initially, about 50 patients will be enrolled in the study.

The researchers will build on what is known about the molecular biology of fragile X. The syndrome is type of genetic mutation called a trinucleotide repeat expansion. A specific combination of DNA building blocks-in this case, the nucleotides CGG-repeats more than a normal number of times. The number of repeats appears to influence the severity of the disease.

The Fragile X mutation also causes inappropriate methylation of DNA. In normal DNA methylation, a methyl group-one carbon and three hydrogen atoms-is added to one of the DNA nucleotides. This chemical event can shut down the activity of a nearby gene. In most cases, this shutdown is necessary for normal function. For example, it inactivates genes on the second X chromosome in females. In fragile-X syndrome, however, there is excess, or hypermethylation, of the FMR1 gene. This hypermethylation leads to abnormal silencing of FMR1. As a result, little or no FMR1 protein is produced. The FMR1 protein is required for normal cognitive function.

With the loving support of his mom and dad Julie and Randy Weaver, Zachary, 13, of Marysville, Wash., enjoys playing middle school football, cooking with his family, and relaxing at home. Zachary has fragile X syndrome. UW research on his condition is looking at the underlying genetic and biochemical mechanisms.