Fragile Syndrome X
An Expanding Gene
In 1991 an international team of scientists identified the gene and mutation that causes fragile X syndrome. They found that in families with fragile X syndrome, there is a piece of the FMR1 gene, called a CGG repeat, which is abnormally expanded.
In the general population, the repeat length can range from about six to fifty-four copies of the CGG, and the repeat is stable, or is passed from parent to child without change. In fragile X families, the premutation form of the repeat contains between fifty and two hundred copies of the CGG repeat, and the repeat is unstable.
Premutation alleles can expand to full mutation alleles (with more than two hundred copies of the CGG repeat) by transmission of the premutation from a mother to her child. A woman's risk of having a child with the full mutation correlates to her own repeat size. The larger her premutation, the more she risks having a child who carries the full mutation.
The CGG repeat is usually interrupted by a single AGG trinucleotide every ten CGG repeats, but this can vary from individual to individual. Because premutation alleles have fewer AGG interruptions compared with normal-size FMR1 alleles, it is believed that the AGG interruptions are important for stability of the CGG repeat.
Individuals with a premutation do not express the clinical symptoms associated with fragile X syndrome, although it has been reported that pre-mutation carrier females can experience premature ovarian failure. Individuals who carry the full mutation can express symptoms of fragile X syndrome because they are missing the protein produced by the FMR1 gene. Males with a full mutation always exhibit some symptoms of the disorder. Due to X inactivation, females with a full mutation may or may not express symptoms.
Although there is currently no cure for fragile X syndrome, scientists are making great progress in understanding the biology of the disorder. In the mid-to late 1990s, Stephen Warren and colleagues determined that the FMR1 gene product, named FMRP, is an RNA-binding protein that shuttles in and out of the nucleus and is involved in binding various messenger RNAs. Moreover, scientists successfully developed mice that lack the FMR1 gene, which will greatly aid research. Symptoms of fragile X mice include learning disabilities, hyperactivity, and, in males, enlarged testicles. Prevailing hypotheses about FMRP suggest that this protein is involved in forming neural connections in the developing brain.
The identification of FMR1 and the expanded CGG repeats was a landmark discovery in human genetics because it established a novel class of human genetic mutations, trinucleotide (or triplet) repeat expansions. Since the discovery of FMR1 and the expanding CGG repeats, scientists have identified more than ten other human genetic disorders that are caused by expansions of trinucleotide repeats, including disorders such as Huntington's disease and myotonic muscular dystrophy.
Hagerman, Randi Jenssen, and Amy Cronister, eds. Fragile X Syndrome: Diagnosis, Treatment, and Research, 2nd ed. Baltimore: Johns Hopkins University Press, 1996.
Online Mendelian Inheritance in Man: Fragile Site Mental Retardation 1; FMR1. JohnsHopkins University and National Center for Biotechnology Information. <http://www.ncbi.nlm.nih.gov/htbin-post/Omim/dispmim?309550>.