Background and Prevalence
Fragile X syndrome (FXS) is the most common known cause of inherited mental impairment. According to the Centers for Disease Control (CDC), between 1 in 4000 and 1 in 6000 males is affected with FXS. Although the prevalence of the disorder in females is less certain, the CDC estimates that at least 1 in 8000 females has FXS.
FXS results from a mutation in a gene on the X chromosome, known as the fragile X mental retardation 1 (FMR1) gene. The mutation involves the expansion of a specific pattern of molecules that make up the gene (i.e., cytosine, guanine, and guanine, or "CGG"). The number of repetitions of the pattern is referred to as "CGG repeats." Individuals with between 6 and 40 CGG repeats have a normal gene. Those with more that 200 CGG repeats have what is called the "full mutation," which causes FXS.
Because the FMR1 gene is on the X chromosome (one of the two chromosomes that determine a person's sex), males and females are affected differently by FXS. Males with the full mutation tend to have significant mental disabilities, ranging from learning disabilities to severe mental retardation. Characteristic physical features of the disorder in males include a long face, prominent ears and chin, arched palate, a horizontal crease across the palms, large testicles, flat feet, and double-jointedness. Males with FXS tend to exhibit hyperactivity, short attention span, hand biting or hand flapping, poor eye contact and social skills, shyness, anxiety, delayed speech and motor development, repetitive speech, and sensitivity to sensory stimulation (including a hypersensitivity to being touched). Approximately 25% of boys with FXS are diagnosed with autism.
Because females have two X chromosomes, one of which generally contains a normal FMR1 gene, they tend to have less severe cognitive, physical, and behavioral symptoms of FXS. About 30% of females with the full mutation have IQ scores in the normal range. These individuals generally experience mild learning disabilities or no discernable mental impairment. The remaining 70% of females with FXS have IQ scores falling below the average range of scores. Most experience mild mental retardation or are considered of borderline intelligence. As with males, physical features such as long face, prominent ears, and arched palate are common among females with the full mutation of the FMR1 gene. Although females with FXS also tend to have some difficulty with attention and concentration, they are less likely than males to exhibit hyperactivity. Females also are less likely to have symptoms similar to those seen in autism. However, shyness, poor social skills, anxiety, and depression are common among females with the full mutation.
There is no known cure for FXS. However, a variety of treatments and interventions are available to address specific symptoms common among individuals with the disorder. Treatment often involves medication and behavioral therapy as well as speech and occupational therapy to address delays in language and motor development. Several web sites, including Online Mendelian Inheritance in Man (OMIM), GeneClinics, and the web site of the National Fragile X Foundation provide information about the treatment of FXS.