Fragile X syndrome is the most common inherited cause of intellectual disability, with males being more commonly affected than females. (Down syndrome is the most common cause of intellectual disability in males; although it is a genetic disorder, most cases occur sporadically and are not inherited.) For more information, see the National Fragile X Foundation.
The symptoms of Fragile X syndrome are caused by an abnormality of the FMR1 gene on the X chromosome. The abnormality is an unstable triplet repeat expansion; unaffected people have < 54 CGG repeats and people with Fragile X syndrome have > 200. People with 55 to 200 CGG repeats are considered to have a premutation because the increased number of repeats increases the likelihood that further mutation will result in > 200 repeats in a subsequent generation.
Fragile X syndrome affects about 1/4000 males and 1/8000 females. The premutation is more common. Females with the disorder are typically less impaired than males. Fragile X is inherited in an X-linked pattern and does not always cause clinical symptoms in females.
In the past, examination of the karyotype revealed a constriction at the end of the long arm of the X chromosome, followed by a thin strand of genetic material, which was why the syndrome was considered a chromosomal anomaly. However, this structural defect does not appear when modern cytogenetic techniques are used, and this is the reason why Fragile X syndrome is now considered a single-gene disorder and not a chromosomal anomaly.
People with Fragile X syndrome may have physical, cognitive, and behavioral abnormalities. Typical features include large, protuberant ears, a prominent chin and forehead, a high arched palate, and, in postpubertal males, macroorchidism. The joints may be hyperextensible, and heart disease (mitral valve prolapse) may occur.
Cognitive abnormalities may include mild to moderate intellectual disability. Features of autism may develop, including perseverative speech and behavior, poor eye contact, and social anxiety.
Women with the premutation may have premature ovarian failure; sometimes menopause occurs in the mid-30s. Males and females with an FMR1 premutation are at risk of Fragile X–associated tremor/ataxia syndrome (FXTAS), which is associated with gait abnormalities, intention tremors, later intellectual impairment, and psychiatric problems.
Fragile X syndrome is frequently not suspected until school age or adolescence, depending on the severity of the symptoms, unless there is a significant family history. Boys with autism and intellectual disability should be tested for Fragile X syndrome, especially when they have similarly affected maternal relatives. Molecular DNA analysis is done to detect the increased number of CGG repeats.
(See also Next-generation sequencing technologies.)
The following is an English-language resource that may be useful. Please note that THE MANUAL is not responsible for the content of this resource.
National Fragile X Foundation: Provides advocacy, education, support, and public and professional awareness programs and services