Fragile X Syndrome

Fragile X syndrome (FXS) is the most common inherited cause of mental retardation. FXS occurs in approximately 1 in 1500 male births and 1 in 1000 female births. Subsequent studies using DNA have demonstrated a lower prevalence of FXS at approximately 1 per 4000 males in the general population. It is a single gene disorder on the X chromosome that occurs in both males and females, but males are typically affected more severely. It is an X-linked disorder and results from a pinched, restricted, or "fragile" location on the long arm of the X chromosome. In 1991, the fragile X mental retardation 1 (FMR1) gene was discovered. With the discovery of FMR1, DNA testing for the presence of fragile X was available. DNA testing provides a more effective means to detect the presence of FXS and carriers of the FMR1 gene by specifically showing the genetic mutation at FMR1. Female carriers can pass down the disorder through generations. FXS minimizes the function of the gene responsible for producing an essential protein necessary for normal brain function.

Because of the divergent impact on males and females, there is a significant degree of phenotypic variability expressed in this disorder. In fact, early in life, there is a limited physical phenotype with a distinctive appearance developing at approximately the time of puberty. That is, males with FXS may have macrocephaly, elongated face, long protruding ears, prominent jaw, macroorchidism in adulthood, large nose, and a prominent forehead. Early in life, the one physical characteristic that may be present is the large ears. Females typically have only a slightly abnormal appearance, with approximately half presenting with protruding ears and other subtle features, such as flexible joints, low muscle tone, or flat feet.

Males and females show disparate patterns of cognitive profiles. Males most often have IQ scores in the moderate to severe range of mental retardation, with some scatter to the borderline to low-average range of intellectual functioning. Behavioral manifestations also include attentional difficulties, hyperac-tivity, tactile defensiveness, hand flapping, seizures, stereotypic behavior, echolalia, and gaze avoidance. Females with FXS are often carriers with little or no expression of the condition they may often be diagnosed as having a learning disability rather than mental retardation.

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