Fragile X syndrome (FXS) is the most common cause of inherited intellectual disability known, and it is the most common single gene disorder associated with autism (Belmonte and Bourgeron 2006; Reddy 2005). It is caused by the lack or deficiency of the FMR1 protein, FMRP (Loesch et al. 2004b). The typical physical features of FXS include prominent ears, hyperextensible finger joints, flat feet, soft skin, and in adolescence and adulthood large testicles (macroorchidism) and a long face (Hagerman 2002b). The behavioral features include poor eye contact, hyperarousal to stimuli, anxiety, hyperactivity, attention deficit, impulsivity, hand stereotypies (such as hand biting and hand flapping), and social deficits including autism and autism spectrum disorder (ASD) (Budimirovic et al. 2006; Clifford et al. 2007; Hall et al. 2008b; Hatton et al. 2006b; Sullivan et al. 2007b). Fragile-X syndrome was first reported by Lubs (1969) in two brothers who had intellectual disability and the appearance of a marker X chromosome, which is a fragile site on their X chromosome. It was later detected that this fragile site on the X chromosome only occurred when the chromosomes were studied in a folate-deficient tissue culture media (Sutherland 1977). Therefore cytogenetic studies were utilized to document cases of FXS throughout the 1980s until the Fragile X Mental Retardation 1 gene (FMR1) was discovered in 1991 (Verkerk et al. 1991). The FMR1 gene was found to have a trinucleotide (CGG) repeat sequence at the 5’ untranslated region, with the normal range later determined to be up to 44 repeats, a gray zone of 45–54 repeats, a premutation of 55–200 repeats, and a full mutation range of more than 200 repeats (Maddalena et al. 2001). Those individuals with the full mutation have a deficit or absence of the FMR1 protein (FMRP) that causes the physical, behavioral, and cognitive features of FXS (Loesch et al. 2004b). Females with the full mutation have another X chromosome that is producing FMRP, depending on the activation ratio (AR) or the percentage of cells that have the normal X chromosome as the active X chromosome.
Proposed Mechanism of Inheritance and Expression of the Human Fragile-X Syndrome of Mental Retardation
