Fragile X Syndrome

Essay by bahau00 May 2004

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Fragile X syndrome is the most common cause of inherited mental retardation, seen in approximately one in 1,200 males and one in 2,500 females. Males with fragile X syndrome usually have mental retardation and often exhibit characteristic physical features and behavior [Hagerman and Silverman, 1991; Warren and Nelson, 1994]. Affected females exhibit a similar, but usually less severe phenotype.

The diagnosis of fragile X syndrome was originally based on the expression of a folate-sensitive fragile site at Xq27.3 (FRAXA) induced in cell culture under conditions of folate deprivation. Cytogenetic analysis of metaphase spreads demonstrates the presence of the fragile site in less than 60% of cells in most affected individuals. The cytogenetic test has limitations, especially in testing for carrier status, and it exhibits a high degree of variability between individuals and laboratories. Also, interpretation of the cytogenetic test for fragile X syndrome is complicated by the presence of other fragile sites in the same region of the X chromosome (FRAXD, FRAXE, and FRAXF).

In 1991, the fragile X gene (FMR1) was characterized and found to contain a tandemly repeated trinucleotide sequence (COG) near its 5' end. The mutation responsible for fragile X syndrome involves expansion of this repeat segment. The number of CGG repeats in the FMR1 genes of the normal population varies from six to approximately 50. There are two main categories of mutation, premutations of approximately 50 to 200 repeats and full mutations of more than approximately 200 repeats. There is no clear boundary between the upper limit of normal and the lower limit of the premutation range. For this reason, alleles with approximately 45-55 copies of the repeat are said to be in the "grey zone." Some alleles in this size range are unstable and expand from generation to generation, while others are stably inherited.