Possible Mechanisms for Trinucleotide Expansion in Fragile X Syndrome

Fragile X syndrome is a major X-linked disease affecting approximately 2,500 individuals and is a common cause of hereditary neurological disorders that cause mental retardation (Kaufmann and Reiss 1999). The single mutant gene FMR1 has been identified and defined as the cause of fragile X. Fragile X sites are highly polymorphic in the chromosome of fragile X individuals and contain long CGG repeats. This CGG polymorphism was identified as the cause of DNA instability (Kremer et al.

Fragile X syndrome is one of the most common forms of hereditary mental retardation, with one in 5,000 affected 1. The majority of cases are caused by amplification of CGG-trinucleotide repeats in the 5 'untranslated region of X-linked FMR1 gene 2. This prevents the gene from being expressed and the encoded FMRP protein is absent. The lack of FMRP causes defects in synaptic transmission. The name of this syndrome is derived from the fact that chromosomes from people with this syndrome appear spreading in the mid-term and appear to have gaps and contractions at the ends of the long arm of the X chromosome. In the past, cytogenetics have diagnosed syndromes through the presence of this vulnerable site, but since then have been replaced by molecular techniques.

Fragile X syndrome is a hereditary disorder that occurs as a result of a fragile X mental retardation 1 (FMR1) gene mutation on the X chromosome, most commonly as a result of an increase in the number of CGG trinucleotide repeats in the 5 'untranslated region of FMR1 . . Mutations in this place were found for about 1 in every 2,000 men and about 1 in 259 females. The incidence of this disease is 1 per person in 3,600 men and 1 in 4,000 to 6 000 women. It accounts for more than 98% of cases, but FXS may also occur due to point mutations affecting FMR1.

The name of the vulnerable X syndrome comes from the X chromosome part of the microscope that contains the genetic abnormality; it usually seems to be broken easily as it is hanging on the line (Figure 3). This syndrome is caused by abnormality of FMR1 (brittle x mental retardation 1) gene. People without fragile X syndrome have 3 to 50 repeat trinucleotide CGG in their FMR1 gene. However, individuals with over 200 iterations have complete mutations, and they usually show symptoms of the syndrome. Too much CGG will methylate CpG islands in the FMR1 gene promoter region, but this is not usually the case. This methylation invalidates the gene and prevents the FMR1 gene from producing important proteins called vulnerable X mental retardation proteins. This specific protein deficiency leads to fragile X syndrome