Background: Fragile X Syndrome (FXS) is the most prevalent inherited cause of mental retardation. The prevalence of FXS in males and females are approximately 1 in 4000 and 1 in 8000 respectively. It is caused by CGG repeat instability in the FMR1 gene, located on chromosome Xq27.3. Normal individuals have CGG repeats ranging from 5 to 53. In premutation carriers, the CGG repeats range from 60 to 200 and shall be more than 200 repeats for full mutation patients. FXS patients have variable clinical features and because of that, an accurate clinical diagnosis is always a problem. Currently, Cytogenetic, PCR and Southern Blot Techniques are widely used for diagnosis of FXS. Case Report: Here we report a pair of brothers suspected to be FXS patients with similar clinical features. However, the cytogenetic result for younger brother did not show fragile site at Xq27.3 of the X chromosome while molecular result was confirmatory for FXS. Conversely, the elder brother showed confirmatory results for Fragile X mutation in both cytogenetic and molecular analysis. Conclusion: We therefore conclude that patient 1 confirms for Fragile X mosaic and patient 2 for Fragile X full mutation. From the result, cytogenetic analysis alone cannot be dependable for the confirmatory diagnosis of FXS.

Fragile X Syndrome (FXS) is the most common form of inherited mental retardation in men. It is caused by abnormalities in the FMR1 gene that are associated with CGG repeat expansion and the hypermethylation status of its promoter. Methylated alleles lead to transcriptional inhibition and consequent loss of Fragile X Mental Retardation Protein. Chemical modification of cytosine to uracil by bisulfite treatment has proved to be an attractive method for DNA methylation studies that precludes labor-intensive Southern blot analysis, which is the gold standard test for FXS. In this report, bisulfite-treated DNA samples were amplified using real-time multiplex methylation-specific polymerase chain reaction followed by melting curve analysis. Our results show that all control samples with known CGG repeat numbers and methylation statuses were correctly diagnosed. The samples from 43 male patients were also successfully diagnosed, which were in complete agreement with the results of Southern blotting. By such means, 39 patients were found to have an unmethylated allele; 3, a fully mutated allele; and 1, both methylated and unmethylated alleles, thus implying a diagnosis of mosaicism. In conclusion, we find our method to be convenient for screening a large number of male patients with FXS, because it is rapid and easy to perform, especially when there is a low quantity of DNA that must be sensitively and accurately assayed.