Fragile-X syndrome (FXS) is the most common cause of inherited mental retardation. Until recently, the diagnosis of FXS has been made based on the cytogenetic expression of the fragile site at Xq27.3 (FRAXA) in the patients' cultured cells and on
the
results of linkage analysis with DNA markers surrounding the fragile X locus. The recent cloning of fragile-X gene(FMR-1) made it possible investigate the molecular defects in FMR-1 gene of individuals at risk. Vast majority of molecular defects
of
FXS
has been known to be an abnormally amplified trinucleotide (cytosine guanine guanine) repeat.
This study aims at establishing the molecular genetic diagnosis of FXS as well as correlating genotype-phenotype analyzing the increment of amplified CGG repeat number, clinical findings, and cytogenetic expression rate in two Korean families
with
FXS
patients. The FXS patients are tested cytogenetically & molecular genetically. The fragile site at Xq27.3 was cytogenetically expressed in folate deficient medium y culturing lymphocytes for 4 days. Molecular diagnostic approaches utilize the
genomic
DNA Soughern blot analysis using genomic probe FXA 241 (ONCOR) and radiolabelled PCR-denaturing polyacrylamide gel elcetrophoresis.
Each patient expressed a FRAXA site in folate deficient medium with the expression rate of 38%, 16% respectively. The molecular genetic study showed that each patient had the CGG amplification 1.6kb, 0.7-0.8kb in the FMR-1 gene respectively. In
addition, this study clarified the carrier status of each familiy members.
In conclusion, molecular genetic studies employed in this study can be utilized for a confimatory diagnostic purpose in FXS patients.
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