A deazaguanine-substituted DNA PCR product from FMR-1 (the fragile X mental retardation syndrome gene) can be efficiently visualized with ethidium bromide on standard agarose gels.
A 2-year-old boy with manifestations of the fragile X syndrome was found to have a cytogenetically visible deletion of Xq27-q28 including deletion of FMR-1.
This deletion is smaller than those previously reported and confirms that FMR1 is the major and probably the only gene implicated in the phenotype of the fragile X syndrome.
The largest of the commonly used probes for Southern blot diagnosis of fragile X mental retardation syndrome spans the CGG repeat cluster in the FMR-1 gene.
We studied the methylation status of the CpG island of the FMR-1 (fragile X syndrome) gene to recognize the possibility of its prenatal diagnosis with early pregnant subjects.
Male patients with fragile X syndrome lack FMR1 protein due to silencing of the FMR1 gene by amplification of a CGG repeat and subsequent methylation of the promoter region.
A total of 27 fragile X pedigrees consisting of over 100 nuclear families were analyzed by Southern blotting methods and probes StB12.3 and StB12.3xx to detect the expansion of the (CGG)n repeat within the FMR-1 gene and the abnormal methylation pattern of the adjacent DNA region responsible for the fragile X syndrome.
The fragile X mental retardation syndrome is associated with the expansion of trinucleotide 5'-d(CGG)-3' repeats within the FMR1 gene and with hypermethylation of the cytosine residues of these repeats.
The results described here indicate that some "nonpenetrant" carrier males may have varying amounts of methylation of the FMR-1 region, which can result in mild expression of the fragile X syndrome.
In the case of fragile X syndrome, sequences adjacent to the repeat are methylated in affected individuals and the FMR1 gene is transcriptionally inactive.
This study was undertaken to determine the relative impact of three molecular characteristics of the FMR1 mutation--number of CGG repeats, methylation status, and X inactivation ratio--on the cognitive involvement of female carriers of fragile X syndrome.
The early transcription of FMR-1 gene and the distribution of FMR-1 mRNAs in human fetuses suggest that alterations of FMR-1 gene expression may contribute to the pathogenesis of fragile-X syndrome and especially the mental retardation.
Fragile X syndrome is associated with massive expansion of a CGG trinucleotide repeat within the FMR-1 gene and transcriptional silencing of the gene due to abnormal methylation.
Our results suggest that mutations in FMR-1 are directly responsible for fragile X syndrome, irrespective of possible secondary effects caused by FRAXA.