Hypomethylation of an expanded FMR1 allele is not associated with a globalDNA methylation defect

The vast majority of fragile-X full mutations are heavily methylated throug hout the expanded CGG repeat and the surrounding CpG island. Hypermethylati on initiates and/or stabilizes transcriptional inactivation of the FMR1 gen e, which causes the fragile X-syndrome phenotype characterized, primarily, by mental retardation. The relation between repeat expansion and hypermethy lation is not well understood nor is it absolute, as demonstrated by the id entification of nonretarded males who carry hypomethylated full mutations. To better characterize the methylation pattern in a patient who carries a h ypomethylated full mutation of similar to 60-700 repeats, we have evaluated methylation with the McrBC endonuclease, which allows analysis of numerous sites in the FMR1 CpG island, including those located within the CGG repea t. We report that the expanded-repeat region is completely free of methylat ion in this full-mutation male. Significantly, this lack of methylation app ears to be specific to the expanded FMR1 CGG-repeat region, because various linked and unlinked repetitive-element loci are methylated normally. This finding demonstrates that the lack of methylation in the expanded CGG-repea t region is not associated with a global defect in methylation of highly re peated DNA sequences. We also report that de novo methylation of the expand ed CGG-repeat region does not occur when it is moved via microcell-mediated chromosome transfer into a de novo methylation-competent mouse embryonal c arcinoma cell line.