DNA methylation represses <i>FMR-1</i> transcription in fragile X syndrome

TLDR

Fragile X syndrome, the most common inherited intellectual disability, is X‑linked dominant with reduced penetrance and is associated with the FMR‑1 gene, which exhibits both CGG repeat expansion and abnormal CpG‑island methylation that correlate with transcriptional repression and the disease phenotype. The study aims to determine whether CGG repeat expansion, CpG‑island methylation, or both are responsible for FMR‑1 transcriptional silencing. The authors performed prenatal diagnosis by analyzing chorionic villi and fetal DNA for CGG repeat expansion and CpG‑island methylation using BssHII digestion. The results show that fetal tissue lacking CpG‑island methylation expresses FMR‑1 while methylated fetal DNA does not, indicating that CpG‑island methylation causes transcriptional silencing and likely occurs early in embryogenesis.

Abstract

Fragile X syndrome is the most frequent form of inherited mental retardation and segregates as an X-linked dominant with reduced penetrance. Recently, we have identified the FMR-1 gene at the fragile X locus. Two molecular differences of the FMR-1 gene have been found in fragile X patients: a size increase of an FMR-1 exon containing a CGG repeat and abnormal methylation of a CpG island 250 bp proximal to this repeat. Penetrant fragile X males who exhibit these changes typically show repression of FMR-1 transcription and the presumptive absence of FMR-1 protein is believed to contribute to the fragile X phenotype. It is unclear, however, if either or both molecular differences in FMR-1 gene is responsible for transcriptional silencing. We report here the prenatal diagnosis of a male fetus with fragile X syndrome by utilizing these molecular differences and show that while the expanded CGG-repeat mutation is observed in both the chorionk villi and fetus, the methylation of the CpG island is limited to the fetal DNA (as assessed by BssHII digestion). We further demonstrate that FMR-1 gene expression is repressed in the fetal tissue, as is characteristic of penetrant males, while the undermethylated chorionk villi expressed FMR-1. Since the genetic background of the tissues studied is identical, including the fragile X chromosome, these data indicate that the abnormal methylation of the FMR-1 CpG-island is responsible for the absence of FMR-1 transcription and suggests that the methylation may be acquired early in embryogenesis.