DNA methylation represses FMR-1 transcription in fragile X syndrome

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 chorionic 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 chorionic 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.

The genetics of autistic disorders and its clinical relevance: a review of the literature

“…55 The molecular basis of the syndrome is an unstable expansion of a CGG repeat ( > 200 repeats) in the 5 0 UTR (untranslated region) of the FMR1 gene located on chromosome Xq27, resulting in a hypermethylation of the CGG sequence and a reduced translation of the FMR1 protein. 56,57 About 2-5% of the children and adolescents diagnosed with AD carry a full FRAXA mutation or FRAXA mosaics. 9,13,16,58 Despite this finding, no linkage or association with FMR1 gene variants 59,60 or the FRAXA mutation has been found in large samples diagnosed with AD by strict criteria.…”

Fragile X Mental Retardation Protein: Nucleocytoplasmic Shuttling and Association with Somatodendritic Ribosomes

“…In fragile X syndrome, the FMR1 repeat is massively expanded to an average of 800 triplets, in contrast to the normal mode of 30 triplets (Brown et al, 1993;Kunst and Warren, 1994;Rousseau et al, 1995). When expanded beyond ϳ230 repeats, the FMR1 gene becomes aberrantly methylated (Sutcliffe et al, 1992;Hornstra et al, 1993) and transcriptionally silent . Thus, the FMR1 repeat expansion results in the absence of the encoded protein, fragile X mental retardation protein (FMRP), which appears to be responsible for the phenotype (Devys et al, 1993).…”

Hotspot for deletions in the CGG repeat region of FMR1 in fragile X patients

“…Premutations are not associated with a clinical phenotype and are found in female carriers and normal transmitting males. During or after passage through female meiosis, a premutation can expand to a full mutation of over 200 repeats resulting in méthylation of the CGG repeat and the CpG island 250 bp proximal to the CGG repeat (5,(8)(9)(10). This méthylation is associated with repression of FMR1 transcription, thereby resulting in the severe reduction of the level of FMRJ protein leading to the fragile X phenotype (11,12).…”