Sutcliffe J S, Nelson D L, Zhang F, Pieretti M, Caskey C T, Saxe D, Warren S T
Howard Hughes Medical Institute, Emory University School of Medicine, Altanta, GA 30322.
Hum Mol Genet. 1992 Sep;1(6):397-400. doi: 10.1093/hmg/1.6.397.
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.
脆性X综合征是遗传性智力迟钝最常见的形式，呈X连锁显性遗传，外显率降低。最近，我们在脆性X位点鉴定出了FMR-1基因。在脆性X患者中发现了FMR-1基因的两种分子差异：一个包含CGG重复序列的FMR-1外显子大小增加，以及该重复序列近端250 bp处的一个CpG岛异常甲基化。表现出这些变化的显性脆性X男性通常显示FMR-1转录受到抑制，推测FMR-1蛋白的缺失被认为是导致脆性X表型的原因。然而，尚不清楚FMR-1基因的这两种分子差异中的一种或两种是否导致转录沉默。我们在此报告利用这些分子差异对一名患有脆性X综合征的男性胎儿进行产前诊断，结果显示虽然在绒毛膜绒毛和胎儿中均观察到了扩展的CGG重复突变，但CpG岛的甲基化仅限于胎儿DNA（通过BssHII消化评估）。我们进一步证明，胎儿组织中的FMR-1基因表达受到抑制，这是显性男性的特征，而未甲基化的绒毛膜绒毛表达FMR-1。由于所研究组织的遗传背景相同，包括脆性X染色体，这些数据表明FMR-1 CpG岛的异常甲基化是FMR-1转录缺失的原因，并提示甲基化可能在胚胎发育早期获得。
