DNA methylation of the fragile X locus in somatic and germ cells during fetal development: Relevance to the fragile X syndrome and X inactivation

To obtain insights into mechanisms responsible for methylation of CpG islands on the inactive X chromosome of normal females, we examined methylation of the fragile X (FraX) locus in a variety of tissues from normal fetuses and adults, and from males with the FraX syndrome. We identified 20 CCGG sites (MspI-HpaII sites M1-M20) within a 12-kb BglII fragment that includes the CpG island. Sites M3-M18, within the 1.2-kb CpG island are unmethylated on the active X in normal males and females at all ages and in all tissues studied. In contrast, these sites are at least partially methylated on the inactive X chromosome in a variety of tissues from normal females by six weeks from conception. The exceptional tissues are chorionic villi and gonads, which are significantly undermethylated. In addition, fetal germ cells are unmethylated at site M3, which is methylated on the inactive X in other tissues; thus, the methylation imprint of the inactive X has been erased. Methylation of the locus on the fragile X chromosome is similar to that of the normal inactive X but is more extensive and less heterogeneous. This suggests that the expansion of the island and the greater number of CpGs that result from amplification of the CGG repeat enhance the methylatibility of the island. Additional studies show that the chromatin of the CpG island is nuclease hypersensitive on the active X but insensitive on both inactive and FraX.