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 me thylation of the fragile X (FraX) locus in a variety of tissues from n ormal fetuses and adults, and from males with the FraX syndrome. We id entified 20 CCGG sites (MspI-HpaH sites M1-M20) within a 12-kb BglII f ragment that includes the CpG island Sites M3-M18, within the 1.2-kb C pG 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 variet y of tissues from normal females by six weeks from conception. The exc eptional tissues are chorionic villi and gonads, which are significant ly 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 no rmal inactive X but is more extensive and less heterogeneous. This sug gests that the expansion of the island and the greater number of CpGs that result from amplification of the CGG repeat enhance the methylati bility 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.