Differential methylation at the 5′ and the 3′ CCGG sites flanking the X chromosomal hypervariable DXS255 locus

The degree of methylation at the 5′ and 3′ CCGG sequences flanking the variable number of tandem repeat (VNTR) region of the DXS255 locus at Xp11.22 was analysed separately in several haematopoietic cell lineages. The 5′ CCGG site on active chromosomes was found to be completely methylated in B and T lymphocytes and granulocytes. Methylation of the 5′ site on inactive X chromosomes differed between females (0%–60%), but was consistent in different cell lineages obtained from individual females. In contrast, methylation at the 3′ CCGG site on active chromosomes was found to vary in B lymphocytes (40%–100%), whereas complete methylation was found in T lymphocytes and granulocytes. The extent of methylation on inactive X chromosomes was found to differ significantly between B lymphocytes (17%), T lymphocytes (54%) and granulocytes (82%). Thus, methylation at the 5′ CCGG site seems to be primarily related to the status of X chromosome inactivation, whereas methylation at the 3′ CCGG site is mainly subject to cell-lineage-specific influences.