X-chromosome methylation ratios as indicators of chromosomal activity: evidence of intraindividual divergencies among tissues of different embryonal origin.

To test whether the differentiation events that lead to the embryonal layers and their derived organs produce divergent X-chromosome activation ratios among the different tissues, the X-chromosome activation ratios in leucocytes and muscle (mesodermal origin), thyroid gland (endodermal origin) and medulla of the suprarenal glands (ectodermal origin) from ten deceased females were surveyed. Analysis of the degree of the methylation of the polymorphic alleles recognized by the probes M27beta and pSPT-PGK showed that the ratios for the medulla of the suprarenals correlated well with those of all other tissues except for leucocytes; the thyroid gland showed limited correlation with muscle, whereas leucocytes showed correlation only with muscle. The results of this preliminary study suggest that differentiation events result in considerable variation in the activation ratios in different tissues. As a consequence caution should be taken in extrapolating from the activation ratios observed in leucocytes or fibroblasts to tissues of endodermal or ectodermal origin.