DNA methylation stabilizes X chromosome inactivation in eutherians but not in marsupials: evidence for multistep maintenance of mammalian X dosage compensation.

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Abstract
In marsupials and eutherian mammals, X chromosome dosage compensation is achieved by inactivating one X chromosome in female cells; however, in marsupials, the inactive X chromosome is always paternal, and some genes on the chromosome are partially expressed. To define the role of DNA methylation in maintenance of X chromosome inactivity, we examined loci for glucose-6-phosphate dehydrogenase and hypoxanthine phosphoribosyltransferase in a North American marsupial, the opossum Didelphis virginiana, by using genomic hybridization probes cloned from this species. We find that these marsupial genes are like their eutherian counterparts, with respect to sex difference in methylation of nuclease-insensitive (nonregulatory) chromatin. However, with respect to methylation of the nuclease-hypersensitive (regulatory) chromatin of the glucose-6-phosphate dehydrogenase locus, the opossum gene differs from those of eutherians, as the 5'' cluster of CpG dinucleotides is hypomethylated in the paternal as well as the maternal gene. Despite hypomethylation of the 5'' CpG cluster, the paternal allele, identified by an enzyme variant, is at best partially expressed; therefore, factors other than methylation are responsible for repression. In light of these results, it seems that the role of DNA methylation in eutherian X dosage compensation is to "lock in" the process initiated by such factors. Because of similarities between dosage compensation in marsupials and trophectoderm derivatives of eutherians, we propose that differences in timing of developmental events.sbd.rather than differences in the basic mechanisms of X inactivation.sbd.account for features of dosage compensation that differ among mammals.