Mechanisms of X-Chromosome Regulation

The mammalian X chromosome is unique in ‬its hemizygous ‬expression ‬in somatic ‬tissues. In ‬males, hemizygous expression ‬is dictated by the XY genotype, but in females, the single active X condition is a result of X-chromosome inactivation. All of the genes on an X chromosome become coordinately inactivated, and, once initiated, the inactive condition becomes a somatically heritable feature that is stably maintained within a cell lineage. The stability ‬of the inactive condition appears to differ between the embryonic and extraembryonic ‬cell ‬lineages of ‬placental mammals and between the somatic lineages ‬of placental mammals and marsupials. Moreover, the relative stability of the inactivation ‬of X-chromosome genes can be altered by changing the genomic environment of the cell in ‬interspecific ‬somatic cell hybrids or by treatment with drugs that ‬impair the DNA cytosine methylation process. In contrast to the single active-X condition of somatic tissues, both X chromosomes are functional in oocytes and in cleavage-stage female embryos. The transitions from single active X expression in primordial germ cells and oogonia to an oocyte ‬with two active X chromosomes, and from embryonic cells with two active Xs to somatic cells with a single ‬X-active, provide a major focus for the ‬interest ‬in the regulation ‬of ‬X-chromosome expression. In general, we would like to know the molecular basis for ‬these changes in X-chromosome expression and how the events that both ‬initiate and maintain inactivation or reactivation relate to the maintenance of those changes in levels of expression. This review attempts to address many of the unresolved issues associated with mammalian X-chromosome inactivation. Whether inactivation, ‬reactivation, or ‬both are active processes is, as yet, ‬unknown. Similarly, whether either condition is fundamentally stable or must be actively maintained is still a matter for speculation. The inactivation process appears to affect virtually the entire X chromosome, and the inactive condition acts as if it ‬becomes a chromosome-autonomous property that is self-perpetuating. By contrast, we know less about the reactivation process and whether active gene products are required to sustain two-X expression during oogenesis and in cleavage stage embryos. Finally, ‬we attempt to discuss X inactivation as a chromosomal process and not merely a regulatory mechanism at ‬the single gene level. The regulation of ‬X-chromosome expression has received ‬ongoing attention as a topic of experimental and speculative interest. Investigation of the role of DNA methylation relative to the inactivation process has been especially prominent in recent years. This review attempts to incorporate much of that data into the refinement of a ‬generalized scheme for regulating X-chromosome expression. Where possible, ‬we have tried to incorporate the special features of X-chromosome expression ‬into ‬a developmental framework with an evolutionary perspective. This view appears especially appropriate when one considers that reactivation occurs at the time of entry into meiotic prophase. Having both X chromosomes in an active state may facilitate pairing and recombination. Conversely, other components of constitutive heterochromatin, such as satellite DNA in rodents, are normally highly methylated in somatic tissues but become hypomethylated during gametogenesis. Thus, it is possible that X-chromosome reactivation is a secondary consequence of generalized demethylation of genomic DNA in heterochromatin during oogenesis. In view of this possibility, we examine other experimental systems that produce demethylation of cytosine in DNA in an attempt to determine the conditions necessary for X-chromosome reactivation. In recent years, the general model that has emerged of X-chromosome expression during development indicates that oocyte and cleavage stage embryos have two active X chromosomes, whereas both embryonic and extraembryonic somatic tissues beyond the initial stages of cytodifferentiation have a single X chromosome active. These aspects of X-chromosome expression have been extensively reviewed.