Methylation and cell memory

Abstract

It has been known for more than 20 years that DNA methylation at CpG dinucleotides is associated with gene silencing in eukaryotes. The preservation of CpG methylation patterns through cell division suggests that they could be important in generating cell ‘memory’ and hence in embryonic cell determination and differentiation. Several lines of evidence support this hypothesis, but a clear example has been lacking. Thomassin et al. 1xGlucocorticoid-induced DNA methylation and gene memory during development. Thomassin, H. et al. EMBO J. 2001; 20: 1974–1983CrossRef | PubMed | Scopus (176)See all References1 now provide an elegant model system in their study of the liver-specific tyrosine aminotransferase (Tat) gene, and Stancheva et al. 2xLoss of the maintenance methyltransferase, xDnmt1, induces apoptosis in Xenopus embryos. Stancheva, I. et al. EMBO J. 2001; 20: 1963–1973CrossRef | PubMed | Scopus (68)See all References2 investigate the role of DNA methylation in amphibian embryonic development.Tat transcription in the liver is triggered at birth by hypoglycemia. This response is mediated by transcription factors that cooperate with the glucocorticoid receptor, interacting with glucocorticoid responsive units (GRUs) located in the Tat enhancer. Thomassin et al. show that glucocorticoid treatment of cultured rat hepatoma cells induces rapid (<1 hour) remodelling of chromatin and recruitment of transcription factors to the Tat enhancer, but also slower (2–3 day) demethylation of CpGs within one of the GRUs. In contrast to transcriptional activation of Tat, which requires continuous glucocorticoid treatment, this CpG demethylation is stable. Furthermore, a second glucocorticoid stimulus, delivered to the cells even one month later, induces Tat transcription more strongly and rapidly. This suggests that demethylation provides the cells with a memory that sensitizes them to the transcription-activating signal.How do these in vitro observations compare with the situation in the embryo? The authors show that the Tat GRU is demethylated, specifically in the liver, following an increase in embryonic corticosteroids a few days before birth. Furthermore, GRU demethylation is prematurely induced by glucocorticoid treatment of cultured fetal hepatocytes. However, unlike hepatoma cells, glucocorticoid stimulation does not immediately trigger Tat transcription in hepatocytes and the embryo. This is induced at birth, when corticosteroids are low, perhaps by factors interacting with the Tat proximal promoter. Thus, this paper suggests that, at least in the Tat gene, DNA demethylation is not sufficient for tissue-specific expression, but contributes to it by preparing the gene for a rapid response to inducing factors. Glucocorticoid receptor knockout mice do not activate the Tat gene at birth, showing the importance of this gene sensitization.Stancheva et al. have previously shown in Xenopus that antisense depletion of the maintenance methyltransferase, xDnmt1, leads to premature expression of certain embryonic transcripts. They now show that phenotypic abnormalities in xDnmt1-depleted embryos are associated with the appearance of apoptotic cells. Experimental inhibition of apoptosis leads to the growth of abnormal undifferentiated cellular masses. These results confirm the importance of DNA methylation for embryogenesis. Cell differentiation activates an apoptotic pathway to eliminate cells in which appropriate methylation is not maintained. An interesting observation is that the genes prematurely activated by xDnmt1-depletion are involved in the early determinative events of embryonic germ-layer patterning. In future, experiments similar to those of Thomassin et al. might shed light on the specific role of DNA methylation in these events.