X chromosome reactivation in mouse embryonal carcinoma cells.
The embryonal carcinoma cell line, C86S1, carries two X chromosomes, one of which replicates late during S phase of the cell cycle and appears to be genetically inactive. C86S1A1 is a mutant which lacks activity of the X-encoded enzyme, hypoxanthine phosphoribosyltransferase (HPRT). Treatment of C86S1A1 cells with DNA-demethylating agents, such as 5-azacytidine (5AC), resulted in (i) the transient expression in almost all cells of elevated levels of HPRT and three other enzymes encoded by X-linked genes and (ii) the stable expression of HPRT in up to 5 to 20% of surviving cells. Most cells which stably expressed HPRT had two X chromosomes which replicated in early S phase. C86S1A1 cells which had lost the inactive X chromosome did not respond to 5AC. These results suggest that DNA demethylation results in the reactivation of genes on the inactive X chromosome and perhaps in the reactivation of the entire X chromosome. No such reactivation occurred in C86S1A1 cells when the cells were differentiated before exposure to 5AC. Thus, the process of X chromosome inactivation may be a sequential one involving, as a first step, methylation of certain DNA sequences and, as a second step, some other mechanism(s) of transcriptional repression.
D000818:Animals; D001374:Azacitidine / Q000494:pharmacology; D002454:Cell Differentiation; D002460:Cell Line; D004261:DNA Replication / Q000187:drug effects; D004303:Dosage Compensation, Genetic; D005786:Gene Expression Regulation / Q000187:drug effects; D007041:Hypoxanthine Phosphoribosyltransferase / Q000235:genetics; D008745:Methylation; D051379:Mice; D013724:Teratoma / Q000473:pathology / Q000503:physiopathology*; D014158:Transcription, Genetic / Q000187:drug effects; D014960:X Chromosome / Q000502:physiology*
D007041:Hypoxanthine Phosphoribosyltransferase; D001374:Azacitidine