Hemimethylation and hypersensitivity are early events in transcriptional reactivation of human inactive X-linked genes in a hamster x human somatic cell hybrid.

Reactivation of the hypoxanthine phosphoribosyltransferase (HPRT) gene on an inactive human X chromosome in a somatic cell hybrid was analyzed following exposure to 5-aza-2'-deoxycytidine. Hemimethylation and chromatin hypersensitivity in the 5' CpG island appeared by 6 h after exposure and continued to increase for 24 h in an exponentially growing cell culture. These results imply that the conformation of inactive chromatin requires a symmetrically methylated 5' G+C-rich promoter region. In addition, quantitative analysis of the time course patterns suggest that chromatin sensitivity changes may depend on strand-specific demethylation. Symmetrically demethylated DNA was first detected at 24 h and continued to increase until 48 h. HPRT mRNA was first detected at 24 h and increased in a biphasic pattern until 48 h. These results suggest that hemimethylation permits nuclease attack but not transcription factor binding, which requires symmetrically demethylated DNA. We also show that in G1-arrested cells, 5-aza-2'-deoxycytidine has no effect on methylation, chromatin conformation, or transcription. We conclude that reactivation of the HPRT gene present on the inactive X chromosome of a somatic cell hybrid involves the initial events of DNA hemimethylation and chromatin hypersensitivity at the 5' CpG island, followed by symmetrical demethylation and transcriptional reactivation.