Replication timing properties of the human HPRT locus on active, inactive and reactivated X chromosomes.
X chromosome inactivation is associated with a highly asynchronous pattern of DNA replication at most X-linked loci in females. We studied the human HPRT locus, which is subject to X inactivation and expressed from only the active homolog, with the goal of comparing replication properties between the active and inactive homologs in this region using a fluorescence in situ hybridization approach. We found that in normal female lymphoblasts this locus is replicated in a highly asynchronous manner across a broad, discrete 500-600 kb zone with earliest replication appearing at the gene coding sequence. This general timing profile is maintained in normal male lymphoblasts, as well as in hamster x human hybrid cells containing the active human X chromosome. However, the inactive human X chromosome in the hamster cell background does not appear to function in a fully equivalent manner to the normal inactive X chromosome in female cells. Furthermore, reactivation of the inactive human X chromosome in a hamster x human hybrid system by 5-azacytidine treatment and HAT selection restores early replication at the HPRT gene itself, but does not change the overall domain behavior.
Subramanian
P S
PS
Department of Molecular and Human Genetics, Baylor College of Medicine, Houston, Texas 77030, USA.
Chinault
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AC
D000818:Animals; D001374:Azacitidine / Q000494:pharmacology; D002461:Cell Line, Transformed; D002843:Chromatin / Q000737:chemistry; D003001:Cloning, Molecular; D006224:Cricetinae; D004261:DNA Replication / Q000187:drug effects; D004789:Enzyme Activation / Q000187:drug effects / Q000235:genetics; D005260:Female; D005786:Gene Expression Regulation / Q000187:drug effects; D006801:Humans; D006822:Hybrid Cells; D007041:Hypoxanthine Phosphoribosyltransferase / Q000187:drug effects / Q000235:genetics*; D008297:Male; D013997:Time Factors; D014960:X Chromosome / Q000187:drug effects / Q000201:enzymology*
D002843:Chromatin; D007041:Hypoxanthine Phosphoribosyltransferase; D001374:Azacitidine