Replication timing properties of the human HPRT locus on active, inactive and reactivated X chromosomes

• Published in: Somatic cell and molecular genetics Vol. 23; no. 2; p. 97
• Main Authors: Subramanian, P S, Chinault, A C
• Format: Journal Article
• Language: English
• Published: United States 01.03.1997
• Subjects: Animals; Azacitidine - pharmacology; Cell Line, Transformed; Chromatin - chemistry; Cloning, Molecular; Cricetinae; DNA Replication - drug effects; Enzyme Activation - drug effects; Enzyme Activation - genetics; Female; Gene Expression Regulation - drug effects; Humans; Hybrid Cells; Hypoxanthine Phosphoribosyltransferase - drug effects; Hypoxanthine Phosphoribosyltransferase - genetics; Male; Time Factors; X Chromosome - drug effects; X Chromosome - enzymology
• ISSN: 0740-7750
• DOI: 10.1007/BF02679969

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.