Maintenance of replication patterns in human-mouse hybrids retaining only one human chromosome

Complete reactivation of X chromosomes from human chorionic villi with a switch to early DNA replication

Mammalian sex-dosage compensation is mediated by maintaining activity of only one X chromosome. The asynchronous DNA synthesis characterizing the silent human X chromosome is thought to be reversible only during ontogeny of oocytes. We have previously shown that the glucose-6-phosphate dehydrogenase (G6PD) locus (G6PD) on the allocyclic X chromosome in chorionic villi is partially expressed. We now show that in hybrids derived from a clone of chorionic villi cells (heterozygous for G6PD A) and mouse A9 cells, the loci for G6PD, hypoxanthine phosphoribosyltransferase (HPRT) and phosphoglycerate kinase are expressed on both human X chromosomes; the human X chromosomes carrying either G6PD A or B replicate synchronously with each other and with murine chromosomes. The X chromosome with G6PD A was identified as the original late-replicating X, because methylation in the body of the HPRT gene on this chromosome remained characteristic of the inactive X chromosome. These results indicate that X-chromosome inactivation is completely reversible in cells of trophoblast origin; induction of full transcriptional activity is accompanied by acquisition of isocyclic replication, showing an intimate relationship between these processes. The molecular events responsible for this reversal may be similar to those occurring during maturation of oocytes. Chorionic villi and derivative hybrids provide in vitro models for exploring early events that program the single active X chromosome.

Chromosome replication in normal and transformed human lymphocytes

We analyzed the late-replication patterns of human B-lymphocyte chromosomes before and after transformation by Epstein-Barr virus. There were no statistically significant differences between normal cells and transformed cells derived from the same male individual; therefore, the order of termination of chromosome replication was unchanged by transformation. We also examined the replication patterns of T lymphocytes from the same donor and found no differences between normal B and T cells.

Patterns of late replication in X chromosomes of human lymphoid cells

Two predominant patterns of late X replication were observed in both short-term and established human lymphoid cultures. One pattern was found in a minority of short-term cultured T-cell metaphases, in most lectin-stimulated B cells, and, with minor variations, in established B-cell lines. In these cells, DNA replication terminated in the distal part of the long arm of the late X. A different pattern was found in the majority of lectin-simulated T cells and in the T-cell line CCRF-CEM. These cells exhibited terminal replication in a region of the long arm of the late X that was nearer to the centromere. It is speculated that the variations in replication patterns correlate with phenotypic and functional characteristics of human lymphoid subsets.