ASYNCHRONOUS REPLICATION OF THE MOUSE SEX CHROMOSOMES

A deletion at the mouse Xist gene exposes trans-effects that alter the heterochromatin of the inactive X chromosome and the replication time and DNA stability of both X chromosomes

The inactive X chromosome of female mammals displays several properties of heterochromatin including late replication, histone H4 hypoacetylation, histone H3 hypomethylation at lysine-4, and methylated CpG islands. We show that cre-Lox-mediated excision of 21 kb from both Xist alleles in female mouse fibroblasts led to the appearance of two histone modifications throughout the inactive X chromosome usually associated with euchromatin: histone H4 acetylation and histone H3 lysine-4 methylation. Despite these euchromatic properties, the inactive X chromosome was replicated even later in S phase than in wild-type female cells. Homozygosity for the deletion also caused regions of the active X chromosome that are associated with very high concentrations of LINE-1 elements to be replicated very late in S phase. Extreme late replication is a property of fragile sites and the 21-kb deletions destabilized the DNA of both X chromosomes, leading to deletions and translocations. This was accompanied by the phosphorylation of p53 at serine-15, an event that occurs in response to DNA damage, and the accumulation of gamma-H2AX, a histone involved in DNA repair, on the X chromosome. The Xist locus therefore maintains the DNA stability of both X chromosomes.

The element(s) at the nontranscribed Xist locus of the active X chromosome controls chromosomal replication timing in the mouse

In female mammalian cells, the inactive X chromosome is replicated late in S phase while the active X chromosome is replicated earlier. The replication times of the X chromosomes reflect a general trend in which late replication is associated with gene repression and earlier replication with transcriptional competence. The X-linked Xist gene is expressed exclusively from the inactive X chromosome where it is involved in the initiation and maintenance of X-inactivation. In contrast, no biological activity has been assigned to the Xist locus of the active X chromosome where the Xist gene is transcriptionally silenced. Here, we provide evidence that the element(s) at the nontranscribed Xist locus of the active X chromosome controls chromosomal replication timing in cis.

The X-autosome translocation in the common shrew (Sorex araneus L.): late replication in female somatic cells and pairing in male meiosis

Common shrews have an XX/XY1Y2 sex chromosome system, with the "X" chromosome being a translocation (tandem fusion) between the "original" X and an autosome; in males this autosome is represented by the Y2 chromosome. From G-banded chromosomes, the Y2 is homologous to the long arm and centromeric part of the short arm of the X. The region of the X that is homologous to the Y2 and also the telomeric region of the short arm of the X were found to be early replicating in somatic cells from a female shrew after 5-bromo-2'-deoxyuridine (BrdU) treatment in vitro. The remainder of the short arm of the X was shown to be late replicating. Electron microscopic examination of synaptonemal complexes in males at pachytene revealed pairing of the Y2 axis with the long arm of the X, and Y1 with the short arm. At early stages of pachytene, there is apparently extensive nonhomologous pairing between the X and Y1. In essence, the short arm of the shrew X chromosome behaves like a typical eutherian X chromosome (it is inactivated in female somatic cells and is paried with the Y1 during male meiosis) while the long arm behaves like an autosome (escapes the inactivation and pairs with the Y2).

Asynchronous replication of constitutive heterochromatin on X chromosomes in female Mus dunni. Possible influence of facultative heterochromatin on the adjacent constitutive heterochromatin

Euchromatin DNA of one X chromosome in mammalian females, which becomes facultatively heterochromatinized, is known to replicate asynchronously late in S phase compared to its active homologue. In the females of a pygmy mouse species Mus dunni, which has prominent segment of constitutive heterochromatin as the short arm of its submetacentric X chromosome, we have observed asynchronous replication of c-heterochromatin arm as well, predominant number of cells showing the segment associated with the facultatively heterochromatic X to be terminating later. The preferential later termination of replication of the c-heterochromatic arm on the "lyonized X" appears to be due to the influence of facultative heterochromatin on the adjacent constitutive heterochromatin.

The relationship between induced mutation frequency and chromosome dosage in established mouse fibroblast lines

The frequencies of induced mutation to ouabain (OUA) and 6-thioguanine (6TG) resistance were compared in two established mouse fibroblast lines with different doses of the active X chromosome. The activity states of the X chromosomes were determined by DNA replication studies. Mutation frequency to 6TG R, an X-linked recessive phenotype, was inversely related to dosage whereas OUA R, a codominant phenotpye, occurred with equal frequency in both lines. 6TG R clones isolated from the line containing a majority of cells with two active X chromosomes were monosomic for the active X chromosome. Enzyme activity studies of these cytologically monosomic lines yieled results that were consistent with the presence of a single active X chromosome. The findings strongly support the hypothesis that mutation frequency is dependent on gene dosage.

Studies on the activity of the X chromosomes in female teratocarcinoma cells in culture

Embryonal carcinoma cells derived from murine teratocarcinomas are able to differentiate into the same variety of tissue types as early embryonic cells. Because embryonal carcinoma cells resemble those of the embryo at a stage before X chromosome inactivation has occurred in females embryonal carcinoma cells containing two X chromosomes were examined to determine whether both are genetically active. The specific activities of X-linked enzymes were measured in embryonal carcinoma cells containing either one or two X chromosomes. The activities in both cell types were similar, suggesting that only one X chromosome was active in the female cells. Further support for this conclusion came from experiments in which azaguanine-resistant mutants were recovered with similar frequencies from embryonal carcinoma cell lines containing one and two X chromosomes. Late replication of an X chromosome DNA was detected in one embryonal carcinoma cell line with two X chromosomes but not in another. This suggests that cells of these two lines were arrested at different developmental stages, and that late DNA replication may not be a necessary adjunct of X inactivation. Evidence is presented which suggests that X chromosome reactivation does not occur during differentiation of the cells in vitro.

Is the centromeric heterochromatin of Mus musculus late replicating?

Quantitative analysis of DNA replication in embryonic cultures of C57BL/6J mice was carried out, using autoradiography after tritiated thymidine incorporation. The centromeric regions are late-replicating.

Chromosomes and DNA of Mus: terminal DNA synthetic sequences in three species

The DNA replication patterns of the terminal S phase of three species of Mus were analyzed by tritiated thymidine autoradiography. The centromeric heterochromatin of M. fulvidiventris is the latest component to finish DNA synthesis. The Y chromosome finishes replication earlier than the centromeric heterochromatin. The centromeric heterochromatin of M. musculus, on the other hand, is not the latest component to finish DNA synthesis. At the very late S phase, grains are found in the euchromatic arms instead of the heterochromatic areas. The "hot X" and the "hot Y" can be identified in the majority of, but not all, cases. The heterochromatic short arms of the autosomes in M. dunni finish DNA replication earlier than many areas in the euchromatic long arms and the heterochromatin of the sex chromosomes. This indicates that in M. dunni there are at least two types of heterochromatin. The late-replicating zones in the euchromatic long arms are distinctly banded. This banded grain pattern can be seen in all Mus species observed, but in M. dunni it is most exaggerated. Late-replicating chromosome segments can be demonstrated also by 2+ cycles of BUdR incorporation and Giemsa staining.

Late DNA replication in male mouse meiotic chromosomes

Parts of the male mouse meiotic complement comprising the Y chromosome, the whole X chromosome, and near-centromeric parts of autosomal bivalents are synthesized late, as judged by tritiated thymidine autoradiography. This confirms the occurrence of end-to-end association between X and Y chromosomes and suggests that paired heterochromatic segments in autosomes must synthesize DNA at the same time.