Human X chromosomes: synchrony of DNA replication in diploid and triploid fibroblasts with multiple active or inactive X chromosomes

Histone acetylation controls the inactive X chromosome replication dynamics

In mammals, dosage compensation between male and female cells is achieved by inactivating one female X chromosome (Xi). Late replication of Xi was proposed to be involved in the maintenance of its silenced state. Here, we show a highly synchronous replication of the Xi within 1 to 2 h during early-mid S-phase by following DNA replication in living mammalian cells with green fluorescent protein-tagged replication proteins. The Xi was replicated before or concomitant with perinuclear or perinucleolar facultative heterochromatin and before constitutive heterochromatin. Ectopic expression of the X-inactive-specific transcript (Xist) gene from an autosome imposed the same synchronous replication pattern. We used mutations and chemical inhibition affecting different epigenetic marks as well as inducible Xist expression and we demonstrate that histone hypoacetylation has a key role in controlling Xi replication. The epigenetically controlled, highly coordinated replication of the Xi is reminiscent of embryonic genome replication in flies and frogs before genome activation and might be a common feature of transcriptionally silent chromatin.

Loss of ABCB7 gene: pathogenesis of mitochondrial iron accumulation in erythroblasts in refractory anemia with ringed sideroblast with isodicentric (X)(q13)

An isodicentric (X)(q13) (idicXq13) is a rare, acquired chromosomal abnormality originated by deletion of the long arm from Xq13 (Xq13-qter), and is found in female patients with hematological disorders involving increased ringed sideroblasts (RSs), which are characterized by mitochondrial iron accumulation around the erythroblast nucleus. The cause of increased RSs in idicXq13 patients is not fully understood. Here, we report the case of a 66-year-old female presenting with refractory anemia with ringed sideroblasts (RARS), and idicXq13 on G-banded analysis. We identify the loss of the ABCB7 (ATP-binding cassette subfamily B member-7) gene, which is located on Xq13 and is involved in mitochondrial iron transport to the cytosol, by fluorescent in situ hybridization (FISH) analysis and the decreased expression level of ABCB7 mRNA in the patient's bone marrow cells. Further FISH analyses showed that the ABCB7 gene is lost only on the active X-chromosome, not on the inactive one. We suggest that loss of ABCB7 due to deletion of Xq13-qter at idicXq13 formation may have contributed to the increased RSs in this patient. These findings suggest that loss of the ABCB7 gene may be a pathogenetic factor underlying mitochondrial iron accumulation in RARS patients with idicXq13.

Mammalian sex chromosomes. IV. Replication heterogeneity in the late replicating facultative- and constitutive-heterochromatic regions in the X chromosomes of the mole rats, Bandicota bengalensis and Nesokia indica

The karyotypes of Nesokia indica and Bandicota bengalensis are identical except for their sex chromosomes, which are much larger in Nesokia due to additional constitutive heterochromatin (C.H.). Replication patterns of their sex chromosomes were studied employing 3H-Tdr autoradiography and BrdUrd-FPG staining techniques. Though the "conservative" part of both early- and late-replicating X chromosomes revealed identical replication patterns in most cells, deviant patterns of only the late replicating X chromosome were encountered in approximately 10% cells. Surprisingly, these late-X variants were similar in the two species. The sex chromosome-associated C.H. segments replicated late in S-phase and, in females, the homologous heterochromatin replicated asynchronously--the later replicating one was predominantly associated with the late X. These results suggest structural and functional conservation of the X chromosomes as well as the possible influence of facultative heterochromatin (F.H.) on the replication of associated C.H. in these two species.

A gene from the region of the human X inactivation centre is expressed exclusively from the inactive X chromosome

X-chromosome inactivation results in the cis-limited dosage compensation of genes on one of the pair of X chromosomes in mammalian females. Although most X-linked genes are believed to be subject to inactivation, several are known to be expressed from both active and inactive X chromosomes. Here we describe an X-linked gene with a novel expression pattern--transcripts are detected only from the inactive X chromosome (Xi) and not from the active X chromosome (Xa). This gene, called XIST (for Xi-specific transcripts), is a candidate for a gene either involved in or uniquely influenced by the process of X inactivation.

Isochromosome X in acute myeloblastic leukemia

An aberrant leukemic clone with a 46,X,i(Xq) karyotype appeared at the first relapse of the disease in a case of acute myeloblastic leukemia (AML). The DNA replication pattern displayed asynchronous late termination within the homologous arms of the isochromosome in all investigated metaphases. The origin of asynchrony in the isochromosome's DNA replication pattern is discussed.

Replication variants of the human inactive X chromosome. II. Frequency and replication rate relative to the other chromosomes of the complement

Replication variants of the inactive X chromosome were investigated in lymphocytes from six donors by means of terminal BrdU or thymidine incorporation. There were interindividual differences in the incidence of particular variants. In endoreduplicated and tetraploid cells both allocyclic X chromosomes showed the same replication sequence. The Xp22 band of the allocyclic X chromosome seemed to replicate later than the homologous material in some cells. Initiation time of DNA synthesis within the inactive X chromosome was found to be stable; termination time, however, varied greatly relative to the other chromosomes. Early completion of replication within the heterochromatic X chromosome could be demonstrated preferentially for the Xq25-27 terminal sequence, but other variants expressed the phenomenon also. A variable replication rate of the inactive X chromosome is believed to be responsible for its asynchronous, independent replication. The biological significance of the phenomenon is discussed with respect to cell differentiation.

The sequence of DNA replication in an iso-dicentric X-chromosome in peripheral blood lymphocytes and skin fibroblasts from the same individual

A comparison of the sequence of DNA replication in an isodicentric (idic) X chromosome was made between peripheral blood lymphocytes and skin fibroblasts from a 33-year-old female with primary amenorrhea, somatic stigmata of Turner syndrome, and normal stature and intelligence. The patient had a karyotype 45,X/46,X,idic(X)(q27.1) to lymphocytes and 46,X,idic(X)(q27.1) in skin fibroblasts. Both centromeric regions of the idic X showed C-staining but only one primary constriction. BrdU-33258 Hoechst-Giemsa techniques were used to analyze regional DNA replication patterns. The idic X chromosome was always late replicating in lymphocytes and skin fibroblasts, except that about 1-2% of cells completed replication simultaneously in both normal and idic X chromosomes. Fifty-six percent of the asymmetric patterns in lymphocytes showed an equal proportion of early and late functional and non-functional centromere halves. In skin fibroblasts, 60.8% of cells were asymmetric: the functional half tended to replicate later than the non-functional half. Some differences were observed between these two cell types. As examples, band q23 was late replicating in lymphocytes, but early replicating in fibroblasts; q25 was intermediate to late replicating in lymphocytes, but one of the last bands to complete replication in fibroblasts. Thus, different cell typed influenced the replication kinetics of the idic(X). Furthermore, several variants of the replication sequence were found in both cell types. The findings support the hypothesis that the control of DNA replication in the inactive X chromosome is multifocal, and suggest that the active idic X chromosome replication may reflect a relative lack of self-control or heterogeneity of cell population.

Specific terminal DNA replication sequence of X chromosomes in different tissues of a live-born triploid infant

Using the thymidine pulse method, DNA replication kinetics were studied on cells derived from cartilage, gonad, lymphocytes, and skin of a live-born triploid (69,XXY) infant with typical clinical findings. Replication studies showed that 3% of the lymphocytes had one early and one late replicating X, and 97% of the lymphocytes, and cartilage, gonad, and skin cells had two early replicating X's. Asynchronous DNA replication between the two early replicating X's was observed in all tissues (range 25-40%). The predominant terminal replication sequence of X chromosomes from chondrocytes, gonad, and skin fibroblast differed from that of the lymphocytes. Thus, a tissue-specific DNA replication pattern of the early-replicating X chromosome may be present. In every tissue, the last band to complete DNA replication was Xq21. Polymorphisms of metaphase chromosomes of parents and the patient were studied by Q-banding. The possible origin of the extra haploid set of chromosomes is discussed.

BrdU-Hoechst-Giemsa analysis of DNA replication in synchronized lymphocyte cultures. Study of human X and Y chromosomes

The combination of a technique of reversible methotrexate (MTX) imposed G1/S block in cultures of human lymphocytes with the BrdU-Hoechst-Giemsa technique permitted the study of DNA replication patterns in individual chromosomes at different intervals of the S phase in a cell cohort with uniform S + G2 duration. The procedure did not increase either the frequency of chromosomal breakage or SCE frequency. The technique applied permitted visualization of the banding pattern in over 90% of mitoses. Examination of mitosis following different times of exposure to BrdU revealed a high degree of synchrony in the progression of the cell cohort examined through the S phase. The presence of two distinct late replication patterns of the allocyclic X chromosome was confirmed in studies on lymphocytes from normal human females by this technique. Interindividual and intercellular differences of the replication pattern have been demonstrated. The replicating patterns from one individual were relatively constant. The analysis of the Y chromosome has revealed marked differences of the termination of replication in individual cells. Euchromatic regions have been shown to complete DNA synthesis first, followed by the distal part of the long arm and, finally, by the region of Yq11/Yq12 junction. Lateral asymmetry was localised at this region.

Derepression with decreased expression of the G6PD locus on the inactive X chromosome in normal human cells

Studies of a unique clone of skin fibroblasts from a normal 46 XX female reveal that the G6PD locus on the inactive X chromosome has been derepressed. The reactivation event occurs spontaneously, and is associated with normal karyotype, including the presence of a late-replicating X chromosome. Analysis of mouse-human hybrids with the relevant chromosome provides evidence that the derepressed locus is on the inactive X, and that reactivation is not extensive (the PGK locus is not derepressed). Nor is any general change in DNA methylation of this chromosome detectable with Hpa II and an X-specific DNA probe. Studies of the glucose-6-phosphate dehydrogenase phenotype in these heterozygous cells indicate that the reactivated X produces only half the enzyme subunits as are produced by the active X. Although this dosage difference may be related to the mutational event responsible for derepression of the locus, these observations along with other evidence suggest that loci on the inactive X, when expressed, have less activity than corresponding loci on the active X.

DNA replication sequence in a dicentric (functionally monocentric) X chromosome formed by the joining of two X chromosomes at region p22

In this study we used densitometry to evaluate DNA replication kinetics in a rearranged chromosome formed by the joining of two X chromosomes at region p22. No 45X mosaicism is present in peripheral blood or fibroblast cultures. The patient has primary amenorrhea, short stature, and gonadal dysgenesis. The sequence of replication in the majority of cells is p11, q11, q13, q22-24, q12, p22, q26, q28, q27, q25, and p21, q21. Thus p11 is the earliest region to replicate, and q21 is the last. In 66% of 127 cells analyzed, the replication pattern is asymmetric, and bands q12, q26, and q28 are most likely to be out of phase on the two sides of the breakpoint. We find that band p22 has a delay of replication compared to an abnormal X derived from two X chromosomes joined at the q23 region previously reported by us. Structural rearrangement may therefore delay replication in the region of the break.

Manufacture of diploid/tetraploid chimeric mice

Tetraploid mouse embryos were produced by cytochalasin B treatment. These embryos usually die before completion of embryonic development and are abnormal morphologically and physiologically. The tetraploid embryos can be rescued to develop to maturity by aggregating them with normal diploid embryos to produce diploid/tetraploid chimeric mice. The diploid/tetraploid chimeric embryos are frequently abnormal: the larger the proportion of tetraploid cells, the greater the abnormality. By karyotype analysis and by the use of appropriate pigment cell markers, we have demonstrated that two of our surviving chimeras are in fact diploid/tetraploid chimeras. One surviving chimera is retarded in growth and displays neurological abnormalities. The coat color chimerism suggests that this chimera is about 50% tetraploid. Another chimera with about 10% tetraploid pigment cells in the coat is only slightly retarded in growth and is a fertile male. Tetraploid cells are distributed in many, if not all, tissues of embryos but evidently are physiologically inadequate to support completely normal development and function in the absence of substantial numbers of normal diploid cells.