Replication kinetics of X chromosomes in fibroblasts and lymphocytes

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.

Chromatin modifications on the inactive X chromosome

In female mammals, one X chromosome is transcriptionally silenced to achieve dosage compensation between XX females and XY males. This process, known as X-inactivation, occurs early in development, such that one X chromosome is silenced in every cell. Once X-inactivation has occurred, the inactive X chromosome is marked by a unique set of epigenetic features that distinguishes it from the active X chromosome and autosomes. These modifications appear sequentially during the transition from a transcriptionally active to an inactive state and, once established, act redundantly to maintain transcriptional silencing. In this review, we survey the unique epigenetic features that characterize the inactive X chromosome, describe the mechanisms by which these marks are established and maintained, and discuss how each contributes to silencing the inactive X chromosome.

Earlier finishing of Xp21.2 subband replication of the inactive X chromosome in Rett syndrome girl but not in her 47,XXX mother

X-inactivation mosaicism has been proposed to explain the origin of Rett syndrome. We present the results of the cytogenetic analysis, including RBG dynamic replication pattern, in a girl with Rett syndrome. The late replicating X chromosome (LRX) showed the earlier replication of subband Xp21.2 in 36% of analysed cells. Unexpectedly the maternal karyotype 47,XXX was found. Replication timing of both maternal LRX chromosomes was normal. The critical region of Xp essential for RS is proposed.

Bromodeoxyuridine: a diagnostic tool in biology and medicine, Part III. Proliferation in normal, injured and diseased tissue, growth factors, differentiation, DNA replication sites and in situ hybridization

This paper is a continuation of parts I (history, methods and cell kinetics) and II (clinical applications and carcinogenesis) published previously (Dolbeare, 1995 Histochem. J. 27, 339, 923). Incorporation of bromodeoxyuridine (BrdUrd) into DNA is used to measure proliferation in normal, diseased and injured tissue and to follow the effect of growth factors. Immunochemical detection of BrdUrd can be used to determine proliferative characteristics of differentiating tissues and to obtain birth dates for actual differentiation events. Studies are also described in which BrdUrd is used to follow the order of DNA replication in specific chromosomes, DNA replication sites in the nucleus and to monitor DNA repair. BrdUrd incorporation has been used as a tool for in situ hybridization experiments.

Escape from X inactivation in human and mouse

Genes that escape X inactivation have been recently found in human and in mouse. Although many of these genes have homologues on the Y chromosome that may compensate for expression from both X alleles in females, some have no Y homologues, and this presumably results in dosage differences between the sexes. Comparisons between human and mouse have revealed that the X-inactivation status of some genes differs significantly between the two species, suggesting continuous evolutionary changes in the sex chromosomes. Questions about the mechanisms of 'escape' are relevant to the understanding of gene regulation by X inactivation.

Premature ovarian failure and ovarian dysgenesis associated with balanced and unbalanced X-6 translocations, respectively: implications for the investigation of ovarian failure

This study reports the effect of an inherited (X;6) translocation which has not previously been described. The proband was intellectually delayed and had ovarian dysgenesis. Karyotyping revealed an unbalanced karyotype: 46,X,der(X)t(X;6)(q22; p11.2)*. Her mother was shown to be a carrier of an apparently balanced translocation between the X chromosome and chromosome 6: 46,X,t(X;6)(q22;p11.2). This finding in the mother raises to 7 the number of cases reported which involve a break within the X chromosome 'critical region', at band Xq22, without causing ovarian dysgenesis, although it was associated with premature ovarian failure. These cases aim to highlight to clinical specialists the range of gonadal and other phenotypic anomalies (apart from those associated with Turner syndrome) which can occur due to partial deletions of the X chromosome. These findings have implications for the investigation of both ovarian dysgenesis and premature ovarian failure.

X-inactivation in girls with Rett syndrome

Cytogenetic studies have been carried out on a series of nine girls with Rett syndrome, six of their mothers and nine normal female controls. No abnormality of the X-chromosome has been observed in any subject. X-inactivation studies using various methods of detecting the timing of individual band replication were performed. The overall pattern seen was essentially the same in all subjects, but in the patients with Rett syndrome there may be an alteration in the timing of the X-inactivation process in the region Xp11.3 or 4-->Xp21.

Delayed replication of Xq27 in individuals with the fragile X syndrome

The timing of late replicating bands on the X chromosome has been studied in individuals with the fragile X [fra(X)] syndrome. Compared to controls both affected individuals and symptomless carriers of the syndrome show delayed replication of the Xq27 region as shown by 2 different methods. The implications of this finding are discussed in relation to the proposal [Laird et al., 1987] that the fraX syndrome is associated with a failure to reactivate the Xq27 band correctly after it has been inactivated in a female.

Replication asynchrony between homologs 15q11.2: cytogenetic evidence for genomic imprinting

Replication kinetics of the Prader-Willi syndrome critical region (15q11.2) was investigated in seven normal healthy adult females using RBG replication bands. Replication asynchrony between homologs 15q11.2 was identified consistently in about 40% of cells in all individuals. It was limited to the stages in which Xp22, Xp11, Xq13 and Xq24/26 were visible in the late-replicating X chromosome. This asynchrony suggested that replication timing overlapped between 15q11.2 and the early replicating R-bands of the late X chromosome in some cells, and that the difference in replication timing between homologs was probably related to genomic imprinting; the latter has been suggested as a pathogenetic basis of Prader-Willi syndrome. As a result of an analysis of the proportions of asynchronous and synchronous cells in each replication stage, two types of cells were deduced providing 1:1 methylation mosaicism of genomic imprinting was assumed. The first type was composed of cells with normal replication in one homolog and delayed replication in the other. The second type was composed of cells with normal replication in both homologs. Our results provide cytogenetic evidence of methylation mosaicism for mammalian genomic imprinting.

Analysis of DNA replication during S-phase by means of dynamic chromosome banding at high resolution

The characteristic patterns of dynamic banding (replication banding) were analysed. Extremely high resolution (850 to 1,250 bands per genome) G- and R-band patterns were obtained after 5-bromo-2'-deoxyuridine (BrdUrd) incorporation either during the early or the late S-phase. We synchronized human lymphocytes with high concentrations of thymidine or BrdUrd as blocking agents, followed by low concentrations of BrdUrd or thymidine respectively as releasing agents, and obtained R- or G-band patterns respectively. The dynamic R- and G-band patterns were complementary for all chromosomes, even for the late-replicating X chromosome. There was no overlapping and every part of each chromosome was positively stained by one of the two banding procedures. The complementarity of the two patterns shows that both high thymidine and high BrdUrd concentrations blocked S-phase progression near the R-band to G-band replication transition in the middle of S-phase. Some bands of the inactive X chromosome replicate before this transition concurrently with R-band replication. The 48 different telomeric regions could be classified into 5 distinct morphotypes based upon the distribution of early and late-replicating DNA in each telomeric region. The dynamic band patterns are particularly useful for the study of the structural and physiological organization of chromosomes at high resolution and should prove invaluable for assessing the replication behavior of rearranged chromosomes.

Analysis of chromosome replication by a BrdU antibody technique

Chromosome replication was studied without synchronization in human lymphocyte and amniotic cell cultures visualizing very short 5-bromodeoxyuridine (BrdU) pulses by an immunologic technique (BAT). The findings agree in general with those facts known from earlier BrdU staining techniques. The very high sensitivity of BAT was shown to allow the detection of replication in a band where 1 in 200 nucleotides is replaced by BrdU. The main observations are: though the replication patterns after BAT appear strange the bands correspond to those described by the Paris Conference (1971). At the beginning of the S-phase a stepwise onset of replication in only a subset of R-bands is confirmed. There is a considerable difference in the sensitivity between early and late S (SE and SL) for the detection of BrdU pulses. This difference probably reflects a different spatial arrangement of chromatin in R-bands as compared with G-bands below the level of cytogenetic analysis. The use of short pulses did not reveal any additional subdivision of SE or SL. The correspondence between chromosomal bands and replicon clusters is discussed briefly with respect to the different time they need for replication.

A pulse BrdU method for SCE

Using 'reverse' harlequin staining (bromouracil-substituted chromatin staining dark), it is possible to detect at metaphase a pulse of bromodeoxyuridine (BrdU) incorporated during S-phase. Provided that this pulse is of reasonable duration, fairly uniform staining along the chromatids of some S-cells is achieved, and no difficulty is encountered in observing and scoring SCE in such cells at second division. Thus, it is possible to define within an asynchronous population a narrow cohort of target cells and recover these for SCE scoring at second division irrespective of treatment induced perturbation. This serves to reduce the heterogeneity found in the usual terminal BrdU SCE protocols for such populations and should lead to more reliable and repeatable quantitative results. The method is illustrated for mitomycin C given to dividing human blood lymphocytes using both simultaneous and delayed pulse modes.

How does inactivation change timing of replication in the human X chromosome?

The kinetics of replication of the inactive (late replicating) X chromosome (LRX) were studied in karyotypically normal lymphocytes and human amniotic fluid cells. Both cell types were successively pulse labeled with 1-h or 1/2-h thymidine pulses in an otherwise BrdU-substituted S phase after partial synchronization of the cultures at G1/S. For the first time with this technique, the entire sequence of replication was analyzed for the LRX from the beginning to the end of the S phase, with special reference to mid S (R-band to G-band transition replication). The inactive X is the last chromosome of the metaphase to start replication, with a delay of 1 or 2 h, after which time a thymidine pulse results in R-type patterns. In mid S, the inactive X is the first chromosome to switch to G-type replication (without overlapping of both types and without any detectable replication pause). Until the end of S, a thymidine pulse results in G-type patterns. To rule out artifacts that might arise by the synchronization of cultures in these experiments, controls were carried out with BrdU pulses and the BrdU antibody technique without synchronization. In the course of replication, no fundamental difference was seen between the two different cell types examined. In contrast to studies using continuous labeling, this study did not reveal an interindividual difference of replication kinetics in the LRXs of the seven individuals studied; thus it is concluded that the inactive X chromosome shows only one characteristic course of replication.