Differential inhibition of sister chromatid condensation induced by 5-azadeoxycytidine in human chromosomes

A historical overview of bromo-substituted DNA and sister chromatid differentiation

The thymidine analogue 5-bromo-2'-deoxyuridine (BrdU) has been widely used to make sister chromatid differentiation (SCD) evident in metaphase chromosomes of cells grown for two cycles in BrdU and, thus, containing varying amounts of the thymidine analogue. A direct consequence was the possibility of making sister chromatid exchange (SCE) evident without using autoradiographic procedures. The latter phenomenon was first discovered in 1953, and its frequency is considered a reliable marker of pathological cell situations, as well as an indicator of mutagenic compounds. Several experimental procedures were found which produced SCD, such as the use of fluorochromes like 33258 Hoechst or acridine orange, whose observation under fluorescence microscopy was directly recorded by photos or stained with Giemsa to make chromosome preparations permanent. Other treatments followed by Giemsa staining required the use of saline hot solutions, acid solutions, nuclease attack and specific monoclonal antibodies. Basically two molecular mechanisms were invoked to explain the different affinity of Giemsa stain for differential BrdU-substituted chromatid DNA. The first implied debromination of chromatid DNA, whose occurrence would be greater in chromatids containing an amount of BrdU greater than that present in sister chromatids. The second mechanism, although not denying the importance of DNA debromination, postulated that chromatin structural organization, in terms of DNA-protein and/or protein-protein DNA interaction, is responsible for SCD production.

Cytogenetic effect of 5-azacytidine in patients with hematological malignancies

Background

Recently, the importance of cytogenetics has grown in the diagnosis, prognosis and treatment of leukemias and myelodysplastic syndromes. 5-azacytidine is a drug that has well-known cytogenetical effects and is approved in the treatment of myelodysplastic syndromes. To date, no studies have been performed to evaluate the impact of 5-azacytidine on the chromosomes of patients with hematological neoplasias. This study aimed to investigate the effects of 5-azacytidine on chromosomes of patients with different hematological malignancies using G-band analyses to identify possible cytogenetical alterations.

Methods

The peripheral blood of 18 patients with hematological malignancies and 18 controls was collected in heparinized tubes. 5-azacytidine was added, at a final concentration of 10-5M, to cultures 7 hours prior to harvest.

Results

Uncoiled centromeric/pericentromeric heterochromatin of chromosomes-1, 9 and 16 occurred more frequently in the patients than in controls. This higher frequency of uncoiled heterochromatin was statistically significant (p-value = 0.004) for chromosome-9. Conversely, we observed that the fragile site at 19q13 was more frequent in controls (p-value = 0.0468).

Conclusions

The results of this study suggest that satellite sequences, located in the heterochromatin of chromosome-9, are hypomethylated in hematological malignancies. This hypomethylation may contribute to the disease, activating transposable elements and/or promoting genomic instability, enabling the loss of heterozygosity of important tumor suppressor genes. An investigation of the 19q13 region may help to understand whether or not the predominant occurrence of the fragile site at 19q13 in controls is due to hypermethylation of this region.

Epigenetic mechanisms for primary differentiation in mammalian embryos

This review examines main developments related to the interface between primary mammalian cell differentiation and various aspects of chromosomal structure changes, such as heterochromatin dynamics, DNA methylation, mitotic recombination, and inter- and intrachromosomal differentiation. In particular, X chromosome difference, imprinting, chromosomal banding, methylation pattern, single-strand DNA breaks, sister chromatid exchanges (SCEs), and sister chromatid asymmetry are considered. A hypothesis is put forward which implies the existence of an epigenetic asymmetry versus mirror symmetry of sister chromatids for any DNA sequences. Such epigenetic asymmetry appears as a result of asymmetry of sister chromatid organization and of SCE and is a necessary (not sufficient) condition for creating cell diversity. The sister chromatid asymmetry arises as a result of consecutive rounds of active and passive demethylation which leads after chromatin assembly events to chromatid difference. Single-strand DNA breaks that emerge during demethylation trigger reparation machinery, provend as sister chromatid exchanges, which are not epigenetically neutral in this case. Taken together, chromatid asymmetry and SCE lead to cell diversity regarding their future fate. Such cells are considered pluripotent stem cells which after interplay between a set of chromosomal domains and certain substances localized within the cytoplasmic compartments (and possibly cell interactions) can cause sister cells to express different gene chains. A model is suggested that may be useful for stem cell technology and studies of carcinogenesis.

Chromosome methylation patterns during mammalian preimplantation development

DNA methylation patterns were evaluated during preimplantation mouse development by analyzing the binding of monoclonal antibody to 5-methylcytosine (5-MeC) on metaphase chromosomes. Specific chromosome patterns were observed in each cell stage. A banding pattern predominated in chromosomes at the one-cell stage. Banding was replaced at the two-cell stage by an asymmetrical labeling of the sister chromatids. Then, the proportion of asymmetrical chromosomes decreased by one-half at each cell division until the blastocyst stage, and chromosomes became progressively symmetrical and weakly labeled. Our results indicate that chromosome demethylation is associated with each DNA replication and suggest that a passive mechanism predominates during early development.

The effects of 5-azacytidine and 5-azadeoxycytidine on chromosome structure and function: implications for methylation-associated cellular processes

5-Azacytidine (5-aza-C) analogs demonstrate a remarkable ability to induce heritable changes in gene and phenotypic expression. These cellular processes are associated with the demethylation of specific DNA sequences. On the other hand, 5-aza-C analogs have dramatic effects on chromosomes, leading to decondensation of chromatin structure, chromosomal instability and an advance in replication timing. Condensation inhibition of genetically inactive chromatin occurs when the DNA is still hemimethylated or fully methylated. In cell cultures prolonged for several replication cycles, chromosomal rearrangements and instability affect the 5-aza-C-sensitive regions. Moreover, the normally late-replicating inactive chromatin undergoes a transient temporal shift to an earlier DNA replication, characteristic of activatable chromatin. zThe induced alterations of chromosome structure and behavior may trigger the 5-aza-C-dependent process of cellular reprogramming. Apart from their differentiating and gene-modifying effects, 5-aza-C analogs can tumorigenically transform cells and modulate their metastatic potential. High doses of 5-aza-C analogs have cytotoxic and antineoplastic activities.

Cytotoxicity of 5-aza-2'-deoxycytidine in a mammalian cell system

After the addition of 5-aza-2'-deoxycytidine, a potent inhibitor of DNA methylation and S-phase-specific cytotoxic agent, metaphase chromosomes of Chinese hamster ovary (CHO) cells exhibited a highly decondensed and extended morphology (numerous "fragile sites") at the first mitotic division. However, when a lethal dose of this drug was added in early G1 phase to cells synchronised by mitotic selection, the majority subsequently divided at the same time as an untreated control cell population with few division abnormalities and with few of the more usual types of chromosome aberrations such as gaps, breaks and exchanges. The drug-treated cells also entered and completed the second S-phase without significant delay and it was only at the second mitosis after addition of 5-azadeoxycytidine that cells showed delays in entering mitosis and significant increases in abnormal divisions concomitant with a modest increase in chromosome aberrations. If cells in a tumour behave similarly, the tumour mass would be expected to double before any reduction in tumour burden could be expected to occur.

5-Azadeoxycytidine induced undercondensation in the giant X chromosomes of Microtus agrestis

Fibroblasts of female Microtus agrestis were treated with 5-azadeoxycytidine (5-aza-dCyd) at a final concentration of 10(-5) M during the last 2 h of culture. This cytidine analogue induces distinct undercondensation of the constitutive heterochromatin in the giant X chromosomes. The undercondensed heterochromatic thread exhibits longitudinal segmentation reminiscent of a chromomere pattern. In the late-replicating X chromosome, 5-aza-dCyd also inhibits condensation of the genetically inactivated euchromatin (facultative heterochromatin). The described effects of 5-aza-dCyd on the X chromosome structure appear to be incorporation independent.

Inhibition of condensation in the late-replicating X chromosome induced by 5-azadeoxycytidine in human lymphocyte cultures

The cytidine analogue 5-azadeoxycytidine (5-azadC) induces a very distinct inhibition of condensation in the genetically inactive, late-replicating X chromosome (XL) when applied to human lymphocyte cultures. One of the two X chromosomes in cytogenetically normal female cells becomes dramatically longer than its homologous partner. The highest rate of metaphases with an undercondensed XL chromosome is achieved when 5-aza-dC is added at a final concentration of 10(-5) M 2 h before cell harvesting. The interactions between 5-aza-dC and chromosomal DNA as well as the factors involved in X chromosome inactivation are discussed.