Detection of G-C rich heterochromatin by 5-azacytidine in mammals

Effect of O6-chloroethylguanine DNA lesions on the kinetics and mechanism of micronucleus induction in vivo

The aim of this work was to determine the kinetics of micronucleus production because of an increase in O(6)-chloroethyl guanine (O6-ChlEt-G) DNA lesions in murine bone marrow cells in vivo. We increased the frequency of O6-ChlEt-G lesions by pretreatment with an inhibitor of O(6)-methylguanine-DNA methyltransferase (MGMT), O(6)-benzylguanine (O6BG), and subsequent treatment with bis-chloroethylnitrosourea (BCNU). The kinetics of micronucleated-polychromatic erythrocyte (MN-PCE) induction was established by scoring the frequency of MN-PCEs per 2000 PCEs in peripheral blood at 8-hr intervals from immediately prior to treatment to 72-hr post-treatment. We examined groups of five mice treated with (i) dimethylsulfoxide (DMSO), (ii) O6BG in DMSO, (iii) BCNU, or (iv) O6BG in DMSO plus BCNU. The data indicate that O6BG pretreatment causes: (i) ían increase in MN-PCEs induced by BCNU, (ii) a delay in the time of maximal MN-PCE induction produced by the different BCNU doses, and (iii) an increase in cytotoxicity. These data confirm that O6-ChlEt-G is a lesion involved in DNA break induction and in the subsequent production of micronuclei, and also that these lesions seem to be stoichiometrically reduced by MGMT. These data also show that induction of MN-PCEs by BCNU is delayed by pretreatment with O6BG for more than 6 hr, perhaps due to the time required for repair of crosslinks derived from O6-ChlEt-G and/or for DNA duplication, which is required for adduct transformation into crosslinks.

Mechanisms of DNA breaks inductionin vivo by 5-azacytidine: paths of micronucleus induction by azaC

The aim of the present study was to correlate the time-response curves of micronucleated polychromatic erythrocyte (MN-PCE) induction by 5-azacytidine (azaC) with the possible processes involved in DNA break production; this is based on the results previously published by other authors. The MN-PCE induction at two different doses of azaC was determined by sampling blood from the tails of mice before the acute treatment and over nine periods of 8 h each afterwards. Both doses caused two peaks of MN-PCE induction, one at 32 h and another at 48 h, approximately; a shoulder was detected that remained high from 56 h up to the end of the study (72 h). These results suggest that azaC induced DNA breaks and subsequently MN (micronucleus) by three different mechanisms, and in agreement with data in the literature, these could be successively the following: (i) during excision of the large adduct comprising the DNA methyl transferase covalently linked to DNA; (ii) failure of recombination repair or mismatch repair; and (iii) persistent chromosome fragility in G-C rich sites due to DNA demethylation and chromatin decondensation.

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.

Cell-cycle dependent micronucleus formation and mitotic disturbances induced by 5-azacytidine in mammalian cells

5-Azacytidine was originally developed to treat human myelogenous leukemia. However, interest in this compound has expanded because of reports of its ability to affect cell differentiation and to alter eukaryotic gene expression. In an ongoing attempt to understand the biochemical effects of this compound, we examined the effects of 5-azacytidine on mitosis and on micronucleus formation in mammalian cells. In L5178Y mouse cells, 5-azacytidine induced micronuclei at concentrations at which we and others have already reported its mutagenicity at the tk locus. Using CREST staining and C-banding studies, we showed that the induced micronuclei contained mostly chromosomal fragments although some may have contained whole chromosomes. By incorporating BrdU into the DNA of SHE cells, we determined that micronuclei were induced only when the compound was added while the cells were in S phase. Microscopically visible effects due to 5-azacytidine treatment were not observed until anaphase of the mitosis following treatment or thereafter. 5-Azacytidine did not induce micronuclei via interference with formation of the metaphase chromosome arrangement in mitosis, a common mechanism leading to aneuploidy. Supravital UV microscopy revealed that chromatid bridges were observed in anaphase and, in some cases, were sustained into interphase. In the first mitosis after 5-azacytidine treatment we observed that many cells were unable to perform anaphase separation. All of these observations indicate that 5-azacytidine is predominantly a clastogen through its incorporation into DNA.

Rare chromosomal aberrations induced by vincristine. Partial endoreduplication and pseudoendoreduplication, segmentally endoreduplicated chromosomes, and segmental premature chromosome condensation

Vincristine (VCR) is capable of inducing a cell containing both conventional chromosomes (monochromosomes) and diplochromosomes. A total of 124 such metaphases were examined by 5-bromodeoxyuridine (BrdU) incorporation and fluorescence plus Giemsa (FPG) technique to analyze cell cycle kinetics. The majority of cells (119 metaphases) showed differential BrdU incorporation between the two kinds of chromosomes, indicating that partial endoreduplication occurred in these cells. In addition, existence of partially endoreduplicated cells with premature chromosome condensation (PCC) in either mono- or diplochromosomes suggests that the timing of monochromosome-replication was very variable in individual cells. On the other hand, the remaining five metaphases showed that both mono- and diplochromosomes incorporated BrdU similarly, indicating that diplochromosomes are formed by pseudoendoreduplication. Two kinds of chromosomal aberrations probably caused by delay of DNA synthesis on chromosome segments, segmental endoreduplication, and segmental PCC were also reported. Segmental endoreduplication was defined as endoreduplication that occurred on some segments of chromosomes. Out of 119 partially endoreduplicated cells, 3 contained a chromosome consisting of both mono- and diplochromosomal segments, indicating that the former segments missed one round of DNA synthesis. Segmental PCC was defined as PCC restricted to only some segments of chromosomes. Two types of segmental PCC, segmental S-PCC and G2-PCC, were observed in VCR-induced ordinary polyploidy. Although both segmental endoreduplication and segmental PCC occurred with very low frequency, these phenomena suggest that DNA synthesis was disturbed in some part of the nucleus.

Chromosome preparation and high resolution banding techniques. A review

High resolution banding techniques enable detection of chromosome rearrangements even within major bands. Banded chromosomes prepared for light microscopic studies of intact metaphase plates are, however, highly modified structures compared with native chromosomes, and the high resolution banding techniques only seem possible because the following methods were standardized and combined. The use of colcemid, which prevents formation of the spindle and thereby collects cells at the metaphase-anaphase border, is routinely used for chromosome preparations. For high resolution banding studies, short exposure time and concentrations near the threshold value have been recommended by several authors. Several agents interfere with chromosome contraction processes, but only a few have had a lasting influence on high resolution banding studies. The most used agents are ethidium bromide, actinomycin D, and Hoechst 33258, which all partially inhibit chromosome contraction. Treatment with hypotonic solutions induces swelling of animal cells, and the methanol in the fixative denatures and precipitates protein by dehydration. The acetic acid coagulates nucleoproteins and causes swelling of the cells. The fixative penetrates the cells rapidly and preserves the chromosome structure. To obtain long segmented chromosomes suitable for high resolution banding hypotonic treatment with .075 M KCl, frequent changes of fixative and overnight fixation at 4 degrees C have been recommended. The use of cell synchronization, 5-bromodeoxyuridine incorporation into DNA, and fluorochrome-photolysis Giemsa (FPG)-staining have improved the quality of high resolution banding. Synchronization techniques, which select for lymphocyte populations in early divisions, provide excellent materials for chromosome preparations and induction of high resolution banding. The banding techniques seem to enhance differences already present in the chromosomes, and the differential Giemsa staining has recently been explained by interactions between the hydrophobic dye complex, the supercoiled DNA helix, and the denaturated histone core of the nucleosomes.

Histological study of cell death in digital malformations induced by 5-azacytidine: suppressive effect of caffeine

The present study investigated microscopically the process of 5-azacytidine (5-AC)-induced digital teratogenesis and caffeine's suppressive effect on this process. Three distinct zones of programmed cell death were observed in control and caffeine-treated embryos 3 hours after 5-AC injection: the preaxial and postaxial ectodermal regions and the central part of the mesodermal regions. 5-AC temporarily suppressed programmed cell death in the ectoderm and mesoderm 3 hours after it was injected. However, caffeine promoted programmed cell death; normal programmed cell death was observed in the limb buds of embryos whose dams were treated with 5-AC and caffeine. The percentage of total cell death in hindlimb buds of embryos treated with 5-AC and caffeine was higher than that from embryos treated with 5-AC, whereas 5-AC-induced digital malformations were reduced by post-treatment with caffeine. Cell death reached a maximum 12 hours after the injection in limb buds from 5-AC and caffeine-treated embryos and at 24 hours in the 5-AC treated embryos. Furthermore, in the 5-AC and caffeine-treated embryos, the frequency of cell deaths at 12 hours increased almost linearly with the doses of caffeine in parallel with the reduction of 5-AC-induced malformation frequency by caffeine. These results suggest that although induced cell death may be one of the factors leading to digital malformations produced by 5-AC, it is not essential, and the existence of other factors affecting the pattern formation of the limb bud is proposed.

Prefixation treatment with ethidium bromide for high resolution banding analysis of chromosomes from cultured human bone marrow cells

Prefixation treatment of cultured human bone marrow cells with a DNA intercalating agent, ethidium bromide (EBr), induced a dose- and time-related elongation of chromosomes. When compared with EBr-free cultures, a 2.9-fold increase in the yield of early mitotic cells with more than 400 bands per haploid set of chromosomes was achieved by simply adding 10 micrograms/ml of EBr during the last 2 hours of culture. The proportion of early mitotic cells was equal to that obtained in methotrexate synchronized cultures. Fluorescence banding methods using base composition specific agents actinomycin D/DAPI for AT base pairs and chromomycin A3/distamycin A for GC suggested that EBr does not have base specificity, because EBr did not alter the banding patterns of chromosomes obtained with these staining procedures.

5-azacytidine-induced decrease in the frequency of Barr body in human fibroblasts

5-azacytidine-treated human fibroblasts exhibit a significant decrease in the frequencies of Barr body+ cells as compared to nontreated cultures. This presumably indicates that 5-azacytidine can induce a change in the degree of condensation of the Barr body. It is suggested that the state of chromatin condensation of the Barr body may be related to the reactivation process by 5-azacytidine of gene loci in the inactive X.

Correspondence between effects of 5-azacytidine on SCE formation, cell cycling and DNA methylation in Chinese hamster cells

The effects of 5-azacytidine (5-Aza-C), alone and in combination with mitomycin C, were measured on sister-chromatid exchange (SCE) formation and DNA methylation in different genomic regions of Chinese hamster ovary cells and in Chinese hamster cells containing amplified, dihydrofolate reductase sequences and resistant to methotrexate. 5-Aza-C, when present for the penultimate preharvest cell cycle, induced SCEs in a manner consistent with a directly measured reduction in deoxycytosine methylation in cellular DNA. At higher 5-Aza-C concentrations, cell cycling was inhibited and both SCE induction and DNA demethylation tended to level off. Under appropriate conditions, 5-Aza-C also potentiated the induction of SCEs by mitomycin C. 5-Aza-C-induced DNA demethylation could also be detected in the vicinity of different DNA sequences with the use of comparative HpaII/MspI digestion, DNA blotting, and molecular probes. The efficiency of an individual demethylation event in inducing SCE induction appeared to be very low, compared with alkylating agents such as 8-methoxypsoralen, suggesting that SCE induction by 5-Aza-C might be an indirect effect from long range changes induced in cellular DNA or chromatin conformation.

New classes of common fragile sites induced by 5-azacytidine and bromodeoxyuridine

Two new classes of common fragile site seen in chromosomes from blood lymphocyte cultures are reported. The first class is induced in bands 1q42 and 19q13 by 5-azacytidine (5-AZA). Maximum induction of these fragile sites occurs when the 5-AZA is added 5-8h prior to harvest. The second class is induced in bands 6q13, 9p21, and 10q21 by bromodeoxyuridine (BrdU). In this instance maximum induction occurred if the BrdU was added 4-6h prior to harvest. The known fragile sites, both rare and common, are summarised.

5-Methyldeoxycytidine in the Physarum minichromosome containing the ribosomal RNA genes

5-Methyldeoxycytidine (5MC) was analyzed by high pressure liquid chromatography (HPLC) and by restriction enzyme digestion in rDNA isolated from Physarum polycephalum. rDNA from Physarum M3C strain microplasmodia has a significant 5MC content (about half that of the whole genomic DNA). This rDNA contains many C5MCGG sites because it is clearly digested further by Msp I than by Hpa II. However, most 5MC is in other sites. In particular, alternating CG sequences appear to be highly methylated. HPLC of deoxyribonucleosides shows tha most of the transcribed regions contain little or no 5MC. Restriction digestion indicates that there is little or no 5MC in any of the transcribed regions including the transcription origin and adjacent sequences. Over 90% of the total 5MC is in or near the central nontranscribed spacer and most methylated restriction sites are in inverted repeats of this spacer. rDNA is very heterogeneous with respect to 5MC. The 5MC pattern doesn't appear to change with inactivation of the rRNA genes during reversible differentiation from microplasmodia (growing) to microsclerotia (dormant), showing that inactivation is due to changes in other chromatin variables. The 5MC pattern is different between Physarum strains. The possible involvement of this 5MC in rDNA chromatin structure and in cruciform and Z-DNA formation is discussed.

Increased satellite association induced by 5' bromodeoxyuridine treatment of phytohemaglutinin-stimulated blood lymphocytes

5'-Bromodeoxyuridine (BrdU) present in the course of late S and G2 phases of the cell cycle in PHA-stimulated human lymphocyte cultures causes the despiralization and elongation of some chromosome regions, including short arms of acrocentric chromosomes. BrdU present at a concentration of 250 microM during the last 10 h in lymphocyte cultures from 19 healthy subjects did not affect the number of silver-stained NORs, but raised significantly the number of satellite associations of acrocentric chromosomes. The mere substitution of thymine by BrdU in DNA strands as a reason for increased number of satellite associations seems a less plausible explanation than the modification of DNA-protein complexes of NOR regions, which could alter the degree of their spiralization and cause the increased tendency of acrocentric chromosomes to associate in the subsequent metaphase.

Z-DNA immunoreactivity in fixed metaphase chromosomes of primates

Antibodies against Z-DNA bind to fixed metaphase chromosomes of man and Cebus albifrons (Platyrrhini, Primate). By indirect immunofluorescence and indirect immunoperoxidase techniques, a heavy staining is detected in some segments of chromosomes of C. albifrons. These segments correspond to R-band-positive heterochromatin, which has a high G + C-base content. Euchromatin of human and Cebus chromosomes show a weak and heterogeneous staining that consistently reproduces an R- and T-banding pattern in both species. Because chromosome homologies previously were demonstrated between these distantly related species by chromosome banding, our results suggest that Z-DNA has been conserved during the course of primate evolution.

Cytogenetical and biochemical characterization of a dG + dC-rich satellite DNA in the primate Cebus capucinus

A very abundant and dG + dC rich DNA satellite from primate Cebus capucinus has been characterized in its cytogenetic and biochemical properties with the purpose of studying the correlation between the staining properties of heterochromatin and the base composition of the corresponding very repetitive DNA. The staining techniques, as well as incorporation of base analogues, show that the heterochromatin segments of C. capucinus chromosomes correspond to a dG + dC-rich satellite. This satellite was detected and isolated by centrifugation in density gradient, radioactively labelled and localized by in situ hybridization on heterochromatin segments.