Azacytidine (ACR) is known to induce uncoiling and somatic association involving the constitutive heterochromatin of human chromosomes 1, 9, 15, and 16 and the Y. These regions are composed of alphoid and classical satellite DNA sequences. Using specific probes for chromosomes 1 and 16, we have performed two-color fluorescence in situ hybridization on human lymphocytes cultured in the presence of ACR. We demonstrate that for these two chromosomes (1) uncoiling and association specifically occur in classical satellite-containing regions at the first cell generation, (2) breakages also affect these regions, and (3) somatic recombinations occur between these regions and lead to translocations at the next cell generation. These results suggest that changes in methylation of repetitive DNA sequences are related to chromosomal instability occurring during cell transformation and tumorigenesis.
To determine possible relationships between DNA hypomethylation and chromosome instability, human lymphoblastoid cell lines from different genetic constitutions were studied with regard to 1) uncoiling and rearrangements, which preferentially affect the heterochromatic segments of chromosomes 1 and 16; 2) the methylation status of the tandemly repetitive sequences (classical satellite and alphoid DNAs) from chromosomes 1 and 16, and of the L1Hs interspersed repetitive sequences. The methylation status largely varied from cell line to cell line, but for a given cell line, the degree of methylation was similar for all the repetitive DNAs studied. Two cell lines, one obtained from a Fanconi anemia patient and the other from an ataxia telangiectasia patient were found to be heavily hypomethylated. The heterochromatic segments of their chromosomes 1 and 16 were more frequently elongated and rearranged than those from other cell lines, which were found to be less hypomethylated. Thus, in these lymphoblastoid cell lines, alterations characterized by uncoiling and rearrangements of heterochromatic segments from chromosomes 1 and 16 seem to correlate with the hypomethylation of their repetitive DNAs. Two-color in situ hybridizations demonstrated that these elongations and rearrangements involved only classical satellite-DNA-containing heterochromatin. This specificity may be related to the excess of breakages affecting the chromosomes carrying these structures in a variety of pathological conditions.
In situ immunofluorescence detection of antibodies against 5-methylcytosine on metaphase chromosomes prepared by a new procedure allows the display of new 5-methylcytosine-rich sites as compared to previously published methods. In short-term culture lymphocytes, the immunofluorescent signals give a recurrent pattern in which four types of binding sites can be distinguished. Type I sites are the secondary constrictions and a few juxtacentromeric regions, type II sites correspond to T-bands. Both types I and II sites emit a strong fluorescence. Type III sites form an R-band pattern and emit a weaker fluorescence. Type IV sites are the short arms of acrocentrics, they emit strong but polymorphic signals. The results obtained from control experiments suggest that the pattern observed is rather the expression of an uneven distribution of 5-methylcytosine-rich sites than a consequence of the various treatments used. In a lymphoblastoid cell line known to have a reduced 5-methylcytosine content, it was possible to demonstrate a heterogeneous hypomethylation among chromosome structures, principally involving type I sites. The method opens the possibility of studying in situ on chromosomes, regional variations of methylation in pathological conditions.
Two-color fluorescent in situ hybridizations using probes for alphoid (alpha) and classical satellite (CS) DNAs from chromosomes 1 and 16 were performed to characterize i(1q), der(1;16), and complex rearrangements observed in breast cancer cells from fresh tumors and established cell lines. Six of seven i(1q) occurred after breakage in the alpha 1 containing region and one of seven was dicentric, with breakage in 1p11.2. The five der(1;16)(q10;p10) studied appeared to result from a variety of breakpoints involving alpha 1, alpha 16, CS1, and CS16 DNAs. All had conserved alpha 16 DNA, suggesting a segregation of the der(1;16) leading to a loss of 16q and a gain of 1q in most cases. One complex rearrangement of chromosome 1 also appeared to involve chromosome 16, suggesting that a der(1;16) occurred first, followed by another rearrangement. Both the apparent preferential involvement of constitutive heterochromatin harboring alpha and CS DNAs and the variety of breakpoints spanning along heterochromatin suggest that the important consequence of the rearrangement is not the breakage per se but the resulting imbalance.
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