Non-random distribution of instability-associated chromosomal rearrangement breakpoints in human lymphoblastoid cells

Association of inter- and intrachromosomal exchanges with the distribution of low- and high-LET radiation-induced breaks in chromosomes

To study the effects of low- and high-linear energy transfer (LET) radiation on break locations within a chromosome, we exposed human epithelial cells in vitro to (137)Cs γ rays at both low and high dose rates, secondary neutrons at a low dose rate, and 600 MeV/u iron ions at a high dose rate. Breakpoints were identified using multicolor banding in situ hybridization (mBAND), which paints chromosome 3 in 23 different colored bands. For all four radiation scenarios, breakpoint distributions were found to be different from the predicted distribution based on band width. Detailed analysis of chromosome fragment ends involved in inter- or intrachromosomal exchanges revealed that the distributions of fragment ends participating in interchromosomal exchanges were similar between the two low-LET radiation dose rates and between the two high-LET radiation types, but the distributions were less similar between low- and high-LET radiations. For fragment ends participating in intrachromosomal exchanges, the distributions for all four radiation scenarios were similar, with clusters of breaks found in three regions. Analysis of the locations of the two fragment ends in chromosome 3 that joined to form an intrachromosomal exchange demonstrated that two breaks with a greater genomic separation can be more likely to rejoin than two closer breaks, indicating that chromatin folding can play an important role in the rejoining of chromosome breaks. Comparison of the breakpoint distributions to the distributions of genes indicated that the gene-rich regions do not necessarily contain more breaks. In general, breakpoint distributions depend on whether a chromosome fragment joins with another fragment in the same chromosome or with a fragment from a different chromosome.

Dose-response prediction for radiation-induced chromosomal instability

Radiation induces chromosome aberrations (CA) that are detected in the first post-irradiation cell cycle and in descendants of irradiated cells. Unstable aberrations in the progeny of exposed cells are referred to as one of the hallmarks of chromosomal instability (CIN). One of the important questions is what is the relationship between the dose response for radiation-induced CA and delayed CA, or CIN. To address this question, a mechanistic model for CIN was developed. Delayed CA are assumed to be formed both by transmission from previous mitotic cycles owing to chromosome breakage-fusion mechanism and by means of generation of DNA/chromosome breakage de novo in each cell cycle of survived cells. Monte Carlo simulation of DNA/chromosome breakage, CA production, cell death due to unstable CA and cell cycle kinetics was performed to predict the dose response for CIN. Different shapes of CIN dose-response curves were predicted for various time points after irradiation and under several assumptions on delayed DNA/chromosome breakage generation. For one of the scenarios studied, the pronounced dose dependence at early time points flattened or even turned into dose independence in a wide dose range after many rounds of replication where a stationary state between CA generation and elimination was achieved. This dose independence was shown to be in concert with the experimental data.

Delayed chromosomal instability in lymphocytes of cancer patients after radiotherapy

PURPOSE

To assess possible delayed chromosomal instability (DCI) expressed as elevated chromatid breakage in cells containing previously formed chromosome type aberrations in cultured blood lymphocytes of cancer patients after radiotherapy (RT).

MATERIALS AND METHODS

Twenty patients treated for uterine cancer with external Co(60) RT, without chemotherapy, were selected. Blood was taken before, 1-2 days after RT and one year later. Lymphocytes were cultured for 50 and 100 h. Metaphases were stained with fluorescence-plus-Giemsa and analysed for chromosome and chromatid aberrations in 1st (M1) and 3rd plus later (M3+) mitoses.

RESULTS

RT caused a significant increase of radiation-specific chromosome aberrations in patients' lymphocytes together with DCI, which was observed as an excessive yield of cells containing both chromosome and chromatid aberrations (defined as C(acs&act)). This DCI passed successfully through mitoses in vitro, and at the end of RT a mean yield of 'extra' C(acs&act) was 3 x 10(-3) x cell(-1) amongst either M1 or M3+ cells. At the end of RT and one year later DCI in M1 lymphocytes appeared at random amongst patients, but some inter-individual variation was found for DCI presence in M3+ cells at both post-irradiation samplings. As time passed, the mean yield of lymphocytes exhibiting DCI decreased in vivo and one year after RT reached the pre-treatment level of 1 x 10(-3) x cell(-1).

CONCLUSIONS

DCI was demonstrated in descendants of human lymphocytes after therapeutic irradiation. The effect diminished one year later, suggesting that the progeny of patients' irradiated stem cells did not produce new daughter lymphocytes exhibiting DCI during the studied post-irradiation period.

The benzene metabolite, hydroquinone and etoposide both induce endoreduplication in human lymphoblastoid TK6 cells

Both occupational exposure to the leukemogen benzene and in vitro exposure to its metabolite hydroquinone (HQ) lead to the induction of numerical and structural chromosome changes. Several studies have shown that HQ can form DNA adducts, disrupt microtubule assembly and inhibit DNA topoisomerase II (topo II) activity. As these are potential mechanisms underlying endoreduplication (END), a phenomenon that involves DNA amplification without corresponding cell division, we hypothesized that HQ could cause END. We measured END in the human lymphoblastoid cell line, TK6, treated with HQ (0-20 microM) and etoposide (0-0.2 microM) for 48 h. Etoposide was used as a positive control as it is a topo II poison and established human leukemogen that has previously been shown to induce END in Chinese hamster ovary cells. Both HQ and etoposide significantly induced END in a dose-dependent manner (P(trend) < 0.0001 and P(trend) = 0.0003, respectively). Since END may underlie the acquisition of high chromosome numbers by tumour cells, it may play a role in inducing genomic instability and subsequent carcinogenesis from HQ and etoposide. In order to further explore the cytogenetic effects of HQ and etoposide, we also examined specific structural changes. HQ did not induce translocations of chromosome 11 [t(11;?)] but significantly induced translocations of chromosome 21 [t(21;?)] and structural chromosome aberrations (SCA) (P(trend) = 0.0415 and P(trend) < 0.0001, respectively). Etoposide potently induced all these structural changes (P(trend) < 0.0001). The lack of an effect of HQ on t(11;?) and the reduced ability of HQ to induce t(21;?) and SCA, compared with etoposide, further suggests that HQ acts primarily as a topo II catalytic inhibitor rather than as a topo II poison in intact human cells.