Ionizing radiation-induced instant pairing of heterochromatin of homologous chromosomes in human cells

1Q12 Loci Movement in the Interphase Nucleus Under the Action of ROS Is an Important Component of the Mechanism That Determines Copy Number Variation of Satellite III (1q12) in Health and Schizophrenia

Introduction: Genome repeat cluster sizes can affect the chromatin spatial configuration and function. Low-dose ionizing radiation (IR) induces an adaptive response (AR) in human cells. AR includes the change in chromatin spatial configuration that is necessary to change the expression profile of the genome in response to stress. The 1q12 heterochromatin loci movement from the periphery to the center of the nucleus is a marker of the chromatin configuration change. We hypothesized that a large 1q12 domain could affect chromatin movement, thereby inhibiting the AR. Materials and Methods: 2D fluorescent in situ hybridization (FISH) method was used for the satellite III fragment from the 1q12 region (f-SatIII) localization analysis in the interphase nuclei of healthy control (HC) lymphocytes, schizophrenia (SZ) patients, and in cultured mesenchymal stem cells (MSCs). The localization of the nucleolus was analyzed by the nucleolus Ag staining. The non-radioactive quantitative hybridization (NQH) technique was used for the f-SatIII fragment content in DNA analysis. Satellite III fragments transcription was analyzed by reverse transcriptase quantitative PCR (RT-qPCR). Results: Low-dose IR induces the small-area 1q12 domains movement from the periphery to the central regions of the nucleus in HC lymphocytes and MSCs. Simultaneously, nucleolus moves from the nucleus center toward the nuclear envelope. The nucleolus in that period increases. The distance between the 1q12 domain and the nucleolus in irradiated cells is significantly reduced. The large-area 1q12 domains do not move in response to stress. During prolonged cultivation, the irradiated cells with a large f-SatIII amount die, and the population is enriched with the cells with low f-SatIII content. IR induces satellite III transcription in HC lymphocytes. Intact SZ patients' lymphocytes have the same signs of nuclei activation as irradiated HC cells. Conclusion: When a cell population responds to stress, cells are selected according to the size of the 1q12 domain (the f-SatIII content). The low content of the f-SatIII repeat in SZ patients may be a consequence of the chronic oxidative stress and of a large copies number of the ribosomal repeats.

Risk of histological transformation and therapy-related myelodysplasia/acute myeloid leukaemia in patients receiving radioimmunotherapy for follicular lymphoma

Histological transformation (HT) of follicular lymphoma (FL) to an aggressive lymphoma after chemotherapy remains a key issue. The incidence of HT after radioimmunotherapy (RIT) is unknown. This single institution study analysed the risk of HT in FL after treatment with yttrium-90 ibritumomab tiuxetan in 115 consecutive patients treated during 1987-2012. RIT was administered for progressive FL in 111 (97%) patients and as first-line therapy in the remaining 4. 28% (n = 32) had HT, occurring at a median of 60 months from diagnosis and 20 months after RIT. 48% (12/25) of patients who received fludarabine developed HT. The estimated 10-year risk of HT in the fludarabine and non-fludarabine groups was 67% and 26% respectively (P = 0·015). Only prior fludarabine was significantly associated with predicting the risk of HT after RIT. 8% (9/115) of patients developed therapy-related myelodysplastic syndrome/acute myeloid leukaemia (tMDS/AML) at a median of 41·4 months (range, 5-89). The estimated 10-year risk of tMDS/AML in non-fludarabine treated patients (n = 90) versus fludarabine treated (n = 25) was 13% and 29%, respectively. The estimated overall risk of FL undergoing HT at 10 years without fludarabine exposure appears similar to patients reported in the literature that have not received RIT. Patients with prior purine-analogue therapy are at significantly higher risk of HT.

Volume increase and spatial shifts of chromosome territories in nuclei of radiation-induced polyploidizing tumour cells

The exposure of tumour cells to high doses of ionizing radiation can induce endopolyploidization as an escape route from cell death. This strategy generally results in mitotic catastrophe during the first few days after irradiation. However, some cells escape mitotic catastrophe, polyploidize and attempt to undergo genome reduction and de-polyploidization in order to create new, viable para-diploid tumour cell sub-clones. In search for the consequences of ionizing radiation induced endopolyploidization, genome and chromosome architecture in nuclei of polyploid tumour cells, and sub-nuclei after division of bi- or multi-nucleated cells were investigated during 7 days following irradiation. Polyploidization was induced in p53-function deficient HeLa cells by exposure to 10Gy of X-irradiation. Chromosome territories #1, #4, #12 and centromeres of chromosomes #6, #10, #X were labelled by FISH and analysed for chromosome numbers, volumes and spatial distribution during 7 days post irradiation. The numbers of interphase chromosome territories or centromeres, respectively, the positions of the most peripherally and centrally located chromosome territories, and the territory volumes were compared to non-irradiated controls over this time course. Nuclei with three copies of several chromosomes (#1, #6, #10, #12, #X) were found in the irradiated as well as non-irradiated specimens. From day 2 to day 5 post irradiation, chromosome territories (#1, #4, #12) shifted towards the nuclear periphery and their volumes increased 16- to 25-fold. Consequently, chromosome territories returned towards the nuclear centre during day 6 and 7 post irradiation. In comparison to non-irradiated cells (∼500μm(3)), the nuclear volume of irradiated cells was increased 8-fold (to ∼4000μm(3)) at day 7 post irradiation. Additionally, smaller cell nuclei with an average volume of about ∼255μm(3) were detected on day 7. The data suggest a radiation-induced generation of large intra-nuclear chromosome territories and their repositioning prior to genome reduction.

Checking the foundation: recent radiobiology and the linear no-threshold theory

The linear no-threshold (LNT) theory has been adopted as the foundation of radiation protection standards and risk estimation for several decades. The "microdosimetric argument" has been offered in support of the LNT theory. This argument postulates that energy is deposited in critical cellular targets by radiation in a linear fashion across all doses down to zero, and that this in turn implies a linear relationship between dose and biological effect across all doses. This paper examines whether the microdosimetric argument holds at the lowest levels of biological organization following low dose, low dose-rate exposures to ionizing radiation. The assumptions of the microdosimetric argument are evaluated in light of recent radiobiological studies on radiation damage in biological molecules and cellular and tissue level responses to radiation damage. There is strong evidence that radiation initially deposits energy in biological molecules (e.g., DNA) in a linear fashion, and that this energy deposition results in various forms of prompt DNA damage that may be produced in a pattern that is distinct from endogenous (e.g., oxidative) damage. However, a large and rapidly growing body of radiobiological evidence indicates that cell and tissue level responses to this damage, particularly at low doses and/or dose-rates, are nonlinear and may exhibit thresholds. To the extent that responses observed at lower levels of biological organization in vitro are predictive of carcinogenesis observed in vivo, this evidence directly contradicts the assumptions upon which the microdosimetric argument is based.

The STRUCTURAL MAINTENANCE OF CHROMOSOMES 5/6 complex promotes sister chromatid alignment and homologous recombination after DNA damage in Arabidopsis thaliana

Sister chromatids are often arranged as incompletely aligned entities in interphase nuclei of Arabidopsis thaliana. The STRUCTURAL MAINTENANCE OF CHROMOSOMES (SMC) 5/6 complex, together with cohesin, is involved in double-strand break (DSB) repair by sister chromatid recombination in yeasts and mammals. Here, we analyzed the function of genes in Arabidopsis. The wild-type allele of SMC5 is essential for seed development. Each of the two SMC6 homologs of Arabidopsis is required for efficient repair of DNA breakage via intermolecular homologous recombination in somatic cells. Alignment of sister chromatids is enhanced transiently after X-irradiation (and mitomycin C treatment) in wild-type nuclei. In the smc5/6 mutants, the x-ray-mediated increase in sister chromatid alignment is much lower and delayed. The reduced S phase-established cohesion caused by a knockout mutation in one of the alpha-kleisin genes, SYN1, also perturbed enhancement of sister chromatid alignment after irradiation, suggesting that the S phase-established cohesion is a prerequisite for correct DSB-dependent cohesion. The radiation-sensitive51 mutant, deficient in heteroduplex formation during DSB repair, showed wild-type frequencies of sister chromatid alignment after X-irradiation, implying that the irradiation-mediated increase in sister chromatid alignment is a prerequisite for, rather than a consequence of, DNA strand exchange between sister chromatids. Our results suggest that the SMC5/6 complex promotes sister chromatid cohesion after DNA breakage and facilitates homologous recombination between sister chromatids.

Full-color painting reveals an excess of radiation-induced dicentrics involving homologous chromosomes

PURPOSE

To determine the ratio of homologous to heterologous dicentric chromosomes induced in human cells by ionizing radiation. This ratio is influenced by, and thus potentially informative about, underlying DNA damage/repair/misrepair processes and also the geometry of individual chromosome domains within the interphase nucleus.

MATERIALS AND METHODS

24-color mFISH (multiplex fluorescent in situ hybridization) was used to determine the ratio of 1-color (homologous) to 2-color (heterologous) dicentrics produced in human lymphocytes or fibroblasts by gamma-rays, alpha particles, or iron ions at various doses. Assuming that randomness independent of homology holds, the expected homologue:heterologue ratio for diploid human male cells is approximately 0.024, as shown by deriving a formula applicable to simple interchanges and then extending the result, via Monte Carlo simulation, to the general situation where complex aberrations are also considered.

RESULTS AND CONCLUSIONS

There was a substantial excess of homologous dicentrics, with probability of occurrence by chance less than 0.02 for each of the three radiations and only about 10(-8) for all the data combined. Overall, approximately 18 homologous dicentrics were expected but 47 were found, including 11 involving chromosome 1. Observed excesses were similar for both sparsely and densely ionizing radiations. Geometric proximity of homologues is a possible explanation for the overabundance; in that case more extensive statistics should eventually uncover a linear energy transfer (LET) dependence. An alternative possibility, not ruled out by the present data, is homology-dependent misrepair.

Random homologous pairing and incomplete sister chromatid alignment are common in angiosperm interphase nuclei

The chromosome arrangement in interphase nuclei is of growing interest, e.g., the spatial vicinity of homologous sequences is decisive for efficient repair of DNA damage by homologous recombination, and close alignment of sister chromatids is considered as a prerequisite for their bipolar orientation and subsequent segregation during nuclear division. To study the degree of homologous pairing and of sister chromatid alignment in plants, we applied fluorescent in situ hybridisation with specific bacterial artificial chromosome inserts to interphase nuclei. Previously we found in Arabidopsis thaliana and in A. lyrata positional homologous pairing at random, and, except for centromere regions, sister chromatids were frequently not aligned. To test whether these features are typical for higher plants or depend on genome size, chromosome organisation and/or phylogenetic affiliation, we investigated distinct individual loci in other species. The positional pairing of these loci was mainly random. The highest frequency of sister alignment (in >93% of homologues) was found for centromeres, some rDNA and a few other high copy loci. Apparently, somatic homologous pairing is not a typical feature of angiosperms, and sister chromatid aligment is not obligatory along chromosome arms. Thus, the high frequency of chromatid exchanges at homologous positions after mutagen treatment needs another explanation than regular somatic pairing of homologues (possibly an active search of damaged sites for homology). For sister chromatid exchanges a continuous sister chromatid alignment is not required. For correct segregation, permanent alignment of sister centromeres is sufficient.

Pairing of heterochromatin in response to cellular stress

We previously reported that exposure of human cells to DNA-damaging agents (X-rays and mitomycin C (MMC)) induces pairing of the homologous paracentromeric heterochromatin of chromosome 9 (9q12-13). Here, we show that UV irradiation and also heat shock treatment of human cells lead to similar effects. Since the various agents induce very different types and frequencies of damage to cellular constituents, the data suggest a general stress response as the underlying mechanism. Moreover, local UV irradiation experiments revealed that pairing of heterochromatin is an event that can be triggered without induction of DNA damage in the heterochromatic sequences. The repair deficient xeroderma pigmentosum cells (group F) previously shown to fail pairing after MMC displayed elevated pairing after heat shock treatment but not after UV exposure. Taken together, the present results indicate that pairing of heterochromatin following exposure to DNA-damaging agents is initiated by a general stress response and that the sensing of stress or the maintenance of the paired status of the heterochromatin might be dependent on DNA repair.

Mitomycin C-induced pairing of heterochromatin reflects initiation of DNA repair and chromatid exchange formation

Chromatid interchanges induced by the DNA cross-linking agent mitomycin C (MMC) are over-represented in human chromosomes containing large heterochromatic regions. We found that nearly all exchange breakpoints of chromosome 9 are located within the paracentromeric heterochromatin and over 70% of exchanges involving chromosome 9 are between its homologues. We provide evidence that the required pairing of chromosome 9 heterochromatic regions occurs in G(0)/G(1) and S-phase cells as a result of an active cellular process initiated upon MMC treatment. By contrast, no pairing was observed for a euchromatic paracentromeric region of the equal-sized chromosome 8. The MMC-induced pairing of chromosome 9 heterochromatin is observed in a subset of cells; its percentage closely mimics the frequency of homologous interchanges found at metaphase. Moreover, the absence of pairing in cells derived from XPF patients correlates with an altered spectrum of MMC-induced exchanges. Together, the data suggest that the heterochromatin-specific pairing following MMC treatment reflects the initiation of DNA cross-link repair and the formation of exchanges.

Spatial association of homologous pericentric regions in human lymphocyte nuclei during repair

Spatial positioning of pericentric chromosome regions in human lymphocyte cell nuclei was investigated during repair after H(2)O(2)/L-histidine treatment. Fifteen to three-hundred minutes after treatment, these regions of chromosomes 1, 15, and X were labeled by fluorescence in situ hybridization. The relative locus distances (LL-distances), the relative distances to the nuclear center (LC-distances), and the locus-nuclear center-locus angles (LCL-angles) were measured in approximately 5000 nuclei after two-dimensional microscopy. Experimental frequency histograms were compared to control data from untreated stimulated and quiescent (G(0)) nuclei and to a theoretical two-dimensional projection from random points. Based on the frequency distributions of the LL-distances and the LCL-angles, an increase of closely associated labeled regions was found shortly after repair activation. For longer repair times this effect decreased. After 300 min the frequency distribution of the LL-distances was found to be compatible with the random distance distribution again. The LL-distance frequency histograms for quiescent nuclei did not significantly differ from the theoretical random distribution, although this was the case for the stimulated control of chromosomes 15 and X. It may be inferred that, concerning the distances, homologous pericentric regions appear not to be randomly distributed during S-phase, and are subjected to dynamic processes during replication and repair.