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

DNA Damage Clustering after Ionizing Radiation and Consequences in the Processing of Chromatin Breaks

Charged-particle radiotherapy (CPRT) utilizing low and high linear energy transfer (low-/high-LET) ionizing radiation (IR) is a promising cancer treatment modality having unique physical energy deposition properties. CPRT enables focused delivery of a desired dose to the tumor, thus achieving a better tumor control and reduced normal tissue toxicity. It increases the overall radiation tolerance and the chances of survival for the patient. Further improvements in CPRT are expected from a better understanding of the mechanisms governing the biological effects of IR and their dependence on LET. There is increasing evidence that high-LET IR induces more complex and even clustered DNA double-strand breaks (DSBs) that are extremely consequential to cellular homeostasis, and which represent a considerable threat to genomic integrity. However, from the perspective of cancer management, the same DSB characteristics underpin the expected therapeutic benefit and are central to the rationale guiding current efforts for increased implementation of heavy ions (HI) in radiotherapy. Here, we review the specific cellular DNA damage responses (DDR) elicited by high-LET IR and compare them to those of low-LET IR. We emphasize differences in the forms of DSBs induced and their impact on DDR. Moreover, we analyze how the distinct initial forms of DSBs modulate the interplay between DSB repair pathways through the activation of DNA end resection. We postulate that at complex DSBs and DSB clusters, increased DNA end resection orchestrates an increased engagement of resection-dependent repair pathways. Furthermore, we summarize evidence that after exposure to high-LET IR, error-prone processes outcompete high fidelity homologous recombination (HR) through mechanisms that remain to be elucidated. Finally, we review the high-LET dependence of specific DDR-related post-translational modifications and the induction of apoptosis in cancer cells. We believe that in-depth characterization of the biological effects that are specific to high-LET IR will help to establish predictive and prognostic signatures for use in future individualized therapeutic strategies, and will enhance the prospects for the development of effective countermeasures for improved radiation protection during space travel.

Cytogenetic follow-up studies on humans with internal and external exposure to ionizing radiation

Cells exposed to ionizing radiation have a wide spectrum of DNA lesions that include DNA single-strand breaks, DNA double-strand breaks (DSBs), oxidative base damage and DNA-protein crosslinks. Among them, DSB is the most critical lesion, which when mis-repaired leads to unstable and stable chromosome aberrations. Currently, chromosome aberration analysis is the preferred method for biological monitoring of radiation-exposed humans. Stable chromosome aberrations, such as inversions and balanced translocations, persist in the peripheral blood lymphocytes of radiation-exposed humans for several years and, therefore, are potentially useful tools to prognosticate the health risks of radiation exposure, particularly in the hematopoietic system. In this review, we summarize the cytogenetic follow-up studies performed by REAC/TS (Radiation Emergency Assistance Center/Training site, Oak Ridge, USA) on humans exposed to internal and external radiation. In the light of our observations as well as the data existing in the literature, this review attempts to highlight the importance of follow-up studies for predicting the extent of genomic instability and its impact on delayed health risks in radiation-exposed victims.

Chromosome Folding Promotes Intrachromosomal Aberrations under Radiation- and Nuclease-Induced DNA Breakage

The long-standing question in radiation and cancer biology is how principles of chromosome organization impact the formation of chromosomal aberrations (CAs). To address this issue, we developed a physical modeling approach and analyzed high-throughput genomic data from chromosome conformation capture (Hi-C) and translocation sequencing (HTGTS) methods. Combining modeling of chromosome structure and of chromosomal aberrations induced by ionizing radiation (IR) and nuclease we made predictions which quantitatively correlated with key experimental findings in mouse chromosomes: chromosome contact maps, high frequency of cis-translocation breakpoints far outside of the site of nuclease-induced DNA double-strand breaks (DSBs), the distinct shape of breakpoint distribution in chromosomes with different 3D organizations. These correlations support the heteropolymer globule principle of chromosome organization in G1-arrested pro-B mouse cells. The joint analysis of Hi-C, HTGTS and physical modeling data offers mechanistic insight into how chromosome structure heterogeneity, globular folding and lesion dynamics drive IR-recurrent CAs. The results provide the biophysical and computational basis for the analysis of chromosome aberration landscape under IR and nuclease-induced DSBs.

Cytogenetic Damage of Human Lymphocytes in Humanized Mice Exposed to Neutrons and X Rays 24 h After Exposure

Detonation of an improvised nuclear device highlights the need to understand the risk of mixed radiation exposure as prompt radiation exposure could produce significant neutron and gamma exposures. Although the neutron component may be a relatively small percentage of the total absorbed dose, the large relative biological effectiveness (RBE) can induce larger biological DNA damage and cell killing. The objective of this study was to use a hematopoietically humanized mouse model to measure chromosomal DNA damage in human lymphocytes 24 h after in vivo exposure to neutrons (0.3 Gy) and X rays (1 Gy). The human dicentric and cytokinesis-block micronucleus assays were performed to measure chromosomal aberrations in human lymphocytes in vivo from the blood and spleen, respectively. The mBAND assay based on fluorescent in situ hybridization labeling was used to detect neutron-induced chromosome 1 inversions in the blood lymphocytes of the neutron-irradiated mice. Cytogenetics endpoints, dicentrics and micronuclei showed that there was no significant difference in yields between the 2 irradiation types at the doses tested, indicating that neutron-induced chromosomal DNA damage in vivo was more biologically effective (RBE ∼3.3) compared to X rays. The mBAND assay, which is considered a specific biomarker of high-LET neutron exposure, confirmed the presence of clustered DNA damage in the neutron-irradiated mice but not in the X-irradiated mice, 24 h after exposure.

History and evolution of cytogenetic techniques: Current and future applications in basic and clinical research

Chromosomes are the vehicles of genes, which are the functional units of a cell's nucleus. In humans, there are more than 20,000 genes that are distributed among 46 chromosomes in somatic cells. The study of chromosome structure and function is known as cytogenetics which is historically a field of hybrid science encompassing cytology and genetics. The field of cytogenetics has undergone rapid developments over the last several decades from classical Giemsa staining of chromosomes to 3-dimensional spatial organization of chromosomes with a high resolution mapping of gene structure at the nucleotide level. Improved molecular cytogenetic techniques have opened up exciting possibilities for understanding the chromosomal/molecular basis of various human diseases including cancer and tissue degeneration. This review summaries the history and evolution of various cytogenetic techniques and their current and future applications in diverse areas of basic research and medical diagnostics.

Dose- and Ion-Dependent Effects in the Oxidative Stress Response to Space-Like Radiation Exposure in the Skeletal System

Space radiation may pose a risk to skeletal health during subsequent aging. Irradiation acutely stimulates bone remodeling in mice, although the long-term influence of space radiation on bone-forming potential (osteoblastogenesis) and possible adaptive mechanisms are not well understood. We hypothesized that ionizing radiation impairs osteoblastogenesis in an ion-type specific manner, with low doses capable of modulating expression of redox-related genes. 16-weeks old, male, C57BL6/J mice were exposed to low linear-energy-transfer (LET) protons (150 MeV/n) or high-LET ⁵⁶Fe ions (600 MeV/n) using either low (5 or 10 cGy) or high (50 or 200 cGy) doses at NASA's Space Radiation Lab. Five weeks or one year after irradiation, tissues were harvested and analyzed by microcomputed tomography for cancellous microarchitecture and cortical geometry. Marrow-derived, adherent cells were grown under osteoblastogenic culture conditions. Cell lysates were analyzed by RT-PCR during the proliferative or mineralizing phase of growth, and differentiation was analyzed by imaging mineralized nodules. As expected, a high dose (200 cGy), but not lower doses, of either ⁵⁶Fe or protons caused a loss of cancellous bone volume/total volume. Marrow cells produced mineralized nodules ex vivo regardless of radiation type or dose; ⁵⁶Fe (200 cGy) inhibited osteoblastogenesis by more than 90% (5 weeks and 1 year post-IR). After 5 weeks, irradiation (protons or ⁵⁶Fe) caused few changes in gene expression levels during osteoblastogenesis, although a high dose ⁵⁶Fe (200 cGy) increased Catalase and Gadd45. The addition of exogenous superoxide dismutase (SOD) protected marrow-derived osteoprogenitors from the damaging effects of exposure to low-LET (¹³⁷Cs γ) when irradiated in vitro, but had limited protective effects on high-LET ⁵⁶Fe-exposed cells. In sum, either protons or ⁵⁶Fe at a relatively high dose (200 cGy) caused persistent bone loss, whereas only high-LET ⁵⁶Fe increased redox-related gene expression, albeit to a limited extent, and inhibited osteoblastogenesis. Doses below 50 cGy did not elicit widespread responses in any parameter measured. We conclude that high-LET irradiation at 200 cGy impaired osteoblastogenesis and regulated steady-state gene expression of select redox-related genes during osteoblastogenesis, which may contribute to persistent bone loss.

Carbon Ion Radiotherapy: A Review of Clinical Experiences and Preclinical Research, with an Emphasis on DNA Damage/Repair

Compared to conventional photon-based external beam radiation (PhXRT), carbon ion radiotherapy (CIRT) has superior dose distribution, higher linear energy transfer (LET), and a higher relative biological effectiveness (RBE). This enhanced RBE is driven by a unique DNA damage signature characterized by clustered lesions that overwhelm the DNA repair capacity of malignant cells. These physical and radiobiological characteristics imbue heavy ions with potent tumoricidal capacity, while having the potential for simultaneously maximally sparing normal tissues. Thus, CIRT could potentially be used to treat some of the most difficult to treat tumors, including those that are hypoxic, radio-resistant, or deep-seated. Clinical data, mostly from Japan and Germany, are promising, with favorable oncologic outcomes and acceptable toxicity. In this manuscript, we review the physical and biological rationales for CIRT, with an emphasis on DNA damage and repair, as well as providing a comprehensive overview of the translational and clinical data using CIRT.

Relative proximity of chromosome territories influences chromosome exchange partners in radiation-induced chromosome rearrangements in primary human bronchial epithelial cells

It is well established that chromosomes exist in discrete territories (CTs) in interphase and are positioned in a cell-type specific probabilistic manner. The relative localisation of individual CTs within cell nuclei remains poorly understood, yet many cancers are associated with specific chromosome rearrangements and there is good evidence that relative territorial position influences their frequency of exchange. To examine this further, we characterised the complexity of radiation-induced chromosome exchanges in normal human bronchial epithelial (NHBE) cells by M-FISH analysis of PCC spreads and correlated the exchanges induced with their preferred interphase position, as determined by 1/2-colour 2D-FISH analysis, at the time of irradiation. We found that the frequency and complexity of aberrations induced were reduced in ellipsoid NHBE cells in comparison to previous observations in spherical cells, consistent with aberration complexity being dependent upon the number and proximity of damaged CTs, i.e. lesion proximity. To ask if particular chromosome neighbourhoods could be identified we analysed all radiation-induced pair-wise exchanges using SCHIP (statistics for chromosome interphase positioning) and found that exchanges between chromosomes (1;13), (9;17), (9;18), (12;18) and (16;21) all occurred more often than expected assuming randomness. All of these pairs were also found to be either sharing similar preferred positions in interphase and/or sharing neighbouring territory boundaries. We also analysed a human small cell lung cancer cell line, DMS53, by M-FISH observing the genome to be highly rearranged, yet possessing rearrangements also involving chromosomes (1;13) and (9;17). Our findings show evidence for the occurrence of non-random exchanges that may reflect the territorial organisation of chromosomes in interphase at time of damage and highlight the importance of cellular geometry for the induction of aberrations of varying complexity after exposure to both low and high-LET radiation.

Repair of DNA damage induced by accelerated heavy ions--a mini review

Increasing use of heavy ions for cancer therapy and concerns from exposure to heavy charged particles in space necessitate the study of the basic biological mechanisms associated with exposure to heavy ions. As the most critical damage induced by ionizing radiation is DNA double strand break (DSB), this review focuses on DSBs induced by heavy ions and their repair processes. Compared with X- or gamma-rays, high-linear energy transfer (LET) heavy ion radiation induces more complex DNA damage, categorized into DSBs and non-DSB oxidative clustered DNA lesions (OCDL). This complexity makes the DNA repair process more difficult, partially due to retarded enzymatic activities, leading to increased chromosome aberrations and cell death. In general, the repair process following heavy ion exposure is LET-dependent, but with nonhomologous end joining defective cells, this trend is less emphasized. The variation in cell survival levels throughout the cell cycle is less prominent in cells exposed to high-LET heavy ions when compared with low LET, but this mechanism has not been well understood until recently. Involvement of several DSB repair proteins is suggested to underlie this interesting phenomenon. Recent improvements in radiation-induced foci studies combined with high-LET heavy ion exposure could provide a useful opportunity for more in depth study of DSB repair processes. Accelerated heavy ions have become valuable tools to investigate the molecular mechanisms underlying repair of DNA DSBs, the most crucial form of DNA damage induced by radiation and various chemotherapeutic agents.

mBAND analysis for high- and low-LET radiation-induced chromosome aberrations: a review

During long-term space travel or cancer therapy, humans are exposed to high linear energy transfer (LET) energetic heavy ions. High-LET radiation is much more effective than low-LET radiation in causing various biological effects, including cell inactivation, genetic mutations, cataracts and cancer induction. Most of these biological endpoints are closely related to chromosomal damage, and cytogenetic damage can be utilized as a biomarker for radiation insults. Epidemiological data, mainly from survivors of the atomic bomb detonations in Japan, have enabled risk estimation from low-LET radiation exposures. The identification of a cytogenetic signature that distinguishes high- from low-LET exposure remains a long-term goal in radiobiology. Recently developed fluorescence in situ hybridization (FISH)-painting methodologies have revealed unique endpoints related to radiation quality. Heavy-ions induce a high fraction of complex-type exchanges, and possibly unique chromosome rearrangements. This review will concentrate on recent data obtained with multicolor banding in situ hybridization (mBAND) methods in mammalian cells exposed to low- and high-LET radiations. Chromosome analysis with mBAND technique allows detection of both inter- and intrachromosomal exchanges, and also distribution of the breakpoints of aberrations.