Evaluation of DNA damage and stress in wildlife chronically exposed to low-dose, low-dose rate radiation from the Fukushima Dai-ichi Nuclear Power Plant accident

The health effects associated with chronic low-dose, low-dose rate (LD-LDR) exposures to environmental radiation are uncertain. All dose-effect studies conducted outside controlled laboratory conditions are challenged by inherent complexities of ecological systems and difficulties quantifying dose to free-ranging organisms in natural environments. Consequently, the effects of chronic LD-LDR radiation exposures on wildlife health remain poorly understood and much debated. Here, samples from wild boar (Sus scrofa leucomystax) and rat snakes (Elaphe spp.) were collected between 2016 and 2018 across a gradient of radiation exposures in Fukushima, Japan. In vivo biomarkers of DNA damage and stress were evaluated as a function of multiple measurements of radiation dose. Specifically, we assessed frequencies of dicentric chromosomes (Telomere-Centromere Fluorescence in situ Hybridization: TC-FISH), telomere length (Telo-FISH, qPCR), and cortisol hormone levels (Enzyme Immunoassay: EIA) in wild boar, and telomere length (qPCR) in snakes. These biological parameters were then correlated to robust calculations of radiation dose rate at the time of capture and plausible upper bound lifetime dose, both of which incorporated internal and external dose. No significant relationships were observed between dicentric chromosome frequencies or telomere length and dose rate at capture or lifetime dose (p value range: 0.20-0.97). Radiation exposure significantly associated only with cortisol, where lower concentrations were associated with higher dose rates (r² = 0.58; p < 0.0001), a relationship that was likely due to other (unmeasured) factors. Our results suggest that wild boar and snakes chronically exposed to LD-LDR radiation sufficient to prohibit human occupancy were not experiencing significant adverse health effects as assessed by biomarkers of DNA damage and stress.

Ad Astra - telomeres in space!

PURPOSE

My journey to the stars began as I - along with the whole world - stood still and watched Neil Armstrong take those first small steps on the Moon. Fast forward 50 years and NASA astronauts Scott Kelly and Christina Koch each spend nearly a year in space aboard the International Space Station (ISS), a remarkable multinational collaborative project and floating U.S. National Laboratory that has supported continuous human presence in low Earth orbit for the past 20 years. Marking a new era of human space exploration, the first commercial rocket, SpaceX Falcon 9, recently launched NASA astronauts Doug Hurley and Bob Behnken in the Crew Dragon spacecraft Endeavor to the ISS and returned safely to Earth. NASA and its commercial partners are rapidly advancing innovative space technologies, and with the recently announced Artemis team of astronauts, plans to send the first woman and next man back to the moon and establish sustainable exploration by the end of the decade. Humankind will then be poised to take the next giant leap - pioneering human exploration of Mars.

CONCLUSIONS

Historically, fewer than 600 individuals have participated in spaceflight, the vast majority of whom have been middle aged males (35-55 years) on short duration missions (less than 20 days). Thus, as the number and diversity of space travelers increase, a better understanding of how long-duration spaceflight affects human health is essential to maintaining individual astronaut performance during, and improving disease and aging trajectories following, future exploration missions. Here, I review findings from our NASA Twins Study and Telomeres investigations, highlighting potential mechanistic roles of chronic space radiation exposure in changes in telomere length and persistent DNA damage responses associated with long-duration spaceflight. Importantly, similar trends were observed in prostate cancer patients undergoing intensity-modulated radiation therapy (IMRT), additional support specifically for the role of radiation exposure. Individual differences in response were also observed in both cohorts, underscoring the importance of developing personalized approaches for evaluating human health effects and long-term outcomes associated with radiation exposures, whether on Earth or living in the extreme environment of space.

Haplotype diversity and sequence heterogeneity of human telomeres

Telomeres are regions of repetitive nucleotide sequences capping the ends of eukaryotic chromosomes that protect against deterioration, and whose lengths can be correlated with age and adverse health risk factors. Yet, given their length and repetitive nature, telomeric regions are not easily reconstructed from short-read sequencing, thus making telomere sequencing, mapping, and variant resolution challenging problems. Recently, long-read sequencing, with read lengths measuring in hundreds of kilobase pairs, has made it possible to routinely read into telomeric regions and inspect their sequence structure. Here, we describe a framework for extracting telomeric reads from whole-genome single-molecule sequencing experiments, including de novo identification of telomere repeat motifs and repeat types, and also describe their sequence variation. We find that long, complex telomeric stretches and repeats can be accurately captured with long-read sequencing, observe extensive sequence heterogeneity of human telomeres, discover and localize noncanonical telomere sequence motifs (both previously reported, as well as novel), and validate them in short-read sequence data. These data reveal extensive intra- and inter-population diversity of repeats in telomeric haplotypes, reveal higher paternal inheritance of telomeric variants, and represent the first motif composition maps of multi-kilobase-pair human telomeric haplotypes across three distinct ancestries (Ashkenazi, Chinese, and Utah), which can aid in future studies of genetic variation, aging, and genome biology.

Telomeric Double Strand Breaks in G1 Human Cells Facilitate Formation of 5' C-Rich Overhangs and Recruitment of TERRA

Telomeres, repetitive nucleoprotein complexes that protect chromosomal termini and prevent them from activating inappropriate DNA damage responses (DDRs), shorten with cell division and thus with aging. Here, we characterized the human cellular response to targeted telomeric double-strand breaks (DSBs) in telomerase-positive and telomerase-independent alternative lengthening of telomere (ALT) cells, specifically in G1 phase. Telomeric DSBs in human G1 cells elicited early signatures of a DDR; however, localization of 53BP1, an important regulator of resection at broken ends, was not observed at telomeric break sites. Consistent with this finding and previously reported repression of classical non-homologous end-joining (c-NHEJ) at telomeres, evidence for c-NHEJ was also lacking. Likewise, no evidence of homologous recombination (HR)-dependent repair of telomeric DSBs in G1 was observed. Rather, and supportive of rapid truncation events, telomeric DSBs in G1 human cells facilitated formation of extensive tracks of resected 5′ C-rich telomeric single-stranded (ss)DNA, a previously proposed marker of the recombination-dependent ALT pathway. Indeed, induction of telomeric DSBs in human ALT cells resulted in significant increases in 5′ C-rich (ss)telomeric DNA in G1, which rather than RPA, was bound by the complementary telomeric RNA, TERRA, presumably to protect these exposed ends so that they persist into S/G2 for telomerase-mediated or HR-dependent elongation, while also circumventing conventional repair pathways. Results demonstrate the remarkable adaptability of telomeres, and thus they have important implications for persistent telomeric DNA damage in normal human G1/G0 cells (e.g., lymphocytes), as well as for therapeutically relevant targets to improve treatment of ALT-positive tumors.

Telomere Length Dynamics and Chromosomal Instability for Predicting Individual Radiosensitivity and Risk via Machine Learning

The ability to predict a cancer patient's response to radiotherapy and risk of developing adverse late health effects would greatly improve personalized treatment regimens and individual outcomes. Telomeres represent a compelling biomarker of individual radiosensitivity and risk, as exposure can result in dysfunctional telomere pathologies that coincidentally overlap with many radiation-induced late effects, ranging from degenerative conditions like fibrosis and cardiovascular disease to proliferative pathologies like cancer. Here, telomere length was longitudinally assessed in a cohort of fifteen prostate cancer patients undergoing Intensity Modulated Radiation Therapy (IMRT) utilizing Telomere Fluorescence in situ Hybridization (Telo-FISH). To evaluate genome instability and enhance predictions for individual patient risk of secondary malignancy, chromosome aberrations were assessed utilizing directional Genomic Hybridization (dGH) for high-resolution inversion detection. We present the first implementation of individual telomere length data in a machine learning model, XGBoost, trained on pre-radiotherapy (baseline) and in vitro exposed (4 Gy γ-rays) telomere length measurements, to predict post radiotherapy telomeric outcomes, which together with chromosomal instability provide insight into individual radiosensitivity and risk for radiation-induced late effects.

Temporal Telomere and DNA Damage Responses in the Space Radiation Environment

Telomeres, repetitive terminal features of chromosomes essential for maintaining genome integrity, shorten with cell division, lifestyle factors and stresses, and environmental exposures, and so they provide a robust biomarker of health, aging, and age-related diseases. We assessed telomere length dynamics (changes over time) in three unrelated astronauts before, during, and after 1-year or 6-month missions aboard the International Space Station (ISS). Similar to our results for National Aeronautics and Space Administration's (NASA's) One-Year Mission twin astronaut (Garrett-Bakelman et al., 2019), significantly longer telomeres were observed during spaceflight for two 6-month mission astronauts. Furthermore, telomere length shortened rapidly after return to Earth for all three crewmembers and, overall, telomere length tended to be shorter after spaceflight than before spaceflight. Consistent with chronic exposure to the space radiation environment, signatures of persistent DNA damage responses were also detected, including mitochondrial and oxidative stress, inflammation, and telomeric and chromosomal aberrations, which together provide potential mechanistic insight into spaceflight-specific telomere elongation.

Telomere Length Dynamics and DNA Damage Responses Associated with Long-Duration Spaceflight

Telomere length dynamics and DNA damage responses were assessed before, during, and after one-year or shorter duration missions aboard the International Space Station (ISS) in a comparatively large cohort of astronauts (n = 11). Although generally healthy individuals, astronauts tended to have significantly shorter telomeres and lower telomerase activity than age- and sex-matched ground controls before and after spaceflight. Although telomeres were longer during spaceflight irrespective of mission duration, telomere length shortened rapidly upon return to Earth, and overall astronauts had shorter telomeres after spaceflight than they did before; inter-individual differences were identified. During spaceflight, all crewmembers experienced oxidative stress, which positively correlated with telomere length dynamics. Significantly increased frequencies of chromosomal inversions were observed during and after spaceflight; changes in cell populations were also detected. We propose a telomeric adaptive response to chronic oxidative damage in extreme environments, whereby the telomerase-independent Alternative Lengthening of Telomeres (ALT) pathway is transiently activated in normal somatic cells.

Abstract 4869: Chromosomal and telomeric biomarkers of normal tissue injury to evaluate risk of secondary malignancy following IMRT for prostate cancer

An overall intent of radiotherapy is to precisely target tumor cells, while minimizing exposures to surrounding normal tissue. Despite successes, there is growing concern that an unacceptably large volume of normal tissue is unavoidably exposed. Chromosome aberrations provide a direct measure of ionizing radiation (IR)-induced DNA damage, as well as an indirect measure of future risk since they are associated with virtually all known cancers. Such structural variants (SVs) include translocations (rearrangements between chromosomes) and inversions (rearrangements within chromosomes), the latter being recently identified as part of a distinctive mutational signature associated with radiation therapy-induced second malignancies. Directional Genomic Hybridization (dGH), is a strand-specific cytogenomics-based methodology for cell-by-cell, high-resolution detection of all SVs, particularly inversions, which when combined with compatible subtelomere probes (Telo-dGH), can be used to distinguish inversions from recombination events (sister chromatid exchange) involving chromosomal termini. Telomeres are critical structural elements that serve to protect the physical ends of chromosomes. Dysfunctional telomeres are associated with instability and carcinogenesis, as well as with a variety of other age-related pathologies (e.g., cardiovascular disease). We are validating Telo-dGH as a prospective “personalized” approach of evaluating normal tissue injury, and therefore future risk, associated with radiation therapy - regardless of tumor type or treatment modality. Here, we report results of monitoring prostate cancer patients before and after intensity-modulated radiation therapy (IMRT) to assess radiosensitivity (toxicity), as well as risk of secondary malignancy and other degenerative late effects. Such a strategy has the potential to better inform patient treatment and management decisions based on predicted individual risk.

Citation Format: Jared J. Luxton, Miles J. McKenna, Lynn Taylor, Gregory P. Swanson, Susan M. Bailey. Chromosomal and telomeric biomarkers of normal tissue injury to evaluate risk of secondary malignancy following IMRT for prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4869.