Influence of radiation quality on mouse chromosome 2 deletions in radiation-induced acute myeloid leukaemia

Influence of Age on Leukemia Mortality Associated with Exposure to γ rays and 2-MeV Fast Neutrons in Male C3H Mice

The relative biological effectiveness (RBE) of densely ionizing radiation can depend on the biological context. From a radiological perspective, age is an important factor affecting health risks of radiation exposure, but little is known about the modifying impact of age on the effects of densely ionizing radiation. Herein, we addressed the influence of age on leukemogenesis induced by accelerator-generated fast neutrons (mean energy, ∼2 MeV). Male C3H/HeNrs mice were exposed to 137Cs γ rays (0.2-3.0 Gy) or neutrons (0.0485-0.97 Gy, γ ray contamination 0.0105-0.21 Gy) at 1, 3, 8, or 35 weeks of age and observed over their lifetimes under specific pathogen-free conditions. Leukemia and lymphoma were diagnosed pathologically. Hazard ratio (HR) and RBE for myeloid leukemia mortality as well as the age dependence of these two parameters were modeled and analyzed using Cox regression. Neutron exposure increased HR concordant with a linear dose response. The increase of HR per dose depended on age at exposure, with no significant dose dependence at age 1 or 3 weeks but a significant increase in HR of 5.5 per Gy (γ rays) and 16 per Gy (neutrons) at 8 weeks and 5.8 per Gy (γ rays) and 9 per Gy (neutrons) at 35 weeks. The RBE of neutrons was 2.1 (95% confidence interval, 1.1-3.7), with no dependence on age. The development of lymphoid neoplasms was not related to radiation exposure. The observed increasing trend of radiation-associated mortality of myeloid leukemia with age at exposure supports previous epidemiological and experimental findings. The results also suggest that exposure at the susceptible age of 8 or 35 weeks does not significantly influence the RBE value for neutrons for induction of leukemia, unlike what has been documented for breast and brain tumors.

Spi1 R235C point mutation confers hypersensitivity to radiation-induced acute myeloid leukemia in mice

Ionizing radiation (IR) is a risk factor for acute myeloid leukemia (rAML). Murine rAMLs feature both hemizygous chromosome 2 deletions (Del2) and point mutations (R235) within the hematopoietic regulatory gene Spi1. We generated a heterozygous CBA Spi1 R235 mouse (CBASpm/+) which develops de novo AML with 100% incidence by ∼12 months old and shows a dose-dependent reduction in latency following X-irradiation. These effects are reduced on an AML-resistant C57Bl6 genetic background. CBASpm/Gfp reporter mice show increased Gfp expression, indicating compensation for Spm-induced Spi1 haploinsufficiency. Del2 is always detected in both de novo and rAMLs, indicating that biallelic Spi1 mutation is required for AML. CBASpm/+ mice show that a single Spm modification is sufficient for initiating AML development with complete penetrance, via the "two-hit" mechanism and this is accelerated by IR exposure. Similar SPI1/PU.1 polymorphisms in humans could potentially lead to enhanced susceptibility to IR following medical or environmental exposure.

International expert group collaboration for developing an adverse outcome pathway for radiation induced leukaemia

PURPOSE

The concept of the adverse outcome pathway (AOP) has recently gained significant attention as to its potential for incorporation of mechanistic biological information into the assessment of adverse health outcomes following ionizing radiation (IR) exposure. This work is an account of the activities of an international expert group formed specifically to develop an AOP for IR-induced leukaemia. Group discussions were held during dedicated sessions at the international AOP workshop jointly organized by the MELODI (Multidisciplinary European Low Dose Initiative) and the ALLIANCE (European Radioecology Alliance) associations to consolidate knowledge into a number of biological key events causally linked by key event relationships and connecting a molecular initiating event with the adverse outcome. Further knowledge review to generate a weight of evidence support for the Key Event Relationships (KERs) was undertaken using a systematic review approach.

CONCLUSIONS

An AOP for IR-induced acute myeloid leukaemia was proposed and submitted for review to the OECD-curated AOP-wiki (aopwiki.org). The systematic review identified over 500 studies that link IR, as a stressor, to leukaemia, as an adverse outcome. Knowledge gap identification, although requiring a substantial effort via systematic review of literature, appears to be one of the major added values of the AOP concept. Further work, both within this leukaemia AOP working group and other similar working groups, is warranted and is anticipated to produce highly demanded products for the radiation protection research community.

Kras mutations and PU.1 promoter methylation are new pathways in murine radiation-induced AML

Therapy-related and more specifically radiotherapy-associated acute myeloid leukaemia (AML) is a well-recognized potential complication of cytotoxic therapy for the treatment of a primary cancer. The CBA mouse model is used to study radiation leukaemogenesis mechanisms with Sfpi1/PU.1 deletion and point mutation already identified as driving events during AML development. To identify new pathways, we analysed 123 mouse radiation-induced AML (rAML) samples for the presence of mutations identified previously in human AML and found three genes to be mutated; Sfpi1 R235 (68%), Flt3-ITD (4%) and Kras G12 (3%), of which G12R was previously unreported. Importantly, a significant decrease in Sfpi1 gene expression is found almost exclusively in rAML samples without an Sfpi1 R235 mutation and is specifically associated with up-regulation of mir-1983 and mir-582-5p. Moreover, this down-regulation of Sfpi1 mRNA is negatively correlated with DNA methylation levels at specific CpG sites upstream of the Sfpi1 transcriptional start site. The down regulation of Sfpi1/PU.1 has also been reported in human AML cases revealing one common pathway of myeloid disruption between mouse and human AML where dysregulation of Sfpi1/PU.1 is a necessary step in AML development.

High Relative Biological Effectiveness of 2 MeV Fast Neutrons for Induction of Medulloblastoma in Ptch1+/- Mice with Radiation-specific Deletion on Chromosome 13

Neutron radiation, a high-linear energy transfer radiation, has a high relative biological effectiveness (RBE) for various end points. The age at exposure is an important modifier of the effects of radiation, including carcinogenesis, with infants being generally more radiosensitive. Ptch1+/- mice offer a unique experimental system for assessing radiation carcinogenesis. Spontaneous development of medulloblastoma tumors occurs in nonirradiated animals that lose their Ptch1+ allele, most frequently by a loss of heterozygosity (LOH) of chromosome 13 via recombination or non-disjunction (referred to as S-type tumors). In contrast, tumors occur in irradiated Ptch1+/- mice as a result of chromosome 13 LOH with an interstitial deletion (R-type), making spontaneous and radiation-induced tumors discernible. To elucidate the influence of age on the effect of fast neutrons, we irradiated Ptch1+/- mice with neutrons (mean energy, ∼2 MeV) or γ rays on embryonic day (E)14 and E17 and on postnatal day (P)1, 4 or 10 and classified the resulting medulloblastomas based on chromosome 13 aberrations. Instead of LOH, some tumors harbored mutations in their Ptch1+ gene via a nonirradiation-associated mechanism such as duplication, insertion, base substitution or deletion with microhomology-mediated end joining; thus, these tumors were classified as S-type. The RBE regarding the induction of R-type tumors was 12.9 (8.6, 17.2), 9.6 (6.9, 12.3), 21.5 (17.2, 25.8), and 7.1 (4.7, 9.5) (mean and 95% confidence interval) for mice irradiated on E14, E17, P1 and P4, respectively, with the highest value seen during the most active development of the tissue and P10 being completely resistant. These results indicate that the developmental stage at exposure of the tissue influences the RBE of neutrons.

Knockdown of SNHG16 suppresses the proliferation and induces the apoptosis of leukemia cells via miR‑193a‑5p/CDK8

Although small nucleolar RNA host gene 16 (SNHG16) is known to exhibit auxo‑action in certain types of tumor, its role in leukemia remains unclear. The present study analyzed the role and mechanisms of action of SNHG16 in leukemia cells in order to identify therapeutic targets for this disease. Reverse transcription‑quantitative polymerase chain reaction (RT‑qPCR) was performed to determine SNHG16 expression in human leukemia cell lines. Using TargetScan 7.2 and dual‑luciferase reporter assay, the target genes of SNHG16 were verified. Following the downregulation of the expression of SNHG16 or its target genes, Cell Counting kit‑8 (CCK‑8) assay was performed to examine the viability of the leukemia cells. In addition, flow cytometry was performed to analyze the cell apoptotic rates, and colony formation assays were used to determine the cell proliferative ability. RT‑qPCR and western blot analysis were used to determine the association between SNHG16 and its target genes. SNHG16 was found to be abnormally highly expressed in acute myeloblastic leukemia cell lines, the knockdown of which weakened the viability of the leukemia cells, suppressed cell proliferation and promoted cell apoptosis. miR‑193a‑5p could bind to SNHG16, and its target gene was CDK8. Moreover, the expression of miR‑193a‑5p increased with the decrease in SNHG16 expression, while the inhibition of miR‑193a‑5p promoted the expression of CDK8. The downregulation of miR‑193a‑5p enhanced the viability of the leukemia cells, accelerated cell cloning and reduced cell apoptosis, which was completely opposite to the effects observed with the silencing of CDK8. The knockdown of SNHG16 suppressed the viability of the leukemia cells, suppressed cell proliferation, and induced cell apoptosis by regulating miR‑193a‑5p/CDK8. Thus, SNHG16 may prove to be a potential therapeutic target for the treatment of leukemia.

Hematopoietic stem and progenitor cell responses to low radiation doses - implications for leukemia risk

Studies of the responses of hematopoietic stem and progenitor cells (HSPCs) to low doses of ionizing radiation formed an important aspect of the RISK-IR project ( www.risk-ir.eu ). A brief overview of these studies is presented here. The findings confirm the sensitivity of HSPCs to radiation even at low doses, and illustrate the substantial impact that differentiation state has upon cell sensitivity. The work provides mechanistic support for epidemiological findings of leukemia risk at dose levels used in diagnostic CT imaging, and further suggests that low-dose irradiation may facilitate bone marrow transplantation, a finding that could lead to refinements in clinical practice.

Risk of second cancer after ion beam radiotherapy: insights from animal carcinogenesis studies

Purpose: To review recent studies to better understand the risk of second cancer after ion beam radiotherapy and to clarify the importance of animal radiobiology therein. Results: Risk of developing second cancer after radiotherapy is a concern, particularly for survivors of childhood tumors. Ion beam radiotherapy is expected to reduce the risk of second cancer by reducing exposure of normal tissues to radiation. Large uncertainty lies, however, in the choice of relative biological effectiveness (RBE) of high linear energy transfer (LET) radiation (e.g. carbon ions and neutrons) in cancer induction, especially for children. Studies have attempted to predict the risk of second cancer after ion beam radiotherapy based on an assessment of radiation dose, the risk of low LET radiation, and assumptions about RBE. Animal experiments have yielded RBE values for selected tissues, radiation types, and age at the time of irradiation; the results indicate potentially variable RBE which depends on tissues, ages, and dose levels. Animal studies have also attempted to identify genetic alterations in tumors induced by high LET radiation. Conclusions: Estimating the RBE value for cancer induction is important for understanding the risk of second cancer after ion beam radiotherapy. More comprehensive animal radiobiology studies are needed.

Dotting the eyes: mouse strain dependency of the lens epithelium to low dose radiation-induced DNA damage

PURPOSE

Epidemiological evidence regarding the radiosensitivity of the lens of the eye and radiation cataract development has led to changes in the EU Basic Safety Standards for protection of the lens against ionizing radiation. However, mechanistic details of lens radiation response pathways and their significance for cataractogenesis remain unclear. Radiation-induced DNA damage and the potential impairment of repair pathways within the lens epithelium, a cell monolayer that covers the anterior hemisphere of the lens, are likely to be involved.

MATERIALS AND METHODS

In this work, the lens epithelium has been analyzed for its DNA double-strand break (DSB) repair response to ionizing radiation. The responses of epithelial cells located at the anterior pole (central region) have been compared to at the very periphery of the monolayer (germinative and transitional zones). Described here are the different responses in the two regions and across four strains (C57BL/6, 129S2, BALB/c and CBA/Ca) over a low dose (0-25 mGy) in-vivo whole body X-irradiation range up to 24 hours post exposure.

RESULTS

DNA damage and repair as visualized through 53BP1 staining was present across the lens epithelium, although repair kinetics appeared non-uniform. Epithelial cells in the central region have significantly more 53BP1 foci. The sensitivities of different mouse strains have also been compared.

CONCLUSIONS

129S2 and BALB/c showed higher levels of DNA damage, with BALB/c showing significantly less inter-individual variability and appearing to be a more robust model for future DNA damage and repair studies. As a result of this study, BALB/c was identified as a suitable radiosensitive lens strain to detect and quantify early low dose ionizing radiation DNA damage effects in the mouse eye lens specifically, as an indicator of cataract formation.

Tracking preleukemic cells in vivo to reveal the sequence of molecular events in radiation leukemogenesis

Epidemiological studies have demonstrated an increased leukemia incidence following ionizing radiation exposure, but to date, the target cells and underlying mechanisms of radiation leukemogenesis remain largely unidentified. We engineered a mouse model carrying a different fluorescent marker on each chromosome 2, located inside the minimum deleted region occurring after radiation exposure and recognized as the first leukemogenic event. Using this tailored model, we report that following radiation exposure, more than half of asymptomatic CBA Sfpi1^GFP/mCh mice presented with expanding clones of preleukemic hematopoietic cells harboring a hemizygous interstitial deletion of chromosome 2. Moreover, following isolation of preleukemic hematopoietic stem and progenitor cells irradiated in their native microenvironment, we identified the presence of Sfpi1 point mutations within a subpopulation of these preleukemic cells expanding rapidly (increasing from 6% to 55% in 21 days in peripheral blood in one case), hence identifying for the first time the presence of such cells within a living animal. Importantly, we also report a previously undescribed gender difference in the phenotype of the preleukemic cells and leukemia, suggesting a gender imbalance in the radiation-induced leukemic target cell. In conclusion, we provide novel insights into the sequence of molecular events occurring during the (radiation-induced) leukemic clonal evolution.

Is Toxoplasma Gondii Infection a Risk Factor for Leukemia? An Evidence-Based Meta-Analysis

BACKGROUND Possible associations of parasite infection with cancer risk have recently attracted much attention. Published studies concerning the association between Toxoplasma gondii (T. gondii) infection and leukemia risk have generated inconsistent results. In the present study, we aimed to address this topic by conducting a quantitative meta-analysis. MATERIAL AND METHODS Relevant publications were searched in electronic databases and eligible studies were rigorously screened and selected. Essential information was extracted and the data were pooled. Subgroup analysis on source of controls and detection target was also performed. RESULTS A total of 6 studies that met the inclusion criteria were selected. The overall data show that T. gondii infection might have an association with increased leukemia risk (OR=3.05; 95%CI=1.83-5.08). Similar results were shown in the subgroups regarding source of controls and detection target. CONCLUSIONS Our results suggest that T. gondii infection might be a risk factor for leukemia, providing new insight into the etiology of leukemia. Future studies with large sample sizes in different geographic areas are needed to confirm this conclusion.