Paula C. Genik

Incidence of acute myeloid leukemia and hepatocellular carcinoma in mice irradiated with 1 GeV/nucleon 56Fe Ions

Abstract Estimates of cancer risks posed to space-flight crews by exposure to high atomic number, high-energy (HZE) ions are subject to considerable uncertainty because epidemiological data do not exist for human populations exposed to similar radiation qualities. We assessed the leukemogenic efficacy of one such HZE species, 1 GeV 56Fe ions, a component of space radiation, in a mouse model for radiation-induced acute myeloid leukemia. CBA/CaJ mice were irradiated with 1 GeV/nucleon 56Fe ions or 137Cs γ rays and followed until they were moribund or to 800 days of age. We found that 1 GeV/nucleon 56Fe ions do not appear to be substantially more effective than γ rays for the induction of acute myeloid leukemia (AML). However, 56Fe-ion-irradiated mice had a much higher incidence of hepatocellular carcinoma (HCC) than γ-irradiated mice, with an estimated RBE of approximately 50. These data suggest a difference in the effects of HZE iron ions on the induction of leukemia compared to solid tumors, suggesting potentially different mechanisms of tumorigenesis.

Radiation Leukemogenesis in Mice : Loss of PU. 1 on Chromosome 2 in CBA and C57BL/6 Mice after Irradiation with 1 GeV/nucleon 56Fe Ions, X Rays or γ Rays. Part I. Experimental Observations

Abstract Peng, Y., Brown, N., Finnon, R., Warner, C. L., Liu, X., Genik, P. C., Callan, M. A., Ray, F. A., Borak, T. B., Badie, C., Bouffler, S. D., Ullrich, R. L., Bedford, J. S. and Weil, M. M. Radiation Leukemogenesis in Mice: Loss of PU.1 on Chromosome 2 in CBA and C57BL/6 Mice after Irradiation with 1 GeV/nucleon 56Fe Ions, X Rays or γ Rays. Part I. Experimental Observations. Radiat. Res. 171, 474–483 (2009). Since deletion of the PU.1 gene on chromosome 2 is a crucial acute myeloid leukemia (AML) initiating step in the mouse model, we quantified PU.1 deleted cells in the bone marrow of γ-, X- and 56Fe-ion-irradiated mice at various times postirradiation. Although 56Fe ions were initially some two to three times more effective than X or γ rays in inducing PU.1 deletions, by 1 month postirradiation, the proportions of cells with PU.1 deletions were similar for the HZE particles and the sparsely ionizing radiations. These results indicate that while 56Fe ions are more effective in inducing PU.1 deletions, they are also more effective in causing collateral damage that removes hit cells from the bone marrow. After X, γ or 56Fe-ion irradiation, AML-resistant C57BL/6 mice have fewer cells with PU.1 deletions than CBA mice, and those cells do not persist in the bone marrow of the C57B6/6 mice. Our findings suggest that quantification of PU.1 deleted bone marrow cells 1 month postirradiation can be used as surrogate for the incidence of radiation-induced AML measured in large-scale mouse studies. If so, PU.1 loss could be used to systematically assess the potential leukemogenic effects of other ions and energies in the space radiation environment.

Effects of 28Si Ions, 56Fe Ions, and Protons on the Induction of Murine Acute Myeloid Leukemia and Hepatocellular Carcinoma

Estimates of cancer risks posed to space-flight crews by exposure to high atomic number, high-energy (HZE) ions are subject to considerable uncertainty because epidemiological data do not exist for human populations exposed to similar radiation qualities. We assessed the carcinogenic effects of 300 MeV/n 28Si or 600 MeV/n 56Fe ions in a mouse model for radiation-induced acute myeloid leukemia and hepatocellular carcinoma. C3H/HeNCrl mice were irradiated with 0.1, 0.2, 0.4, or 1 Gy of 300 MeV/n 28Si ions, 600 MeV/n 56Fe ions or 1 or 2 Gy of protons simulating the 1972 solar particle event (1972SPE) at the NASA Space Radiation Laboratory. Additional mice were irradiated with 137Cs gamma rays at doses of 1, 2, or 3 Gy. All groups were followed until they were moribund or reached 800 days of age. We found that 28Si or 56Fe ions do not appear to be substantially more effective than gamma rays for the induction of acute myeloid leukemia. However, 28Si or 56Fe ion irradiated mice had a much higher incidence of hepatocellular carcinoma than gamma ray irradiated or proton irradiated mice. These data demonstrate a clear difference in the effects of these HZE ions on the induction of leukemia compared to solid tumors, suggesting potentially different mechanisms of tumorigenesis. Also seen in this study was an increase in metastatic hepatocellular carcinoma in the 28Si and 56Fe ion irradiated mice compared with those exposed to gamma rays or 1972SPE protons, a finding with important implications for setting radiation exposure limits for space-flight crew members.

γ-H2AX Foci after Low-Dose-Rate Irradiation Reveal Atm Haploinsufficiency in Mice

Abstract Kato, T. A., Nagasawa, H., Weil, M. M., Genik, P. C., Little, J. B. and Bedford, J. S. γ-H2AX Foci after Low-Dose-Rate Irradiation Reveal Atm Haploinsufficiency in Mice. Radiat. Res. 166, 47–54 (2006). We have investigated the use of the γ-H2AX assay, reflecting the presence of DNA double-strand breaks (DSBs), as a possible means for identifying individuals who may be intermediate with respect to the extremes of hyper-radiosensitivity phenotypes. In this case, cells were studied from mice that were normal (Atm+/+), heterozygous (Atm+/−), or homozygous recessive (Atm−/−) for a truncating mutation in the Atm gene. After single acute (high-dose-rate) exposures, differences in mean numbers of γ-H2AX foci per cell between samples from Atm+/+ and Atm−/− mice were clear at nearly all sampling times, but at no sampling time was there a clear distinction for cells from Atm+/+ and Atm+/− mice. In contrast, under conditions of low-dose-rate irradiation at 10 cGy/h, appreciable differences in the levels of γ-H2AX foci per cell were observed in synchronized G1 cells derived from Atm+/− mice relative to cells from Atm+/+ mice. The levels were intermediate between those for cells from Atm+/+ and Atm−/− mice. After 24 h exposure at this dose rate, measurements in cells from four different mice for each genotype yielded mean frequencies of foci per cell of 1.77 ± 0.13 (SEM) for Atm+/+ cells, 4.75 ± 0.20 for the Atm+/− cells, and 11.10 ± 0.33 for the Atm−/−cells. The distributions of foci per G1 cell were not significantly different from Poisson. To the extent that variations in sensitivity with respect to γ-H2AX focus formation reflect variations in radiosensitivity for biological effects of concern, such as carcinogenesis, and that similar differences are seen for other genetic DNA DSB processing defects in general, this assay may provide a relatively straightforward means for distinguishing individuals who may be mildly hypersensitive to radiation such as we observed for Atm heterozygous mice.

Animal studies of charged particle-induced carcinogenesis

The distribution of energy deposition in cells and tissues by high-charge, high-energy (HZE) nuclei differs considerably from that of low linear energy transfer (LET) radiation, raising concerns that charged particle exposure may be more efficient in inducing radiogenic cancers or may induce a different spectrum of tumors. The authors have performed a review of charged particle carcinogenesis in animals with the following observations. A limited number of animal studies with carcinogenesis endpoints have been performed to evaluate the effectiveness of HZE ions. These include the induction of skin and mammary tumors in the rat and Harderian gland tumors, acute myeloid leukemia (AML), and hepatocellular carcinomas in the mouse. In general, high relative biological effectiveness (RBE) has been reported for solid tumor induction. RBE dependence on HZE radiation quality has been most extensively characterized in studies of mouse Harderian gland tumorigenesis. In this model, the RBE increases with LET and plateaus in the 193-953 keV μm(-1) range. Unlike the results of solid tumor studies, a leukemogenesis study found 1 GeV nucleon(-1) 56Fe ions no more efficient than gamma-rays for AML induction. No novel tumor types have been observed in HZE irradiated animals as compared with those that occur spontaneously or following low-LET radiation exposures. Genetic background of the irradiated animals is critical; the tumor types induced in HZE irradiated mice depend on their strain background, and the incidence of HZE ion-induced mammary carcinogenesis in the rat is also strain dependent.

Molecular characterisation of murine acute myeloid leukaemia induced by 56Fe ion and 137Cs gamma ray irradiation

Exposure to sparsely ionising gamma- or X-ray irradiation is known to increase the risk of leukaemia in humans. However, heavy ion radiotherapy and extended space exploration will expose humans to densely ionising high linear energy transfer (LET) radiation for which there is currently no understanding of leukaemia risk. Murine models have implicated chromosomal deletion that includes the hematopoietic transcription factor gene, PU.1 (Sfpi1), and point mutation of the second PU.1 allele as the primary cause of low-LET radiation-induced murine acute myeloid leukaemia (rAML). Using array comparative genomic hybridisation, fluorescence in situ hybridisation and high resolution melt analysis, we have confirmed that biallelic PU.1 mutations are common in low-LET rAML, occurring in 88% of samples. Biallelic PU.1 mutations were also detected in the majority of high-LET rAML samples. Microsatellite instability was identified in 42% of all rAML samples, and 89% of samples carried increased microsatellite mutant frequencies at the single-cell level, indicative of ongoing instability. Instability was also observed cytogenetically as a 2-fold increase in chromatid-type aberrations. These data highlight the similarities in molecular characteristics of high-LET and low-LET rAML and confirm the presence of ongoing chromosomal and microsatellite instability in murine rAML.

Comparison of cellular lethality in DNA repair-proficient or -deficient cell lines resulting from exposure to 70 MeV/n protons or 290 MeV/n carbon ions

Charged particle therapy utilizing protons or carbon ions has been rapidly intensifying over recent years. The present study was designed to jointly investigate these two charged particle treatment modalities with respect to modeled anatomical depth-dependent dose and linear energy transfer (LET) deliveries to cells with either normal or compromised DNA repair phenotypes. We compared cellular lethality in response to dose, LET and Bragg peak location for accelerated protons and carbon ions at 70 and 290 MeV/n, respectively. A novel experimental live cell irradiation OptiCell™ in vitro culture system using three different Chinese hamster ovary (CHO) cells as a mammalian model was conducted. A wild-type DNA repair-competent CHO cell line (CHO 10B2) was compared to two other CHO cell lines (51D1 and xrs5), each genetically deficient with respect to one of the two major DNA repair pathways (homologous recombination and non-homologous end joining pathways, respectively) following genotoxic insults. We found that wild-type and homologous recombination-deficient (Rad51D) cellular lethality was dependent on both the dose and LET of the carbon ions, whereas it was only dependent on dose for protons. The non-homologous end joining deficient cell line (Ku80 mutant) showed nearly identical dose-response profiles for both carbon ions and protons. Our results show that the increasingly used modality of carbon ions as charged particle therapy is advantageous to protons in a radiotherapeutic context, primarily for tumor cells proficient in non-homologous end joining DNA repair where cellular lethality is dependent not only on the dose as in the case of more common photon therapeutic modalities, but more importantly on the carbon ion LETs. Genetic characterization of patient tumors would be key to individualize and optimize the selection of radiation modality, clinical outcome and treatment cost.

Leukemogenesis in heterozygous PU.1 knockout mice

Most murine radiation-induced acute myeloid leukemias involve biallelic inactivation of the PU.1 gene, with one allele being lost through a radiation-induced chromosomal deletion and the other allele affected by a recurrent point mutation in codon 235 that is likely to be spontaneous. The short latencies of acute myeloid leukemias occurring in nonirradiated mice engineered with PU.1 conditional knockout or knockdown alleles suggest that once both copies of PU.1 have been lost any other steps involved in leukemogenesis occur rapidly. Yet, spontaneous acute myeloid leukemias have not been reported in mice heterozygous for a PU.1 knockout allele, an observation that conflicts with the understanding that the PU.1 codon 235 mutation is spontaneous. Here we describe experiments that show that the lack of spontaneous leukemia in PU.1 heterozygous knockout mice is not due to insufficient monitoring times or mouse numbers or the genetic background of the knockout mice. The results reveal that spontaneous leukemias that develop in mice of the mixed 129S2/SvPas and C57BL/6 background of knockout mice arise by a pathway that does not involve biallelic PU.1 mutation. In addition, the latency of radiation-induced leukemia in PU.1 heterozygous mice on a genetic background susceptible to radiation-induced leukemia indicates that the codon 235 mutation is not a rate-limiting step in radiation leukemogenesis driven by PU.1 loss.

Potentially Lethal Damage Repair in Drug Arrested G2-Phase Cells after Radiation Exposure

Potentially lethal damage (PLD) repair has been defined as that property conferring the ability of cells to recover from DNA damage depending on the postirradiation environment. Using a novel cyclin dependent kinase 1 inhibitor RO-3306 to arrest cells in the G2 phase of the cell cycle, examined PLD repair in G2 in cultured Chinese hamster ovary (CHO) cells. Several CHO-derived DNA repair mutant cell lines were used in this study to elucidate the mechanism of DNA double-strand break repair and to examine PLD repair during the G2 phase of the cell cycle. While arrested in G2 phase, wild-type CHO cells displayed significant PLD repair and improved cell survival compared with cells released immediately from G2 after irradiation. Both the radiation-induced chromosomal aberrations and the delayed entry into mitosis were also reduced by G2-holding PLD recovery. The PLD repair observed in G2 was observed in nonhomologous end-joining (NHEJ) mutant cell lines but absent in homologous recombination mutant cell lines. From the survival curves, G2-NHEJ mutant cell lines were found to be very sensitive to gamma-ray exposure when compared to G2/homologous recombination mutant cell lines. Our findings suggest that after exposure to ionizing radiation during G2, NHEJ is responsible for the majority of non-PLD repair, and conversely, that the homologous recombination is responsible for PLD repair in G2.

Strain Background Determines Lymphoma Incidence in Atm Knockout Mice

About 10% to 30% of patients with ataxia-telangiectasia (A-T) develop leukemias or lymphomas. There is considerable interpatient variation in the age of onset and leukemia/lymphoma type. The incomplete penetrance and variable age of onset may be attributable to several factors. These include competing mortality from other A-T-associated pathologies, particularly neurodegeneration and interstitial lung disease, allele-specific effects of ataxia-telangiectasia mutated (ATM) gene mutations. There is also limited evidence from clinical observations and studies using Atm knockout mice that modifier genes may account for some variation in leukemia/lymphoma susceptibility. We have introgressed the Atm(tm1Awb) knockout allele (Atm(-)) onto several inbred murine strains and observed differences in thymic lymphoma incidence and latency between Atm(-/-) mice on the different strain backgrounds and between their F1 hybrids. The lymphomas that arose in these mice had a pattern of sequence gains and losses that were similar to those previously described by others. These results provide further evidence for the existence of modifier genes controlling lymphomagenesis in individuals carrying defective copies of Atm, at least in mice, the characterized Atm(-) congenic strain set provides a resource with which to identify these genes. In addition, we found that fewer than expected Atm(-/-) pups were weaned on two strain backgrounds and that there was no correlation between body weight of young Atm-/- mice and lymphoma incidence or latency.