A chromosome 15 quantitative trait locus controls levels of radiation-induced jejunal crypt cell apoptosis in mice

Genetic modifiers regulating DNA replication and double-strand break repair are associated with differences in mammary tumors in mouse models of Li-Fraumeni syndrome

Breast cancer is the most common tumor among women with inherited variants in the TP53 tumor suppressor, but onset varies widely suggesting interactions with genetic or environmental factors. Rodent models haploinsufficent for Trp53 also develop a wide variety of malignancies associated with Li-Fraumeni Syndrome, but BALB/c mice are uniquely susceptible to mammary tumors and is genetically linked to the Suprmam1 locus on chromosome 7. To define mechanisms that interact with deficiencies in p53 to alter susceptibility to mammary tumors, we fine-mapped the Suprmam1 locus in females from an N2 backcross of BALB/cMed and C57BL/6J mice. A major modifier was localized within a 10 cM interval on chromosome 7. The effect of the locus on DNA damage responses was examined in the parental strains and mice that are congenic for C57BL/6J alleles on the BALB/cMed background (SM1-Trp53^+/−). The mammary epithelium of C57BL/6J-Trp53^+/− females exhibited little radiation-induced apoptosis compared to BALB/cMed-Trp53^+/− and SM1-Trp53^+/− females indicating that the Suprmam1^B6/B6 alleles could not rescue repair of radiation-induced DNA double-strand breaks mostly relying on non-homologous end joining. In contrast, the Suprmam1^B6/B6 alleles in SM1-Trp53+/− mice were sufficient to confer the C57BL/6J-Trp53^+/− phenotypes in homology-directed repair and replication fork progression. The Suprmam1^B6/B6 alleles in SM1-Trp53^+/− mice appear to act in trans to regulate a panel of DNA repair and replication genes which lie outside the locus.

Ionizing radiation, inflammation, and their interactions in colon carcinogenesis in Mlh1-deficient mice

Genetic, physiological and environmental factors are implicated in colorectal carcinogenesis. Mutations in the mutL homolog 1 (MLH1) gene, one of the DNA mismatch repair genes, are a main cause of hereditary colon cancer syndromes such as Lynch syndrome. Long-term chronic inflammation is also a key risk factor, responsible for colitis-associated colorectal cancer; radiation exposure is also known to increase colorectal cancer risk. Here, we studied the effects of radiation exposure on inflammation-induced colon carcinogenesis in DNA mismatch repair-proficient and repair-deficient mice. Male and female Mlh1^−/− and Mlh1^+/+ mice were irradiated with 2 Gy X-rays when aged 2 weeks or 7 weeks and/or were treated with 1% dextran sodium sulfate (DSS) in drinking water for 7 days at 10 weeks old to induce mild inflammatory colitis. No colon tumors developed after X-rays and/or DSS treatment in Mlh1^+/+ mice. Colon tumors developed after DSS treatment alone in Mlh1^−/− mice, and exposure to radiation prior to DSS treatment increased the number of tumors. Histologically, colon tumors in the mice resembled the subtype of well-to-moderately differentiated adenocarcinomas with tumor-infiltrating lymphocytes of human Lynch syndrome. Immunohistochemistry revealed that expression of both p53 and β-catenin and loss of p21 and adenomatosis polyposis coli proteins were observed at the later stages of carcinogenesis, suggesting a course of molecular pathogenesis distinct from typical sporadic or colitis-associated colon cancer in humans. In conclusion, radiation exposure could further increase the risk of colorectal carcinogenesis induced by inflammation under the conditions of Mlh1 deficiency.

Animal models for medical countermeasures to radiation exposure

Since September 11, 2001, there has been the recognition of a plausible threat from acts of terrorism, including radiological or nuclear attacks. A network of Centers for Medical Countermeasures against Radiation (CMCRs) has been established across the U.S.; one of the missions of this network is to identify and develop mitigating agents that can be used to treat the civilian population after a radiological event. The development of such agents requires comparison of data from many sources and accumulation of information consistent with the "Animal Rule" from the Food and Drug Administration (FDA). Given the necessity for a consensus on appropriate animal model use across the network to allow for comparative studies to be performed across institutions, and to identify pivotal studies and facilitate FDA approval, in early 2008, investigators from each of the CMCRs organized and met for an Animal Models Workshop. Working groups deliberated and discussed the wide range of animal models available for assessing agent efficacy in a number of relevant tissues and organs, including the immune and hematopoietic systems, gastrointestinal tract, lung, kidney and skin. Discussions covered the most appropriate species and strains available as well as other factors that may affect differential findings between groups and institutions. This report provides the workshop findings.

Differential susceptibility of C57BL/6NCr and B6.Cg-Ptprca mice to commensal bacteria after whole body irradiation in translational bone marrow transplant studies

Background

The mouse is an important and widely utilized animal model for bone marrow transplant (BMT) translational studies. Here, we document the course of an unexpected increase in mortality of congenic mice that underwent BMT.

Methods

Thirty five BMTs were analyzed for survival differences utilizing the Log Rank test. Affected animals were evaluated by physical examination, necropsy, histopathology, serology for antibodies to infectious disease, and bacterial cultures.

Results

Severe bacteremia was identified as the main cause of death. Gastrointestinal (GI) damage was observed in histopathology. The bacteremia was most likely caused by the translocation of bacteria from the GI tract and immunosuppression caused by the myeloablative irradiation. Variability in groups of animals affected was caused by increased levels of gamma and X-ray radiation and the differing sensitivity of the two nearly genetically identical mouse strains used in the studies.

Conclusion

Our retrospective analysis of thirty five murine BMTs performed in three different laboratories, identified C57BL/6NCr (Ly5.1) as being more radiation sensitive than B6.Cg-Ptprc^a/NCr (Ly5.2). This is the first report documenting a measurable difference in radiation sensitivity and its effects between an inbred strain of mice and its congenic counterpart eventually succumbing to sepsis after BMT.

Correlation between single nucleotide polymorphisms and jejunal crypt cell apoptosis after whole body irradiation

PURPOSE

To identify loci concerned with radiosensitivity in a mouse model using single nucleotide polymorphism (SNP) markers.

MATERIALS AND METHODS

We subjected 276 second filial generation (F2) mice descended from two inbred mouse strains, radiation-induced apoptosis sensitive C57BL/6JNrs (B6) and radiation-induced apoptosis resistant C3H/HeNrs (C3H), to 2.5 Gy whole-body irradiation. We quantified jejunal crypt apoptosis, performed a genome-wide survey, and identified quantitative trait loci (QTL) associated with radiation sensitivity. We expressed apoptosis levels as an apoptotic score (AS), which was equal to the number of apoptotic bodies divided by the number of crypts. We genotyped the mice for 109 SNP markers.

RESULTS

AS values were 97.7+/-32.9 in B6 mice and 49.0+/-24.9 in C3H mice (p < 0.01). Genome-wide analysis revealed 8 markers (2 on chromosome 9, 4 on 15, 1 on 17, and 1 on 18) affecting radiation-induced jejunal apoptosis with log odds (LOD) scores ranging from 2.11+/-3.91. We found a significant locus on chromosome 15, which was previously reported by Weil and colleagues.

CONCLUSION

These findings support the view that the radiosensitivity of clinically normal tissue depends on variations in several genes.

Cell and tissue responses to genotoxic stress

Cancers arise as a consequence of the accumulation of multiple genetic mutations in a susceptible cell, resulting in perturbation of regulatory networks that control proliferation, survival, and cellular function. Here, the sources of cellular stress that can cause oncogenic mutations and the responses of cells to DNA damage are reviewed. The role of different repair pathways and the potential for cell- and tissue-specific reliance on individual repair mechanisms are discussed. Evidence for cell- and tissue-specific activation of p53-mediated growth arrest and apoptosis after exposure to an individual genotoxin is assessed and some of the potential mediators of these different responses are provided. These cell- and tissue-specific responses to particular forms of DNA damage are likely to be key determinants of tissue-specific tumour susceptibility, and there is good evidence for genetic variations in these responses. The role that genotoxic agents play in altering the microenvironment to produce indirect effects on tumourigenesis through altered production of free radicals and cytokines that are characteristic of inflammatory-type processes is also evaluated. Changes to the microenvironment as direct or indirect effects of genotoxic stress can be involved in both tumour initiation and progression and may even be a prerequisite for tumourigenesis. Therefore, tumour susceptibility after endogenous or exogenous genotoxic stress represents a balance between cell-intrinsic responses of target cells and changes to the microenvironment. A fuller understanding of cell- and tissue-specific responses, alterations to the microenvironment, and genetic modifiers of these responses could lead to novel prevention and therapeutic strategies for common forms of human malignancy.

Radiation-induced strain differences in mouse alveolar inflammatory cell apoptosis

Whole-thorax irradiation results in the development of the diffuse inflammatory response alveolitis in C3H/HeJ (C3H) mice and a milder alveolitis with fibrosis in C57BL/6J (B6) mice. In this study, we investigate if this mouse strain difference in response to radiation is due to differences in lung inflammatory cell apoptosis. Mice of the C3H and B6 strains were given a radiation dose of 18 Gy to the thorax and the animals were sacrificed at 11 or 18 weeks following exposure or when they were moribund. Active caspase-3 staining was used to identify apoptotic cells in the alveolar space of histological lung sections from the mice. The apoptotic index of B6 mice was greater than that of C3H mice at 11 weeks postirradiation (17.8% of airspace cells vs. 7.8%, p = 0.028) and in mice sacrificed because of illness (27.3% vs. 14.4%, p = 0.036). No C3H mice survived to the later time point. The inflammatory cells undergoing apoptosis in the mouse lungs were morphologically consistent with alveolar macrophages. We conclude that a difference in inflammatory cell apoptosis may contribute to the disparate pulmonary radiation response of these mouse strains.Key words: mouse, lung, radiation, apoptosis, fibrosis, caspase-3.

Intrinsic susceptibility to radiation-induced apoptosis of human lymphocyte subpopulations

PURPOSE

With the aim to evaluate intrinsic radiosensitivity, the susceptibility of lymphocyte subpopulations to radiation-induced apoptosis was determined. The investigated parameters included measurement reliability, phenotypic variance, intra- and inter-individual variability, and correlations between radiation-induced and spontaneous apoptosis.

METHODS AND MATERIALS

Quiescent lymphocytes of 63 healthy volunteers, sampled up to four times over a 1-year period were gamma-irradiated in vitro. Subsequent apoptosis (annexin V) was measured for T4-, T8-, and B-lymphocyte subpopulations using 6-color flow cytometry. Spontaneous apoptosis was measured and radiosensitivity was quantified from the dose-effect curves.

RESULTS

After thawing and short-term culture, both spontaneous apoptosis as well as radiation-induced apoptosis (radiosensitivity) differed among the three lymphocyte subpopulations, with T4 being most resistant, and B most sensitive. Spontaneous and radiation-induced apoptosis were correlated in all cell types, and variance between individuals was considerably higher than variance within individuals for both. A small but highly significant increase of both spontaneous and radiation-induced apoptosis was observed with age for T8, but not for T4 and B. Radiosensitivity of T8 and B proved to be sex-independent, whereas female T4 lymphocytes were less radiosensitive than those from males. T4 and T8 radiosensitivities were loosely correlated, and neither of them was related to B radiosensitivity.

CONCLUSION

Tendency to spontaneous and radiation-induced apoptosis of lymphocyte subpopulations differs among individuals. In addition, depending on the cell types, age and sex are factors influencing these parameters.

Different radiation susceptibility among five strains of mice detected by a skin reaction

Published reports about skin reactions to radiotherapy, especially among breast-cancer patients, suggest that there are interindividual differences in the normal tissue response, and genetic factors are thought to be involved in this variation. An analysis of murine strain differences may reveal the mechanism of genetic factors in the extent of normal tissue damage from irradiation for several endpoints. The variation in the radiation susceptibility was observed when the skin of mice from strains A/J, C3H/HeMs, C57BL/6J, C.B.17/Icr-scid and C3H-scid was irradiated with a single dose ranging from 10 to 60 Gy, using Cs-137 gamma rays. The active skin reaction of A/J mice lasted for months. C3H/HeMs mice showed dose-dependent skin damage, and consequently recovered to a state of mild damage within 40 days after local irradiation. The time course of the response in C57BL/6J mice was shorter than in A/J mice. The 2 strains of scid mice exhibited severe damage after irradiation at any dose from 20 to 50 Gy, and did not show any dose dependency. The variation between murine strains in macroscopic and histopathological changes in skin during the progression and resolution of damage caused by irradiation suggests an inter-strain variation in the expression of genes involved in injury, apoptosis, repair, and remodeling.

Tissue-specific p53 responses to ionizing radiation and their genetic modification: the key to tissue-specific tumour susceptibility?

Although little is understood of the underlying mechanisms, there are tissue-specific responses to tumourigenic and therapeutic agents and these responses are influenced by genetic factors. Ionizing radiation is an important tumourigenic and therapeutic agent for which there is substantial evidence for such tissue-dependent and genotype-dependent responses. Because the p53 tumour suppressor protein is a major determinant of cellular responses to radiation, the present study has investigated whether modification of the p53 pathway contributes to tissue-dependent and genotype-dependent responses using inbred strains of mice. Comparison of responses in haemopoietic and epithelial cells in irradiated C57BL/6 and DBA/2 mice revealed significant differences in p53 and apoptotic responses in different cell types and in different cells of the same type, reflecting the complexity of damage responses operating in the whole organism. The data suggest that p53-mediated up-regulation of Bax is a major determinant of apoptosis in the spleen, but not in the intestine, whereas p53-mediated induction of p21(waf1) plays an anti-apoptotic role in the spleen, but not in the intestine. It is also shown that p53 stabilization and differential transactivational activities towards Bax or p21(waf1) are influenced by genetic factors that act in a tissue-specific manner. Analysis of ATM, a potential mediator of differential p53 activation, indicates that this key regulator of radiation responses is preferentially induced in epithelial cells, but is unlikely to account for genetic modification of p53 or apoptotic responses in the mouse strains studied. Polymorphisms in the p53 or DNA-PKcs genes are also unlikely to account for the genetic modifications that are reported here. There are numerous further potential modifiers of the p53 pathway, but analysis of backcross and inter-cross mice demonstrates that genes responsible for the complex modification of these in vivo responses can be identified by linkage analysis. This approach has the potential to reveal new or unexpected interactions involving the p53 pathway that determine both short-term and long-term effects of radiation exposure and the basis of tissue-specific responses and tumour susceptibility.