Myeloid, B and T lymphoid and mixed lineage thymic lymphomas in the irradiated mouse

Irradiated Blm-deficient mice are a highly tumor prone model for analysis of a broad spectrum of hematologic malignancies

Mutations in the BLM gene cause human Bloom syndrome (BS), an autosomal recessive disorder of growth retardation, immunodeficiency and cancer predisposition. Homozygous null Blm(m3/m3) mice are cancer prone with a 5-fold increased risk of cancer compared with Blm(m3/+) and Blm(+/+) mice. Irradiation of Blm(m3/m3) mice increased the risk to 28-fold. Tumors occurred mainly in the hematopoietic system and were similar to those in BS based on detailed histologic and immunohistochemical analyses. Irradiated Blm-deficient mice thus provide a novel model for understanding accelerated malignancies in BS and a new platform for investigating the molecular basis for a wide range of hematopoietic neoplasms.

Hypomethylation and genome instability in the germline of exposed parents and their progeny is associated with altered miRNA expression

Recent studies suggest that transgenerational genome instability may be epigenetic in nature and mediated via altered DNA methylation and microRNAome. Here, we investigated the nature and mechanisms underlying the disruption of DNA methylation and microRNA expression status in the germline and progeny of exposed parents. We have found that paternal irradiation leads to upregulation of the miR-29 family in the exposed male germline, which causes decreased expression of de novo methyltransferase, DNA methyltransferase 3a, and profound hypomethylation of long interspersed nuclear elements 1 (LINE1) and short interspersed nuclear elements B2 (SINE B2). Epigenetic changes in the male germline further resulted in deleterious effects in the somatic thymus tissue from the progeny of exposed animals, including hypomethylation of LINE1 and SINE B2. Hypomethylation of LINE1 and SINE B2 in the thymus tissue from the progeny was associated with a significant decrease in the levels of lymphoid-specific helicase (LSH) that is crucial for the maintenance of methylation and silencing of repetitive elements. Furthermore, we noted a significant upregulation of miR-468 that targets LSH and leads to its decreased expression in thymus in the progeny of exposed parents. We suggest that miR-468-mediated suppression of LSH leads to aberrant methylation of LINE1 and SINE B2. In summary, altered microRNAome and hypomethylation of retroelements constitute deleterious effects that may significantly influence genome stability of the parental germline and consequently cause genome instability in the progeny.

Radiation-induced bystander effects in vivo are epigenetically regulated in a tissue-specific manner

Exposure of animal body parts to ionizing radiation (IR) can lead to molecular changes in distant shielded "bystander" tissues and organs. Nevertheless, tissue specificity of bystander responses within the same organism has not been examined in detail. Studies on in vivo bystander effect conducted so far analyzed changes induced by single-dose exposure. The potential of fractionated irradiation to induce bystander effects in vivo has never been studied. We analyzed changes in global DNA methylation and microRNAome in skin and spleen of animals subjected to single-dose (acute or fractionated) whole-body or cranial exposure to 0.5 Gy of X-rays. We found that IR-induced DNA methylation changes in bystander spleen and skin were distinct. Acute radiation exposure resulted in a significant loss of global DNA methylation in the exposed and bystander spleen 6 hr, 96 hr, and 14 days after irradiation. Fractionated irradiation led to hypomethylation in bystander spleen 6 hr after whole-body exposure, and 6 hr, 96 hr, and 14 days after cranial irradiation. Contrarily, changes in the skin of the same animals were seen only 6 hr after acute whole-body and head exposure. DNA hypomethylation observed in spleen was paralleled by a reduction of methyl-binding protein MeCP2 expression. Irradiation also induced tissue-specific microRNAome alterations in skin and spleen. For the first time, we have shown that IR-induced epigenetic bystander effects that occur in the same organism are triggered by both acute and fractionated exposure and are very distinct in different bystander organs. Future studies are clearly needed to address organismal and carcinogenic repercussions of those changes.

Spaceflight effects on T lymphocyte distribution, function and gene expression

The immune system is highly sensitive to stressors present during spaceflight. The major emphasis of this study was on the T lymphocytes in C57BL/6NTac mice after return from a 13-day space shuttle mission (STS-118). Spleens and thymuses from flight animals (FLT) and ground controls similarly housed in animal enclosure modules (AEM) were evaluated within 3-6 h after landing. Phytohemagglutinin-induced splenocyte DNA synthesis was significantly reduced in FLT mice when based on both counts per minute and stimulation indexes (P < 0.05). Flow cytometry showed that CD3(+) T and CD19(+) B cell counts were low in spleens from the FLT group, whereas the number of NK1.1(+) natural killer (NK) cells was increased (P < 0.01 for all three populations vs. AEM). The numerical changes resulted in a low percentage of T cells and high percentage of NK cells in FLT animals (P < 0.05). After activation of spleen cells with anti-CD3 monoclonal antibody, interleukin-2 (IL-2) was decreased, but IL-10, interferon-gamma, and macrophage inflammatory protein-1alpha were increased in FLT mice (P < 0.05). Analysis of cancer-related genes in the thymus showed that the expression of 30 of 84 genes was significantly affected by flight (P < 0.05). Genes that differed from AEM controls by at least 1.5-fold were Birc5, Figf, Grb2, and Tert (upregulated) and Fos, Ifnb1, Itgb3, Mmp9, Myc, Pdgfb, S100a4, Thbs, and Tnf (downregulated). Collectively, the data show that T cell distribution, function, and gene expression are significantly modified shortly after return from the spaceflight environment.

DNA methylation during mouse hemopoietic differentiation and radiation-induced leukemia

OBJECTIVE

To examine DNA methylation in mouse hemopoiesis before and after in vivo exposure to a leukemogenic dose of x-rays, and address whether methylation levels are associated with the relative radiosensitivity of tissues in vivo.

METHODS

The methylation status of control CBA/H and C57BL/6 mouse tissues before and after exposure to 3-Gy x-rays, and myeloid and lymphoid leukemias and lymphomas, was assessed by the direct analysis of the 5-methylcytosine (5-(Me)C) content of DNA, and by Southern blot analysis of genomic repeat sequences.

RESULTS

The DNA 5-(Me)C content of bone marrow is 15% lower than spleen. Together with the analyses of stem (myeloid) and progenitor (lymphoid) leukemias and lymphomas, we found a trend of increasing methylation during hemopoietic differentiation. Exposure to x-rays induced greater cell death in the hypomethylated bone marrow (>80%) than spleen (50%) in vivo, supporting the observed correlation found between methylation status and radiosensitivity of other high-turnover hierarchical tissues. Furthermore, there was an 8% DNA 5-(Me)C content decrease in bone marrow after in vivo exposure to 3-Gy x-rays, but this was genotype dependent, being observed in AML-susceptible (CBA/H) but not AML-resistant (C57BL/6) inbred mice.

CONCLUSION

Together these data suggest that methylation status may be related to the relative radiosensitivity of high-turnover hierarchical tissues such as bone marrow and that radiation-induced DNA hypomethylation has a role in radiation leukemogenesis.

Target cell frequency is a genetically determined risk factor in radiation leukaemogenesis

Whole body exposure to ionizing radiation increases the risk of radiation-induced acute myeloid leukaemia (r-AML). r-AML is the result of the accumulation of mutations in a single haemopoietic stem cell, so risk is therefore a function of the number of mutations required to transform the stem cell and the mutation rate. There is a genetic component to the risk of AML within the general population, and low penetrance variant alleles encoding DNA repair enzymes have been genetically implicated in therapy-related AML susceptibility. However, what is largely ignored is that target cell number, which defines the number of genomes at risk from DNA damaging agents, is also part of the equation that defines risk. We will review the evidence from genetic studies of inbred mouse models that target cell frequency is a risk factor in radiation leukaemogenesis. Inbred mouse strains that differ in their susceptibility to radiation-induced r-AML and thymic lymphoma (r-TL), spontaneous TL and pristane-induced plasmacytoma (PCT) have been exploited to identify susceptibility loci. The target cell in AML is the haemopoietic stem cell, whereas TLs and PCT arise from more mature lymphoid progenitor cells. Inbred mice also differ significantly in all aspects of haemopoiesis, and these differences have been used to identify quantitative trait loci (QTL) that determine the frequency of specific haemopoietic stem, progenitor or mature blood cells. The co-localization of QTL that determine risk and target cell frequency in all three haemopoietic malignancies is strong evidence that target cell frequency is a risk factor in radiation leukaemogenesis.

Evidence for clustered tumour suppressor gene loci on mouse chromosomes 2 and 4 in radiation-induced acute myeloid leukaemia

PURPOSE

To investigate the influence of genetic and epigenetic factors on allelic loss on chromosomes 2 and 4 in mouse radiation-induced acute myeloid leukaemia (r-AML).

METHODS

r-AML that arose in (CBA/HxC57BL/6)F1xCBA/H and F1xC57BL/6 mice were screened for transcription factor PU1 (also known as SPI-1) gene mutations and methylation of the paired box gene 5 (Pax5) gene promoter. We have increased the statistical significance of a genetic linkage analysis of affected F1xCBA/H mice to test for linkage to loci implicated directly or indirectly with r-AML-susceptibility.

RESULTS

There was a statistically significant difference ( p < 10-4) in the frequency of PU1 gene mutations in F1xCBA/H and F1xC57BL/6 r-AML, implicating a second linked but genotype-dependent myeloid leukaemia suppressor gene on chromosome 2. A suggestive CBA/H r-AML-resistance locus maps within 10 cM of the minimally deleted region on chromosome 4. The Pax5 gene promoter is subject to ongoing subclonal promoter methylation in the r-AML, evidence that Pax5 gene silencing confers a selective advantage during clonal expansion in vivo.

CONCLUSIONS

Allelic loss in mouse r-AML and subsequent tumour suppressor gene mutation (PU1) or silencing (Pax5) is strongly influenced by genetic background and/or epigenetic factors, and driven by in vivo clonal selection.

Fractionated low-dose radiation exposure leads to accumulation of DNA damage and profound alterations in DNA and histone methylation in the murine thymus

Thymus, an important component of hematopoietic tissue, is a well-documented "target" of radiation carcinogenesis. Both acute and fractionated irradiation result in a high risk of leukemia and thymic lymphoma. However, the exact mechanisms underlying radiation-induced predisposition to leukemia and lymphoma are still unknown, and the contributions of genetic and epigenetic mechanisms in particular have yet to be defined. Global DNA hypomethylation is a well-known characteristic of cancer cells. Recent studies have also shown that tumor cells undergo prominent changes in histone methylation, particularly a substantial loss of trimethylation of histone H4-Lys20 and demethylation of genomic DNA. These losses are considered a universal marker of malignant transformation. In the present study, we investigated the effect of low-dose radiation exposure on the accumulation of DNA lesions and alterations of DNA methylation and histone H4-Lys20 trimethylation in the thymus tissue using an in vivo murine model. For the first time, we show that fractionated whole-body application of 0.5 Gy X-ray leads to decrease in histone H4-Lys20 trimethylation in the thymus. The loss of histone H4-Lys20 trimethylation was accompanied by a significant decrease in global DNA methylation as well as the accumulation of DNA damage as monitored by persistence of histone gammaH2AX foci in the thymus tissue of mice exposed to fractionated irradiation. Altered DNA methylation was associated with reduced expression of maintenance (DNMT1) and, to a lesser extent, de novo DNA methyltransferase DNMT3a in exposed animals. Expression of another de novo DNA methyltransferase DNMT3b was decreased only in males. Irradiation also resulted in approximately 20% reduction in the levels of methyl-binding proteins MeCP2 and MBD2. Our results show the involvement of epigenetic alterations in radiation-induced responses in vivo. These changes may play a role in genome destabilization that ultimately leads to cancer.

Stable loss of global DNA methylation in the radiation-target tissue—A possible mechanism contributing to radiation carcinogenesis?

Radiation-induced lymphomagenesis and leukemogenesis are complex processes involving both genetic and epigenetic changes. Although genetic alterations during radiation-induced lymphoma- and leukemogenesis are fairly well studied, the role of epigenetic changes has been largely overlooked. Rodent models are valuable tools for identifying molecular mechanisms of lymphoma and leukemogenesis. A widely used mouse model of radiation-induced thymic lymphoma is characterized by a lengthy "pre-lymphoma" period. Delineating molecular changes occurring during the pre-lymphoma period is crucial for understanding the mechanisms of radiation-induced leukemia/lymphoma development. In the present study, we investigated the role of radiation-induced DNA methylation changes in the radiation carcinogenesis target organ--thymus, and non-target organ--muscle. This study is the first report on the radiation-induced epigenetic changes in radiation-target murine thymus during the pre-lymphoma period. We have demonstrated that acute and fractionated whole-body irradiation significantly altered DNA methylation pattern in murine thymus leading to a massive loss of global DNA methylation. We have also observed that irradiation led to increased levels of DNA strand breaks 6 h following the initial exposure. The majority of radiation-induced DNA strand breaks were repaired 1 month after exposure. DNA methylation changes, though, were persistent and significant radiation-induced DNA hypomethylation was observed in thymus 1 month after exposure. In sharp contrast to thymus, no significant persistent changes were noted in the non-target muscle tissue. The presence of stable DNA hypomethylation in the radiation-target tissue, even though DNA damage resulting from initial genotoxic radiation insult was repaired, suggests of the importance of epigenetic mechanisms in the development of radiation-related pathologies. The possible role of radiation-induced DNA hypomethylation in radiation-induced genome instability and aberrant gene expression in molecular etiology of thymic lymphomas is discussed.

Risk factors for the development of post-transplant lymphoproliferative disorder in a large animal model

A high incidence of a post-transplant lymphoproliferative disorder (PTLD) is observed in miniature swine conditioned for allogeneic hematopoietic cell transplantation using a protocol involving T-cell depletion and cyclosporine therapy. This study was designed to assess contributing factors to disease development. Forty-six animals were studied including 12 (26%) that developed PTLD. A number of risk factors for PTLD were examined, including degree of immunosuppression, degree of MHC mismatch and infection by a porcine lymphotrophic herpesvirus (PLHV-1). Flow cytometry was used to measure host and donor T- and B-cell levels in the peripheral blood. Porcine lymphotrophic herpesvirus viral load was determined by quantitative PCR. Animals developing PTLD had significantly lower levels of T cells on the day of transplant. Cyclosporine levels did not differ significantly between animals with and without PTLD. Animals receiving transplants across a two-haplotype mismatch barrier showed an increased incidence of PTLD. All animals with PTLD had significant increases in PLHV-1 viral loads. Porcine lymphotrophic herpesvirus viral copy numbers remained at low levels in the absence of disease. The availability of a preclinical large-animal model with similarities to PTLD of humans may allow studies of the pathogenesis and treatment of that disorder.