An improved micro-method for obtaining chromosome preparations from individual haemopoietic colonies

Long-lived inflammatory signaling in irradiated bone marrow is genome dependent

Ionizing radiation is carcinogenic, but genotype is a key determinant of susceptibility. Mutational DNA damage is generally attributed to cause disease, but irradiation also affects multicellular interactions as a result of poorly understood bystander effects that may influence carcinogenic susceptibility. In this study, we show that the bone marrow of irradiated mice will retain the ability to kill hemopoietic clonogenic stem cells and to induce chromosomal instability for up to 3 months after irradiation. Chromosomal instability was induced in bone marrow cells derived from CBA/Ca mice, a strain that is susceptible to radiation-induced acute myeloid leukemia (r-AML), but not in C57BL6 mice that are resistant to r-AML. Similarly, clonogenic cell lethality was exhibited in C57BL/6 mice but not CBA/Ca mice. Mechanistic investigations revealed that these genotype-dependent effects involved cytokine-mediated signaling and were mediated by a cyclooxygenase-2-dependent mechanism. Thus, our results suggested that inflammatory processes were responsible for mediating and sustaining the durable effects of ionizing radiation observed on bone marrow cells. Because most exposures to ionizing radiation are directed to only part of the body, our findings imply that genotype-directed tissue responses may be important determinants of understanding the specific consequence of radiation exposure in different individuals.

Chromosomal instability in unirradiated hemaopoietic cells induced by macrophages exposed in vivo to ionizing radiation

The tumorigenic potential of ionizing radiation has conventionally been attributed to DNA damage in irradiated cells induced at the time of exposure. Recently, there have been an increasing number of reports of damage in unirradiated cells that are either neighbors or descendants of irradiated cells, respectively, regarded as bystander effects and genomic instability and collectively termed nontargeted effects. In this study, we show that descendants of normal murine hemaopoietic clonogenic stem cells exposed to bone marrow-conditioned medium derived from gamma-irradiated mice exhibit chromosomal instability unlike the descendants of directly gamma-irradiated cells. The instability is expressed in bone marrow cells of the radiation-induced acute myeloid leukemia (r-AML) susceptible strain (CBA/Ca) but not in mice resistant to r-AML (C57BL/6). Furthermore, crossgenetic experiments show the induction of the instability phenotype requires both the producer and responder cells to be of the susceptible CBA/Ca genotype. Macrophages are the source of the bystander signals, and the signaling mechanism involves tumor necrosis factor-alpha, nitric oxide, and superoxide. The findings show a genotype-dependent chromosomal instability phenotype induced by radiation-induced macrophage-mediated bystander signaling. As the majority of accidental, occupational, and therapeutic exposures to ionizing radiation are partial body exposures, the findings have implications for understanding the consequences of such exposure.

Role of genetic background in induced instability

Genomic instability is effectively induced by ionizing radiation. Recently, evidence has accumulated supporting a relationship between genetic background and the radiation-induced genomic instability phenotype. This is possibly due to alterations in proteins responsible for maintenance of genomic integrity or altered oxidative metabolism. Studies in human cell lines, human primary cells, and mouse models have been performed predominantly using high linear energy transfer (LET) radiation, or high doses of low LET radiation. The interplay between genetics, radiation response, and genomic instability has not been fully determined at low doses of low LET radiation. However, recent studies using low doses of low LET radiation suggest that the relationship between genetic background and radiation-induced genomic instability may be more complicated than these same relationships at high LET or high doses of low LET radiation. The complexity of this relationship at low doses of low LET radiation suggests that more of the population may be at risk than previously recognized and may have implications for radiation risk assessment.