E. Yin1, M.A. Coleman1, L.E. Peterson2,
L. Tomascik-Cheeseman1, L. Mascio-Kegelmeyer1, and
A.J. Wyrobek1
1. Biology and Biotechnology Research Program, Lawrence Livermore
National Laboratory, Livermore, CA. 94551. 2. Department
of Medicine, Baylor College of Medicine, Houston, TX 77030
Exposure of mammalian cells to high-doses of ionizing radiation (IR) results in a range of tissue, cellular, and genetic defects. But, little is known of the relative cellular mechanisms and responses after low-dose exposures. The objectives of this research were to apply custom and commercial microarray technologies to investigate transcription profiles after low (10cGy) versus high doses (2Gy) of gamma irradiation of mammalian cells and to identify genes and pathways that are associated with low-dose responses to IR. Two model systems were evaluated: brain tissue of mice irradiated in vivo and human lymphoblastoid (HLB) cell lines irradiated in vitro. Brain cells are relatively quiescent and tolerant to irradiation. Using custom microarrays, we found that brain tissue of unirradiated mice had relatively lower transcription levels of cell-cycle and DNA-damage-repair-related genes than other tissues. We hypothesized that genes which show modulated transcription within the first few hours after low-dose IR are pivotal in determining the fate of irradiated cells. We termed such genes: Radiation-modulated, Early Onset, and Sensitive (i.e., REOS genes). Employing larger expression arrays and a variety of statistical and clustering methods, we identified hundreds of genes whose transcript levels were modulated after 10cGy and 2Gy exposures. Four hours after irradiating HLB cells, there were 218 and 332 genes with modulated expression after 2Gy and 10cGy exposures, respectively, of which 58 genes overlapped between the doses. After irradiating mouse brain, there were 556 and 589 genes with modulated expression after 2Gy and 10cGy exposures, respectively, of which 289 genes overlapped between the doses. Examples of REOS genes in HLB cells irradiated at 10cGy include genes involved in cell cycle (MDM2, PAK3, NEK2, PLK-1, CYCLIN G1), cell proliferation (C-MYC, N-RAS), and DNA damage repair (BRCA2). In mouse brain, REOS genes include transcription factors (Egr-1), as well as those involved in a number of pathways closely related to IR response, including UV and heat-shock response (e.g. Chaperonin 10), oxidative stress response (Icam-5), and inflammatory response (Ly-6, membrane type-three matrix metalloproteinase). These findings indicate that: (a) there are differences in baselines of stress-response and DNA repair-related genes among unirradiated mouse tissues, (b) doses as low as 10cGy induce changes in gene expression in both mouse and human cells, (c) there are numerous genes for which transcriptional modulation is limited to low-dose exposure, and (d) that transcriptional profiles vary qualitatively and quantitatively across doses and cell types. [This work was conducted under the auspices of DOE and LLNL under contract W-7405-ENG-48 with support from NIH (ES09117-02) and DOE (KP110202).]