Joint research towards a better radiation protection-highlights of the Fifth MELODI Workshop.

MELODI is the European platform dedicated to low-dose radiation risk research. From 7 October through 10 October 2013 the Fifth MELODI Workshop took place in Brussels, Belgium. The workshop offered the opportunity to 221 unique participants originating from 22 countries worldwide to update their knowledge and discuss radiation research issues through 118 oral and 44 poster presentations. In addition, the MELODI 2013 workshop was reaching out to the broader radiation protection community, rather than only the low-dose community, with contributions from the fields of radioecology, emergency and recovery preparedness, and dosimetry. In this review, we summarise the major scientific conclusions of the workshop, which are important to keep the MELODI strategic research agenda up-to-date and which will serve to establish a joint radiation protection research roadmap for the future.

The role of Trp53 in the transcriptional response to ionizing radiation in the developing brain.

Brain formation results from a series of well-timed consecutive waves of cellular proliferation, migration and differentiation. Acute irradiation during pregnancy selectively interferes with these events to result in malformations such as microcephaly, reduced cortical thickness and mental retardation. In the present study we performed a straight-through cDNA-microarray analysis of the developing mouse brain at embryonic day E13, 3 h after in utero exposure to 50 cGy X-radiation. This dataset was used as an indication of genes involved in different pathways that are activated upon early radiation exposure, and for further evaluation using quantitative PCR (qPCR). Microarray and qPCR data revealed that the main activated pathways in irradiated wild-type embryos are involved in the regulation of a p53-mediated pathway that may lead to cell cycle delay/arrest and increased levels of apoptosis. To define whether the transcriptional radiation response was solely p53 mediated, we analysed the expression of cell cycle regulating genes in a Trp53 null mutant. The modulated expression of cell cycle regulating genes such as cyclins and Cdk genes indicated the induction of a cell cycle arrest, without evidence for the onset of apoptosis. Additional gene-expression studies have shown that various E2F transcription factors may be involved in this event. Together, these results provide a detailed view of the different p53-related mechanisms that are triggered in response to ionizing radiation in the developing brain.

Transcriptional response to ionizing radiation in lymphocyte subsets.

Human lymphocyte subpopulations differ in their cellular responses to ionizing radiation. To shed light on the molecular basis of this effect, we characterized the transcriptional response to 1 Gy X-rays of CD4+ T lymphocytes. Of 18,433 genes tested, 102 were modulated more than 1.5-fold. The majority of the strongly activated genes were p53 targets involved in DNA repair and apoptosis. The expression of three of these genes was further tested by quantitative RT-PCR in lymphocyte subpopulations [CD4+ and CD8+ T, CD19+ B, CD56+ natural killer cells and peripheral blood lymphocytes (PBLs)] from ten adult donors. In contrast to DDB2, TNFRSF10B and BAX were differentially modulated among the subpopulations and the PBLs, being more activated in irradiated CD19+ B and CD8+ T lymphocytes. The level of BAX activation in the various subpopulations correlated with the sensitivity of the cells to radiation, suggesting its possible role in the differential radiosensitivity of hematopoietic cell subsets.

Effect of ionizing radiation on gene expression in CD4+ T lymphocytes and in Jurkat cells: unraveling novel pathways in radiation response.

To better understand at the molecular level the effect of ionizing radiation in leukocytes, the global transcriptional response to X-ray irradiation was studied in human CD4+ T lymphocytes and in Jurkat cells. Microarray analysis performed on freshly isolated human CD4+ lymphocytes 8 h after an LD50 irradiation dose of 1 Gy revealed that out of 13,825 genes, 1084 were modulated more than 1.5-fold. The most strongly up-regulated genes were predominantly p53 targets. In contrast, exposure of the CD4+ T lymphocyte-derived Jurkat leukemic cell line (with no functional p53 gene) to an equivalent LD50 dose (0.5 Gy) induced a partly different and more limited set of genes. Interestingly, this set of genes belonged to the Rho and cytokine signaling pathways regulated by low-dose ionizing radiation.

Molecular aspects of individual radiosensitivity.

Radiation therapy is a clinical treatment modality where ionizing radiation is used to treat patients with malignant neoplasms. The goal is to deliver a measured dose of radiation to a defined volume with minimal damage to surrounding normal tissue, resulting in eradication of the tumor. Radiotherapy is generally given in divided doses or fractionated. Molecular biology methods have enhanced our ability to investigate the response of cells to ionizing radiation. These methods can be applied to tissue-culture systems or to biopsies from patients both to develop a quick and easy way to predict the radiosensitivity of a patient and to understand how cells respond to stress produced by ionizing radiation. In this review we will mainly explain two major mechanisms involved in human individual radiosensitivity: the DNA-damage repair defect mechanism and the DNA-repair signaling via cell cycle checkpoint defect.

Gene expression induced by X-ray irradiation in human blood cell lines.

The response to X-ray irradiation of three different human hematopoietic cell lines originating from T (Jurkat), B (Raji), and promyelocytic (HL60) leukemia was analyzed. The survival after irradiation differed among the three cell lines, with Jurkat cells being the most vulnerable and HL60 being the least sensitive. The profile of gene expression was studied with the microarray technique in both Jurkat and HL60 cell lines. Out of the 13,800 different genes spotted on microarrays, very few genes (<0.5%) appeared to be induced more than 2-fold or repressed more than 2.5-fold in both cell lines.