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# Biological responses after low doses X-ray exposure: gene expression and mechanistic studies

(2014)
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## Citation

Chicago
El Saghire, Houssein. 2014. “Biological Responses After Low Doses X-ray Exposure: Gene Expression and Mechanistic Studies”. Ghent, Belgium: Ghent University. Faculty of Medicine and Health Sciences.
APA
El Saghire, H. (2014). Biological responses after low doses X-ray exposure: gene expression and mechanistic studies. Ghent University. Faculty of Medicine and Health Sciences, Ghent, Belgium.
Vancouver
1.
El Saghire H. Biological responses after low doses X-ray exposure: gene expression and mechanistic studies. [Ghent, Belgium]: Ghent University. Faculty of Medicine and Health Sciences; 2014.
MLA
El Saghire, Houssein. “Biological Responses After Low Doses X-ray Exposure: Gene Expression and Mechanistic Studies.” 2014 : n. pag. Print.
@phdthesis{4337625,
abstract     = {The health effects arising from exposure to low doses of ionizing radiation are increasing due to the extensive use of medical and diagnostic applications. One of the central questions in the field of radiation biology is understanding the health consequences of these low doses, where several factors like genetics, life-style, and lack of robust epidemiological studies are impeding well-validated conclusions. Within the context of this PhD, we aim to understand the biological responses after low dose exposures in both in vitro and in vivo studies.
The first part of this PhD is composed of five different chapters that aim to introduce the main objectives of our research and discuss the complexity of low dose research.
Ionizing radiation contributes to the development in many of the sectors like medical diagnosis, therapy, industry, and agriculture. However, exposure to these radiations can carry serious health consequences, like cancer or non-cancer diseases. In Chapter 1, we give a short overview about the physical properties of ionizing radiation and basic concepts of dosimetry. Furthermore, we discuss the main sources of ionizing radiation and the epidemiological health studies that aimed to investigate the effects of ionizing radiation. Moreover, we discuss both deterministic and stochastic effects. On the other hand, we give a thorough overview about the risk assessment models used to estimate the risk after ionizing radiation exposures. In particular, we explain the linear-no threshold (LNT) hypothesis, which is based on linear extrapolation from high to low doses; however, several controversies challenge this hypothesis, mainly due to mechanisms involved in cellular communications involved in the low dose range.
DNA double strand breaks may induce mutations and contribute to the development of cancers if misrepaired or unrepaired. In Chapter 2, we give a general overview about the cellular and molecular mechanisms involved in radiation responses. These include DNA damage signalling and repair. Furthermore, we discuss the different mechanisms involved in radiation-induced cell death, such as apoptosis and senescence.
One of the challenging objectives within the context of low dose studies is the characterization of low dose specific biomarkers that are sensitive and robust. In Chapter 3, we highlight the basic considerations for choosing a biomarker for epidemiological studies. On the other hand, we discuss possible biomarkers that can be of use in the low dose range. DNA double strand break studies, via scoring of the Ser 139 phosphorylated form of the histone H2AX (\ensuremath{\gamma}-H2AX) is one of the most sensitive biomarkers for DNA damage and has been shown to be sensitive for low doses; but also, oxidative stress biomarkers must be investigated as well. Along with DNA damage studies, transcriptional and translational changes could provide {\textacutedbl}rich{\textacutedbl} information about the mechanisms involved in low dose responses, in particular whole genome analysis and cytokine measurement were shown to be reliable and sensitive techniques for radiation low dose research.
Our research has shown that immune responses play a central role after low dose exposures. In Chapter 4, we give an overview about the different immune modulatory responses involved in radiation responses. First, we discuss the radiosensitivity of the different immune blood cells followed by an overview about the basic immune pathways, such as NF-\ensuremath{\kappa}B, MAPK and toll-like receptor (TLR) signalling and their involvement in radiation responses.
The introduction part is complemented with Chapter 5 that discusses the aims of the thesis and the outline of the research.
The second part of this PhD is composed of three different scientific papers that discuss the results combined for this dissertation.
To understand better the differences in responses between low and high doses of ionizing radiation, aliquots of whole blood samples were collected from ten healthy donors and irradiated in vitro with high and low doses of ionizing radiation. Using whole genome analysis and different bioinformatics approaches, we unravelled that low doses of ionizing radiation are characterized by the induction of an immune response by the activation of chemokine and cytokine signalling; furthermore, there they exhibit a pro-inflammatory response due to the activation of NF-\ensuremath{\kappa}B, MAPK and TLR signalling. On other hand, the high dose response is characterized by damage signalling, mainly by the involvement of p53. Several genes belonging to immune and damaging responses were confirmed with quantitative RT-PCR. The results of this study are discussed in Chapter 6.
In an aim to confirm our in vitro whole genome analysis, we investigated the response to low doses of ionizing radiation in vivo. Because samples collected from human would be more reliable than animal studies, we chose prostate cancer patients undergoing intensity modulated radiotherapy (IMRT) as target population. These patients are adequate for low dose studies because they receive a high dose to the tumour; however, large volumes of normal tissues receive low doses. In an attempt to unravel the biological responses, whole blood samples were collected before and after the first fraction of IMRT. DNA double strand breaks and whole genome analysis were performed. Using different bioinformatics analysis, we showed that the main response after low dose exposure is the induction of immune-related response, which is composed of DNA damage and inflammatory responses, growth factor signalling and positive regulation of cell cycle. Immune responses and growth factors were shown to be related to the positive regulation of cell cycle progression; this indicates that cell cycle arrest was not activated; nevertheless, DNA damage signalling was not completed. Using quantitative RT-PCR, various genes belonging to different biological pathways were confirmed. The results of this study are presented in Chapter 7.
Monocytes are central players in the induction of immune response via the secretion of immune-stimulatory cytokines and the activation of immune-related pathways, mainly Toll-like receptors (TLRs), NF-\ensuremath{\kappa}B and mitogen activated protein kinases (MAPKs). In an attempt to confirm the immune-stimulatory nature of the responses combined from both the in vitro and in vivo studies, we investigated the activation of immune pathways in monocytes isolated from human donors. Our results showed that low doses positively regulate the TLR, NF-\ensuremath{\kappa}B and MAPKs signalling; however high doses did show less involvement of these pathways, which might suggest immune-suppressive responses. The results of this study are presented in Chapter 8.
The third part of this PhD thesis comprises the general discussion. It is composed of two different chapters that intend to explain all the results combined from the different studies with conclusions and perspectives.
Taking into account the results presented in Chapters 6, 7, and 8, we provide in Chapter 9 a thorough explanation about the differences between responses after low and high dose exposures. These are divided into immune responses, growth factor signalling, and damage responses. Furthermore, we discuss the involvement of the different pathways, namely MAPKs, NF-\ensuremath{\kappa}B and growth factors in cancer development and inflammation. On the other hand, we discuss the role of NF-\ensuremath{\kappa}B, MAPKs and growth factors as demonstrated bystander effects players. Finally, we give an overview about the different bioinformatics approaches that could be useful for whole genome analysis of low dose responses used in our studies as well as those described in the literature.
In addition to that, in Chapter 10, we conclude that low and high doses of ionizing radiation are characterized by different responses, thus extrapolation is not an accurate approach. Furthermore, we suggest that radiation protection measures and dose reducing techniques should be applied in the medical field. On the other hand, and in an attempt to develop our understanding to low dose responses, we recommend performing mechanistic studies and molecular epidemiology for clear-cut answers concerning low dose health effects. However, we discuss the many challenges that hinder the fast progression of low dose research, mainly because there is a difficulty in choosing a model for biological studies.},
author       = {El Saghire, Houssein},
isbn         = {9789038222981},
language     = {eng},
pages        = {XXXI, 171 + annexe},
publisher    = {Ghent University. Faculty of Medicine and Health Sciences},
school       = {Ghent University},
title        = {Biological responses after low doses X-ray exposure: gene expression and mechanistic studies},
year         = {2014},
}