电磁辐射中低能电子诱导的DNA簇损伤及其与能量沉积的关联性研究

发布时间：2018-08-24 21:24

【摘要】：在辐射生物效应的研究中,探索DNA损伤与效应之间的关联性是一个重要的研究课题。DNA损伤与效应之间的关联,将辐射生物效应的机理指向了原初的DNA损伤信息,而探索本源的损伤机理,在DNA分子水平上建立细致的损伤信息与辐射终点效应的关联,将具有根本的索源意义。早期基于靶理论提出的L-Q模型建立了一个初略的辐射所致DNA双链断裂数与细胞存活的关联,该模型本质上反映的是剂量和效应的关联。细胞响应阈值模型建立了靶元中能量沉积与辐射终点效应的关联,该模型仍是反映剂量和效应的关联。显然,以上两类关联模型没有将损伤的机理指向原初的DNA损伤谱,不能揭示基本的DNA损伤信息与终点效应的关联性。因此,研究DNA损伤谱与靶元能量沉积的关联性,是搭建辐射生物效应和原初损伤关联的重要环节,是理论上解释辐射生物效应机理的关键,因而具有重要的科学意义。实验和理论研究已表明,几乎所有类型的电离辐射都会产生大量的低能电子(能量低于几keV),也称"δ射线",它们会进一步与生物分子相互作用,使之激发或电离。而DNA分子作为遗传信息的携带者,是最重要的生物靶分子,DNA损伤,可能会导致基因突变、细胞死亡以及其他严重的生物学后果。因此,低能电子诱导的DNA损伤一直是辐射生物学研究的一个重要课题。获得DNA损伤谱是对辐射生物效应机理解释并预测生物效应的第一步。低能电子辐照DNA时,会产生大量不同类型的DNA损伤,包括单链断裂(SSB)、双链断裂(DSB)、碱基损伤以及链断裂与碱基损伤结合形成的簇损伤。DNA簇损伤是辐射所致细胞死亡和突变的关键损伤,被认为是难以修复且对细胞是致命的。因此,DNA簇损伤谱分布的研究具有更重要的生物学意义。然而,由于实验条件和理论计算的局限性,目前有关的研究基本上是针对高能粒子所诱导的简单DNA簇损伤,未能给出各种不同复杂类型DNA簇损伤的定量分析。本文应用Monte Carlo径迹结构模拟法,对包括亚电离电子作用的低能电子诱导的DNA簇损伤及其与DNA靶元和核小体靶元能量沉积的关联性作系统深入的研究。建立了一个更为严格的低能电子在液态水中的径迹结构模拟方法,计算了低能电子诱导的DNA直接损伤谱,定量分析了不同初始能量下,亚电离电子对于DNA单链断裂、双链断裂以及碱基损伤产额的贡献,研究不同类型DNA簇损伤以及核小体DNA损伤与靶元能量沉积的关联性。本文的研究工作,主要包含以下方面的内容和结果:1、论文的第一章,简要介绍了低能电子诱导DNA损伤与能量沉积关联性研究的背景及意义,综述了国内外在该领域的研究现状和研究方法。2、论文的第二章,描述了低能电子与液态水相互作用的两个弹性散射的模型,即Tan模型和Champion模型。前者主要应用基于解相对论性Dirac方程的Mott模型的平均散射截面方法,后者使用解非相对论性Schrodinger方程的分波法,并考虑了液态水的凝聚态相效应。应用Emfietzoglou等发展的基于介电响应理论的光学数据模型计算低能电子在液态水中的非弹性散射,比较研究了基于两个弹性散射模型的低能电子在液态水中径迹结构的模拟,计算分析了液态水凝聚态相效应对表征径迹结构的能量沉积和非弹性散射事件空间分布的影响。结果表明,液态水凝聚态相效应的影响主要发生在较低的电子能量。基于此,并由于电子能量较高时Mott模型考虑了电子的相对论效应,提出并建立了一个更为严格的低能电子在液态水中径迹结构模拟方法。本章建立的模型可为辐射诱导DNA损伤的研究提供更为可靠的电子径迹结构。3、论文的第三章,建立了一个计及亚电离电子作用的低能电子诱导DNA直接损伤谱的模拟方法。尤其,这一方法中对低能电子与DNA各组分(四个碱基:腺嘌呤-Adenine(A)、胸腺嘧啶-Thymine(T)、鸟嘌呤-Guanine(G)、胞嘧啶-Cytosine(C),糖环-sugar moiety和磷酸基团-phosphate group)之间的弹性相互作用,使用了最新的理论计算截面。基于建立的模拟方法,系统地模拟研究了计及亚电离电子作用的低能电子诱导DNA碱基损伤、DNA链断裂及相应的簇损伤,定量分析了亚电离电子对不同复杂类型DNA链断裂和碱基损伤产额的贡献。结果表明:亚电离电子对DNA链断裂产额的贡献约为40-70%,且SSB为最主要的链断裂类型,随着初始能量的增高,SSB的相对产额逐渐增大;双链断裂类型的链断裂所占比重较小,并随着初始能量的升高而减小;亚电离电子诱导的DSB产额比相应的SSB产额要小约230-290%;亚电离电子对DNA碱基损伤产额的贡献约为20-40%,且A-T碱基对的损伤产额要比G-C碱基对的明显的高;由亚电离电子诱导的SSB和A-T碱基对损伤之间具有较强的关联性。本章的结果,尤其是亚电离电子的贡献,为辐射生物效应的研究提供了原初的损伤信息,是研究低能电子诱导的各类DNA簇损伤与能量沉积关联性研究的基础。4、论文的第四章,提出并建立确定六种类型的DNA簇损伤靶单元的方法,将DNA簇损伤分为简单簇损伤和复杂簇损伤两类,前者由每种类型的单链断裂与邻近碱基损伤结合构成,后者包括每种类型的双链断裂与邻近碱基损伤的结合。应用Monte Carlo径迹结构模拟法,系统地模拟不同初始能量下低能电子诱导的DNA簇损伤谱,定量研究简单簇损伤和复杂簇损伤关联的能量沉积分布特征,定量研究能量沉积与DNA簇损伤的关联规律。本章的研究获得了如下的结果:(1)不同初始能量下,总的簇损伤相对产额随能量沉积的变化规律一致,约90%簇损伤的能量沉积分布在约低于150 eV的范围,简单簇损伤为最主要的簇损伤,约占全部簇损伤的90%;(2)不同初始能量下,简单簇损伤的能量沉积分布规律相似,能量沉积主要分布在约低于150 eV的范围,峰值出现在约50 eV处;(3)在考虑的电子初始能量范围内(≤4.5keV),SSB+BD(单链断裂与邻近的碱基损伤结合)簇损伤谱由1个单链断裂分别结合1到5个碱基损伤构成,SSB+BD类型的簇损伤约占简单簇损伤的75-90%。随着碱基损伤数目的增加,SSB+BD簇损伤靶元内的平均能量沉积逐渐增大。碱基损伤数一定,不同初始能量下的SSB+BD簇损伤靶元内的平均能量沉积变化不大,即靶元内的能量沉积主要取决于DNA损伤的复杂性,对初始能量的依赖很小,这是DNA靶元能量沉积与DNA簇损伤关联的一个重要特征。此外,1个单链断裂结合1个碱基损伤是最主要的SSB+BD簇损伤,约占SSB+BD簇损伤总产额的80%,SSB+BD簇损伤复杂性越高,能量沉积越大。(4)在复杂簇损伤中,DSB+BD(双链断裂与邻近的碱基损伤结合)簇损伤占主导地位。在考虑的电子初始能量范围,DSB+BD损伤谱由1个双链断裂分别结合1到5个碱基损伤构成。其中,1个双链链断裂结合1个碱基损伤构成的DSB+BD是最主要的复杂簇损伤,约占全部DSB+BD簇损伤的83%,其平均能量沉积约为106 eV。随着复杂性增加,能量沉积逐渐增大,平均能量沉积亦明显的大,但很难形成。然而,尽管复杂簇损伤的产额很小,但它们的生物效应不可忽略。本章的工作定量地研究了不同复杂性DNA簇损伤与DNA靶元能量沉积的关联性,揭示了相应的关联特征,为辐射生物效应和原初损伤的关联搭建起关键的环节,从而使辐射生物效应机理的研究能够指向原初的损伤谱。5、论文的第五章,建立了核小体的体积模型以及模拟核小体DNA损伤谱的Monte Carlo方法,并提出DNA链断裂关联损伤的概念。应用建立的Monte Carlo方法,模拟获得了核小体靶元中的DNA链断裂关联损伤和DNA簇损伤谱,定量研究了核小体靶元能量沉积与其上的DNA损伤的关联规律,获得了如下的结果:(1)不同初始能量下,核小体靶元DNA链断裂关联损伤的相对产额随靶元能量沉积的变化规律一致,具有DNA链断裂关联损伤的核小体靶元中90%的能量沉积分布在约低于180 eV的范围。(2)简单的单链断裂SSB,是核小体DNA中最主要的链断裂类型,约占全部链断裂产额的80-90%。不同初始能量下,SSB关联损伤的能量沉积分布规律相似,主要分布在约低于180eV的范围,且SSB关联损伤的谱分布峰值出现在约30 eV处。SSB关联损伤中,碱基损伤数为0和1的SSB关联损伤是核小体DNA的SSB关联损伤最主要的损伤类型,约分别占全部核小体DNA的SSB关联损伤的70-90%和10-20%。(3)DSB是核小体DNA最主要的双链断裂类型,约占全部双链断裂产额的85-95%。在所考虑的初始能量范围(≤3keV),核小体DNA发生DSB关联损伤时,结合的碱基损伤数从0到3,所对应的核小体靶元内的平均能量沉积分别为101.86 eV、122.79 eV、159.80 eV和229.28 eV。这表明了结合的碱基损伤数越多,损伤复杂性越高,能量沉积越大。其中,碱基损伤数为0的核小体DSB关联损伤是最主要的DSB关联损伤类型,约占全部核小体DSB链断裂关联损伤的 70-80%。(4)簇损伤是复杂性较高的链断裂关联损伤,其产额很小,占核小体DNA链断裂关联损伤的12.48%。不同初始能量下,SSB+BD簇损伤是最主要的DNA簇损伤。在核小体靶元中,当DNA分别发生简单簇损伤和复杂簇损伤时,核小体靶元的平均能量沉积约分别为112.68 eV和170.88 eV,明显高于相应的链断裂关联损伤的平均能量沉积。本章对于核小体DNA损伤谱与相应靶元能量沉积的关联性研究,为辐射生物效应机理研究提供了相应的理论参考。
[Abstract]:In the study of biological effects of radiation, it is an important research topic to explore the relationship between DNA damage and effect. The relationship between DNA damage and effect points the mechanism of biological effects of radiation to the original DNA damage information, and explores the original damage mechanism, and establishes detailed damage information and radiation end point at the DNA molecular level. The early L-Q model based on target theory established a preliminary association between the number of DNA double strand breaks induced by radiation and cell survival, which essentially reflects the correlation between dose and effect. It is obvious that the two models do not point the damage mechanism to the original DNA damage spectrum and can not reveal the correlation between the basic DNA damage information and the end effect. Experimental and theoretical studies have shown that almost all types of ionizing radiation produce large quantities of low-energy electrons (energies less than a few keV), also known as "delta-rays", which further interact with biological molecules to make them interact. DNA molecule, as the carrier of genetic information, is the most important biological target molecule. DNA damage may lead to gene mutation, cell death and other serious biological consequences. Therefore, DNA damage induced by low-energy electrons has always been an important subject in radiation biology. The first step in explaining and predicting biological effects is to irradiate DNA with low-energy electrons, resulting in a large number of different types of DNA damage, including single-strand breaks (SSB), double-strand breaks (DSB), base damage, and cluster damage resulting from the combination of strand breaks and base damage. However, due to the limitations of experimental conditions and theoretical calculations, the current studies are basically aimed at the simple DNA cluster damage induced by high-energy particles, and fail to provide a variety of complex types of DNA cluster damage. Quantitative analysis. A more rigorous method for simulating the track structure of low-energy electrons in liquid water has been developed by using Monte Carlo method to study the damage of DNA clusters induced by low-energy electrons including subionized electrons and their correlation with the energy deposition of DNA and nucleosome targets. The direct DNA damage spectra induced by low energy electrons were calculated. The contribution of subionized electrons to the yield of DNA single-strand breakage, double-strand breakage and base damage at different initial energies was quantitatively analyzed. The relationship between different types of DNA cluster damage and DNA damage in nucleosomes and target energy deposition was studied. The contents and results are as follows: 1. In the first chapter, the background and significance of the study on the correlation between low-energy electron-induced DNA damage and energy deposition are briefly introduced. The research status and methods in this field at home and abroad are reviewed. 2. In the second chapter, two elastic scattering models of the interaction between low-energy electrons and liquid water, namely T, are described. The former is mainly based on the average scattering cross section method of the Mott model for solving the relativistic Dirac equation, while the latter is based on the wave-splitting method for solving the non-relativistic Schrodinger equation, taking into account the condensed phase effect of liquid water. The inelastic scattering of low-energy electrons in liquid water is calculated. The simulation of the track structure of low-energy electrons in liquid water based on two elastic scattering models is compared. The effect of condensed phase effect of liquid water on the spatial distribution of energy deposition and inelastic scattering events is calculated and analyzed. Based on this, and considering the relativistic effect of electrons in the Mott model when the electron energy is high, a more rigorous simulation method for the trajectory structure of low energy electrons in liquid water is proposed and established. The model established in this chapter can provide more information for the study of radiation-induced DNA damage. In order to obtain a reliable electron track structure, the third chapter of this paper establishes a simulation method for the direct DNA damage spectra induced by low-energy electrons, taking into account the effect of subionized electrons. In particular, this method is applied to the low-energy electrons and DNA components (four bases: adenine-Adenine (A), thymine-Thymine (T), guanine (G), cytosine-Cytosine (G). (C), the elastic interaction between the glycocycle-sugar moiety and the phosphate group, using the latest theoretical calculation cross section. Based on the established simulation method, the low-energy electron-induced DNA base damage, DNA strand breakage and the corresponding cluster damage were systematically simulated and analyzed quantitatively. The results show that the contribution of sub-ionized electrons to the yield of DNA strand breakage is about 40-70%, and SSB is the most important type of strand breakage. Subionization electron-induced DSB yields were about 230-290% less than the corresponding SSB yields; the contribution of subionization electron to DNA base damage yields was about 20-40%, and the damage yields of A-T base pairs were significantly higher than those of G-C base pairs; there was a strong correlation between SSB and A-T base pairs induced by subionization electron and DNA damage. The results of this chapter, especially the contribution of subionized electrons, provide the original damage information for the study of biological effects of radiation, and are the basis for the study of the correlation between various DNA cluster damage induced by low-energy electrons and energy deposition. 4. Chapter 4 of this paper proposes and establishes a method for identifying six types of DNA cluster damage target units, namely, DN. Cluster A damage can be divided into two types: simple cluster damage and complex cluster damage. The former consists of each type of single-strand breakage combined with adjacent base damage, and the latter includes each type of double-strand breakage combined with adjacent base damage. Damage spectra are used to quantitatively study the distribution of energy deposition associated with simple and complex cluster damage, and to quantitatively study the correlation between energy deposition and DNA cluster damage. Deposition is distributed in the range of about 150 eV, and simple cluster damage is the main cluster damage, accounting for about 90% of the total cluster damage; (2) The distribution of energy deposition of simple cluster damage is similar under different initial energies, the energy deposition mainly distributes in the range of about 150 eV, and the peak value appears at about 50 eV; (3) The initial electron energy norm is considered. In the periphery (< 4.5 keV), the damage spectra of SSB+BD clusters consist of one single strand break combined with one or five base damage respectively. The damage of SSB+BD clusters accounts for 75-90% of the damage of simple clusters. With the increase of the number of base damage, the average energy deposition in the damage targets of SSB+BD clusters increases gradually. The average energy deposition in SSB+BD cluster damage target cells at different initial energies does not change much, that is, the energy deposition in the target cells mainly depends on the complexity of DNA damage and has little dependence on the initial energy. This is an important feature of the association between DNA target energy deposition and DNA cluster damage. Injury is the main damage of SSB+BD cluster, accounting for about 80% of the total damage yield of SSB+BD cluster. The higher the damage complexity of SSB+BD cluster, the greater the energy deposition. Among them, DSB+BD composed of one double-strand break and one base break is the most important complex cluster damage, accounting for about 83% of all DSB+BD cluster damage, and its average energy deposition is about 106 eV. Although the yield of complex cluster damage is very small, its biological effects should not be neglected. In this chapter, we quantitatively studied the correlation between DNA cluster damage and DNA target energy deposition, and revealed the corresponding correlation characteristics, which is the key link for the correlation between radiation biological effects and primary damage, thus making the radiation biological effects possible. In chapter 5, the volume model of nucleosome and Monte Carlo method to simulate the DNA damage spectrum of nucleosome are established, and the concept of DNA strand breakage associated damage is proposed. The results are as follows: (1) At different initial energies, the relative yield of DNA strand breakage associated damage of nucleosome target element is consistent with the variation of energy deposition of target element, and 90% of the energy of nucleosome target element with DNA strand breakage associated damage is obtained. (2) Simple SSB is the most important type of strand breakage in nucleosome DNA, accounting for 80-90% of the total strand breakage yield. Under different initial energies, the energy deposition pattern of SSB associated damage is similar, mainly distributed in the range of less than 180 eV, and the peak value of spectrum distribution of SSB associated damage. SSB-related damage is the most important type of SSB-related damage in nucleosome DNA, accounting for 70-90% and 10-20% of SSB-related damage in nucleosome DNA, respectively. (3) DSB is the most important type of double-strand breakage in nucleosome DNA, accounting for 85-95% of the total double-strand breakage yield. In the initial energy range considered (< 3 keV), the number of binding base damage ranged from 0 to 3, and the average energy deposition in the corresponding nucleosome target cells were 101.86 eV, 122.79 eV, 159.80 eV and 229.28 eV, respectively. Among them, nucleosome DSB-related damage with zero base damage is the most important type of DSB-related damage, accounting for 70-80% of all nucleosome DSB-related damage. (4) Cluster damage is a more complex chain breakage-related damage, and its yield is very small, accounting for 12.48% of nucleosome DNA-chain breakage-related damage at different initial energies. The average energy deposition of nucleosome target cells is about 112.68 eV and 170.88 eV respectively, which is significantly higher than that of the corresponding chain breakage associated damage. The study of product correlation provides a theoretical reference for studying the mechanism of biological effects of radiation.
【学位授予单位】：山东大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：Q691

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