Mechanisms of RET/PTC Rearrangement in Thyroid Cancer

说明(申请人提供)：电离辐射是一种众所周知的人类致癌物质，与多种癌症有关，包括甲状腺癌、白血病、乳腺癌和软组织肉瘤。辐射致癌的分子机制尚不清楚。已有大量证据支持染色体重排在辐射致癌中的主导作用。由于电离辐射引起双链DNA断裂，可以想象，染色体重排是由辐射产生的、位于细胞核中彼此接近的自由DNA末端错接而直接形成的。在甲状腺癌中，RET/PTC重排通常通过染色体10q的倒位形成，是放射相关肿瘤的分子标志。最近，我们建立了辐射后人甲状腺细胞产生RET/PTC的体外模型。该系统将成为进一步研究辐射诱导的DNA损伤和染色体重排的重要工具。在这个提案中，我们将检验RET/PTC重排是辐射诱导的双链DNA断裂错误重新连接的直接结果的假设。具体地说，我们将表征RET基因区域放射性断裂和重接动力学的频率和频谱，测试重排是否需要一个或两个DNA断裂，并比较辐射和1-131对RET/PTC重排产生的影响。我们还将确定细胞获得重排后的命运，并将表征由各种RET/PTC类型启动的辐射诱导致癌的早期阶段。最后，我们将发现这些重排是否是由于在体外双链断裂修复中发挥核心作用的几个基因的功能改变而易于发生的。然后，我们将测试这些基因的改变在人类人群中辐射诱发甲状腺癌易感性中的作用。这些研究将扩大我们对辐射致癌机制的理解，并为适用于各种领域的辐射风险评估和防护提供重要信息，如体外放射治疗和I-131的临床应用、职业辐射照射、空间探索以及针对涉及核电反应堆或放射性恐怖主义的潜在事故的辐射防护。

DESCRIPTION (provided by applicant): Ionizing radiation is a well-known human carcinogen linked to a variety of cancers including thyroid cancer, leukemia, breast cancer and soft tissue sarcomas. The molecular mechanisms of radiation-induced carcinogenesis remain poorly understood. Significant evidence has been accumulated supporting the dominant role of chromosomal rearrangements in the carcinogenesis initiated by radiation exposure. Since ionizing radiation induces double strand DNA breaks, it is conceivable that chromosomal rearrangements are formed directly by mis-rejoining of free DNA ends produced by radiation and located close to each other in the nucleus. In thyroid cancer, RET/PTC rearrangements, which usually form via an inversion in chromosome 10q, are a molecular signature of radiation-associated tumors. Recently, we have established an in vitro model of dose-dependent generation of RET/PTC in human thyroid cells after exposure to radiation. This system will serve as an important tool for further studies of radiation-induced DNA damage and chromosomal rearrangements. In this proposal, we will test the hypotheses that RET/PTC rearrangement is a direct result of mis-rejoining of double-strand DNA breaks induced by radiation exposure. Specifically, we will characterize the frequency and spectrum of radiogenic breaks and rejoining kinetics in the RET gene region, test if one or two DNA breaks are required for the rearrangement, and compare the effects of irradiation and 1-131 on the generation of RET/PTC rearrangements. We will also determine the fate of cells after they acquire the rearrangement and will characterize the earliest stages of radiation-induced carcinogenesis initiated by various RET/PTC types. Finally, we will find whether these rearrangements are predisposed by altered function of several genes playing a central role in double-strand break repair in vitro. Then, we will test the role of alterations in these genes in predisposition to radiation-induced thyroid cancer in human populations. These studies will expand our understanding of the mechanisms of radiation-induced carcinogenesis and provide important information for radiation risk assessment and protection applicable to a variety of areas such as clinical use of external radiotherapy and I-131, occupational radiation exposure, space exploration, and radioprotection from potential accidents involving nuclear power reactors or radiological terrorism.