Maier J, van Steeg H, van Oostrom C, Karger S, Paschke R, Krohn K
III Medical Department, University of Leipzig, Inselstrasse 22, D-04103 Leipzig, Germany.
Endocrinology. 2006 Jul;147(7):3391-7. doi: 10.1210/en.2005-1669. Epub 2006 Apr 20.
Thyroid tumors are a frequent finding not only in iodine-deficient regions. They are predominantly characterized by somatic genetic changes (e.g. point mutations or rearrangements). Because slow thyroid proliferation is a apparent contradiction to a high frequency of tumor initiation, we characterized mutational events in thyroid. First we studied the frequency of certain base exchanges in somatic TSH receptor (TSHR) mutations and determined the spontaneous mutation rate in thyroid and liver. Then we applied different protocols of the comet assay to quantify genomic DNA damage and conducted immunohistochemistry for 8-oxoguanine as a molecular marker for oxidative stress. Among 184 somatic mutations of the human TSHR found in thyroid tumors, C-->T transitions had a unexpectedly high frequency (>32%). The mutation rate in thyroid is 8-10 times higher than in other organs. The comet assay detected increased levels of oxidized pyrimidine (2- to 3-fold) and purine (2- to 4-fold) in thyroid, compared with liver and lung, and a 1.6-fold increase of oxidized purine, compared with spleen. Immunohistochemistry revealed high levels of 8-oxoguanine in thyroid epithelial cells. We have shown a strikingly high mutation rate in the thyroid. Furthermore, results of the comet assay as well as immunohistochemistry suggest that oxidative DNA modifications are a likely cause of the higher mutation rate. It is possible that free radicals resulting from reactive oxygen species in the thyroid generate mutations more frequently. This is also supported by the spectrum of somatic mutations in the TSHR because more frequent base changes could stem from oxidized base adducts that we detected in the comet assay and with immunohistochemistry.
甲状腺肿瘤不仅在缺碘地区很常见。它们主要由体细胞遗传变化（如点突变或重排）所表征。由于甲状腺增殖缓慢与肿瘤起始的高频率明显矛盾，我们对甲状腺中的突变事件进行了表征。首先，我们研究了体细胞促甲状腺激素受体（TSHR）突变中某些碱基交换的频率，并确定了甲状腺和肝脏中的自发突变率。然后，我们应用不同的彗星试验方案来量化基因组DNA损伤，并对作为氧化应激分子标记的8-氧鸟嘌呤进行免疫组织化学检测。在甲状腺肿瘤中发现的184个体细胞TSHR突变中，C→T转换的频率出乎意料地高（>32%）。甲状腺中的突变率比其他器官高8到10倍。与肝脏和肺相比，彗星试验检测到甲状腺中氧化嘧啶（2至3倍）和嘌呤（2至4倍）水平升高，与脾脏相比，氧化嘌呤水平升高1.6倍。免疫组织化学显示甲状腺上皮细胞中8-氧鸟嘌呤水平较高。我们已经证明甲状腺中的突变率极高。此外，彗星试验和免疫组织化学的结果表明，氧化性DNA修饰可能是突变率较高的原因。甲状腺中活性氧产生的自由基可能更频繁地产生突变。这也得到了TSHR体细胞突变谱的支持，因为更频繁的碱基变化可能源于我们在彗星试验和免疫组织化学中检测到的氧化碱基加合物。
