Deoxyribonucleic Acid Damage and Spontaneous Mutagenesis in the Thyroid Gland of Rats and Mice

Abstract

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.

Role of H2O2 in RET/PTC1 Chromosomal Rearrangement Produced by Ionizing Radiation in Human Thyroid Cells

“…For many years, H 2 O 2 , considered a DNA genotoxic agent, was suspected to play a mutagenic role in the thyroid gland and to be responsible for the thyroid nodules that were frequently found (24). Several observations strengthen this hypothesis: in mice, the spontaneous mutation frequency is higher in the thyroid gland than in any other organ (25), somatic mutations of the TSH receptor are frequent in human thyroid tissue (24), and thyroid glands of mice transgenic for the mutant a 1B -adrenergic receptor (Tg-a 1B AR mice) that have increased H 2 O 2 generation develop malignant nodules with a high frequency (26). H 2 O 2 causes DNA damage, such as guanine oxidation, single-strand breaks and DSBs leading to mutagenesis and genomic instability.…”

Sexual dimorphism and thyroid dysfunction: a matter of oxidative stress?

“…Maier et al (2006) showed that the rat thyroid gland has a high level of DNA oxidative damage in comparison with other tissues, such as liver, spleen, and lung, indicating that the high frequency of somatic mutations and tumor initiation found in this organ is due to the oxidative environment that thyroid cells are subjected to during their long lives (Maier et al 2006 In fact, any disturbance in ROS generation or consumption will promote ROS accumulation that can contribute to thyroid dysfunction. In this context, it is important to note that the site of ROS formation is closely linked to the type and extent of the damage.…”

Oestrogen action on thyroid progenitor cells: relevant for the pathogenesis of thyroid nodules?

“…However, tissues with a high cell turnover are more sensitive to mutagenesis and other molecular mechanisms that initiate tumour formation, whereas in resting tissues such as the thyroid gland, these mechanisms are less operative. To explain this discrepancy, it has been suggested that free radicals resulting from reactive oxygen species in the thyroid gland generate mutations more frequently (Maier et al 2006).…”