8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) and 8-hydroxy-2'-deoxyguanosine (8-OHdG) as a Cause of Autoimmune Thyroid Diseases (AITD) During Pregnancy?

Abstract

The thyroid is not necessary to sustain life. However, thyroid hormones (TH) strongly affect the human body. Functioning of the thyroid gland affects the reproductive capabilities of women and men, as well as fertilization and maintaining a pregnancy. For the synthesis of TH, hydrogen peroxide (H₂O₂) is necessary. From the chemical point of view, TH is a reactive oxygen species (ROS) and serves as an oxidative stress (OS) promoter. H₂O₂ concentration in the thyroid gland is much higher than in other tissues. Therefore, the thyroid is highly exposed to OS. 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) and 8-hydroxy-2'-deoxyguanosine (8-OHdG) are DNA lesions resulting from ROS action onto guanine moiety. Due to their abundance, they are recognized as biomarkers of OS. As thyroid function is correlated with the level of OS, 8-oxodG and 8-OHdG has been taken under consideration. Studies correlate the oxidative DNA damage with various thyroid diseases (TD) such as Hashimoto's thyroiditis (HT), Graves' disease (GD), and thyroid cancer. Human sexual function and fertility are also affected by OS and TD. Hypothyroidism and hyperthyroidism diagnosed in pregnant women have a negative effect on pregnancy as it may increase the risk of miscarriage or fetus mortality. In the case of TD in the mother, fetal health is also at risk - neurodevelopment and cognitive function of the child may be impaired in its future life. This review presents thyroid function in the context of TD during pregnancy. The authors introduce OS and describe oxidative DNA lesions as a crucial marker of thyroid pathologies.

Keywords: DNA damage; deoxyguanosine; oxidative stress; pregnancy; thyroid diseases; thyroid hormones.

Figure 1

Synthesis of TH. TG is synthesized in the endoplasmic reticulum and secreted into the colloid in the process of exocytosis. The active transport of I^- to follicular cells is carried out by sodium-iodide TSH-dependent symporter protein. The Na/I symporter draws 2 Na⁺ and 1 I^- into the cell. I^- are secreted into the colloid by the pendrin transporter. I^-, which is necessary for TH biosynthesis, is oxidized by thyroid peroxidase (TPO) in the presence of high concentrations H₂O₂ and forms molecular iodine (I₂). It reacts with Tyr residues in organification reaction leading to MIT and DIT coupling. They subsequently conjugate to form T4 with 2xDIT combination and T3 with MIT+DIT combination. These reactions also need TPO and H₂O₂. H₂O₂ is necessary for TPO action and is formed in the colloid through NADPH oxidases (DUOX subfamily). Next, TG is transported by endocytosis into the cell where it undergoes proteolysis and releases T3 and T4. TG is “recycled” and used for further TH synthesis. The TH are transported from the cell to the blood by the monocarboxylate transporter (MCT).

Figure 2

Mechanism of ROS production and neutralization in eukaryotic cells. Redox homeostasis of eukaryotic cell is presented. ROS are generated in the cell by enzymatic systems which include endothelial NOS, cyclooxygenase (COX), xanthine oxidase (XOD), P450 enzymes and mitochondrial ETC. eNOS is involved in the generation of O₂^•- and its transformation into ONOO^- with the participation of NO^•. COX, XOD and P450 enzymes also generate peroxide anion. In the VI complex of OXPHOS, a sequential univalent reduction of O₂ occurs, which during the exchange of 4 electrons is reduced to H₂O. However, about 5% of O₂ is transformed to H₂O₂ in a process of electron leakage when only 2 electron reduction occurs. Moreover, extracellular H₂O₂ may enter the cell through membrane transporters e.g. aquaporins (AQP). H₂O₂ in specific conditions is transformed to ^•OH through Fenton reactions. ^•OH is highly reactive and potentially mutagenic to the cell. Antioxidant defense systems include SOD, TRX, GPX, PRX and chemical antioxidants (e.g. vitamins (C, A, E), β-carotene, GSH). Chemical antioxidants inhibit reaction cascades of ROS formation. Enzymatic antioxidants transform ROS into inactive molecules, e.g. H₂O. GSH is treated as a non-enzymatic antioxidant as it is a substrate for the GPX. It reduces H₂O₂ and oxidizes GSH to form glutathione disulfide (GSSG). GSSG is then reduced to GSH by GR. CAT reduces H₂O₂ directly to H₂O. PRX is another enzyme reducing reactive H₂O₂ molecules. PRX becomes its oxidized form (ox-PRX), reducing H₂O₂ to H₂O. PRX is regenerated by TRX which becomes oxidized in this process (ox-TRX).

Figure 3

Formation of 8-oxodG and 8-OHdG. The lesions are generated through interaction between ¹O₂ or ^•OH and G or dG. As a result of ^•OH addition, different radical adducts are formed (1). Subsequently, electron abstraction generates 8-OHdG which undergoes keto-enol tautomerism forming an oxidized product: 8-oxodG. 8-oxodG and 8-OHdG lesions can also be generated by cycloaddition of ¹O₂ into the imidazole ring of dG. This reaction generates (5) which is later rearranged into (6). (1) 2′-deoxyguanosine, (2) 8-Hydroxy-7,8-dihydro-2′-deoxyguanosyl radical, (3) 8-oxodG, (4) 8-OHdG, (5) 4,8-endoperoxide-2′-deoxyguanosine, (6) 8-hydroperoxy-2′-deoxyguanosie.

Figure 4

Overview of oxidative DNA damage generation in thyrocytes. In the thyroid cell, TPO needs H₂O₂ in order to produce TH. H₂O₂ is generated by DUOX1 and DUOX2 which are NADPH oxidases. In the case of TD, extracellular H₂O₂ may cross the cellular membrane. It has a potential of damaging DNA directly or through NOX4 (NADPH oxidase 4) located in the nucleus and endoplasmic reticulum. Moreover, H₂O₂ generated by mitochondria may affect mitochondrial and nuclear DNA. H₂O₂ in specific conditions is transformed to ^•OH, which is highly reactive and mutagenic ROS.