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?

Over the past decade, increase in the incidence of TD, including thyroid cancer, has been noted [ 1 ]. TD along with gestational diabetes are the most frequently diagnosed endocrinopathies during pregnancy [ 2 ]. World Health Organization’s research (1994-2006) estimated that about 31% of the world population has insufficient iodine (I - ) intake [ 3 ]. Iodine deficiency (ID) is a common problem in many regions of the world where it concerns a significant part of the population (30% in Southeast Asia, 42% in Africa, 47.2% in the East Mediterranean region and 52% in Europe) [ 4 ]. The situation is different in the US. The estimated average I - intake (138-353 mcg/day) fulfills demand for I - in the US population [ 5 ]. Insufficient I - level causes e.g. hypothyroidism – a condition in which not enough TH is produced for maintaining optimal body function [ 6 ]. The incidence of diagnosed hypothyroidism reaches 5.3% in the European population, while in the US it is 0.3% for clinical hypothyroidism and about 4% for subclinical form [ 7 , 8 ]. Incidence of overt hyperthyroidism in the US and Europe is 0.5% and 0.7%, respectively. However, data show that 1 in 20 people in US, including 1 in 8 women, will develop TD [ 9 ]. I - is used by the thyroid to synthesize TH. Its deficiency may result in the hypothyroidism and negative effects on reproductive functions, pregnancy, lactation, and may impact the fetus. The American Thyroid Association (ATA), American Endocrine Society (AES), and European Thyroid Association (ETA) recommended thyrotropin (TSH) ranges for pregnant women ( Table 1 ). The guidelines are similar for the US and Europe. However, clinical trials from China and India show that ethnicity has an impact on reference values, which are higher than Western guidelines [ 10 - 13 ]. In the case of maternal TD, developmental disorders of the nervous system in the child may occur and cognitive functions may be weakened ( e.g. congenital ID syndrome) during the offspring’s life. In addition, there is a higher risk of miscarriage, stillbirth, mortality, and impaired somatic development [ 14 , 15 ]. The negative effects of TD on the fetus have been confirmed in China in over 1000 pregnant women and their children [ 16 ]. The study shows that maternal hypo- or hyperthyroxinemia increases risk of fetal loss, congenital circulation system malformations, poor vision development, and neurodevelopmental delay. Therefore, it is important to control I - levels, especially in pregnant and lactating women [ 17 ]. Table 2 presents recommended I - intake for US and Europe [ 18 , 19 ]. TD are divided according to its underactive and overactive physiological status [ 20 ]. The two most common autoimmune thyroid diseases (AITD) are HT, a thyroid-degrading inflammation associated with hypothyroidism, and GD, associated with hyperthyroidism and gland enlargement [ 21 ]. AITD are associated with an increase of OS level. 8-oxodG and 8-OHdG are markers of OS due to its frequent formation. Therefore, they are worth considering as diagnostic markers in the field of TD [ 22 - 24 ]. Proper thyroid function is also important for female and male fertility. In women, hypothyroidism causes changes in menstrual cycle length and bleeding, reduces the likelihood of conception, and negatively affects the miscarriage rate [ 25 ]. The topic of female fertility is discussed further in the text. As for men, AITD can cause a decrease in semen quality, sexual behavior, and impotence disorders. Studies indicate that sperm density, morphology, and motility were unsatisfactory in patients with hyperthyroidism [ 26 ]. Hyperthyroidism is also associated with a decrease in testosterone/estradiol ratio which contributes to libido disorders [ 27 ]. Moreover, hypothyroidism appears to be correlated with fewer spermatozoa and their reduced motility, which can significantly affect fertility as the female body may even reject impaired semen [ 25 , 28 - 30 ]. AITD are also associated with male sexual dysfunctions – sexual coldness, erectile dysfunction, or premature ejaculation. According to a study from 2008, 84% of patients diagnosed with hypothyroidism have problems with sexual function [ 25 , 26 , 31 ]. Clearly, poor quality of sperm may affect the success of conception and possibility of pregnancy. Furthermore, it is worth considering that DNA damage such as 8-oxodG (which form more often in AITD) may be present also in sperm’s genetic material. It impairs the quality of sperm and carries the potential of passing on mutated DNA onto a child. In the first part of this review, we present the thyroid gland, its hormones and autoimmune diseases. Next, we describe alterations in thyroid functions during pregnancy and its influence on fertility. The second part focuses on introducing the concept of OS, oxidative DNA damage and its correlation with TH in order to discuss the influence of OS biomarkers (8-oxodG and 8-OHdG) on the TD in pregnant women and to discuss other environmental factors that may impact rate of repair of those lesions in DNA.

Sedentary lifestyle and exposure to toxins predispose to increased OS and might affect ova, sperm, and embryo development [ 98 ]. During uncomplicated gestation, OS is mainly stimulated by the mitochondria-rich placenta and purines metabolism. However, cells correctly neutralize or reduce the negative effects/products of ROS and maintain their concentration at the safe level. ROS seem to play a pivotal role in right placenta development. Study on mice shows that 4-5 days after fertilization, the developing blastocyst produces O 2 •- (>8 nmol/embryo h -1 ) and H 2 O 2 (ca. 4 nmol/embryo h -1 ). Moreover, cytochemical evidence of H 2 O 2 entails the appearance of • OH [ 99 , 100 ]. In the first trimester, level of placental O 2 is low – the embryo is protected from ROS which favors its development, placental angiogenesis, and cell proliferation. At the end of the first trimester, O 2 levels increase due to stabilization of maternal intraplacental circulation. The possibility of normal fetal development is ensured by modulation of hypoxia-inducible factor 1α (HIF-1α) and antioxidant defense systems [ 101 ]. A study by Hung et al . shows that among healthy pregnant women, the urinary level of 8-oxodG increases in the third trimester and returns to physiological level after delivery. Moreover, other biomarkers of OS also increase (GPX and SOD) [ 102 ]. A subsequent study by Hung et al . presents that increased level of OS among pregnant women may implicate pregnancy complications [ 103 ]. Excessive OS may induce polycystic ovary syndrome, endometriosis, preeclampsia, idiopathic infertility, premature birth, recurrent pregnancy loss, and intrauterine growth restriction [ 104 ]. OS plays role in the course of pregnancy and thyroid disorders, therefore it is interesting to explore the scenario where both conditions are present simultaneously. Hence, the next sections attempt to explore this connection of oxidative stress and thyroid disease in pregnant women and also discuss factors which influence their incidence such as diet ( e.g. antioxidant intake). AITD are common endocrine dysfunctions during gestation which affect about 1% of all pregnant women [ 105 ]. H 2 O 2 is a ROS and OS inducer but is also essential for TH synthesis. Studies show that H 2 O 2 production is dependent on I - and TSH levels [ 106 ]. AITD are associated with TSH secretion, caused by ID and affect the H 2 O 2 production in the thyroid gland. H 2 O 2 does not react directly with genetic material but is a precursor of highly reactive compounds: 1 O 2 and • OH [ 107 ]. AITD are correlated with OS and its markers – 8-oxodG and 8-OHdG. These lesions are formed directly in the reaction of G moiety with • OH and 1 O 2 and, in the case of AITD, their accumulation is observed [ 24 ]. Hyperthyroidism is linked with overproduction of OS markers while hypothyroidism is linked with reduced availability of antioxidants [ 23 ]. Connections between AITD and the DNA damage is studied worldwide but still needs more extensive exploration. In 2013, studies assessed the level of 8-OHdG in people with TD. The results show that patients suffering from toxic multifollicular goiter, GD, and HT have a higher concentration of 8-OHdG in urine. For each disease, 8-OHdG is on average 22.26 ng/ml (5.11 ng/ml for the control) [ 108 ]. In addition, plasma 8-OHdG levels (1.23 ng/ml and 0.67 ng/ml in the study and control group, respectively) are useful markers of carcinogenic potential for multinodular goiter. Thyroid nodules occur in 68% of the general population (with 7-15% malignant) thus, testing 8-OHdG plasma levels may improve early diagnosis of thyroid cancer [ 24 ]. Impact of AITD in pregnant women on fetal OS is considered. A study from 2018 shows that subclinical form of hypothyroidism increases the level of OS in amniotic fluid – O 2 •- level increased from 0.1 to 0.2 nmol/10 6 cells. The authors suggest that diet and supplementation of antioxidants may counteract effects of OS [ 109 ]. A different study on pregnant women with clinical hypothyroidism shows that the level of O 2 •- in the amniotic fluid doubles (from 3.5 to 8.0 nmol/mL). Additionally, it describes a positive correlation between O 2 •- and a reduced body weight in women and reduced Apgar scores in newborns [ 110 ]. As suggested by those authors, environmental factors, such as diet and weight, impact the level of OS. In the case of pregnant women, diet and overall health is crucial for the mother and the fetus. However, not only as a mean to the well-nourished child and healthy women, but as it turns out also in terms of maintaining genetic integrity, preventing DNA damage formation, and DNA repair systems operation. DNA damage and its aftermath may cause cancer, neurodegenerative diseases, and others [ 111 ]. The accumulation of DNA lesions induce deregulation of cell functions e.g. DNA replication and transcription, proliferation, or immune response [ 112 ]. Fortunately, cells are equipped with several DNA repair mechanisms including direct repair, excision repairs, and recombination systems [ 113 ]. The fundamental mechanism for oxidative DNA damage repair is the BER system. Fragment of 1-20 nucleobases can be excised from DNA strand depending on the damage type [ 114 ]. The impact of a plant-rich diet on DNA repair mechanisms has been studied for over 20 years. While at first results were contradictory, currently it is known that a diet rich in vegetables and fruit has genoprotective properties and positive influence on DNA repair mechanisms, including BER. Clinical studies also stress the fact that poor dietary choices and subsequent problems, such as obesity, which induces ROS generation, are correlated with the thyroid and its disorders [ 115 ]. As diet provides antioxidants that restore the balance between the formation and removal of ROS it may reduce DNA damage level in the genome [ 116 ]. The most widely considered antioxidant is vitamin C. Its influence on oxidative lesions was identified for the first time in 1998 [ 117 ]. Subsequent years brought new results, where fruit and vegetable consumption and its influence on 8-oxodG, 8-OHdG, and in some cases DNA repair gene expression levels were examined [ 118 - 123 ]. Typically used in such studies are broccoli, kiwifruit, or vitamin C supplements. However, results were contradictory for a long time and hard to compare probably due to testing variable subject groups such as smokers, healthy people, or patients with different pathologies. However, subsequent years brought new insights and confirmed that diet rich in fruit and antioxidants protects from DNA damage and stimulates DNA repair [ 120 , 124 - 128 ]. Moreover, recent in vitro studies seem to confirm the beneficial properties of antioxidants on expression of proteins involved in DNA repair mechanisms ( e.g. GPX, OGG1) [ 119 , 129 - 131 ]. Green tea is also widely considered as beneficial in the field of genome protection. Camellia sinensis increases OGG1 activity after only 7 days of regular intake. Moreover, DNA damage level decreases by 30% just after 1h from drinking the tea [ 129 , 132 ]. According to Lalonde’s theory, who already in 1976 determined that the overall health status of humans depend, in more than 50% on environmental factors, diet seems to be a major factor influencing our well-being [ 133 ]. These facts are especially important for women, as proper dietary choices and healthy lifestyle are crucial before, during, and after pregnancy.

AITD, as a growing health problem of developing countries, became widely investigated in relation to pregnancy and early fetus development as a crucial part of human procreation. Increasing numbers of studies indicate a connection between ROS overgeneration and the regulation of the immune system during pregnancy. During uncomplicated gestation ROS levels are higher, mainly due to mitochondria-rich placenta [ 99 ]. OS is an important factor in predicting complications during pregnancy. It may cause dysfunction in cells and lead to a generation of DNA lesions such as 8-oxodG and 8-OHdG [ 102 ]. Elevated level of these lesions during pregnancy may indicate pathological states e.g. AITD. These disorders may cause complications for mother and fetus. Current evidence shows that patients with GD and HT have elevated levels of 8-OHdG in urine samples [ 108 ]. Considering novel data level of urine 8-OHdG can be an important biomarker of pregnancy complications, including those concerning thyroid. Moreover, increased level of ROS during gestation stimulates dysfunction of the endocrine system. This might induce doubling of O 2 •- levelsin the amniotic fluid or provoke reduced body weight among mothers and reduced Apgar scores in newborns [ 110 ]. Further studies in the field of redox biology are highly demanded. ROS formation and identification of possible cut-off values cannot be yet precisely monitored. If possible, this would help to predict negative consequence of ROS generation and develop personalized treatments. As 8-oxodG and 8-OHdG are connected with AITD, studies should continue to determine their levels in various pathologies. Oxidative lesions may also indicate the condition of pregnant women and fetuses, but more data is needed concerning correlation of newborn health and level of oxidative DNA damage in the mother. In order to improve the care of pregnant mothers, future studies should focus on deepening knowledge about endocrine system dysfunction during pregnancy and possible cut-off values of 8-OHdG and 8-oxodG for later clinical application. We believe it would help to better understand the etiology of AITD, select high-risk patients, and avoid passing on risk of health complications onto a child. Therefore, the oxidative DNA damage as a potential AITD biomarker are worth exploring in order to advance personalized treatment options and early diagnosis, especially for pregnant women. Furthermore, AITD have a direct impact on human reproductive capacity [ 25 , 27 ]. Apart from sex hormone disorders, men experience sexual dysfunction, such as impotence or frigidity [ 26 , 31 ]. In women, AITD result mostly in menstrual disorders as well as limited fertility and/or ability to maintain pregnancy [ 67 , 70 , 71 ]. Information presented in this review allows us to assume that different kind of DNA lesions may be the missing link between AITD and complications of pregnancy, including miscarriages. Therefore, we believe that studies should be undertaken to examine how cellular systems of DNA damage control impact pregnant women with AITD and the condition of the fetus. As mentioned in the previous section, nutrients and antioxidant intake influence to some extent oxidative DNA lesion levels and efficiency of BER mechanism, which in turn may impact all reproductive processes – female and male fertility, conception, pregnancy, and even proper development of the fetus. Recently, more studies confirm a connection between the diet and cellular capacity to prevent and/or repair DNA lesions, including 8-oxodG. Antioxidants present in e.g. broccoli or green tea, are important factors in maintaining cellular redox homeostasis, hence, increasing the defense capabilities of cells. All species are not supposed to be immortal (long living). The main evolutionary goal is to sustain life, reproduce, and pass on the genetic material to ensure the survival of the entire species. In this context, DNA is the most important particle of life and any damage or impairment to its integrity may have severe consequences for the survival of the species. Therefore, it is of high importance that the genome remains intact or properly repaired by specialized systems, so the whole organism (human being) may serve its purpose in evolution. Referring to Lalonde’s theory once more, if 50% of our health depends on environmental factors, and the major factor in everyday life is the diet, we have a vast possibility to influence our own well-being. Due to the fact that both pregnancy and AITD are related to increased DNA damage formation, a healthy plant-rich diet should be integral part of the strategy to protect our organism from destructive influence of OS and oxidative DNA damage. It especially concerns pregnant women with higher risk or already diagnosed AITD [ 134 ].