Cigarette smoking is a common practice amongst women of a wide range of ages and social statuses [46]. Unfortunately, the exposure to cigarette smoke has been associated with several diseases [35, 47, 48] because of their deleterious effects on carbohydrates, lipids, proteins, cellular structures, or DNA [49, 50], owing to the enormous quantity of free radicals (FRs) that are produced during cigarette burning. Due to their lower reduction potential, guanines are the main adductor of FRs, and such interaction produces 8-OHdG [36]. 8-OHdG level has been used widely as a marker of DNA damage in smokers [51].
As previously mentioned, we determined the plasma levels of 8-OHdG in the mother-child pairs in the smoking and non-smoking groups to determine whether the effect of smoking during the first trimester of pregnancy remains until the end of pregnancy. In this work, we detected levels of 8-OHdG in umbilical blood and plasma similar as reported by other authors [52–54], who employed the same methodology. In addition, we found significantly lower 8-OHdG levels in the smoking women group, consistent with previously reported findings [55, 56]. However, the 8-OHdG levels were significantly lower in the new-borns of the smoking women, contrary to the report of Ebina et al., where the 8-OHdG levels of the new-borns of smoking women were high. Nevertheless, they did not specify the time of smoking during pregnancy [52], which is important since 8-OHdG higher levels have been observed in active adult smoker [57, 58] than in pregnant women [55, 56].
In this work, the smoking women displayed significantly higher consumptions of calories, micronutrients, and macronutrients. One reason is that once the women stopped smoking during the first trimester of pregnancy, the cessation of smoking itself could increase food consumption [59, 60] in two ways: first, nicotine inhibits food desire, and second, cigarette reward is substituted with food-reward (Table 2) [61]. Even though the higher estimated consumption of micronutrients of the smoking group, several studies have associated lower plasma levels of vitamin A [62, 63], vitamin C [62, 64, 65], vitamin E [62, 63, 65], folic acid, and vitamin B12 [66] with smokers, which could explain the lower global metDNA levels as we have proposed (Figure 3). This could be explained by lower absorption rate [67], altered vitamin metabolism [68–70], or increased elimination rate [68, 71] in the smoking women group. However, investigating whether the plasma vitamin levels of the smoking women in our study were deficient is beyond the scope of this work.
Owing to the association of overweight and obesity with oxidative stress in adults and pregnant women [72], and oxidative stress with DNA damage [73], we further analysed the nutritional status of the study groups on the basis of their 8-OHdG levels. Thereby, we found no correlation between 8-OHdG and BMI (Rho = 0.08, p = 0.426), which is in agreement with other studies [40, 56, 58, 74]. Nevertheless, we observed that smoking women had greater food intakes than the non-smoking women (Table 2). This behaviour could provide higher amounts of vitamins with protective or preventive effects on 8-OHdG formation in the foetus. In addition, it may explain the significantly lower level of 8-OHdG in the smoking pair (Table 3), and the inverse correlation of vitamins with antioxidant potential and the 8-OHdG levels (Table 4). On the other hand, we cannot discard the possibility that 8-OHdG works antioxidant in smoking binomials [75], or be due to a buffering effect of the placenta [76–78], which could be additional explanations of the lower levels detected in our smoking binomials.
8-OHdG is a modified nucleotide product of the interaction between guanines and FRs and owing to its structure, 8-OHdG more easily pairs with thymine than with cytosine during DNA replication or deficient repair reversion [36, 73, 79]. The transversion or 8-OHdG expression itself in CpG islands (CpGIs) can inhibit the methylation by DNA methyl-transferase 3a [80, 81] or 3b [82]. Human genomes up to 60% of the CpGIs are located in gene promotors [83] of which 80% are associated with gene expression because cytosine methylation suppresses or decreases gene expression [84, 85].
The placenta connects the child to the mother for imperative functions, including nutritional support, waste elimination, gas transference, hormone secretion and synthesis, immune tolerance to the product, and acts as a barrier against pathogenic agents [86]. Thus, placenta study is important for understanding the mechanisms of deleterious agents. We assessed the overall metDNA percentage on the basis of its association with 8-OHdG and metDNA levels (Figure 3). We found a negative correlation between the percentage of metDNA and the number of cigarettes consumed per day, which could be due to the mechanism we described earlier [87]. Our study was different from other studies that aimed to identify specific differential methylated CpGIs in the placentas of smoking women [29, 88–91]. In this study we observed a significantly lower global methylation status in the placenta. However, other studies did not take into consideration nutritional status, it is important because nutritional status can modify metDNA patterns [92]. One plausible explanation of the oxidative stress-induced significant decrease in metDNA level in the smokers could be that the saturation levels of the antioxidant enzymatic systems such as the superoxide dismutase (SOD) enzyme activities, which is one of the most important antioxidant systems [93], were decreased in smokers as compared with non-smokers [94, 95]. Although, the SOD levels of smokers were normal [96], the levels of their cofactors were high [94]. The cofactors, together with reactive oxygen species, nitric oxide synthase, and metals from tobacco smoke [97, 98], could inhibit SOD enzyme activities in smokers. Although, several studies report that consumption of ≥ 10 cigarettes per day have deep effects [40, 58, 99], we found biological effects of consuming 1 to 5 cigarettes per day until late pregnancy even though women stopped smoking during the first trimester of pregnancy. However, we found that the mothers’ and new-borns’ DNA damage maker levels (8-OHdG) showed no significant correlations with the percentage of metDNA (p > 0.05).
Lastly, studies on epigenetic changes induced by cigarette smoking have associated these changes with the development of several diseases (Figure 3). Therefore, studying the epigenome in new-borns exposed to cigarette smoke in early life stages, even without any pathological evidence, could be useful for identifying new-borns at higher risk of the aforementioned conditions and for establishing preventive health programs focusing on this population.
Limitations: First, we cannot assure women stop smoking during the first trimester of pregnancy since we can only take into consideration what they mentioned during the interview. Second, because of the cross-sectional design of this study, it is hard to determine the levels of 8-OHdG at begin of pregnancy. Third, due to vitamins estimation was determined by the query, it is hard to estimate a direct link among them and the levels of 8-OHdG. Last, we did not have record of the men smoking history since paternal smoking has been associated with health affectations in the offspring [100] .