## 4 Discussion
### VDR mRNA Expression  
In our study we analyzed VDR, CYP27B1 and CYP2R1 genes involved in vitamin D metabolism. The most interesting observation was the lower VDR mRNA expression (\(p\) = 0.030) in placentas of preeclampsia women. This finding was maintained after stratification of the preeclampsia group into early- and late-onset subgroups, both of which differed significantly from the control group.  
It was also noteworthy that in placentas of the PE group higher mRNA expression of CYP27B1 (\(p\) = 0.006) and CYP2R1 (\(p\) = 0.039) genes compared to the healthy pregnant women was observed. In addition, a reduced placental level of CYP2R1 protein in the PE group (CYP2R1 median 1.32 vs. 1.43 in controls, \(p\) = 0.019) compared to healthy women was noted. This difference was also observed after dividing the whole PE group into early- and late-onset preeclampsia.  
Numerous studies have assessed the gene expression in placenta from preeclampsia women. Our results are consistent with several previous analyses performed in women with preeclampsia and fetal growth restriction (FGR).  
In an interesting study, [PERSON] et al. analyzed the expression of proteins related to VD metabolism--vitamin D binding protein (VDBP), 25-hydroxylase (CYP2R1), 1\(\alpha\)-hydroxylase (CYP27B1), 24-hydroxylase (CYP24A1), and vitamin D receptor (VDR)--in placenta of preeclampsia women and healthy pregnant controls. The expression of VDR and CYP2R1 proteins was lower, while CYP27B1 and CYP24A1 were higher, in the group of women with PE. The authors explain these inconsistent results as an effect of oxidative stress, which could have disturbed the expression of genes related to VD metabolism in the placenta of women with PE [21].  
Less encouraging results were obtained in a study analyzing both methylation and expression of genes related to VD metabolism: VDR, CYP27B1, and RXR. Hypermethylation of DNA for VDR, CYP27B1 and RXR was observed in the placenta of women with PE, assuming that hypermethylation reduces gene expression. Only decreased expression of RXR was observed in the group of preeclampsia women, without statistically significant differences for other proteins [44].  
In contrast to the above-mentioned studies, [PERSON] et al. observed higher placenta VDR mRNA expression and lower VD maternal concentration in women with preeclampsia compared to healthy controls. The authors stated that this may be due to the placental compensation mechanism in conditions of vitamin D deficiency [45].  
No less exciting results were obtained in pregnant women with FGR, which is a condition associated with impaired invasion of the placenta and repeatedly occurs in preeclampsia. In the study by [PERSON] et al., the VDR mRNA expression in placenta was reduced in FGR pregnancies compared to the healthy control group. In addition, this study also attempted to assess the influence of VDR mRNA expression on the trophoblast function. For this purpose, BeWo cell lines were used, where VDR mRNA inactivation led to increased levels of markers for cell differentiation, adhesion and apoptosis, and in this way to uncontrolled premature maturation and trophoblast apoptosis [46]. Further studies by [PERSON] et al. showed that decreased VDR mRNA expression in the placenta may lead to disturbed expression of genes regulating the cell cycle and directly lead to placenta insufficiency with FGR and PE development [47]. Furthermore, it was found that vitamin D deficiency in the mother may be connected with lower VDR mRNA expression in the placenta in FGR and PE women, which could be associated with the trophoblast persistence capacity in these disorders [48].