Downregulation of Syncytin-2 expression in preeclamptic placentas is associated with DNA hypermethylation of the downstream CpG-rich region.

Syncytin-2 is an endogenous retroviral envelope protein constitutively expressed in human placental trophoblasts. As a membrane glycoprotein, Syncytin-2 together with Syncytin-1 mediates the fusion of mononucleated cytotrophoblasts to form multinucleated syncytiotrophoblasts. Syncytiotrophoblasts constitute the fetal-maternal interface important for fetal-maternal exchange, barrier and endocrine functions of the placenta. Besides the fusogenic function, Syncytin-2 also possesses an immunosuppressive activity. In this study, the results of quantitative PCR indicated that Syncytin-2 expression was downregulated in third-trimester preeclamptic placentas, which is consistent with the result of previous studies. Importantly, the results of Combined Bisulfite Restriction Assay (COBRA) suggested hypermethylation of the downstream CpG-rich region, but not the promoter/exon1/intron1 and exon2 CpG- rich regions of SYN-2 gene in third-trimester preeclamptic placentas. Subsequent bisulfite conversion and PCR amplification, cloning and sequencing of the downstream CpG- rich region confirmed hypermethylation of the 4 CpGs in this region in preeclamptic placentas. Moreover, treatment of human choriocarcinoma BeWo cells with DNMT inhibitor ADC (5-aza-deoxycytidine) resulted in a dose-responsive demethylation of the downstream CpG-rich region and an increased SYN-2 mRNA level. Thus, the hypermethylation of the downstream CpG-rich region closely correlated with the downregulation of Syncytin-2 expression in preeclamptic placentas. These new findings underscore the significance of epigenetic alterations in preeclamptic placentas, and facilitate a better understanding on the pathological mechanism of preeclampsia.