Fetal membrane activation at labour includes the upregulation of proinflammatory genes. We have found previously that the promoters of key inflammatory genes in the amnion are marked by the activating histone methylation H3K4me3 and the repressive histone methylation H3K27me3. This suggested that the promoters were bivalent and the labour-associated regulation was epigenetic. Here we explored these possibilities further and determined the expression of histone modifying enzymes that control histone methylation levels.
Amnion was collected in early pregnancy (16-18 weeks) and at term after elective caesarean section or following spontaneous labour. Chromatin immunoprecipitation (ChIP) and sequential ChIP (ChIP-reChIP) was used to determine level and co-occurrence, respectively, of the H3K4me3 and H3K27me3 marks at the promoters of the labour-associated inflammatory genes prostaglandin endoperoxide synthase-2 (PTGS2), bone morphogenetic protein-2 (BMP2) and nicotinamide phosphoribosyltransferase (NAMPT). Histone methyltransferase and demethylase expression was determined by quantitative RT-PCR.
There was a significant rise in the H3K4me3:H3K27me3 ratio in the proximal promoter of the three genes at term prior to labour, compared to early gestation, due to increased H3K4me3 levels. The ratio decreased at the PTGS2 promoter with labour. Moreover, co-occurrence of the H3K4me3 and the H3K27me3 modifications was found at the PTGS2 and BMP2 promoters throughout gestation. The H3K4-methyltransferases KMT2A-D, 2F and 2G were expressed in the amnion with KMT2F and KMT2G mRNA levels increasing prior to term labour. Both H3K27me3-demethylases were expressed and KDM6B, but not KDM6A, mRNA abundance increased during pregnancy and at term labour. KDM6A mRNA level increased at preterm labour, and H3K27 methyltransferase (EZH2) expression decreased sharply with advancing pregnancy.
Our data strongly suggests that histone-3-K4 and -K27 methylation epigenetically regulate inflammatory gene activity in the fetal membranes at term labour. Bivalent promoters are present and their role in the timing of gene activation during pregnancy is the subject of ongoing work.