DNA methylation variation in lean and obese placenta
One in five pregnant women is obese in the UK. Obesity in pregnancy is associated with risks of complications for the mother and child. Mechanisms linking maternal obesity with adverse outcomes for the offspring are not entirely understood though recent studies have suggested epigenetic modifications may be important. As the placenta plays a key role in fetal nutrition, metabolism and protection we hypothesized that there would be DNA methylation changes between lean and obese placenta. DNA methylation array (Infinium HD Assay) was carried out on placentas collected from n=31 obese (BMI>40 kg/m2) and n=29 lean (BMI<25 kg/m2) women. Three genes miR-411, HSD17B4 and FABP1, with false discovery rate (FDR) adjusted P<0.05, and potentially relevant to maternal obesity, gestational diabetes or fat metabolism after literature review, were selected for validation of DNA methylation by pyrosequencing and measurement of mRNA levels by RT-qPCR. The mean (sd) miR-411 and FABP1 DNA methylation percentage was significantly higher in obese placenta vs lean group (68.9 (13.4)% vs 58.9 (15.8)%, P=0.01 and 89.7(2.76)% vs 85.8(7.16)%, P=0.01, respectively), in accord with the array findings. There were no differences in HSD17B4 DNA methylation between groups. MiR-411 DNA methylation was significantly higher in samples from non-smoking obese vs non-smoking lean (70.42(9.6)% vs 58.63(17.4)%, P=0.02). MiR-411 DNA methylation percentage was highest in placentas from male babies born to obese mothers. There were no significant differences in mRNA levels of miR-411 between obese and lean groups and there were no correlations between methylation levels and mRNA levels of miR-411 in either obese or lean placentas (miR-411 obese r=0.21, P=0.32; lean r=-0.32, P=0.14). MiR-411 mRNA levels were significantly higher in current smokers vs non-smokers and ex-smokers (3.61(3.2) vs 1.3(1.2) vs 1.13(0.7), P≤0.05) in all placenta samples. Infant BMI was also positively correlated with mRNA levels of miR-411 in lean but not obese group (lean r=0.636, P=0.003; obese r=0.021, P=0.931). Maternal age was negatively correlated with miR-411 mRNA levels in obese placenta (r= -0.42, P=0.05). We were unable to detect FABP1 mRNA in either lean or obese placenta. Hence, we conclude that there are differences of DNA methylation and gene expression between lean and obese placenta, and these differences are also influenced by maternal environment, fetal sex and infant BMI. The explanation for the lack of association of DNA methylation changes and gene expression changes is not known but may be due to the small magnitude of the DNA methylation changes. Further studies are needed to understand the functional outcomes of these DNA methylation changes. Exploration of the regulation pathway, down-stream genes and function of miR-411 is a potential avenue for future work.