This work was supported by grants from the National Natural Science Foundation of China (81503074, 81270420, 81470434, and 81670265), Hunan Provincial Natural Science Foundation (14JJ3102
2017JJ2227), China Postdoctoral Science Founda- tion (2014M560647). We especially thank Bing-bing WANG (Assistant Professor, Perinatal Biology Laboratory, Division of Maternal-Fetal Medicine, Rutgers University-Robert Wood Johnson Medical School, USA) for the English language cor- rection.