Epigenetic regulation and role of LINE-1 retrotransposon in embryogenesis

LINE-1 retrotransposon is the most common mobile genetic element in the genomes of various mammals, including humans. Its genes are represented by the greatest number of copies. For a long time, it has been considered that the presence of LINE-1 in genome reflects the limited ability of cells to eliminate it, and the retrotransposon activity is negative owing to the insertional mutagenesis. In recent years, the increased expression of LINE-1 retrotransposon and the activity of their encoded proteins observed in mammalian cells at different stages of development and, first of all, in early embryogenesis have been discussed in the literature. Is early embryogenesis the stage of development when the organism is more susceptible to the activity of retrotransposons, or does LINE-1 play some positive role in early embryonic development? This review is aimed at classifying the available data on the epigenetic regulation and the role of LINE-1 retrotransposon in embryogenesis of mammals. The link between the mechanisms of regulation of LINE-1 expression and the waves of epigenetic reprogramming is tracked in germ cells, during fertilization, and in blastocyst, as well as during the differentiation of embryonic and extraembryonic tissues.