Activity of Retrotransposons in Stem Cells and Differentiated Cells

Transposable elements (TEs) are major components of mammalian genomes, accounting for approximately half of the human genome. These elements have contributed to the current structure of the human genome in a myriad of ways. Indeed, the activity of TEs is increasingly recognized as a major force in the evolution of eukaryotic genomes. However, TE insertions and TE-mediated genomic rearrangements can become a serious threat to genome stability. TEs have all the characteristics associated with “selfish DNA” due to their inherent ability to mobilize and amplify within genomes. Nonetheless, there are numerous cases where a TE is co-opted by the host to create a new gene or modify genomic regulation. Hence, a new TE insertion could have a mutagenic, neutral, or beneficial effect. In recent years, there has been an emergence of interest in mobile elements in mammals, specially their biology in humans. However, we still have an incomplete picture of their biology and impact. In this chapter, we will focus on a type of human TE, Long INterspersed Elements class 1 (LINE-1 or L1), a widespread distributed retrotransposon capable of mobilization in humans. LINE-1s, as a prototype of selfish DNA, can generate new copies of themselves that can be transmitted to the next generation. Indeed, LINE-1 mobilization during early embryonic developmental stages or in germ cells ensures its transmission to the next generation. Surprisingly, and despite not being a characteristic associated with selfish DNA, there is growing evidence demonstrating that the activity of L1s can further impact the somatic human genome.