Lien DOI – 10.1007/978-94-007-4899-6_17
Witzany G. (eds) Viruses: Essential Agents of Life. Springer, Dordrecht
The content of 5–90 million years old retroviruses and even older retrotransposons of animal genomes and the wide variety of modern retroviruses infecting the same range of species suggest that these elements can be assimilated to shuttle across evolution. A snapshot taken a few decades ago showed us the capture of cellular proto-oncogenes by infectious elements, representing the dark side of the communication between the worlds of viruses and animals. Another snapshot we took more recently shows multiple captures by animal genomes of envelope genes originating from infectious retroviruses, illustrating a phenomenon of convergent evolution. This could be seen as the bright side of these relations as those envelopes were shown to be involved in the earlier steps of human development, i.e. fusion of placental syncytiotrophoblastic layer, therefore they were dubbed Syncytins. Sequencing of more and more animal genomes allowed comparative genomic analyses that revealed how these envelopes have been domesticated in human, mouse, goat, rabbit, etc. More generally, we illustrate in this chapter how close are the viral and animal genome worlds and, focusing mainly on the hominoid ERVWE1 locus encoding Synctin-1, how the different proviruses encoding Syncytins have been domesticated to achieve placental functions. Influence of the chromosomal integration context, the epigenetic control and the splicing strategy upon transcription, and protein maturation processes as well will be discussed in order to illustrate what makes these nowadays genes different from their ancestral infectious counterpart. The price to pay for this beneficial invasion will be illustrated by the possible implications of Syncytin-1 in a wide range of diseases. Last, the apparent stringency of placental regulation will await to be challenged as regard to the evidence of expression in other physiological fusogenic contexts such as myoblasts and osteoclasts.