Scientists have long known that a significant portion of the human genome is made up of the remnants of ancient viruses; these sequences are a subset of transposable elements known as endogenous retroviruses. They are estimated to make up about 8% of the human genome. They are a crucial part of evolution and can be found in an array of organisms. For example, they are thought to have been crucial to the evolution of the placenta. It’s been thought that most transposable elements, which once had the ability to move around the genome in a ‘cut-and-paste’ fashion, are now inactive.
But some recent work has revealed more about how some of these transposable elements are reactivated during an extraordinary time for mammalian cells: the earliest stages of development.
Reporting in Cell earlier this year, researchers studied an array of organisms at their earliest stages, including cow, mouse, and rhesus macaque. This effort showed that the ancient viral elements were active and expressed in these embryos. Certain elements seem to control the expression of a huge number of other genes.
“Our research uncovered that transposable element activation is a distinctive feature of early embryos in several mammalian species. This finding is significant because these early-stage cells can differentiate into all body cell types. By understanding how these cells regulate ancient viral elements, we gain crucial insights into the mechanisms of cellular plasticity,” said Professor Maria-Elena Torres-Padilla of Helmholtz Munich, among many other appointments. “This study sets the stage for future research into specific regulatory elements, with broad implications for health, disease, and how manipulating these elements could impact cellular processes.”
Unrelated research reported in Science identified a retroviral protein that is expressed during mammalian development (the researchers used a mouse model in this case). Thisnewly identified retroviral protein seems to be crucial to changes in cellular states that are required for the proper timing and progression of mammalian development.
And recent work reported in Nature has indicated that endogenous retroviruses called HERVK LTR5Hs, which entered the human genome after we diverged from Old World monkeys, are required for proper development. When it was not expressed, blastocysts became nothing but clumps instead of orderly groups of cells that were growing properly. The research showed that HERVK LTR5Hs seem to boost the activity of nearby genes. Blastoid formation is shown in the video above.
Advances in genetic sequencing and analytical technologies will surely continue to reveal more about how ancient viruses have influenced human biology.
Sources: Helmholtz Munich, Cell, Science, Nature
