Viruses are tricky, but we humans may be trickier still. Stanford stem cell biologists Vittorio Sebastiano, PhD, and Jens Durruthy-Durruthy, PhD, published a study today in Nature Genetics indicating that the genetic remnants of ancient viral infections that still linger in our genome are essential to early human embryonic development.
As Sebastiano explained in our release:
We’re starting to accumulate evidence that these viral sequences, which originally may have threatened the survival of our species, were co-opted by our genomes for their own benefit. In this manner, they may even have contributed species-specific characteristics and fundamental cell processes, even in humans.
The researchers, who talk about their work in the video above, relied on a new RNA sequencing technique to investigate the expression of what are called long-intergenic noncoding, or lincRNAs. These molecules don't contain protein-making instructions, but instead affect the expression of other genes. They've been implicated in many important biological processes, including the acquisition of a developmental state called pluripotency that is necessary for a fertilized egg to develop into the cells and tissues of a growing fetus.
More from our release:
They identified more than 2,000 previously unknown RNA sequences, and found that 146 are specifically expressed in embryonic stem cells. They homed in on the 23 most highly expressed sequences, which they termed HPAT1-23, for further study. Thirteen of these, they found, were made up almost entirely of genetic material left behind after an eons-ago infection by a virus called HERV-H.
[...] After identifying HPAT1-23 in embryonic stem cells, Sebastiano and his colleagues studied their expression in human blastocysts — the hollow clump of cells that arises from the egg in the first days after fertilization. They found that HPAT2, HPAT3 and HPAT5 were expressed only in the inner cell mass of the blastocyst, which becomes the developing fetus. Blocking their expression in one cell of a two-celled embryo stopped the affected cell from contributing to the embryo’s inner cell mass. Further studies showed that the expression of the three genes is also required for efficient reprogramming of adult cells into induced pluripotent stem cells.
I can't stop marveling at the close ties we have with viruses. It makes me think of the words of Michael Corleone in The Godfather: "Keep your friends close, and your enemies closer." As Durruthy-Durruthy told me, "It’s fascinating to imagine how, during the course of evolution, primates began to recycle these viral leftovers into something that’s beneficial and necessary to our development."
Previously: My baby, my… virus? Stanford researchers find viral proteins in human embryonic cells, Mastermind or freeloader? Viral proteins in early human embryos leave researchers puzzled and Species-specific differences among placentas due to long-ago viral infection, say Stanford researchers
Video by Christopher Vaughan/Stanford Institute for Stem Cell Biology and Regenerative Medicine