Viruses reproduce by infiltrating living cells and taking over the biological machinery inside. It’s an insidious process that can leave the host with a life-threatening illness, a mild fever, or no ill effects at all. Recent advances in medical science have allowed humans to combat viruses like never before, but a new study from researchers at Rockefeller University shows how our primate ancestors may have waged war on a virus with only the weapon of evolution.
Viruses don’t leave behind fossil evidence, but sometimes their DNA can survive as part of their victims, and that’s where the team went looking in this case. The study focuses on an ancient virus known as HERV-T, which began infecting primates some 32 to 43 million years ago. HERV-T is a retrovirus (just like HIV), which means it carried its genetic material around as RNA. Eukaryotic cells (like ours) are DNA-based, so one of the first things HERV-T did upon gaining access to a cell was turn its RNA into DNA, then it stuffed it into the cell’s DNA to be duplicated.
HERV-T has long since gone extinct, but the researchers were able to find its remains in the genetic material of various primates. The germline cells like fetal cells, sperm progenitors, and eggs that were infected with HERV-T passed the viral genes down over the eons. This is what allowed the team to construct a timeline for the rise and fall of HERV-T, and find out how our ancestors might have killed it.
When HERV-T began popping up in primates around 40 million years ago, it used a protein on host cells called MCT1 to gain access. The virus used a protein called ancHTenv to link up with that protein, like a key in a lock. The team also found a remarkably well-preserved version of that protein hiding in the DNA of primates (including humans), which they’ve named hsaHTenv. It’s not uncommon for organisms to pick up a bit of nucleic acid here and there from viruses, but the nature of this gene suggests some interesting possibilities. The team used the remaining genetic code to reconstruct the ancient virus protein and study its function, which was a scientific first.
Scientists postulate that hsaHTenv was captured by our hominid ancestors’ cells around 19 million years ago. This gene was used by cells to produce the “key” protein of the virus independent of the virus itself. This in turn allowed cells to immunize themselves. Basically, hsaHTenv could bind to MCT1 particles in the cell, preventing them from being added to the cell’s membrane. With no MCT1 on the surface, the virus had no way to infect the cell. In the space of a few million years, HERV-T died out as its pool of hosts shrunk.
Not all researchers are convinced by the data, but such is the nature of science. It’s possible the presence of hsaHTenv in primate genomes is due to some other factor, not its use as a weapon against ancient retroviruses. Still, it’s a fascinating hypothesis that warrants further study.