Our Inner Parasites

Endogenous Retroviruses

The idea that a host’s genes could have come from viruses is almost philosophical in its
weirdness. We like to think of genomes as our ultimate identity. We know who our biological parents
are because they gave us our DNA. In our DNA are not just the instructions for the color of our skin
or our susceptibility to diabetes. Our very nature lurks there. That’s why the idea of cloning is so
abhorrent: no one should have to carry secondhand genes.

But if most of an organism’s genes arrived in its genome in a virus, does it even have a distinct
identity of its own? Or is it just a mishmash of genes, cobbled together by evolution? It’s as if the
world was filled with hybrid monsters, with clear lines of identity blurred away.

Microbiologists have been getting used to the viral roots of the microbes they study for decades
now. And as long as microbes were the only organisms with much evidence of virus-imported genes,
we could try to ignore this philosophical weirdness by thinking of it merely as a fluke of “lower” life
forms. But now we can no longer find comfort this way. If we look inside our own genome, we now
see viruses. Thousands of them.

We have the jackalope to thank for this realization. The myth of the jackalope was one of the clues
that led virologists to discover that some viruses cause cancer. In the 1960s, one of the most intensely
studied cancer-causing viruses was avian leukosis virus. At the time, the virus was sweeping across
chicken farms and threatening the entire poultry industry. Scientists found that avian leukosis virus
belonged to a group of species known as retroviruses. Retroviruses insert their genetic material into
their host cell’s DNA. When the host cell divides, it copies the virus’s DNA along with its own.
Under the certain conditions, the cell is forced to produce new viruses—complete with genes and a
protein shell—which can then escape to infect a new cell. Retroviruses sometimes trigger cells to
turn cancerous if their genetic material is accidentally inserted in the wrong place in their host’s
genome. Retroviruses have genetic “on switches” that prompt their host cell to make proteins out of
nearby genes. Sometimes their switches turn on host genes that ought to be kept shut off, and cancer
can result.

Avian leukosis virus proved to be a very strange retrovirus. At the time, scientists tested for the
presence of the virus by screening chicken blood for one of the virus’s proteins. Sometimes they
would find the avian leukosis virus protein in the blood of chickens that were perfectly healthy and
never developed cancer. Stranger still, healthy hens carrying the protein could produce chicks that
were also healthy and also carried the protein.

Robin Weiss, a virologist then working at the University of Washington, wondered if the virus had
become a permanent, harmless part of the chicken DNA. He and his colleagues treated cells from
healthy chickens with mutation-triggering chemicals and radiation to see if they could flush the virus
out from its hiding place. Just as they had suspected, the mutant cells started to churn out the avian
leukosis virus. In other words, these healthy chickens were not simply infected with avian leukosis
virus in some of their cells; the genetic instructions for making the virus were implanted in all of their