29 February 2020 | New Scientist | 37>viruses came from within an animal. By the
mid-1980s, we had found them in humans, too.
The advent of genome sequencing in the
1990s revealed just how common these
viruses are. Ever since they first evolved about
500 million years ago, countless retroviruses
have buried themselves in the DNA of their
hosts, to the extent that this ancient viral
material now occupies about 8 per cent of the
human genome. “You have to consider these
viruses as a very, very old thing that happened
to our ancestors millions of years ago,” says
Patrick Küry, a neuroscientist at Heinrich
Heine University Düsseldorf in Germany.
Over the millennia, most of these viral genes
have become so riddled with mutations that
they have become the genetic equivalent of
fossils: inert and semi-degraded. There are a
couple of exceptions. In humans, two families
of retroviruses have been identified that, under
certain circumstances, can reawaken and start
producing small pieces of viral proteins that
can activate the immune system. Not long after
this discovery, signs started to emerge that
these enemies within might be contributing
to some relatively common human diseases.
Some of the first evidence came from people
with MS, an autoimmune condition in which
the body’s own immune cells start attacking
the protective sheath that wraps around nerve
cells, disrupting the messages they transmit.
In 1989, Hervé Perron at the University of Lyon
in France discovered an unknown retrovirus
in brain tissue taken from people with the
condition. Further experiments showed
that the source of this virus was the human
genome itself. Perron initially named the virus
MS-associated retrovirus, but later sequencing
of its genome revealed that it belonged to a
new family of human endogenous retroviruses
(HERVs) that became called HERV-W.
Perron’s work caught the eye of virologist
Antonina Dolei at the University of Sassari
in Sardinia, Italy. She began testing people