36 | New Scientist | 29 February 2020
cancer-causing virus provided the first clues
that viruses can become resident in our DNA.
The discovery began in 1910, when a woman
knocked on his door at the Rockefeller Institute
in New York, clutching her prized Plymouth
Rock hen, which had a tumour called a sarcoma
growing on its chest. Curious about its cause,
Rous transplanted a small piece of the tumour
into other chickens, and found that they
developed a highly invasive cancer – even
when the cancer cells and any accompanying
bacteria were filtered out. The culprit was Rous
sarcoma virus (RSV), a member of a previously
unknown group of viruses called retroviruses,
which insert a copy of their genome into theDNA of the cells they infect. This means they can
reproduce without making infectious particles
that could tip off the host’s immune system –
something other viruses can’t do.
The discovery of retroviruses raised an
intriguing possibility: if one were to infect
a sperm or egg cell (see diagram, page 38),
then viral DNA could be passed from parent
to offspring through successive generations.
Although scientists found no evidence that
this happened with RSV, they soon identified
several other retroviruses tucked away in
the chicken genome. They named these
endogenous retroviruses, because theEnemies within
Ancient invaders hidden in our DNA may cause some
of our most devastating illnesses, finds Carrie Arnold,
suggesting a path to new treatments
S
TRANGE fevers and unusual infections
are common among the people with HIV
who come to Avindra Nath’s clinic for
treatment. But when one young man showed
up in 2005 struggling to move his arms and
legs, Nath was baffled. Although the man had
been diagnosed with HIV a few years earlier, his
new symptoms matched those of amyotrophic
lateral sclerosis (ALS), also known as motor
neuron disease. In an attempt to get his HIV
under control, Nath convinced him to start
taking antiretroviral drugs. Much to everyone’s
surprise, his ALS symptoms improved too.
ALS is caused by progressive deterioration
and death of the nerve cells that control
voluntary movement. What triggers this
destruction is unclear, but recovery is rare.
Puzzled, Nath, who ran an immunology clinic
at Johns Hopkins University in Baltimore,
began searching the medical literature. There
he found other people with HIV and ALS whose
ALS symptoms improved with antiretrovirals –
drugs that stop viruses replicating. Could this
neurological condition be triggered by a
dormant virus hiding in our DNA, brought
back to life by HIV?
This question doesn’t only hover over
ALS. Increasingly, we are waking up to the
possibility that conditions including multiple
sclerosis (MS), schizophrenia and even type 1
diabetes may in some cases be triggered by
ancient viruses buried in our genomes. Under
certain circumstances, they revive and start
producing mutated versions of themselves,
triggering the immune system to attack and
destroy neighbouring tissues.
“It’s a wild new theory of disease,” says
Cedric Feschotte, a molecular biologist at
Cornell University in New York. And already
it is pointing the way to new treatments.
Most viruses are only temporary visitors.
They make us sick, but soon we either get
better or we die. A century ago, however,
biologist Peyton Rous’s discovery of a BRIAN LAROSSAFeatures
“ Viruses that have
buried themselves
in our DNA now
occupy about
8 per cent of our
genome”