Do You Really Think
that your DNA is all human? Think again.
A new discovery
suggests that it's even less human than scientists thought.
Nineteen newly found
pieces of non-human DNA – which were left by viruses infected our ancestors over
hundreds and thousands of years ago – Those Links have just been found, lurking
in between our very own genes.
And one stretch of
newfound DNA, found in about 50 of the 2,500 people studied, contains an
intact, full genetic recipe for an entire virus, say the scientists who
published their findings in the Proceedings of the National Academy of
Sciences.
Whether or not it can
replicate, or reproduce, it isn't yet known. But other studies of ancient virus
DNA have shown it can affect the humans who carry it.
In addition to finding
these new stretches, the scientists also confirmed 17 other pieces of virus DNA
found in human genomes by other scientists in recent years.
The study looked at
the entire span of DNA, or genome, from people from around the world, including
a large number from Africa -- where the ancestors of modern humans originated
before migrating around the world. The team used sophisticated techniques to
compare key areas of each person's genome to the "reference" human
genome.
Working at Tufts
University and the University of Michigan Medical School, the researchers made
the findings with funding from the National Institutes of Health.
HERV-enly find
The findings add to
what science already knows about human endogenous retroviruses, or HERVs.
That's the name for the ancient infectious viruses that inserted a DNA-based
copy of their own RNA genetic material into our ancestors' genomes. They're
part of the same type of virus that includes the modern human immunodeficiency
virus, which causes AIDS.
Over generations, the
virus-generated DNA kept getting copied and handed down when humans reproduced.
That's how it ended up in our DNA today. In fact, about 8 percent of what we
think of as our "human" DNA actually came from viruses. In some
cases, HERV sequences have been adopted by the human body to serve a useful
purpose, such as one that helps pregnant women's bodies build a cell layer
around a developing fetus to protect it from toxins in the mother's blood.
The new HERVs are part
of the family called HERV-K. The intact whole viral genome, or provirus, just
found was on the X chromosome; it's been dubbed Xq21. It's only the second
intact provirus found to be hiding in human DNA.
In the researchers'
own words: "This one looks like it is capable of making infectious virus,
which would be very exciting if true, as it would allow us to study a viral
epidemic that took place long ago," says senior author and virologist John
Coffin, Ph.D. of the Tufts University School of Medicine. "This research
provides important information necessary for understanding how retroviruses and
humans have evolved together in relatively recent times."
"Many studies
have tried to link these endogenous viral elements to cancer and other
diseases, but a major difficulty has been that we haven't actually found all of
them yet," says co-first author Zachary H. Williams, a Ph.D. student at
the Sackler School of Graduate Biomedical Sciences at Tufts University in Boston.
"A lot of the most interesting elements are only found in a small
percentage of people, which means you have to screen a large number of people
to find them."
"This is a
thrilling discovery," says co-first author Julia Wildschutte, Ph.D., who
began the work as a Ph.D. student in Coffin's lab at Tufts. "It will open
up many doors to research. What's more, we have confirmed in this paper that we
can use genomic data from multiple individuals compared to the reference human
genome to detect new HERVs. But this has also shown us that some people carry
insertions that we can't map back to the reference."
U-M genetics
researcher Jeffrey Kidd, Ph.D., worked with Wildschutte when she was a member
of his laboratory team. "These are remnants of ancient events that have
not been fixed in the population as a whole, but rather happened in the
ancestors of some people alive today," Kidd says. "There have been a
number of examples of other HERVs that insert themselves next to human genes or
near them, and have impact on their expression. We're interested in applying
these methods to find other types of viral or mobile element insertions."
Genetic teamwork
The Michigan team used
methods for characterizing repetitive DNA sequences that Kidd and his team had
developed, while Coffin and Williams used complementary techniques. Wildschutte
is now at Bowling Green State University.
Many of the genomes
they examined were from the 1000 Genomes Project, an international
collaboration. Another set of genomes came from work Kidd and colleagues at
Stanford University had done as part of the Human Genome Diversity Project,
with a focus on DNA samples from African volunteers.
These latter samples
showed more signs of HERVs, in line with the high level of genetic diversity in
African populations. That diversity stems from the longtime stability and
intermixing of the continent's population -- as opposed to other populations in
Europe, Asia and the Americas that stem from specific out-migrations in ancient
times.
Cataloging all the
HERV insertions in humans will require even more scanning of whole human
genomes, which are becoming easier to come by as technology improves and
becomes less expensive. And although intact proviruses lurking in our DNA may
be rare, the impact of other HERV sequences on our health or disease is
probably not.
The research was
funded by the National Institutes of Health (OD009154, CA089441, GM112339) as
well as the American Cancer Society and the F.M. Kirby Foundation.
Story Source:
The above post is
reprinted from materials provided by University of Michigan Health System. Note:
Materials may be edited for content and length.
Journal Reference:
1. Julia Halo Wildschutte, Zachary H. Williams,
Meagan Montesion, Ravi P. Subramanian, Jeffrey M. Kidd, John M. Coffin. Discovery
of unfixed endogenous retrovirus insertions in diverse human populations.Proceedings
of the National Academy of Sciences, 2016; 201602336 DOI: 10.1073/pnas.1602336113
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