DNA of ancient virus found in human genome

If you've ever wondered what it is like to be a bornavirus, you may not have to for much longer. A recent study published in Nature found that the bornavirus genome comprises parts of the human genome, and has been a part of our ancestry for the past 40 million years.

The bornavirus is an RNA virus that dwells in the nucleus of a cell. While there, it gains access to the cell's genetic material and inserts its own in order to produce new copies of itself.

In the study, American and Japanese scientists found the bornavirus genome in four locations throughout the human genome. This finding is the first to suggest that the human genome consists of RNA viruses other than retroviruses.

A retrovirus is a virus that stores its genetic information in RNA, a one stranded chain of nucleotides corresponding to DNA. The virus undergoes a process called reverse transcription, in which it synthesizes DNA parallel to its RNA strand. (Normally, RNA is produced from DNA, hence the term reverse transcription). The DNA is then inserted into the host's nucleus, tricking it to produce new retroviruses.

"We now know that a significant portion of our genome consists of genomes of ancient viruses, mainly retroviruses," said Keizo Tomonaga, a virologist from Osaka University and one of the authors of the study.

Retroviruses have been located in 100,000 components of human DNA. Typically, when these viruses are incorporated into a host's genome, the effects are detrimental, as seen in the cases of HIV and tumor-causing viruses.

Yet, there are also retroviruses that can eradicate infection and retroviruses whose genes are now crucial for placental development in primates.

Scientists have been trying to examine retrovirus fossils in order to gain a better understanding of virus evolution. Especially important is studying HIV, with the hope that fossils will help unfold its history.

Virus fossils are formed when a virus unsuccessfully invades a host's cell and essentially becomes trapped inside the cell. If the chosen cell of invasion is a sperm or egg cell, the virus can be transferred to subsequent generations. It is usually unable to multiply and can cause infection due to DNA mutations that occur with time.

As a result of these unsuccessful invasions, scientists are left with generations of fossils from viruses that are stuck in their hosts' genomes. This is known as the fossil virus hypothesis.

In order to test this hypothesis, scientists gathered a group of genetically similar virus fossils from the human genome. By studying each virus's mutations, they were able to decipher the original virus's gene sequence from before the mutations had occurred.

Once they located the original gene sequence, they wanted to test whether this initial, un-mutated virus would still have the power to infect cells.

Surprisingly, when they replicated the initial DNA and injected it into cells, the cells became infected and manufactured new viruses.

By examining retrovirus fossils, scientists have formed a comprehensive view of retrovirus history in relation to human DNA. However, no such history exists for the bornavirus.

"[The study provides] a fossil record of bornavirus that was previously only available for retroviruses" said John Coffin, a virologist at Tufts University School of Medicine, in Wired Science. "It tells us that virus evolution doesn't proceed the way many people have viewed it."

Tomonaga discovered the bornavirus accidentally. He was studying virus genes in comparison to human genes, when he stumbled upon four human DNA segments that evolved from the bornavirus. "I was very surprised when I found them, because I never expected that bornavirus-related genes [existed] in the human genome."

The discovery of the bornavirus is crucial to the understanding virus evolution. More importantly, it may change our understanding of ourselves.

"We now know that a significant portion of our genome consists of genomes of ancient viruses, mainly retroviruses," Tomonaga said.

"Our results suggested that bornaviruses exist at least 40 million years with us. This may indicate a shrewd trick of bornaviruses for survival and also that the battle between viruses and man will never end."