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Reviving ancient viruses
A recent discovery1 of a 30,000 year old giant virus in the permafrost of Siberia marks the fourth such discovery in the last decade. Scientists from the Aix-Marseille Universite published the discovery of Mollivirus sibericum in PNAS, and it was quickly picked up by news agencies and science blogs. The group of scientists will attempt to reanimate the virus, which was found buried at a depth of 100 feet, after determining the host species of the virus. Although the term "giant virus" may be somewhat misleading, it refers to the relative size of these viruses compared to others. Giant viruses can typically be seen with a light microscope, and can be as big as 1.5 µm in size. A typical virus is smaller than 0.2 µm, and an average bacterium is about 5 µm in diameter. M. sibericum is 0.6 µm and has over 500 genes, placing it firmly in the giant virus category. Influenza virus, for example, has only 8 genes.

islandcrisis.net.
The first giant virus to be discovered was Mimivirus2, long thought to be a bacterial parasite of amoebae. Since the days of Pasteur, it was always thought that viruses could only be up to 0.2 microns in size, and anything that passed through a 0.2 micron filter was classified as a virus. Other giant viruses have since joined Mimivirus, including Pandoraviruses and Marseilleviruses. These viruses are actually chimeras – they exchange genetic material with their hosts and other microbes that infect their hosts. Giant viruses have mostly been isolated from algae, flagellates, and amoebae. More recently, scientists from San Diego State University and the Washington University School of Medicine sequenced the virome of human stool3 and found DNA from Mimivirus and Marseillevirus, suggesting that they can be found in humans as well. It isn’t clear if these viruses came from the diet of the human host or whether they are harbored in the gastrointestinal tract. When the same group of scientists studied the virome of healthy blood donors4 without prior filtration, they also found Marseillevirus. They were able to grow the virus in human T-cells, and found circulating viral DNA in the blood, as well as antibodies against viral proteins. Antibodies against Mimivirus have also been reported in cases of pneumonia5.

It is widely debated among scientists whether viruses are really "alive", because they cannot replicate on their own and rely on their host’s replication machinery to make copies of themselves. However, Mimivirus carries genes that encode a "viral factory"6, which is a system that does not contain ribosomes but can copy the viral genome and assemble viral particles. The nucleotides for the genome come from the host’s own DNA that is digested and reassembled into viral sequences. This kills the amoeba host, and releases 10,000 viral particles for every one infectious particle within 16 hours.

Microbes, Molecules, and Mysteries of the Universe

The host of M. sibericum is not known at this time, and the group that discovered the virus is doing experiments to determine the hosts. This will involve reanimating the virus, and there is some controversy in doing that. People are concerned that the virus may be able to infect humans or animals, and that reanimating it may cause an outbreak of the virus in a susceptible population. However, the other viruses that were discovered in the vicinity infect amoeba so it is likely that this virus is similar. This doesn’t mean that all viruses found in the same general area will infect the same host, or that even if a virus does infect a human or animal host that it will be deadly or even cause infection. Poliovirus, for example, causes symptoms in only 0.5-1% of people that it infects. The rest of the 99% of people infected never show any symptoms. Smallpox is an example of a larger virus (it contains ~200 genes) that infects humans and has a mortality rate of about 30%. It has been eradicated since the late 1970s, but there are concerns of it resurfacing from frozen areas that are thawing due to climate change, such as the area in Siberia where M. sibericum was discovered.

The reanimation of M. sibericum will be done in safe laboratory conditions, with many safeguards put in place to prevent any possibility of infection of the scientists that are working on this experiment. It is their hope to learn more about the origin of this virus and others like it, and to better understand the mode of evolution of these viruses. If you have any comments on giant viruses, please share them with us at tech@biolegend.com.
References:
  1. In-depth study of Mollivirus sibericum, a new 30,000-y-old giant virus infecting Acanthamoeba
  2. A Giant Virus in Amoebae
  3. Viruses in the faecal microbiota of monozygotic twins and their mothers
  4. Giant Blood Marseillevirus recovered from asymptomatic blood donors
  5. The 1.2-megabase genome sequence of Mimivirus
  6. Ultrastructural Characterization of the Giant Volcano-like Virus Factory of Acanthamoeba polyphaga Mimivirus
Contributed by Rea Dabelic, PhD.
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