Ancient virus defends koalas against new viral attacks

Date:  August 6, 2018

Source:  University of Illinois College of Agricultural, Consumer and Environmental Sciences

Summary:

A new study in koalas uncovers how virulent retroviruses become harmless bits of 'junk DNA' over time.

The human genome is riddled with endogenous retroviruses -- little pieces of degraded and generally harmless retrovirus DNA passed down through the generations, along with our own genetic information. Because most endogenous retroviruses have been part of our DNA for millions of years, scientists can't explain how they went from their virulent, disease-causing forms to the inert bits of "junk DNA" most of them are today. A new study published today in the Proceedings of the National Academy of Sciences looks to koalas for an answer.

"In humans, the youngest known endogenous retrovirus groups are around 5 million years old. That makes it very hard to tell what happened. But the koala is one of the few species known to have an ongoing invasion of the germline by a retrovirus," says Alfred Roca, one of the authors of the study and an associate professor in the Department of Animal Sciences at the University of Illinois.

Like other viruses, retroviruses first attack from the outside. They enter the body, fuse with cells to release their contents, and insert pieces of their DNA into the genetic code of the host, hijacking the host's DNA-replicating machinery to make more copies of themselves.

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Koala study reveals clues about origins of the human genome

Date:

November 6, 2014

Source:

Institute for Genomic Biology, University of Illinois at Urbana-Champaign

Summary:

Eight percent of your genome derives from retroviruses that inserted themselves into human sex cells millions of years ago. In a recent study, scientists discovered that 39 different koala retroviruses in a koala's genome were all endogenous, which means passed down to the koala from one parent or the other; one of the koala retroviruses was found in both parents.

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New Retroviruses Found in Polar Bear 'Knut' and Panda 'Bao Bao'

July 15, 2013 — Endogenous retroviruses (ERVs) are viruses that at some point in the past inserted themselves into the nuclear genome of a host's germ cell. Once integrated in a germ cell the virus would be passed on from one generation to the next and the endogenous retroviral genome would therefore be inherited to new species that evolve from the original host. 'ERV sequences and fragments make up about eight per cent of the human genome,' explains Professor Jens Mayer from the Department of Human Genetics at Saarland University. Endogenous retroviruses are found not only in humans, but also in other mammals such as horses, cattle, apes, koalas and, as has now been shown, in polar bears and giant pandas.

Working in collaboration with Professor Alex D Greenwood and Kyriakos Tsangaras from the Leibniz Institute for Zoo and Wildlife Research in Berlin, Jens Mayer has been taking a closer look at the DNA sequences from polar bears and great pandas. 'We have characterised endogenous retroviral sequences in both bear species and found a strong similarity between the two, which indicates that these two virus species are closely related,' says human geneticist Mayer. The researchers also identified ERV sequences in other bear species such as the brown bear, the black bear and the spectacled bear. 'Using molecular dating methods we have now been able to show that the retrovirus became integrated into the genetic material of an ancestor of today's bear species around 45 million years ago,' explains Greenwood. The research team also showed that the original retrovirus was closely related to those found in the genomes of bats and cattle. Interestingly, the viruses found in bears exhibit strong similarity with several endogenous retroviruses found in the human genome. 'Some of these sequences are suspected of playing a role in the occurrence of cancer, neurodegenerative or autoimmune diseases,' says Mayer.

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Genome-Invading Retroviruses Have a Nasty Surprise for Koalas: Serious Pathological Impact for Centuries

ScienceDaily (Sep. 20, 2012) — By integrating themselves into the germ line of their host, retroviruses change the genetic code of their host. The only known case where this process can be currently observed is in Koalas. As an international team of scientists from Australia, Europe and North America just found out, this process may take longer than expected, with the virus continuing to have a serious pathological impact on the host which may go on for centuries.

These findings have been just published online in Molecular Biology and Evolution.

"The process by which a retrovirus invades the host germ line appears to be quite drawn out in this case, so that the koala population has suffered the strongly pathological effects of the virus for many generations during the process of retroviral endogenisation," said Alfred Roca of the University of Illinois at Urbana-Champaign, one of the authors of the study. Koala retrovirus is associated with Chlamydia infection and leukemia in modern koala populations. Given the similarity between ancient and modern koala retrovirus (KoRV) sequences, ancient KoRV would have caused similar pathologies.

Continued:  http://www.sciencedaily.com/releases/2012/09/120920135313.htm