Cancer in Koalas Linked to Retrovirus Spread Through Genome

(CN) --- Eight percent of your genome is made up of RNA inserted by retroviruses millions of years ago. Research published in the journal Nature Communications on Friday uses the relatively recent infection of koalas with the KoRV retrovirus to understand the relationship between these DNA-altering parasites and the spread of cancer among their hosts.

Retroviruses, like HIV and KoRV, insert their RNA into the DNA of their hosts, and can be passed on through generations. Eventually, the endogenous retroviruses can become fixed and inactive in the population.

Scientists hypothesize that the genomic disruption caused by retrovirus insertion leads to higher rates of cancers. While this idea is supported by studies on lab rats, evidence of this occurring the wild remains difficult to collect.

KoRV began infecting koalas within the last 50,000 years and has slowly spread from northern to southern populations. Today, all koalas in Australia’s Queensland and New South Whales regions are believed to carry the retrovirus. The koala retrovirus lineage is associated with cancers that are common among koalas, including lymphoma which infects 3% of northern koalas and lymphoid neoplasms which infects 7%.

"Each koala carries around 80--100 inherited copies of KoRV in its genome," explained study author Alex Greenwood in a statement. "The genomic locations of most of these are not shared between koalas, indicating a rapid expansion and accumulation of KoRV copies in the population."

Greenwood, the head of the department of wildlife diseases at the Leibniz Institute for Zoo and Wildlife Research in Germany added: "Each time a retrovirus copies and re-inserts itself into the genome, it causes a mutation, potentially disrupting gene expression, which could be detrimental to the host."

Among 10 koalas studied, researchers identified 1,002 infection sites from the retrovirus, an average of 100 per animal. Researchers found retrovirus genes in both healthy tissue and tumors.

The international team of researchers additionally identified “hotspots” in koala genomes, where the retrovirus inserted itself in regions containing genes typically associated with cancer.

“All koalas in the northern population likely carry similar numbers of KoRV-A proviruses with some proviral infection site possibly conferring a higher cancer risk than others," the researchers found.

"Although other factors may also contribute to cancer in koalas, the mutational burden from endogenous KoRV likely increases the frequency of tumorigenesis and may potentially shorten the time for cancer to develop,” the study concludes.

Tissue was collected from koalas that were checked into wildlife treatment centers for illnesses like tumors and wasting. In addition to assigning identification numbers to koalas, many veterinary teams also name the animals. This study included tissues samples from Ralph, Elise, Lisa, Donovan, Butler, Kathy, Bonnie and Buster as well as Bilbo.

The University of Queensland Animal Ethics Committee approved the sampling of tissue from the koalas. Researchers from Germany, the United Kingdom, Australia, and the U.S. contributed to this study, which was funded in part by the U.S. National Institute of General Medical Sciences and the National Institutes of Health.

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