Viral invasion of the koala genome

University of Queensland researchers may have discovered why the koala is susceptible to certain infections and cancers.

Rachael Tarlinton, PhD student from University of Queensland's School of Molecular and Microbial Sciences, and supervisors associate professor Paul Young and Dr Joanne Meers have found the koala genome is currently being invaded by a virus called koala retrovirus (KoRV). They studied the presence of the virus both in captive and wild koala populations throughout Australia.

Retroviruses are a group of RNA viruses that insert a DNA copy of their genome into the chromosomes of a host cell as part of their natural life cycle. Some retroviruses become permanently integrated into the host genome and are passed on from one generation to the next, gradually become inactive over time such that they no longer produce harmful infections. These are referred to as endogenous retroviruses and are widespread in the animal kingdom with most having invaded their hosts many thousands to millions of years ago.

KoRV is still an active virus that appears to have entered the koala genome relatively recently, perhaps within the past 100-200 years.

Young said what surprised researchers was, unlike other endogenous retroviruses of other species which are fairly stable and ancient inserts in the host genome of all members of the species, they found some koala populations were either free of this virus or showed a mixed prevalence.

This finding, combined with high levels of viral activity and variability between individual koalas, suggested the virus was in transition between infectious and endogenous forms.

The work has important implications for the conservation of Australia's koala populations as the research has also shown an association between this virus and a high incidence of cancer in both captive and wild koalas.

The discovery of this virus-host interaction in the wild has additional significance. "While many endogenous retroviruses appear to simply be inactive passengers or constitute "junk' within their host genome, some can be disruptive, with several human cancers being linked to the activity of selected retroviral elements," Young said.

"Coming to grips with how the koala handles this initial viral onslaught may give us insights into the dynamic events that occurred millions of years ago when retroviruses first invaded the human genome."

The findings were published in a recent issue of Nature (Vol. 442, no. 7098).