A virus is attacking koalas genes – but their DNA is fighting back

A virus that infects koalas is steadily integrating itself into their DNA, ensuring that it is passed down from generation to generation. But the koala genome is defending itself, revealing that DNA has its own immune system to shut down invaders.

The virus, called koala retrovirus (KoRV), is linked to weakened immunity, cancer, and chlamydia infection in koalas. All retroviruses hijack the DNA in some cells of their host's body, but not all of them manage to be transmitted to the host's offspring.

Almost all known retrovirus genome invasions happened millions of years ago. However, KoRV is a recently identified exception. The virus spreads between individuals, but is also infecting sperm and egg cells, so many koalas are born with this pathogen as part of their genome.

Findings of team from University of Queensland, published today in Cell, suggest these cells mount a novel "innate genome immune response" to viral infection, which may help control the spread of infectious KoRV.

Within this project, the team analysed DNA and RNA from different tissue samples from deceased wild koalas from South East Queensland. (Like DNA, RNA also contains genetic information about the koalas – but it is also what KoRV's own genome is made of.)

The team specifically looked for short sequences of RNA, between 23 and 35 nucleotides long, known as PIWI Interacting RNAs (piRNAs). Clusters of piRNA sequences are retained within the genome and serve as a kind of memory bank of undesirable sequences – signatures of invading viruses – to be targeted.