World's hardiest animal has evolved radiation shield for its DNA

They are the toughest known animals on Earth and now the secret to one of their superpowers – resistance to radiation – is out.

Tardigrades, also known as water bears or moss piglets, are tiny, eight-legged creatures that live in small bodies of water in habitats such as moss across the planet and are renowned for their extreme survival skills.

They can survive in the vacuum of outer space, withstand temperatures ranging from close to absolute zero to nearly 100°C, cope with pressures six times greater than those at the bottom of the deepest ocean and survive dehydration and being frozen for years on end.

They can also defy hefty amounts of radiation that would be lethal to most other life on the planet – and now we know how they do it.

It is mainly down to a bizarre protective protein they evolved that somehow shields their DNA from radiation damage. Short for “Damage suppressor”, Dsup appears to work by physically cuddling up to DNA and cocooning it from harm, but without disrupting its normal functions.

It may also help by somehow mopping up DNA-damaging agents called reactive oxygen species.

“We guess that Dsup binds densely to DNA to provide a shield against environmental stress, somehow making DNA inaccessible to any damaging agents,” says Takekazu Kunieda at the University of Tokyo. “To our knowledge, this is the first identification of a DNA-associating protein which confers DNA protection and improved tolerance to radioactivity in animal cells.”

Kunieda and his colleagues discovered Dsup after sequencing the genome of Ramazzottius varieornatus, one of the most stress-tolerant tardigrade species. To their surprise, the protein also protected human kidney cells against radiation damage when the cells were genetically engineered to make Dsup themselves.

“The human cells that made Dsup saw a reduction of around 40 to 50 per cent in the DNA damage caused by X-rays compared with control cells,” says Kunieda. This protection disappeared almost completely when his team used RNA to sabotage the Dsup gene, demonstrating that it is the key protective factor.

Kunieda says that transferring the Dsup gene into animals through genetic engineering might increase their resistance to radiation damage, although this would be trickier to do in a whole animal than in lab cultures of individual cells.

“As Dsup improved the radiation-tolerance of human cultured cells, I hope it might be possible to improve the radiation-tolerance of individual animals,” he says.

Applications in the distant future might protect healthy human cells from cancer treatment or cosmic rays. “It could be helpful for space flight, radiotherapy and radiation workers in the far future,” says Kunieda.

But just because more cells could theoretically survive in a Dsup-producing animal, this would not guarantee the animal’s survival, as some vital cells and organs might be lost despite the improved DNA protection, he says.

Trawling through the tardigrade genome, Kunieda also found that the animals have extra copies of other protective genes. They have 16 copies of enzymes that neutralise reactive oxygen species compared with just 10 in most other creatures, and four copies of MRE11 genes that repair DNA instead of the single copy normally found in animal cells.

A 2015 study concluded that tardigrades had scavenged up to one-sixth of their DNA — and many of their protective genes — from bacteria and other organisms by a process called horizontal gene transfer.

It was unclear how they would have done that, but we do know that their DNA breaks into small pieces when they are desiccated, while their nucleus becomes leaky when they rehydrate, perhaps allowing the entry of foreign DNA that could then mix with the water bear’s own genes.

We know horizontal transfer happens occasionally , but most cases involve less than 1 per cent of an organism’s genes. If water bears could really steal such a large proportion of DNA from other organisms, this would change how we think about evolution and the inheritance of genetic material: the tree of life would become a web with genes crossing between branches.

But that evidence was disputed earlier this year — the putative foreign DNA may have just been the result of sample contamination. That study found instead that water bears have only around 1 per cent of foreign genes, as one would expect.

Now, Kunieda’s team have also found that, while some protective genes were imported — such as those producing enzymes that detoxify reactive oxygen species — most were “home-grown”.

“It lays to rest the proposal that tardigrades acquired their extreme survival biology through massive acquisition of genes from other species,” says Mark Blaxter at the University of Edinburgh, UK.

Read more: ‘Water bears’ are first animal to survive space vacuum