On 24th April 1986, a reactor at the Chernobyl power plant in the Ukraine exploded. It was the worst nuclear accident the world has ever seen.
Radioactive material plumed, contaminating the air, soil and water. Effects are still seen today, 32 years later and hundreds of miles away, where land in parts of Europe is deemed unsafe for farming.
The number of people who died as a result of the disaster may never be known. It is thought that thousands of lives have been lost. Some died from acute radiation sickness immediately after the blast; others, later, from thyroid cancer caused by exposure to radiation.
After the explosion, over 90,000 people were evacuated from hundreds of towns and communities in the vicinity of the plant. They had to leave everything behind, including their pets. The area lay barren for many years as high radiation levels meant nothing could grow.
Chernobyl’s Exclusion Zone
Today, a 200 km chain-link metal fence surrounds the plant – creating an ‘exclusion zone’ stretching for 30km in every direction. Radiation levels have returned to near normal in most places, though certain ‘hot spots’ remain. Forests have re-established themselves, swallowing the abandoned towns and villages.
People have returned to the zone too - some 3,500 work there, as security guards or in offices that, remarkably, still surround the power plant. Animals have also returned, including bears, wolves and smaller mammals. Some animals never left, including the descendants of the pet dogs that people had to leave behind when they fled.
The dogs, resembling German Shepherds, are fed by visitors, workers and the security guards. Despite this kindness, the animals face many challenges. During the harsh winters, they seek shelter in the abandoned buildings. They are hunted by wolves in the forests and exposed to rabies by wild animals. Though puppies seem to thrive, it is hard to find a dog more than a few years old.
Alex Cagan, post-doctoral researcher at the Wellcome Sanger Institute, joined a trip to Chernobyl in June 2018. Run by the Clean Futures Fund (CFF), he travelled with a team of vets to visit the abandoned dogs. He was looking for an unusual type of cancer.
Catching cancer
Transmissible cancer is a strange form of the disease. Unlike any other type of cancer it is not caused by an individual’s own cells growing uncontrollably. It’s an infectious cancer – it’s a cancer dogs can catch.
It first arose in an animal who lived about 8,000 years ago. Cells from this animal, termed the ‘founder dog’, were passed on to other dogs. The cells somehow survive, evading the immune system of new animals, continuing to grow and form tumours.
The tumours are contagious, sexually transmitted, and have spread around the world. Genomic analysis has shown that wherever there are populations of stray dogs - from the deserts of Africa to the Himalayas to the Australian Outback – there are these tumours. Each one carries descendants of the cells from the founder dog.
In essence, the tumour is a parasite, being passed from host to host. Little is understood about its biology – for example no-one knows exactly how it escapes the immune system. Alex is working with Elizabeth Murchison, a group leader at the University of Cambridge who studies transmissible cancers. The aim was to collect samples for genomic analysis – to see if the radiation has any effects on the cancers.
Caring for the dogs of Chernobyl
Clean Futures Fund make several trips a year to care for the animals. The vets check the dogs, treat any injuries and neuter them, to help control the population. Researchers from around the world, studying the unique environment and the effects of radiation on animal populations, join them.
Alex joined the team in the town of Slavutych, just outside the exclusion zone. Travelling by train, and then in an old Soviet truck, they entered the exclusion zone every day, setting up makeshift clinics in old barracks or buildings.
Despite the 30 degree summer heat, everyone entering the zone is required to wear long sleeves and trousers to cover as much skin as possible, as a precaution. All visitors are given a small Geiger-counter to wear around their neck – constantly monitoring the radiation levels. There’s no eating or drinking allowed out in the open, to minimise the risk of radiation exposure. People are screened on the way out of the zone to check radiation levels on their body. If they’re too high, the advice is to take a shower.
“I was worried at first,” said Alex. “But I was with radiation experts and they weren’t, so my mind was put at ease. The amount of background radiation there now, in most places, is the same as you’d get taking a trans-Atlantic flight. The risks are really low.”
“You do see the odd anomaly. There was a sink in one of the buildings with a mirror above it. Someone had written ‘Danger: Do not touch sink bowls. Handles ok’. If you pointed your Geiger counter at it, the readings were massive.”
Each person in the team had a clear job. While the dog catchers set off to find the strays, the vets set up the clinics. Alex took tissue samples from the testes of the neutered animals.
The curious case of the contagious cancer
In the 200 dogs he saw over two weeks, Alex didn’t find a single case of transmissible cancer. Elizabeth was surprised to hear the news. “We see transmissible cancer in dogs all around the world. We find it almost everywhere there are free roaming dog populations. We don’t know why the dogs in Chernobyl don’t have it.”
“There are several theories. Most likely it is probably by chance. They are an isolated population, so perhaps they’ve never come across it, or maybe the disease used to be in the population but has now disappeared. This is really just speculation, but it might be something unique about this dog population, perhaps their immune systems are more able to fight it off somehow. But as far as we know there is nothing different about them compared to other Ukrainian dog populations.”
“We know the cancers are very sensitive to radiotherapy – and so the wildest theory is that maybe the exposure to radiation over the years have been protective.”
“The wildest theory is that maybe the exposure to radiation over the years have been protective”
The dog transmissible cancer normally doesn’t grow if its DNA is broken, or exposed to DNA damaging agents. This is good news for any dogs who do have it – it is easily treated with chemotherapy and the vets had doses ready.
“We will probably never know why it’s not there.” Elizabeth is going to keep in touch with CFF, in case the vets do spot any cases.
If they find it, her team is particularly interested in the ‘mutational signature’ of the DNA in the tumour cells. These are patterns of change in a DNA sequence. Anything that causes damage to DNA, like tobacco smoke, or radiation, causes a unique pattern of change. Radiation causes a particular kind of damage to DNA – double stranded breaks.
The team were particularly keen to study cancers that might have been exposed to radiation. Would they be able to spot tell-tale signs of the Chernobyl explosion in the DNA? It would tell them how the cancer responds to DNA damage and different levels of radiation.
From Chernobyl to Tasmania (via Cambridge)
Elizabeth’s research into transmissible cancers continues back in Cambridge. Her goal is to look at the genomic diversity of transmissible cancers around the world. As well as being affected by external agents, the transmissible cancer genome evolves over time, accumulating changes. By tracking these changes, her team is able to construct an evolutionary tree – showing how related each cancer is to another, and when it was passed on.
Her team are also studying the effects of another infectious cancer - Tasmania Devil Facial Tumour Disease. Spread in the animals' saliva when they bite each other's faces, the cancer is a huge threat to the devils. It has decimated their numbers, affecting up to 65 per cent of the population in Tasmania, Australia.
When Elizabeth was based at the Sanger Institute, she sequenced the genome of the Tasmanian devil transmissible cancer. She found that, again, the disease first arose from the cells of a single animal - in this case, a female Tasmanian devil. The animal has been dubbed 'The Immortal Devil', because although she died over 20 years ago, her DNA lives on in the contagious cancer cells she spawned. Elizabeth’s aim is to eliminate the disease.
Mutational DNA Signatures
Alex’s visit was driven by curiosity and he hopes it will deliver some useful insights. He took 20 DNA samples from the dogs for his group’s cancer work back at the Sanger Institute. Their team is interested in mutational signatures – although they will be looking for DNA damage in healthy, non-cancerous cells. DNA analysis of the healthy dogs will also give information about the population as a whole. For example, it may reveal if the dogs have mated with the wolf population. And it may give clues as to why the dogs only live for a few years.
Chernobyl is a unique location for tragic reasons. But it might be able to help Alex’s team find out more about the impact of radiation on the genome. A huge amount is unknown, said Alex. “It is the first time that complete genomes have been sequenced from any animals living in the exclusion zone. We don’t know what we will find.”
Find out more
Alex Cagan is spoke about his work at a 'Genome Lates' event on Friday 7th December 2018 at the Wellcome Genome Campus in Cambridge.
