While human cancers cannot be transmitted from person to person, scientists have recently identified two types of contagious cancers in animals. In Tasmanian devils and domestic dogs, cancer cells have evolved into independent parasites, jumping from animal to animal like an infectious virus.
Cancer cells are, for all intents and purposes, immortal. Having broken free of the rules and strictures that govern other cells, they are free to grow and divide as they please. In a short space of time, a lone cancer cell can form a mass of identical clones – a tumour. Theoretically, cancers could exist indefinitely, but as always, there is a catch. Those that spread quickly and aggressively do so at the expense of their host, who usually ends up dying, taking the tumour with it.
But there is one way a cancer could escape this fate and carry on its selfish reproduction – by finding another host. It could become contagious.
In humans, cancers are definitely not contagious. You can’t catch cancer from someone who has it. At most, you can inherit a higher risk of developing cancer, because of faulty genes passed on from your parents. But recently, scientists have found some startling exceptions to this rule.
Even devils get cancer
Earlier this year, Australian researchers Anne-Marie Pearse and Kate Swift found that a facial cancer plaguing the local Tasmanian devils (right) was caused by contagious cancers.
The condition, known as devil facial tumour disease, is spread when an infected devil bites another. The devils’ boisterous temperaments and their propensity for squabbling over carcasses mean that such bites are common.
Once infected, the animals develop grotesque tumours that stop them from feeding properly, and they usually die of starvation within six months. As a result, the cancer is decimating the already small population.
Going to the dogs
At University College London, Robin Weiss and Claudio Murgia have found another example of an infectious cancer – a disease called canine transmissible venereal tumour (CTVT), or Sticker’s sarcoma.
CTVT is transmitted through sex or close contact between infected dogs. It was first described 130 years ago by a German scientist called Novinski and its origins have been debated ever since. Some scientists suggested that it was caused by a virus, much like human papillomavirus (HPV) causes cervical cancer in humans today.
But in a study published this month, Weiss and Murgia have put these theories to rest. They and their colleagues analysed tumour samples from 40 dogs across five continents. All these samples shared identical and distinctive genetic markers that uninfected tissues from the same dogs did not.
The explanation was clear – these cancers had not developed in the usual way from the cells of the host animals. The cancer cells themselves were spreading from dog to dog.
These rogue cells have become parasites in their own right, evolving from a single ancestor into a dynasty that has colonised the globe aboard canine vessels. How this process began is still a mystery, but Weiss’s analysis provides some hints as to where and when.
The original cancer cell must have developed in either a wolf or an old Asian dog lineage, such as a Husky (left). It evolved anywhere between 200 and 2500 years ago and may well have been around for even longer.
In fact, the CTVT cancer cell is very likely to be the oldest lineage of mammalian cells still in existence. The cells that Weiss is studying today are most probably direct clone descendants of the same cells that Novinski identified 130 years ago – genetically identical great-granddaughters of the original tumour.
To kill or not to kill
When that original cell gained independence, it became truly immortal, long outliving its original body and lasting for centuries. So far, we don’t know of any human cancer cells that have pulled off a similar trick. But Weiss feels that if they did, the best place to look for them would be in people with weaker immune systems including transplant patients and those with HIV.
His group have found evidence that evading the host’s immune defences is a key part of CTVT’s strategy for finding another dog to infect. The cells accomplish this by switching off some key immune system players – a group of genes collectively called dog leukocyte antigens (DLAs). They also secrete a protein called TGF-β1 that strongly blocks any immune responses.
But slipping past immune sentries would do the cells no good if the host died before infecting another dog. Infection requires sex, which may not happen for some time. So CTVT is a merciful parasite.
At the start of infection, it grows rapidly, but within 3-9 months, it regresses of its own accord. By never killing its carrier, the cells ensure that they can spread to as many new hosts as possible.
The Tasmanian devils are not so lucky. Their cancers are spread through biting, a more frequent event than sex, and as such, they can afford to be more aggressive. But the devils’ population is small and their genetic diversity is low. This combination may spell the end for both devils and cancer cells, unless some vaccine can be found.
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