Venezuelan officials announced this week that they would investigate whether enemies could have deliberately infected late President Hugo Chávez with cancer. Chávez died on March 5, apparently of a heart attack, after battling cancer for two years.
When the former Venezuelan president was diagnosed with an undisclosed form of cancer in 2011, he speculated that his enemies could have given him the disease. He also implied that U.S. agents could have developed a technology to induce cancer, according to a CNN news story at the time. The U.S. State Department called the accusation “absurd.”
The theory that someone could be infected with cancer is not biologically impossible, but it is unlikely. A healthy immune system will combat any foreign cells, including cancerous ones. Only three types of contagious cancers have been identified, and all occur in non-primates.
Scientific American spoke with Katherine Belov, professor of comparative genomics at the University of Sydney who studies a contagious cancer called Tasmanian devil facial tumor disease. She explains why contagious cancers are rare and whether cancer could infect another person.
[An edited transcript of the interview follows.]
What are contagious cancers?
In humans, we know that you can catch viruses, like the human papillomavirus, which make you more likely to get cancer. [HPV can cause cervical cancer in women, and genital warts and anal cancer in men.] In humans, environmental causes play an important role, too—cigarette smoke and radiation exposure can cause cancer. However, we don't have any clear examples of [naturally occurring] transmissible cancers in humans.
There is a transmissible cancer in dogs. It’s a sexually transmitted disease called canine transmissible venereal tumor, or CTVT. And there is also the Tasmanian devil facial tumor disease, which I work on. The devil’s cancer causes large ulcerations in their mouth and around their jaw. When they fight—and they fight a lot—they are biting other animals, and the cancerous cells are implanting in other animals’ wounds.
In both the Tasmanian devils and in the case of CTVT, the tumor evolved in really inbred populations of animals. There was a lack of diversity and so the cancer is genetically very similar to the animals it passes to.
Why does lack of diversity help the cancer jump from animal to animal?
The cancer is transmitted to animals that are genetically similar to one another and also to the tumor. The immune system doesn't "see" it and doesn't mount an immune response. The cancer can then grow until it kills the animal.
Over time the devil’s facial tumor disease would have encountered animals that were genetically dissimilar to it. But the cancer found a way to down-regulate [or produce fewer] cell-surface molecules, which are sort of red flags to the immune system in genetically different animals. These flags are part of the major histocompatibility complex [a set of molecules attached to cells that regulate interactions with immune cells]—they are MHC molecules. Without those special immune molecules the cancer is able to fly under the radar of the immune system and pass from animal to animal.