Cancer places a tremendous burden on our society, killing more than half a million Americans every year. Fortunately, over the last few decades, scientists have made considerable advances in understanding the causes of different types of cancer and in developing therapeutic options. We now know that cancer cells are basically the body’s own cells that have lost control of their growth programs. Whereas normal cells may occasionally divide and replicate for a while before stopping (for example in wound healing), cancer cells continue to divide, possibly establishing a tumor and migrating to other parts of the body. In general, this loss of growth control has two fundamental causes. First, mutations in the genome may cause the brakes on the cells’ growth program to fail. Anything that damages our DNA can lead to cancer, such as a strong dose of UV light when out in the sun without sunscreen, or other mutagens, such as those found in cigarette smoke. Second, infection with certain viruses and bacteria can induce uncontrolled cell division leading to cancer. A classic example of this is the human papilloma virus which can cause cervical cancer, although this particular virally induced cancer can now be prevented by vaccination.
One of the hallmarks of cancer has been that it’s unique to each individual. In other words, you can’t catch cancer directly from another person since your body’s immune system would recognize the cancer cells as foreign and immediately destroy them, much as it would do with an infectious agent. However, three striking animal examples have shown that this rule of thumb is not hard and fast. These cancers can actually spread from individual to individual, just like an infectious disease, yet are not rejected by their new host.
The first example is a facial tumor disease caused by transmissible parasitic cancer in the Tasmanian devil, a marsupial carnivore native to the island of Tasmania. The disease, which emerged in the 1990s, causes face, neck and mouth tumors which can grow to considerable size and cause death within months. It is thought to be transmitted by biting during fights among the animals; the current rate at which the disease is spreading will result in the extinction of Tasmanian devils in the next few decades. The second example is a genital tumor sexually transmitted in dogs. Known as Sticker’s sarcoma, it is the oldest known tumor cell line. In fact, very recent studies show this cancer first arose in a single dog around 11,000 years ago and has been propagated continuously ever since. The tumor results in lesions in the genitals of both male and female dogs and can grow quite large. However, the tumors generally regress with time and are not usually lethal. The third example of a transmissible tumor is not naturally transmitted. A spontaneously arising cancer emerged in laboratory colonies of Syrian hamsters in the 1960s. Although this cancer was transmitted into new host animals by experimental transplantation, it later emerged that this cancer could also be experimentally transferred between hamsters by mosquitoes.
The key feature of these transmissible tumors is that the cancer cells themselves are transferred, not a cancer-causing virus. So cancer cells must have mechanisms to avoid rejection by their new host’s immune system. Scientists speculate that transmissible cancers can arise when there is a bottleneck in the evolution of a species whereby the number of individuals, and hence genetic variability, is greatly reduced for a while. This temporary reduction in genetic variability may allow cancer cells to not appear foreign and thereby avoid rejection. Much more research needs to be done to understand the underlying biology.
What about transmissible cancer in other animals including humans? There is evidence that transmissible cancers may also occur in newts and fish, and some scientists believe this form of disease is underestimated in wildlife. Yet there is little evidence that transmissible tumors occur in humans. There are cases of some cancers, such as Kaposi’s sarcoma, developing after organ transplantation. However, it is not clear whether this is really due to the specific outgrowth of cancerous donor cells or the introduction of a cancer-inducing virus via the donor cells. Fortunately, even if these transmissible cancers do exist, they are exceedingly rare and only transferred during surgery. So while examples of transmissible cancers highlight the amazing interplay of immune systems and infectious agents, they are not really a concern for us. But the scientific lessons might help researchers better understand much more common, intrinsic cancers that cause so much human suffering.
David L. “Woody” Woodland, Ph.D. is the chief scientific officer of Silverthorne-based Keystone Symposia on Molecular and Cellular Biology, a nonprofit dedicated to accelerating life science discovery by convening internationally renowned research conferences in Summit County and worldwide. Woody can be reached at (970) 262-1230 ext. 131 or firstname.lastname@example.org.