Imagine a deadly disease like cancer spreading almost as easily as the common cold or flu, with the potential to cause an epidemic comparable to HIV/AIDS.
Sounds terrifying, right?
Generally, cancer is contained within one organism, and can metastasize throughout that organism’s body, but does not spread to another being. However, researchers point to a new discovery referred to as ‘transmissible cancer’, cancer that can spread from an organism to another organism like any other contagion. Transmissible cancers have not been documented in humans so far and are exclusive to certain species such as dogs, Tasmanian devils, and soft-shelled clams. In dogs, the cancer is called canine transmissible venereal tumor (CTVT), in Tasmanian devils -devil facial tumor disease (DFTD), and disseminated neoplasia in soft-shelled clams.
CTVT in dogs mainly spreads when dogs mate, but can also be transferred through licking, sniffing or during birth. Similar to HIV/AIDS, this cancer can be classified as a sexually transmitted disease (STD) since it mainly spreads through mating.
DFTD in Tasmanian devils spreads when the marsupials bite each other, which they often do when fighting over mates or food. DTFD is also more widespread among devils than CTVT is among dogs, wiping out almost 95% of devil colonies. Unlike CTVT, which can be cured relatively easily with chemotherapy, DTFD does not respond to chemotherapy and has pushed the species to endangered levels. Because of its alarming rate of spread DTFD can be recognized as an epidemic.
A good way to discover more about a process or thought is to learn about its origin. The origin of such cancers could also potentially point to a viable treatment. The CTVT cancer originated in a “founder dog” around 11,000 years ago, but did not spread extensively until the 15th century, when it presumably was introduced to new lands by European colonizers and explorers traveling with dogs across the Earth, especially to the Americas. Because of its historic origin, CTVT is officially classified as the oldest cancer to exist in nature.
DFTD, discovered in the mid-1990s, originated on the island of Tasmania, south of Australia, and most likely began when a Schwann cell in a single Tasmanian devil metastasized. A Schwann cell is a type of neural cell that protects nerve cells by secreting the lipid myelin as a sheath.
Transmissible cancers like viruses and bacteria also have to deal with the organism’s main defense- the immune system. Usually, pathogens get ousted by the immune response of the organism. On the other hand, the tumor cells have managed to bypass the immune system.
This is achieved by masking the “scent” of the major histocompatibility complex (MHC) proteins. MHC is a set of genes that translate into cell surface proteins through which the immune system can detect pathogens. MHC proteins present antigens, which then attract T-cells to come and destroy the cells. DFTD cells do not express MHC molecules and therefore escape the immune system.
An interesting observation in transmissible cancers is that tumors always contain nearly the same DNA sequence. For example, in CTVT, each tumor in every dog will have the same genes. Of course, the DNA will get slightly mutated when passing from dog to dog, but the major coding segments of DNA remain unchanged. After discovering this anomaly in an already peculiar cancer, a group of researchers from the UK sequenced the genome of two dogs, to try and recreate what the original founding dog might have looked like, possibly leading to an explanation for this unique characteristic. The dogs were chosen from two very different parts of the world, just to make sure they were genetically different.
The researchers declared the reason why this disease has been able to prevail
with limited amounts of mutations is because
it exists in extremely isolated populations.
Because the populations are isolated, most of the dogs become inbred. This results in limited genetic variation in the population, and thus the dogs were more susceptible to diseases as they are all genetically closely related. This is why CTVT was able to persist with minimal mutations.
On a related note, one may be thinking, can’t viruses like Hepatitis C or HPV cause cancer, and since they are viruses that can spread, aren’t these cancers technically transmissible?
An excellent and intriguing proposition, but let’s break it down.
Yes, HPV and Hepatitis C can spread from person to person, but that’s essentially where the similarity ends.
You see, the viruses are transmissible themselves, yes, but cancer itself does not spread. This is why a small percentage of cases of HPV actually progress to the cancer stage. If cancer itself spread, then everyone who got HPV would suffer from cancer, which obviously, is not true. But this does not deny the fact that the majority of cervical cancer cases are caused by HPV, it’s simply the fact that the HPV virus acts as a sort of intermediate between cancers.
Although these diseases can seem as if they’re ravaging the population of their respective species, there is general optimism toward the future populations of dogs and Tasmanian devils. Most dogs do make full recoveries from CTVT and a new study from Dr. Rodrigo Hamede and his group, show that DTFD has the potential to fade out within the next 100 years with Tasmanian Devils developing a resistance to the disease.
Just because transmissible cancers haven’t been documented in humans does not mean we should not worry about it. The studies in dogs and devils provides an insight into the evolution of cancer and how it adapts to its environment. This information will be extremely crucial as we continue our battle against one of the world’s intriguing and deadly diseases.
Author: Srikar
Srikar is a high school sophomore attending Carnegie Vanguard High School in Houston, Texas. He has been fascinated with biology and science in general from a very young age and loves to read about anything science-related. In his free time, he participates in Quiz Bowl-a trivia competition-and loves to play the piano. He has recently indulged in investing, managing a hedge fund he and other classmates fundraised.
Illustrator: Arghya Manna
Arghya Manna
is a comics artist, illustrator, and a Ph.D. dropout. He began his career as a doctoral student at Bose Institute, India. He had been working on Tumor Cell migration in a 3D environment. Along with this, he was an active participant in several projects related to tumor immunology and cancer stem cell. After leaving the lab without bagging the degree Arghya found refuge in art and got involved in drawing comics. He is an enthusiast in History of Science and has been running a blog named “Drawing History of Science”. Arghya wishes to engage the readers of history and science with the amalgamation of images and texts.
Editors: Rajamani Selvam, Arunima Singh, and Roopsha Sengupta
Blog design: Rajamani Selvam
Srikar trained as a summer intern in science writing with Roopsha Sengupta, this article is the outcome of the internship program.
The contents of Club SciWri are the copyright of Ph.D. Career Support Group for STEM PhDs {A US Non-Profit 501(c)3}. (PhDCSG is an initiative of the alumni of the Indian Institute of Science, Bangalore. The primary aim of this group is to build a NETWORK among scientists, engineers, and entrepreneurs).
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