Oncology is the area of medicine that deals with the diagnoses, treatment, and prevention of cancer. Thanks to dedicated oncologists, rates of lung cancer and melanoma have declined rapidly in the past twenty years.
This is due to improved preventative measures such as reducing alcohol and tobacco consumption (links to these carcinogens weren’t obvious until scientists made the connections), better diagnostic tools like blood tests that can detect DNA abnormalities allowing for the earlier identification of cancer, and improved treatments like Image Guided Radiation Therapy (IMGT) that can target tumors without damaging neighboring tissues.
Comparative Oncology is the field of study that translates animal cancer research and treatment to humans. This includes the origin and progress of cancers in animals, as well as their treatment and management. In other words, by studying and treating cancers in animal models, we can better understand and treat the cancers that appear in humans.
What is Cancer?
Cells in our body grow, divide, then die before being replaced by new cells. Each cell must undergo “programmed cell death”, also known as apoptosis. When cells skip this process, they divide uncontrollably and spread to other parts of the body. This is characteristically known as cancer.
Cancer is caused by damage to genes (segments of DNA that code for proteins), resulting in misshapen and malfunctional proteins. Normally, our bodies have a multitude of effective processes to repair damage and ensure cells die when they’re supposed to, such as cancer suppression genes.
While these mutations don’t always result in cancer, if errors occur in a gene that regulates programmed cell death or that code for proteins that repair DNA damage and suppress cancer, tumors will take hold, grow, and spread.
Cancer Affects Different Sized Animals in Different Ways
Cancer doesn’t affect every animal the same way, and Comparative Oncologists are taking lessons from the multitude of ways different species confront cancer.
Since every cell has a chance of becoming a tumor, you might just think that having more cells in the body would increase your chances of developing tumors. However, large mammals have surprisingly low rates of cancer. In fact, humans die of cancer 2-5 times more often than elephants. This revelation was so surprising to the scientist who made the discovery, Richard Peto, that he named it Peto’s paradox.
Between Species and Within
And it’s especially astonishing since if we look within species there is a correlation between size and rates of cancer. A 25-year-long observational study of men found that incidents of cancer increased consistently with height, even when factoring in lifestyle influences like smoking and drinking. And a study conducted by the University of Georgia found that the biggest breeds of dogs (despite how they look, all dogs are part of the canine species) have the highest rates of cancer compared to smaller breeds. But if you look across species, from tiny mice to gargantuan elephants, you find no correlation between the incidents of cancer and the size of an animal’s body.
Why Do Bigger Animals Have Lower Rates of Cancer?
One reason proposed for these lower cancer risks is that larger animals have slower metabolisms and rates of cell division, reducing the chances of cancers.
Another hypothesis is that since size affects incidents of cancers within a species, but not between species, scientists believe that animals that evolved to be larger on average developed greater methods of suppressing and avoiding cancer. For instance, elephants have twenty copies of the tumor suppressor gene TP53 whereas humans and most other mammals have a single copy.
Some scientists have begun testing these genes in smaller species. A study published in Nature found that mice who were genetically altered to express a more active version of the PT53 tumor suppressor gene had reduced instances of cancer. However, let’s not all have our DNA altered just yet.
This study found that while these mice had less cancer, they also appeared to age faster. We also might not want to have all of these cancer-suppressing genes as other evidence suggests there are drawbacks such as reduced fertility.
Studying Cancer in Animals
Despite the risks, it’s such discoveries that spur research into Comparative Oncology. Mammals are of particular interest to scientists since we share so much of our anatomy and physiology. After all, many of us know that we share 99% of our DNA with chimpanzees and bonobos, our closest relatives. But not everyone knows that we also share 90% of our DNA with cats, 88% with mice, 84% with dogs, and 80% with cows.
Of these mammals, dogs are the most prevalent in Comparative Oncological research. In fact, dogs are used so often in this field that it is sometimes referred to as Canine Oncology. This is for several reasons; they share physical and physiological similarities to humans, they have a long history of use in drug trials, and—unfortunately—there is an unusually high number of dogs who are diagnosed with cancer each year. A quarter of dogs die of cancer before their third birthday, a rate that is increasing.
Canine cancers also share features with human cancers. They appear anatomically similar and behave biologically in similar ways, they attack the same molecular targets, share similar tumor genetics, and respond to the same therapeutics. In fact, at least ten cancer drugs have been developed in the past decade using cancer research intended for man’s best friend.
The Future of Comparative Oncology
All this data is coming in from people’s beloved pets being treated with cutting-edge immunotherapies. This information is being collated into data banks that are shared between scientists and doctors around the world.
Such databases inform scientists on how these drugs work, and how canine and human patients respond to treatments. By working with many species, scientists, doctors, and veterinarians are combining forces to better understand the origins and development of cancer and develop novel therapies to treat humans and animals.