Catching Cancer

Viruses that cause it and a form that can spread from dog to dog
Cancer is a scary disease. Sometimes it seems to strike people out of nowhere. And sometimes it runs in families. But at least you can't catch it from someone. Or can you? The vast majority of cancers can't be passed to anyone. This is because cancers originate from your own cells gone bad. If your cells are passed to someone else, their body will destroy the foreign cells. That is one of the big advantages of cancer (at least from the cancer's point of view). The cancer can escape the body's immune system because it looks very similar to other cells in the body. This advantage would obviously be lost if the cancer moved to a new body. And yet, there are examples where cancers can be spread. Cervical cancer is one of these. The actual cancer isn't spread but the cause of the cancer, the human papillomavirus (HPV), is. An even weirder example is a cancer found in dogs that can be spread through intimate contact. This is a cancer that can be passed from dog to dog.
A virus that can cause cancer
Cervical cancer has been in the news lately because of a new vaccine that is said to prevent it by protecting women from human papillomavirus (HPV). But how can HPV cause cancer? HPV doesn't cause cancer in the usual way. Most of the over 200 different diseases we call cancer are caused by mistakes in the DNA of a single cell (click here for details). The mistakes can be caused by something in the environment like sunlight or something we eat, breathe, or drink. Or sometimes our cells make a mistake when copying their DNA. Whatever their origin, the changes in DNA cause the cell to grow uncontrollably. HPV works a little differently. To understand how, we need to go over a bit about how DNA changes can turn a cell cancerous. Not all DNA changes cause cancer (thank goodness). So what is special about the one that does? Location, location, location. Where the DNA change happens determines if it will cause cancer. One example is when a DNA change hits a gene whose job it is to keep a cell's growth under control. These genes are called tumor suppressor genes. If a DNA change makes a tumor suppressor stop working, then the cell will become a cancer cell. Luckily we have two copies of most of our genes so just hitting one copy usually won't cause problems (click here to learn more). But if both get hit, then the cell can grow and grow. It is thought that the forms of HPV that cause cervical cancer work by affecting tumor suppressors. These forms of HPV make proteins called E6 and E7 that turn off some tumor suppressors. This has the same effect as a DNA change in the tumor suppressor gene. If this form of HPV is passed to another person, then they may develop cancer as well. But most likely not. It is estimated that 75-80% of the adult population has been infected by some form of HPV. And there are around 10,000 new cases of cervical cancer in the U.S. each year. What this means is that very few infected people end up with cancer. The reason for this is unclear. Certainly some of it has to do with the fact that most forms of HPV don't make E6 or E7. Also, many people clear the virus before any harm can be done. In fact, people usually need to have an unresolved infection for a decade or more before the cancer appears. So we don't know why so few people develop cervical cancer when infected with HPV. But I don't want to minimize the importance of the vaccine. Even though only a small percentage of infected people end up with cervical cancer, lots of people have the virus. Which translates to 10,000 new cases of cervical cancer each year. Any vaccine that will bring those numbers down is a breakthrough.

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This virus, HPV, can
cause cancer
Infectious cancer
So one way to catch cancer from someone is by getting a form of HPV that can cause cervical cancer. But you aren't getting the cancer from the person. You are getting the cause. This is more like the second hand smoke from a smoker rather than the smoker's lung cancer. But there is a cancer that can actually move from dog to dog through intimate physical contact. Now that is a scary thought. Scientists weren't sure if canine transmissible venereal tumor (CTVT) worked through a virus like HPV or spread on its own. They found the answer by looking at the DNA of the cancer cells. If the disease was caused by a virus, then the cancer cells of one dog would look different than the cancer cells of another dog. The DNA in cancer cells from a German Shepherd would look like German Shepherd DNA. And the DNA in cancer cells from a Schnauzer would look like Schnauzer DNA. But if the cancers all came from the same source, then the Schnauzer cancer would look like the German Shepherd cancer. So scientists looked closely at cancers from 16 unrelated dogs from five different continents. And the DNA was all very similar. In fact, it looked like an older Asian breed like a husky. Or maybe a wolf. DNA testing isn't good enough yet to distinguish between the two. But it is good enough to tell that these cells all looked very similar. And that they looked very different from the infected dogs. So this is a cancer that can move from dog to dog—a truly infectious cancer. They could also tell that the disease started around 250-1000 years ago. So sometime between 1000 C.E. and 1750 C.E., a dog developed this cancer and since then, the cancer has spread throughout the world. Lots of dogs get the cancer but most survive. This is one of the reasons why this cancer has stayed around so long. But we still don't know how it manages to spread from dog to dog. Or why it is so stable. Most cancers would burn out after 250-1000 years but this one is still going strong. These are the next questions for scientists to answer. Luckily we don't know of any human cancer that can spread like this. But it is scary enough to know that it is possible for a cancer to do this.
There is a cancer that
can spread from dog to dog