It can be confusing to say that cancer is genetic but also that most of the time it is not hereditary. It is genetic in the sense that it involves our genes, but most of the time there are gene changes that occur long after we are born, that were not passed down to us by our parents. This also means that we will generally not pass them on to our children. So contrary to popular belief, the fact that you may have a relative or two with cancer does not in any way mean that you are doomed to develop it as well. Before we go further, let's fill in some background:
Genes are the structures in our cells that carry the instructions to make proteins. We all have something like 20,000 genes in each of our cells. We all have the same genes, and that's the way it is supposed to be. The issue is whether there is a gene alteration that does not allow it to work properly. For example, you will hear people ask "do you have that breast cancer gene?"The question really should be "do you have a change in a gene that makes you more susceptible to breast cancer?" Bottom line: certain gene changes cause cells to escape normal growth controls and become cancer. Cancer cells tend to have more genetic changes than normal cells. Some of these changes can result from the cancer, rather than be the cause of it. As a cancer grows, additional changes can occur.
Generally, genetic testing for cancer risk is recommended when someone has a personal or family history suggestive of an inherited cancer syndrome, as long as the test results can be adequately interpreted and may help inform the person's medical care.
Janice Berliner, 2019
When these gene changes are present in our egg or sperm cells, we may pass them on to our children. Any child who inherits this type of genetic change will have it in every cell of his or her body. This does not mean that the child will go on to develop cancer. It is known as a predisposition, meaning there will be a greater likelihood of developing cancer, but certainly no guarantee. Inherited gene changes play a major role in about 10% of all cancers, a much lower number than most people think. I know this because almost every time I told a patient that most cancers are not hereditary, I heard surprise and relief from them. It seems to be a common misunderstanding that all cancers are hereditary. Whether a particular family has a hereditary cancer syndrome is sometimes difficult to determine. This is where the genetic counselor comes in!
Generally, genetic testing for cancer risk is recommended when someone has a personal or family history suggestive of an inherited cancer syndrome, as long as the test results can be adequately interpreted and may help inform the person's medical care. Whether someone should pursue genetic testing, and what types of hereditary cancer syndromes are appropriate to test for, are the topics of another blog. Stay tuned!
When genetic counselors are asked to consider whether a person's family history may suggest a hereditary cancer syndrome, they will look for certain patterns in a family, including the types of cancer that may be present, the ages at which the cancers developed, how many family members are affected, and how the affected family members are related to each other, including whether they are on the same side of the family.
Cancers that are not caused by inherited gene mutations may appear to run in families. Sometimes a shared environment or lifestyle can cause similar cancers to develop among family members. Sometimes there are several different cancers within a family, and the cancers are not related to each other in a way that would represent a hereditary cancer syndrome. For example, if there is a lung cancer in Grandma, who was a smoker, a breast cancer in a post-menopausal aunt, and a thyroid cancer in a cousin, these do not indicate a pattern of inheritance that would be caused by a hereditary cancer syndrome. Even if a cancer-predisposing mutation is present in a family, not everyone who inherits the mutation will necessarily develop cancer.
All of these considerations are taken into account by a genetic counselor. In gathering the family history, he or she can guide a patient as to whether and which genetic testing to pursue, what the results might mean, and whether any action can be taken once the results are known. In the next blog post, we will address some hereditary cancer syndromes, the testing available for them, and potential issues regarding instances where test results are difficult to interpret.
To find a genetic counselor in your area, go to nsgc.org and click on "find a counselor." In the drop down box you can specifically request someone who is an expert in cancer genetics.