We all know that cancer is, unfortunately, a very common disease. You may have also noticed that certain types, like breast or colon cancer, are more common in some families than others. Some of this family clustering is caused by various hereditary cancer syndromes that are caused by gene mutations that can be found with genetic testing. We know that individuals and families with hereditary cancer syndromes are much more likely to develop certain kinds of cancer than those without.
So…how common are these hereditary cancer syndromes, and how likely is it that you or your family could have one? Well, that depends on which syndrome we’re talking about. The best known, and one of the most common, causes of hereditary cancer are mutations in the BRCA1 and BRCA2 genes that cause hereditary breast and ovarian cancer (HBOC) - thanks, Angelina Jolie, for raising awareness about this! (Visit our Mutation Carriers page for more information on the cancer risks for men and women with HBOC). While around 1 out of every 8 women (or 12%) will develop breast cancer at some point in their lives, only around 1 out of 400-800 people (less than 1%) has HBOC. The exception is in the Ashkenazi Jewish population where this number is closer to 1 in 40(2.5%). Only about 10% of all breast cancer is hereditary, but it is important to know about hereditary cancer syndromes, especially if breast cancer seems to be running in your family- especially at younger ages or if you have any male relatives with breast cancer.
When you hear that 1/400-800 number, do you think HBOC is “common”? In the genetics world, we deal with a lot of really rare diseases, like 1 in 10,000-100,000 rare, so these hereditary cancer syndromes seem common to most of us. I know many of the patients I saw with hereditary cancer felt very differently about these numbers. Some felt extremely unlucky that they and their family were in the <1% of people affected by hereditary cancer. I had a handful of patients who commented that they felt they should go out and buy a lottery ticket because if something as rare as a hereditary cancer syndrome could happen to them, then maybe they could beat the odds and win the lottery too. Whether you work with hereditary cancer every day and think it is fairly common, or this is the first time you’ve heard of it and feel that it is super rare, finding out that you or someone in your family has a hereditary cancer syndrome can change what you know about your risk to develop cancer. On a more helpful note, finding out that you have hereditary cancer in your family can help affected relatives start screening earlier or possibly consider preventative surgery in the hope of preventing cancer or at least catching it early.
Now that we’ve covered the general population chance to have one of these genetic mutations, let’s switch gears and talk about how the likelihood to have a mutation changes once someone in your family tests positive. Most hereditary cancer syndromes are inherited in an “autosomal dominant” fashion. That bit of fancy genetics jargon just means that people who carry a mutation have a 50% chance (1/2) to pass it on each time they have a child, regardless of their gender or the child’s. This also means that, if one of your parents has a mutation, then you would have a 50% chance to inherit it from them. If you have a grandparent, aunt, or uncle who tests positive (aka a 2nd degree relative), then you would have a 25% (1/4) chance to also carry the mutation. If the mutation is found in a 3rd degree relative (a first cousin, great aunt/uncle, etc.), then you would have a 12.5% (1/8) chance to carry that mutation. Even when a fairly distant family member is found to have a mutation, your chances to also carry that mutation jump up drastically from the <1% population incidence, so these hereditary cancer syndromes can feel a lot more “real” once someone in your family tests positive for a gene mutation. It is important to share the news of a mutation in the family with as many relatives as possible so that everyone can be well informed of their possible cancer risk and have the opportunity to meet with a genetic counselor or other trained healthcare professional to discuss genetic testing for themselves.
