Bowel cancer is the third most common cancer in the UK for men and the second most common cancer for women. Every year more than 40,000 people develop it. So if you just have one elderly relative who had bowel cancer, it’s unlikely that you will have a significantly increased risk.
About 5–10% of bowel and womb cancers are thought to occur in people who have a mutation in one of the known bowel cancer susceptibility genes. This means about 95% of people with bowel cancer (19 out of every 20) don’t have an inherited type of bowel cancer. In families who have a change in one of the known bowel cancer susceptibility genes, there will be a number of family members with bowel cancer. There will probably be two or more relatives with bowel cancer in at least two generations.
There are three main types of inherited bowel cancer susceptibility.
This is also known as hereditary non-polyposis colorectal cancer (or HNPCC). It is the most common cause of a hereditary susceptibility to bowel cancer. Women with Lynch syndrome also have an increased risk of womb (uterus) cancer and a slightly increased risk of ovarian cancer. For men and women, there’s also an increased risk of cancers of the stomach, kidney and ureter.
If a bowel cancer or other cancer develops this is usually at a younger age than people who haven’t inherited a cancer risk. People with Lynch syndrome may get a few polyps (growths) in the bowel, which can develop into a cancer if not removed.
Familial adenomatous polyposis (FAP)
With FAP, hundreds to thousands of non-cancerous (benign) growths, called polyps, develop in the large bowel. These polyps start to develop from the teenage years. If the condition is not treated, some of these polyps will eventually develop into cancer.
Family members who are at risk may be screened yearly from their early teens.
MYH associated polyposis (MAP)
MAP causes polyps to develop in the bowel. It’s caused by a fault in a tumour suppressor gene called the MYH gene. We have two copies of each gene – one from each parent. A person needs two faulty copies of the MYH gene to be at increased risk of MAP. This makes it far less likely for children to inherit the susceptibility from their parents. Both their parents would need to carry the MYH genetic mutation, and the child would need to inherit the daulty copy from both parents.
More genes may be identified in the future, which together with factors such as diet, contribute to the development of bowel cancer.