Most bowel cancers are not caused by inherited cancer genes and most people who get bowel cancer don’t have a family history of it.
In general, the chance of there being a family link is greater when:
- a number of family members have been diagnosed with bowel cancer (or related cancers such as womb or ovarian cancer)
- the family members were diagnosed at a young age
- the family members are closely related.
If you have one relative who developed bowel cancer at an older age, this doesn’t significantly increase your risk.
Your risk of developing bowel cancer may be increased significantly if:
- one of your first-degree relatives was diagnosed with bowel cancer before the age of 50 (first-degree relatives are your parents, siblings and children)
- two first-degree relatives from the same side of the family were diagnosed with bowel cancer at any age (for example, your father and his sister)
- you have relatives with bowel and womb cancer on the same side of the family
- you have relatives who have multiple growths or polyps in the bowel (known as familial adenomatous polyposis or FAP).
Inherited genes and bowel cancer
Some people have a high risk of developing bowel cancer because they have inherited an altered gene (cancer gene) that greatly increases their risk. If there is a cancer gene running in a family, several relatives on the same side of the family may be diagnosed with bowel cancer.
The main conditions linked to inherited cancer genes are:
- FAP – also known as familial adenomatous polyposis.
- Lynch syndrome – also known as hereditary non-polyposis colorectal cancer, or HNPCC.
In Lynch syndrome, there may also be other cancers in the family that are related to bowel cancer, such as womb or ovarian cancer. People in the family may also be diagnosed with cancer at a particularly young age.
If you are worried about a history of bowel cancer in your family, talk to your GP. They may be able to reassure you or refer you to a clinical genetics service or family cancer clinic.
A genetics specialist will talk to you about the history of cancer in your family. You may have a genetic test to look for inherited cancer genes in your family. These tests look for changes (alterations) in genes that increase the risk of getting bowel cancer.
Bowel cancer under 50
People who get bowel cancer before the age of 50 may have another type of genetic test when they are diagnosed. Doctors may test cancer cells from the bowel tumour for:
- a genetic trait called microsatellite instability (MSI)
- changes in mismatch repair (MMR) genes.
If either of these are found, the cancer may be due to an inherited gene mutation.
The doctor will then offer genetic testing to look for an inherited bowel cancer gene. This test is done on a blood sample.
Other rare gene mutations
There are other rare inherited conditions that can increase bowel cancer risk. These include:
- MYH-associated polyposis (MAP)
- juvenile polyposis
- Peutz Jeghers syndrome.
MYH-associated polyposis (MAP) is inherited in a different way from other cancer gene mutations covered in this booklet. A person needs two copies of the faulty MYH gene, one from each parent, to develop MAP. This makes it far less likely for children to inherit MAP. Both their parents would need to carry the MYH gene mutation, and the child would need to inherit the faulty copy from both parents.
People with MAP, juvenile polyposis and Peutz Jeghers syndrome are offered regular bowel cancer screening. This is done every 1–3 years depending on the condition. Most people will have begun bowel screening by the age of 25.