The exact cause of most bowel cancers is unknown. However, genetic mutations are thought to account for an estimated 5–10% of cases of bowel cancer, and a further 2–5% occur alongside other inherited syndromes.

Approximately 70% of people with bowel cancer have no family history of the disease. However, a family history or hereditary factors may be identified in approximately 30% of patients diagnosed with bowel cancer. In general, the more members of a family affected by bowel cancer, and the younger they were when diagnosed, the greater the possibility of a hereditary link.

There are several genetic conditions with a known link to bowel cancer, including:

• Lynch syndrome (hereditary non-polyposis colorectal cancer)
• Familial adenomatous polyposis (FAP)
• MYH-Associated Polyposis (MAP)

If you have a family history of bowel cancer, genetic counselling may also be of value.

Lynch Syndrome (Hereditary Non-polyposis Colorectal Cancer)

Lynch syndrome is associated with a fault in a gene associated with DNA repair. This means that day-to-day damage to DNA (the blueprint for normal cell function) may not be adequately repaired, potentially resulting in cells in the bowel and other organs malfunctioning and becoming cancerous.

This increased risk of DNA damage means approximately 50% of people with Lynch syndrome will develop bowel cancer before the age of 70 years. As a comparison, 6% of the general population in New Zealand develop bowel cancer by the age of 75. Lynch syndrome is associated with 3% of bowel cancers diagnosed in New Zealand, and the average age of diagnosis for these patients is 45 years old compared with 67 years in the general population.

The children of people with Lynch syndrome have a 50% probability of inheriting a faulty gene from their parent. Therefore, yearly screening with a colonoscopy is recommended from the age of 25 years, or at least 5 years earlier than when a person’s youngest relative was diagnosed, for individuals with a family history of Lynch syndrome.

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Familial Adenomatous Polyposis (FAP)

FAP is a rare condition characterised by the presence of hundreds to thousands of adenomatous polyps in the large bowel of affected individuals, which often start in adolescence, and is associated with an almost 100% lifetime risk of developing bowel cancer. FAP is usually diagnosed following colonoscopy to confirm the presence of polyposis.

Testing for a mutation in the APC gene to confirm FAP currently detects 95% of mutations present. In patients with a confirmed diagnosis, the affected part of the colon is surgically removed, often in the patient’s late teens or early twenties.

An affected person has a 50% chance of passing the gene for FAP on to each of their children. Therefore, screening for FAP using a colonoscopy generally commences around age 12–15 years for children of people with FAP.

MYH-associated Polyposis (MAP)

MAP is a rare condition characterised by the presence of between ten and a few hundred polyps in the large bowel of affected individuals and is associated with an almost 100% lifetime risk of developing bowel cancer by the age of 65 years. FAP is usually diagnosed following colonoscopy to confirm the presence of polyposis.

If both parents are carriers of a gene associated with MAP, their children will have a 25% chance of developing MAP. Faulty copies of the MYH gene must be passed on by both parents for a person to develop MAP. Testing for MAP may also be performed if genetic tests in patients with suspected FAP are negative.

Genetic Counselling

If you aware of anyone in your family that has had bowel or any other kind of cancer, talk to them and make sure you all know your family history.

If you think that you have a strong family history of bowel cancer, you should make an appointment with your doctor to talk about your concerns. If your doctor agrees with you, they may refer you to the New Zealand Familial Gastrointestinal Cancer Registry.

A genetic counsellor will assess your family history with you in great detail and ask you to provide information about who has been affected, how old they were when they were diagnosed, and the site where their cancer developed. You may also have blood tests performed as part of this investigation.

If the genetic counsellor agrees you have an increased risk of bowel cancer, you will be referred to a specialist to discuss what types of screening and/or surveillance tools they recommend, and how these should best be used for you and your family.

Regular screening and/or surveillance for anyone with a family history of bowel cancer helps identify any signs of bowel cancer are identified and treated quickly. However, it is important to note that screening and surveillance may be dependent on resource availability within each District Health Board.