Genetic mutations

Around 70% of people diagnosed with bowel cancer have no family history of the disease. A family history or hereditary factors may be identified in approximately 30% of those diagnosed.

Generally speaking, the more members of a family diagnosed with bowel cancer, and the younger they are when diagnosed, the greater the possibility of a hereditary link. If there is a history of bowel cancer in your family, genetic counselling may prove valuable.

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)

Lynch syndrome is a type of inherited cancer syndrome associated with a genetic predisposition to different cancer types. It is among the most common hereditary cancer syndromes.

DNA is the blueprint for normal cell function and Lynch syndrome is associated with a fault in a gene involved with DNA repair. The fault may inhibit repair of the day-to-day damage to DNA, causing cells in the bowel and other organs to malfunction and become cancerous.

This increased risk of DNA damage means approximately 50% of people with Lynch syndrome will develop bowel cancer before the age of 70. By comparison, 6% of the general population in New Zealand develop bowel cancer by the age of 75.

FAP is a rare condition characterised by the presence of hundreds to thousands of adenomatous polyps in the large bowel of affected individuals. FAP can often start in adolescence and is associated with an almost 100% lifetime risk of developing bowel cancer.

Testing for a mutation in the APC gene to confirm FAP currently detects 95% of mutations present. 

MAP is a rare condition characterised by the presence of ten to a few hundred polyps in the large bowel of affected individuals. MAP is associated with an almost 100% lifetime risk of developing bowel cancer by the age of 65 and is usually diagnosed following a 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 the condition. Faulty copies of the MYH gene must be passed on by both parents for a child to develop MAP. Genetic testing for MAP may also be performed in patients who receive a negative result for suspected FAP.

Knowing your family history of bowel, or any other cancer, can be an important step in determining your risk of developing the disease.

If you believe there is a strong history of bowel cancer within your family, make an appointment with your GP to discuss your concerns. If your doctor agrees with you, he or she may refer you to the New Zealand Familial Gastrointestinal Cancer Registry.

A genetic counsellor will assess your family history based on information you provide about who has been affected, how old they were at the time of diagnosis, and the site where their cancer developed. With your consent, blood tests may be taken as part of the investigation.