Genetics of Colo-rectal Cancer

Bowel cancer is unfortunately a very common cancer. Here in Australia, a person’s lifetime risk of developing bowel cancer is approximately 1 in 20 (1 in 18 for man and 1 in 26 for woman)

There are many genes involved in the development of bowel cancer. Some of the major genes are now known. However many other genes and their role in the development in colorectal cancer are still to be discovered.

Relevance of a family history to Risk of Developing Bowel Cancer

Generally, having a family history would increase one’s risk of bowel cancer.

If one has a first or second-degree relative who has had bowel cancer at the aged of 55 or older, one’s risk of having bowel cancer would be double (X2)

If one has a first-degree relative who has at bowel cancer diagnosed younger than the age of 55, one’s risk of bowel cancer would be increased three to six-fold (X3-6)

If one has two first or second-degree relatives on the same side of the family with bowel cancer (at any age), one’s risk of bowel cancer would be increased three to six-fold (X3-6)

(Note: the above calculations are based on the base rate of 1 in 50 in someone without any family history)

Effects of Age on Risk of  Developing Bowel Cancer

In addition one’s risk of bowel cancer increases with age. One’s risk of bowel cancer is quite low prior to 40 years of age. This risk increases sharply around the age or 50-55, doubling with each subsequent decade.

It is best that you see a doctor to discuss your risk of bowel cancer.

Familial/hereditary Bowel Cancer

Hereditary colorectal cancer has 2 well-known forms:

1. Familial Adenomatous Polyposis (FAP) – this is due to mutations in the APC gene. This disease is characterized by the development of hundreds of polyps in the bowel. FAP also associated with desmoid tumour (this tumour can grown around the bowels and cause bowel blockage), benign cysts in the limbs/face/scalp,  benign bony tumours in the jawbone, pigmentation of the retina and cancer in the duodenum.
2. Hereditary Non Polyposis Colorectal Cancer (HNPCC) – this is due to mutations in the genes that repair damage to our DNA. This condition is also know as the Lynch syndrome. About 2-5% of all colon cancers are attributed to HNPCC. This can also cause cancers in the small bowel, uterus and urinary tract.

If you have 2 or more family members with bowel cancer or other cancers associated with familial bowel cancer (e.g. uterine, small bowel and urinary tract cancers) it is important that you discuss this with a doctor. A detailed family tree needs to be taken and further information would also need to be obtained in regards to cancers in the other family members.