The findings could lead to a new blood test

About 4% of bowel cancers are caused by mutations in single genes.

But Cancer Research UK scientists found another 20% are caused by the combined effect of many genetic mutations - most insignificant alone.

They hope the research, published in Proceedings of the National Academy of Sciences, could lead to a sophisticated blood test for bowel cancer.

I believe we could eventually exploit this knowledge to develop a blood test to identify those at high risk of the disease. Sir Walter Bodmer

This could enable doctors to identify and monitor people at high risk of the disease.

Bowel cancers can develop from small growths in the gut, called polyps. So having multiple polyps puts individuals at higher risk of developing the disease.

In conditions such as familial adenomatous polyposis, or FAP, there is an inherited gene that is the clear culprit for the hundreds of polyps that develop.

Now scientists have found rare genes may come together to cause some other cases of bowel cancer.

The team compared blood samples from 124 patients who had multiple polyps in their gut with samples from 483 members of the general population.

They found a range of genetic variants in the group with multiple polyps, each twice as frequent on average as in the control group.

Each of these variants appears to increase the likelihood that individuals will develop polyps.

Active monitoring

Researcher Sir Walter Bodmer said: "Patients who had the genetic variants we studied were twice as likely to have multiple polyps in their gut than those who did not.

"Further work will identify additional similar key variant genes that raise a person's risk of bowel cancer.

"I believe we could eventually exploit this knowledge to develop a blood test to identify those at high risk of the disease."

Doctors could actively monitor such patients for cancerous changes in their polyps, and remove them before they develop into bowel cancer.

Further research using samples from a larger number of patients is now planned to confirm associations between particular genetic mutations and multiple polyps.

This work could also lead to the discovery of other mutations responsible for inherited susceptibility to bowel cancer.

Sir Walter adds: "Cancer susceptibility is most probably due to the cumulative effect of many individually infrequent genetic variants, rather than a modest effect of one or more common variants.

"This could help explain why it has been difficult to identify specific genes that confer genetic susceptibility to cancer, and also to conditions as varied as schizophrenia and Alzheimer's disease."

Jola Gore-Booth, of Colon Cancer Concern, said: "Bowel cancer is very treatable when caught in its early stages, and so being able to identify people with increased risk of developing the disease can only be a good thing."

Introduction of a national screening programme for bowel cancer was announced last week.

Tara MacDowel, of Beating Bowel Cancer, said: "Early diagnosis of bowel cancer can mean the difference between life and death, so we would welcome any developments, such as a blood test, that could help to identify people at risk and remove polyps before they become cancerous."