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FAP is an inherited condition caused by a mutation in a gene that is inherited in an autosomal dominant way. This means that half the children of an affected parent will have FAP, but the remaining unaffected children do not have the gene to pass on to their offspring.
The condition is characterised by the formation of polyps, also known as adenomas (because they are at a pre-cancerous stage, where they may or may not develop into cancerous cells). By the time an affected individual is 15-20 years old, hundreds of adenomatous polyps will have developed in the colon.
Although each individual polyp has a fairly low risk of becoming cancer, the patient has so many polyps that colon cancer is almost inevitable by the age of 50. Commonly, this cancer occurs much earlier in adult life unless prophylactic (preventive) colectomy (the removal of the colon) is done.
Although the colon is the main site of the disease, other organs are also affected.
 Individuals affected with FAP have approximately a 10 per cent lifetime risk of developing cancer in the duodenum, a cancer that is very rare in the general population.
Desmoid tumours, which are solid tumours of the connective tissues, particularly within the abdomen, occur in 5-10 per cent of people with FAP. Although they are not malignant, they can cause major problems by invading surrounding tissues, and they can be very difficult to cut out. Individuals with desmoid tumours have a one in five risk of death at an average age of about 35.
Benign (non-cancerous) cysts can occur on the limbs, face or scalp, and benign bony tumours of the skull and lower jawbone can also occur. Occurrence of these cysts in an individual with bowel polyps used to be known as 'Gardner's syndrome', but this is now acknowledged to be due to a mutation in the same gene that is defective in FAP, the APC (adenomatous polyposis coli) gene.
Harmless pigmented (coloured) lesions occur in the retina. These have been used as a non-invasive way of screening relatives for the disease since they tend to appear before polyps develop.
A rare form of FAP exists, known as attenuated FAP. In this condition, the mutation occurs at the extreme end of the APC gene, which, presumably, causes only a relatively minor defect in the function of the APC protein. The onset of this disease is about 10 years later than FAP, with fewer polyps (less than 100), but the risk of developing colon cancer is still very high.
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