With familial adenomatous polyposis, or simply FAP, familial refers to the fact that the disease runs in the family, and adenomatous polyposis refers to the fact that people affected develop multiple polyps that arise from the glands in the large intestine, which includes the colon and the rectum.
Now, the walls of the gastrointestinal tract are composed of four layers.
The outermost layer is called serosa.
Then there’s the muscular layer, which contracts in a synchronized way to move food through the bowel.
Then there is the submucosa, which consists of a dense layer of tissue through which blood vessels, lymphatics, and nerves run and branch into the mucosa and the muscular layer.
Finally, the inner lining of the intestine is called the mucosa; it surrounds the lumen of the gastrointestinal tract, and comes into direct contact with digested food.
The mucosa is organized as invaginations called the intestinal glands or colonic crypts, lined with large cells that are specialized in absorption.
Familial adenomatous polyposis is caused by an autosomal dominant mutation in the adenomatous polyposis coli gene or APC gene on chromosome 5q, which is a tumor suppressor gene.
Tumor suppressor genes stop cells from dividing uncontrollably.
But if the gene is mutated and the cell is without a functioning APC, the intestinal gland cells are more likely to accumulate mutations and start dividing faster than usual - ultimately giving rise to polyps, which are benign outgrowths of intestinal gland tissue.
Even though for any single polyp the chance that it evolves into cancer is generally quite low, polyps might accumulate additional mutations in other genes like the p53 gene (another tumor suppressor) or K-ras gene (a proto-oncogene), and with enough mutations, a cell might become completely unregulated and might start invading nearby tissue and become malignant.