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Signs and Symptoms
Familial adenomatous polyposis (colloquially known as FAP) is a heritable disorder which causes a predisposition for the development of colon cancer. Individuals affected by the disorder develop benign, noncancerous polyps in the colon, which eventually lead to colon cancer if left untreated. Depending on the age of onset, or the number of polyps, three types of this disorder can be discerned.
- Classic familial adenomatous polyposis: This type is characterized by the development of polyps in the teenage years of the affected individuals, who will develop colon cancer at an average age of 39 years.
- Attenuated familial andematous polyposis: The onset is delayed in this type of the disorder. The average age of colon cancer onset is 55.
- Autosomal recessive familial adenomatous polyposis: In this type of the disorder, fewer polyps are present, mostly fewer than hundred. This contrasts greatly with the several hundreds or thousands that develop in the other types.
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Genetics of FAP
Mutations in two genes have been thus far identified as cause for FAP:
- APC gene, which is associated with classic and attenuated FAP. The mutation in this gene affects the ability of the cell to grow and function normally, which results in cell overgrowth, which, in turn, leads to the development of colon polyps.
- MUTYH gene, which is associated with autosomal recessive FAP. The mutation in this gene leads to a prevention of the correction of mistakes that are made when DNA is copied in a cell. As these mistakes build up, the likelihood of developing polyps increases.
The inheritance pattern of FAP depends on which gene carries a mutation.
- APC: When there is a mutation in this gene, FAP is inherited through an autosomal dominant pattern.
- MUTYH: A mutation in this gene is inherited through an autosomal recessive pattern (hence the name: autosomal recessive FAP).
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Prevalence and Prognosis
The estimates for the incidence of FAP vary from 1 in every 7,000 to 22,000 individuals.
If FAP goes untreated, the average lifespan is roughly 42 years of age. Since screening techniques and drug therapies are constantly improving, however, this average rises to over 60 years when the symptoms are recognized and diagnosis is made early on.
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Diagnosis and Treatment
The diagnosis starts with a family history, since the disorder has a strong genetic component. If a risk is identified, a genetic test will be taken. Should the mutation in the APC or MUTYH gene be noticed, the colon of the affected individual will be examined to determine the stage of the polyp formation.
When one or more polyp(s) is noticed during the examinations, the only effective treatment will be surgical removal of the affected part of the colon. Three techniques are currently used to do this. Which one is used depends on the severity of the polyp ‘infection’.
- Total Colectomy and Ileorectal Anastomosis (IRA): Used in the case of few polyps. Colon is removed, but rectum remains. Normal bowel function after surgery.
- Colectomy with Ileorectal Pouch (or restorative proctocolectomy): More polyps are noticed. Colon and rectum are removed, but a new rectum is made from the small intestine. Normal bowel function after surgery.
- Proctocolectomy and Ileostomy: Used when the other techniques are not an option. Colon and rectum are removed and a permanent ileostomy is created. Patients have to wear bags permanently to collect wastes that leave the body.
While surgery is the most common treatment of FAP, it does not cure the disease. A variety of medications has been developed to prevent polyps from returning.
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- Cleveland Clinic: http://www.clevelandclinic.org/registries/inherited/fap.htm
- Genetics Home Reference: http://ghr.nlm.nih.gov/condition/familial-adenomatous-polyposis
- Stanford Medicine Cancer Center: http://cancer.stanford.edu/information/geneticsAndCancer/types/fap.html
- Zua, M.S. (1999) Familial Adenomatous Polyposis Syndrome. Hospital Physician. pp. 61 – 68.