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PubMed This is a summary of 20 peer-reviewed journal articles Updated
Gastroenterology

Navigating the Different Faces of FAP

At a Glance

Familial Adenomatous Polyposis (FAP) appears in several forms based on specific gene mutations. Classic FAP causes hundreds of polyps early in life, while Attenuated FAP is milder. Variations like Gardner and Turcot syndromes can also cause bone growths or brain tumors.

While everyone with Familial Adenomatous Polyposis (FAP) shares a common risk, the way the condition shows up can vary significantly from person to person. Doctors use two specific terms to describe this: genotype, which is the specific genetic “typo” you have in your DNA, and phenotype, which is the physical way those genetic changes appear in your body, such as how many polyps you have or how early they started [1][2].

Because of these variations, FAP is often viewed as a spectrum of related syndromes. Understanding which subtype you have helps your care team create a roadmap tailored to your specific needs [2][3].

Classic vs. Attenuated FAP

The most common way doctors categorize FAP is by the “polyp burden”—the total number of polyps in the colon.

  • Classic FAP: This is the most common form. It is characterized by the development of hundreds to thousands of polyps, usually starting in the teenage years [4][5]. Without treatment, the risk of colon cancer is nearly 100% by age 40 [5].
  • Attenuated FAP (AFAP): This is a milder version of the condition. People with AFAP typically have fewer than 100 polyps [6][5]. The polyps often appear later in life, and the risk of colon cancer, while still very high, tends to develop about 10 to 15 years later than in the classic form [6][7].

Gardner and Turcot Syndromes

Sometimes FAP includes symptoms that affect parts of the body other than the colon. These are often named after the doctors who first described them.

Gardner Syndrome

Gardner syndrome is a version of FAP where a person develops polyps in the colon plus several specific “extra” features [8][9]:

  • Osteomas: Non-cancerous bony growths, most commonly on the jaw or skull [10][9].
  • Epidermoid cysts: Small, harmless lumps under the skin [8].
  • Desmoid tumors: Firm, scar-like growths that do not spread like cancer but can grow large and press on nearby organs [8][11].

Turcot Syndrome

Turcot syndrome describes a rare situation where a person has both colon polyps and a primary brain tumor [12][13]. There are two types:

  • Type 1: Often linked to Lynch Syndrome (a different genetic condition) and typically involves a brain tumor called a glioblastoma [12][14].
  • Type 2: Linked directly to FAP (the APC gene) and is usually associated with a brain tumor called a medulloblastoma [12][13].

MUTYH-Associated Polyposis (MAP)

There is another condition called MUTYH-Associated Polyposis (MAP) that can look almost exactly like FAP but is caused by a different gene (MUTYH) and follows a different inheritance pattern [3][15].

Feature FAP MAP
Gene Involved APC [1] MUTYH [15]
Inheritance Autosomal Dominant: Only one parent needs the gene to pass it on [5]. Autosomal Recessive: A person must inherit a mutated gene from both parents [15][16].
Polyp Count Usually 100s to 1,000s [4]. Usually 10 to a few hundred [17][18].

If you have polyps but no family history of the disease, your doctor may test for both the APC and MUTYH genes to determine which condition you have, as this changes how your family members should be screened [19][20]. Regardless of the subtype, the goal remains the same: early detection and proactive management to keep you healthy.

Common questions in this guide

What is the difference between Classic FAP and Attenuated FAP?
Classic FAP typically involves hundreds to thousands of colon polyps that begin growing in the teenage years. Attenuated FAP is a milder form where people usually develop fewer than 100 polyps, and they tend to appear later in life. Both conditions carry a high risk for colon cancer if left unmanaged.
What are Gardner syndrome and Turcot syndrome?
Gardner and Turcot syndromes are variations of FAP that affect other parts of the body alongside the colon. Gardner syndrome involves additional physical features like harmless bone growths, skin cysts, and desmoid tumors. Turcot syndrome is a rare condition where a person develops both colon polyps and a primary brain tumor.
How is MUTYH-Associated Polyposis (MAP) different from FAP?
While MAP looks very similar to FAP, it is caused by mutations in the MUTYH gene rather than the APC gene. It also follows a recessive inheritance pattern, meaning a person must inherit the mutated gene from both parents to develop the condition, whereas FAP only requires the gene from one parent.
How does my specific FAP subtype affect my screening schedule?
Your specific FAP subtype and polyp count will dictate how often you need colonoscopies and at what age they should start. Depending on your genetic classification, your doctor may also recommend additional baseline imaging to monitor for extra-intestinal features like desmoid tumors or brain tumors.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Does my specific APC mutation location (my genotype) suggest a higher risk for desmoid tumors or other Gardner syndrome features?
  2. 2.Am I currently classified as having 'Classic' or 'Attenuated' FAP based on my polyp count and age?
  3. 3.Since MAP is recessive, do my siblings need to be tested even if our parents don't have the condition?
  4. 4.Should I have any baseline imaging for brain tumors if my diagnosis leans toward the Turcot syndrome spectrum?
  5. 5.How does my specific subtype change the timing or frequency of my colonoscopies and other screenings?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References

References (20)
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This page explains the different subtypes of Familial Adenomatous Polyposis (FAP) for educational purposes only. Your gastroenterologist and genetic counselor are the best sources for interpreting your specific genetic test results and diagnosis.

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