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PubMed This is a summary of 18 peer-reviewed journal articles Updated
Gastroenterology

Understanding the Basics of FAP

At a Glance

Familial Adenomatous Polyposis (FAP) is a rare genetic condition caused by a mutation in the APC gene, which leads to hundreds of colon polyps. While unmanaged FAP almost always leads to colorectal cancer, personalized and proactive medical care can successfully prevent cancer from developing.

Receiving a diagnosis of Familial Adenomatous Polyposis (FAP) can feel overwhelming and frightening. It is natural to feel a sense of uncertainty when learning about a genetic condition that carries a high risk of cancer [1][2]. However, FAP is a well-understood condition with a clear, established roadmap for medical care. Because doctors know exactly what causes the disease and how it progresses, they can provide proactive monitoring and treatments designed to prevent cancer before it starts [3][4].

What is FAP?

Familial Adenomatous Polyposis (FAP) is a genetic condition that causes hundreds or even thousands of small growths, called adenomas (polyps), to form in the colon and rectum [1][5]. While these polyps start out as non-cancerous, if they are not monitored and managed, they have a nearly 100% chance of developing into colorectal cancer over time [1][2].

FAP is rare, but it is one of the most clearly defined genetic cancer syndromes. Most people are diagnosed in their teens or twenties when polyps begin to appear, though the exact timing can vary [5][3].

The Role of the APC Gene

The “instruction manual” for your body is made of genes. In FAP, the core issue is a mutation (a change or “typo”) in a specific gene called APC (Adenomatous Polyposis Coli) [2][6].

The “Brake” System

Under normal circumstances, the APC gene acts as a tumor suppressor [7][8]. You can think of it as a brake system for your cells. It produces a protein that tells cells when to stop dividing. Specifically, it helps break down a substance called beta-catenin [7][9]. When the APC gene is working correctly, it keeps beta-catenin levels low, which prevents cells from growing out of control [8][9].

Why Polyps Form

In someone with FAP, the “brakes” are broken. Because the APC protein isn’t working right, beta-catenin builds up inside the cells [10][8]. This buildup sends a constant signal to the cells to divide and multiply, even when they shouldn’t [9]. This rapid, uncontrolled cell growth is what leads to the formation of the numerous polyps characteristic of FAP [9][11].

How FAP is Inherited

FAP follows a pattern called autosomal dominant inheritance. This medical term describes how the condition is passed down through families:

  • Autosomal: The gene is located on one of the numbered chromosomes (specifically chromosome 5), meaning it affects men and women equally [1][5].
  • Dominant: You only need one copy of the mutated gene (from either parent) to have the condition [1].

If a parent has FAP, there is a 50% chance with each pregnancy that the mutation will be passed on to the child [1][5]. It is important to note that about 25% to 30% of people with FAP have a de novo mutation, meaning they are the first person in their family to have the condition [12][13]. In these cases, the mutation happened spontaneously during early development [12].

Looking Beyond the Colon

While the colon is the primary focus of FAP, the APC gene mutation can sometimes cause signs in other parts of the body. These “extracolonic manifestations” can actually help doctors confirm a diagnosis [14][15]:

  • CHRPE: Small, harmless dark spots on the retina of the eye [14].
  • Osteomas: Non-cancerous bony growths, often on the jaw [16].
  • Dental Anomalies: Extra teeth or impacted teeth [16].
  • Desmoid Tumors: Thick, scar-like growths that are non-cancerous but can be aggressive [16][17].

A Path Forward

While a diagnosis of FAP is life-changing, it is not a surprise to the medical community. Because the genetic cause is so well-defined, your care team can use genotype-phenotype correlation—matching your specific mutation to known patterns—to personalize your care [3][18]. This allows for precise surveillance (like regular colonoscopies) and timely interventions that have successfully saved many lives by preventing cancer [4][14].

Common questions in this guide

What causes Familial Adenomatous Polyposis (FAP)?
FAP is caused by a mutation in the APC gene, which normally acts as a 'brake' to stop cells from dividing too quickly. When this gene is mutated, cells in the colon and rectum multiply out of control, leading to the formation of hundreds or thousands of polyps.
How is FAP inherited?
FAP is an autosomal dominant condition, meaning you only need one copy of the mutated gene to have the disease. If a parent has FAP, there is a 50% chance they will pass it to their child. However, up to 30% of patients are the first in their family to have FAP due to a spontaneous mutation.
Does FAP always cause colon cancer?
Yes, if left unmonitored and untreated, the polyps associated with FAP have a nearly 100% chance of turning into colorectal cancer. Fortunately, regular medical monitoring and timely interventions are highly successful at preventing cancer before it starts.
What are the signs of FAP outside of the colon?
While FAP primarily affects the colon, it can cause signs in other areas of the body. These include harmless dark spots on the retina of the eye (CHRPE), non-cancerous bone growths called osteomas, extra or impacted teeth, and scar-like desmoid tumors.
How does knowing my specific genetic mutation help my treatment?
Doctors can match your specific APC gene mutation to known patterns of the disease in a process called genotype-phenotype correlation. This allows your care team to create a highly personalized screening schedule, ensuring you get the right monitoring at the right time.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Based on my specific APC gene mutation, what is the expected severity of my polyposis?
  2. 2.Does my genetic test result suggest I have 'classic' or 'attenuated' FAP, and how does that change my monitoring schedule?
  3. 3.How do I share this information with my siblings or children, and at what age should they begin genetic testing?
  4. 4.Are there specific extracolonic markers, like retinal spots (CHRPE) or dental anomalies, that you've noticed in my case?
  5. 5.What are the next steps in my 'roadmap' for care over the next year?

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

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This page provides general information about Familial Adenomatous Polyposis (FAP) and the APC gene for educational purposes. It does not replace professional medical advice, diagnosis, or genetic counseling from your healthcare team.

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