Can FAP Occur Without a Family History? APC Genetics

Yes, you can have Familial Adenomatous Polyposis (FAP) even if no one else in your family has ever been diagnosed with it. While the word “familial” suggests that the disease is always passed down from generation to generation, it actually means that the condition can be inherited ^[1]. Being the first person in your family to be diagnosed is a recognized occurrence ^[2][3].

Why might I be the first in my family?

There are two main reasons someone might be diagnosed with FAP without a known family history:

1. A ‘De Novo’ Mutation: In about 15% to 30% of people with FAP, the condition is caused by a de novo (new) pathogenic variant in the APC gene ^[2][3][4]. This means the genetic alteration happened by chance in a single sperm or egg cell before conception, or very early in embryonic development ^[5]. No one caused this to happen. If parental testing confirms this is a new mutation, it means neither parent carries the mutation in their blood, and you are the first to have it.
2. Hidden Family History: Sometimes, the mutation was inherited, but it wasn’t recognized. Parents or grandparents might have had a milder form of the disease (attenuated FAP), died before symptoms appeared, or simply never underwent genetic testing ^[6].

Additionally, in some cases of unexplained polyposis, a person may have mosaicism ^[7][8]. This means the genetic change happened after conception, and only some cells in the body carry the mutation. Mosaicism can sometimes make the mutation harder to detect in standard blood tests and may require individualized medical advice ^[8][9].

What This Means for Your Family

If a genetic test confirms you have a standard germline pathogenic (disease-causing) APC variant, FAP is an autosomal dominant condition ^[1][10]. This means:

• Your children: You have a 50% chance of passing the mutation to each biological child, regardless of their sex, in every pregnancy ^[1][10]. (Note: If you have mosaicism instead of a standard germline mutation, this transmission risk may be different ^[7].)
• Your parents and siblings: To confirm if your mutation is truly de novo, your parents should be tested. If their blood tests are negative, the risk to your siblings is very low, but not exactly zero, due to a small chance that a parent has the mutation only in their reproductive cells ^[11]. If a parent tests positive, your siblings also have a 50% chance of inheriting the mutation.

Next Steps for Testing and Care

If you have been diagnosed with a pathogenic APC variant, it is highly recommended to work with a genetic counselor ^[6][1]. They can help coordinate targeted testing for your first-degree relatives (parents, siblings, and children).

For children who might inherit the mutation, testing should be arranged early enough to start regular bowel examinations and, when needed, preventive treatment. Predictive genetic testing is often considered around ages 10 to 12. If a child tests positive for the family’s specific APC variant, medical guidelines generally recommend beginning colonoscopic surveillance (regular bowel examinations using a colonoscope or sigmoidoscope) around age 10 to 15 for classic FAP, with personalized timing for attenuated forms of the disease ^[12][6]. Pre- and post-test genetic counseling can help families make informed decisions about when and how to proceed with testing ^[6].