Familial adenomatous polyposis

Inheritance

How is familial adenomatous polyposis inherited?

Familial adenomatous polyposis is usually inherited in an autosomal dominant manner. This means that an individual only needs one mutation in a gene in order to have an increased risk for colon polyps and cancer. If a parent has a mutation, there is a 50% chance that each of their children will inherit the mutation. If an individual has a mutation there is a 50% chance that their siblings also have the mutation. Men and women are equally likely to have these mutations and sons and daughters are equally likely to inherit them.

Sometimes there may not be a family history of FAP and approximately 20-25% of the time, the change in the APC gene is only found in one person who has FAP and not found in any other of their family members. This is called a de novo mutation, which means the change happened for the first time in that individual, and it was not inherited from a family member. However, the mutation can still be passed on to that individual's children.

MUTYH-associated polyposis is inherited differently. It is inherited in an autosomal recessive manner. This means, an individual needs two mutations in order to have an increased for developing polyps. For someone to inherit two mutations, they would inherit one mutation from their mother and one mutation from their father. In this case, the parents would be considered carriers of MUTYH-associated polyposis. This means that each parent is not at risk to develop colon polyps, however, they are at a slightly increased risk for developing colon cancer. If both parents carry a mutation, each of their children have a 25% chance of having two mutations and having an increased risk for developing colon polyps. Men and women are equally likely to have these mutations and sons and daughters are equally likely to inherit them.

References
  • Bisgaard, ML, Fenger, K., Bulow, S., Niebuhr, E., Moh, J. (1994). Familial adenomatous polyposis (FAP): frequency, penetrance, and mutation rate. Hum Mutat: 3:121-5.
  • Nielsen, M. (2019, October 10). MUTYH Polyposis. Retrieved from http://www.ncbi.nlm.nih.gov/books/NBK107219/.
  • Jasperson, K. W. (2017, February 2). APC-Associated Polyposis Conditions. Retrieved from http://www.ncbi.nlm.nih.gov/books/NBK1345/.
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What does it mean to have inherited a "variant of uncertain significance" in the gene for familial adenomatous polyposis?

Are there forms of familial adenomatous polyposis that aren't genetic?

I have familial adenomatous polyposis. Will my children have it?

What does it mean to have inherited a "variant of uncertain significance" in the gene for familial adenomatous polyposis?

A variant of uncertain significance (VUS), means that there is a change in the DNA sequence of a gene, but it is not yet known whether this variant impacts the ability of the protein to do its job or is associated with risk for disease.

References
  • Lindor, N.M. et al. (2007). Concise handbook of familial cancer susceptibility syndromes. Journal of the National Cancer Institute. Monographs, (38) 1-93.
Are there forms of familial adenomatous polyposis that aren't genetic?

Typically, if an individual has a personal history of more than 100 adenomatous polyps they will test positive for a mutation in the APC gene. If an individual has attenuated FAP, mutations in the APC gene are not always identified, and this suggests that there may be another gene, currently unknown, which may be responsible for the multiple polyps.

References
  • Jasperson, K. W. (2017, February 2). APC-Associated Polyposis Conditions. Retrieved from http://www.ncbi.nlm.nih.gov/books/NBK1345/.
I have familial adenomatous polyposis. Will my children have it?

Because familial adenomatous polyposis (FAP) is inherited in an autosomal dominant inheritance pattern, it means that every first degree relative of an affected individual has a 50% chance of also having it. First degree relatives are siblings, children, and parents. Second degree relatives should have testing after first degree relatives are all tested. in a specific example, let's say 27-year-old Andrea has been diagnosed with FAP. Andrea has 3 children. EACH of her children has an individual 50% chance to also have it. Andrea has 2 siblings: one brother and one sister. EACH of her siblings has an individual 50% chance to also have it. Andrea's mother has testing and is negative. This means that Andrea's mothers side of the family does NOT need to have any more testing. Andrea's father has testing and is positive. Each of Andrea's father's siblings should have testing as well, because they have an individual 50% chance of being positive as well.

References
  • Jasperson, K. W. (2017, February 2). APC-Associated Polyposis Conditions. Retrieved from http://www.ncbi.nlm.nih.gov/books/NBK1345/.

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