Confused about FAP inheritance (why is it dominant)

[nm825]

So I'm a bit confused as to why FAP is autosomal dominant if you still have a healthy APC gene on the other chromosome.

Is it because that after you have the inherited mutation, you are nearly 100% bound to get a second-hit to the second, normal APC gene, thereby resulting in adenomatous polyp formation?

When we say autosomal dominant here, are we essentially saying that the phenotype is "increased risk for cancer" meaning a mutation of just one APC gene puts you at increased risk for CA?

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[stop2stop]

Yes, as far as I know you've basically got it. AD inheritance refers to the fact that you're already missing one allele of APC, which is a tumor suppressor gene. Thus it takes very little to get "Another hit" of that gene, lose the remaining allele, and thus develop a polyp. The process is no different than diseases like Li-Fraumeni (Same idea but with p53), and is understood through the "Two-Hit" Hypothesis which is a fairly big deal in cancer genetics. Read more below:

https://en.wikipedia.org/wiki/Knudson_hypothesis

 

[worldbeater]

In general, autosomal dominant refers to a structural problem while autosomal recessive refers to a missing enzyme. By that logic, you can see why a condition that has multiple polyps like FAP would be AD.

 

[fw5tape6kq]

Yes, as far as I know you've basically got it. AD inheritance refers to the fact that you're already missing one allele of APC, which is a tumor suppressor gene. Thus it takes very little to get "Another hit" of that gene, lose the remaining allele, and thus develop a polyp. The process is no different than diseases like Li-Fraumeni (Same idea but with p53), and is understood through the "Two-Hit" Hypothesis which is a fairly big deal in cancer genetics. Read more below:

https://en.wikipedia.org/wiki/Knudson_hypothesis

Agreed. It's all about the loss of heterozygosity once the other functioning allele undergoes a mutation, which is pretty much inevitable at some point in one's lifetime. Remember a colonic polyp need only develop from one cell.