Is there variable expression or incomplete penetrance in familial Alzheimer Disease?
The question of whether or not there is variable expression or incomplete penetrance in familial Alzheimer Disease is dependent on which gene has a change (mutation) that causes it. There are three genes that cause early onset Alzheimer Disease: PSEN1, PSEN2, and APP. There is another gene, APOE, that is associated with an increased risk for late onset Alzheimer Disease.
PSEN1 gene: Familial AD caused by mutations to the PSEN1 gene has nearly complete penetrance. This means that nearly all people with a disease-causing change in this gene will develop AD.
PSEN2 gene: Familial AD caused by mutations in the PSEN2 gene has around 95% penetrance. This means that 95% of people with a disease-causing change in this gene will develop AD. This means 5% of people who have a change in this gene will not develop symptoms of Alzheimer Disease.
APP gene: Familial AD caused by mutations in the APP gene does not have complete penetrance although the exact numbers are not known at this time. This means not everyone will an APP disease causing change will develop Alzheimer's disease although the chances are very high.
APOE gene: Having a change to APOE does not confirm or rule out that a person will have Alzheimer Disease. Certain changes to APOE (mainly APOe4 allele) can raise the risk to develop Alzheimer Disease. There is no way to accurately predict whether someone with an APOe4 allele will develop Alzheimer disease or not. This is why genetic testing of the APOE gene is not recommended at this time.
All of the genes can show variable expression. This means that different people with the same genetic change will show different symptoms at different ages. Even within the same family people can develop Alzheimer Disease at different ages. In some people the disease progresses quickly and in others it progresses more slowly. Some people present with different symptoms than other family members.