Genetic Testing

Blood tests are available that can determine if you carry one of the genetic mutations or variants associated with Alzheimer's disease or one of the other extremely rare genetically transmitted degenerative disorders. In people with a family history of early-onset familial Alzheimer's disease, a rare form of Alzheimer's that can appear as early as the fourth decade of life, genetic testing will identify mutations in three genes: presenilin 1, presenilin 2, and the amyloid precursor protein gene. Mutations in these genes are associated with early-onset familial Alzheimer's, which accounts for about 5 percent of all cases of the disease.

In contrast, the vast majority of Alzheimer's disease is designated sporadic, meaning that there is no known cause. However, variants of one gene—ApoE—influence your risk of developing the disease at some point during your lifetime. The ApoE gene has three alleles, or forms: e2, e3, and e4. Because you inherit one allele from each parent, there are six possible allele combinations: e2/e2, e2/e3, e2/e4, e3/e3, e3/e4, and e4/e4. The e4 allele is associated with the greatest probability of developing Alzheimer's disease; having two e4 alleles places you in the highest risk group. On the other hand, possessing two e2 alleles places you in a reduced risk category.

Between 35 and 50 percent of people with Alzheimer's disease have at least one copy of the ApoE e4 allele. That said, inheriting a copy of the e4 allele doesn't mean that you will definitely get Alzheimer's disease, and not inheriting the allele doesn't mean that you won't, so this genetic information may not be highly meaningful for most people. However, for people who are currently exhibiting symptoms of a dementia, ApoE testing can lend weight for or against Alzheimer's disease as the underlying cause.

I want to emphasize that even if you do have a family history of early-onset familial Alzheimer's, that doesn't mean that you will absolutely develop the disorder yourself or that you should have a genetic test. Even if you knew that you had one of the disease-producing mutations, we do not yet possess the therapy to change your odds of manifesting the disorder. If you don't want ,111

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