Your DNA

Alzheimer's disease is believed to be caused by a mix of genetic and environmental factors, with inherited factors accounting for about 62 percent of the risk, according to studies of twins. Many other risk factors have been proposed, but only advanced age and family history are verified. The common, or late-onset, form of Alzheimer's generally begins after age 65.

Early onset Alzheimer's disease, which begins before age 60, is even more strongly genetic in its cause, but very rare — it represents less than 2 percent of cases. Navigenics does not test for this rare kind of Alzheimer's. If you have family members who developed the disease at an early age, you should consult your physician, who may refer you to a genetic counselor.

More than 1,000 scientific papers have reported associations between various genetic markers and late-onset Alzheimer's disease, but most of these studies do not meet Navigenics' rigorous standards. However, there is a strong and well-established association between the disease and a gene called APOE. APOE comes in three different versions: APOE-2, APOE-3 and APOE-4. Having one or more copies of APOE-4 raises your chances of getting Alzheimer's disease. Additional genetic factors that contribute to Alzheimer’s will undoubtedly be validated in the future, but APOE is currently, and is likely to continue to be, the single most important genetic factor related to late-onset Alzheimer’s.

In several published studies, more than half of the subjects with late-onset Alzheimer's had at least one copy of APOE-4. But having the APOE-4 variant does not necessarily mean that you will develop the disease. About 3 percent of the Caucasian population has two copies of APOE-4, but not all of them develop Alzheimer's.

Everyone has some chance of developing Alzheimer's disease. Each risk marker increases your odds by a different amount — some a little, some a lot. The "odds ratio" column in the chart above shows their relative impact.

More than 99 percent of our genetic code is identical from person to person. The remainder is unique among individuals and gives rise to our diversity. DNA, the chemical instructions that determine our genetic code, is inherited from our parents and is composed of four biochemical bases that are represented by the DNA alphabet: A, T, G and C.

SNPs, or single nucleotide polymorphisms, are one-letter variations in our DNA. For most conditions in the Navigenics Health Compass, each SNP (pronounced "snip") consists of two markers or variants, one inherited from each of our parents, and each marker has two possible versions, a risk version and a non-risk version. For example, at a given SNP, the base A may be the risk marker and the base G may be the non-risk marker. At another SNP, G may be the risk marker and T the non-risk marker.

For Alzheimer's disease, the situation is a little different. Two SNPs in the APOE gene code for three different versions of the APOE protein: E2, E3, and E4. E4 is the risk version.

The SNPs included in your Navigenics risk estimate have been reliably shown to be associated with diseases, although it is not known exactly how they contribute to the conditions. Just because you may have one or even two risk markers does not mean that you will definitely develop a given health condition, but it can raise your risk, especially if other lifestyle or environmental risk factors are present.