Genetics and Alzheimer's disease

Most Alzheimer's disease has a complex cause

Age is the strongest risk factor for late-onset Alzheimer's disease. Genetics also matters, along with vascular health, other biology, environment, and factors that have not yet been fully defined.

Late-onset Alzheimer's may occur in several members of a family without a single disease-causing variant. Relatives share many genetic differences, but they may also share aspects of health, environment, and longevity. A family history is a meaningful clue. It is not proof of autosomal dominant inheritance.

Having one first-degree relative with late-onset Alzheimer's generally raises a person's risk compared with someone who has no affected first-degree relatives. It still does not answer whether that person will develop the disease.

APOE changes risk rather than determining the future

APOE provides instructions for a protein involved in transporting fats and in several processes relevant to the brain. The common forms are called e2, e3, and e4. A person inherits one APOE copy from each biological parent.

APOE e4 is associated with a higher probability of Alzheimer's disease, and two copies are generally associated with higher risk than one. APOE e2 is associated with lower risk in many populations. Neither form gives a yes-or-no answer.

APOE testing does not show whether amyloid or tau is present in the brain. It does not diagnose the cause of current symptoms. Risk estimates differ across age, ancestry, sex, and study populations, so a genotype should not be converted into one universal personal percentage.

APOE has acquired a separate clinical role in amyloid-lowering treatment. APOE e4, especially two copies, is associated with a higher risk of ARIA, a potential side effect involving brain swelling or bleeding. Current prescribing information recommends APOE testing before lecanemab or donanemab treatment to inform that risk discussion.

Rare variants can cause inherited Alzheimer's disease

Pathogenic variants in APP, PSEN1, and PSEN2 can cause rare forms of autosomal dominant Alzheimer's disease. These families often have several affected people across successive generations, frequently with symptoms beginning before age 65.

If a biological parent carries a confirmed pathogenic variant in one of these genes, each child has a 50 percent chance of inheriting the variant. The result may clarify the cause of disease in a family, but it does not perfectly predict the age symptoms will begin or the clinical course.

These rare inherited forms account for only a small proportion of Alzheimer's disease. Early onset alone does not prove that one of these genes is involved, and late onset does not exclude every genetic contribution.

Down syndrome is a different genetic context

Most people with Down syndrome have an extra copy of chromosome 21, which includes the APP gene. The additional gene dosage contributes to the earlier development of Alzheimer's-related amyloid and tau changes and a substantially higher risk of dementia with aging.

Many, but not all, adults with Down syndrome develop dementia. A diagnosis depends on a decline from that individual's previous abilities and should not be made from age or chromosome status alone.

When a genetic evaluation may be useful

A clinician may consider referral for genetic counseling when:

• cognitive or neurologic symptoms began unusually early;
• similar symptoms occurred in multiple generations;
• several relatives had early-onset dementia, FTD, ALS, parkinsonism, or another related neurologic condition;
• a pathogenic variant is already known in the family;
• the clinical pattern suggests a specific inherited disorder;
• a person is considering predictive testing or needs help interpreting an existing result.

When possible, testing a relative who has symptoms is more informative than starting with an unaffected relative.

What genetic testing can add

A confirmed pathogenic variant can identify the cause of a rare familial form, clarify which adult relatives may consider targeted testing, and open access to some research studies. APOE status can inform a treatment-safety discussion in someone being evaluated for amyloid-lowering therapy.

Genetic testing cannot replace an evaluation of symptoms. It does not measure memory, daily function, amyloid, tau, or the amount of injury in the brain. A negative panel does not rule out Alzheimer's disease and does not prove that the family pattern is non-genetic.

The useful next step

Begin with the clinical and family question, not with the largest available panel. Record which relatives were affected, their ages when symptoms began, and what changed first. Bring copies of any family genetic reports rather than only the name of a gene.

A neurologist or genetic counselor can then determine whether the question is about diagnosis, inherited disease, future risk, or treatment safety. Each requires a different kind of answer.