Genetic tests for dementia answer two very different kinds of questions, and most people asking are dealing with the kind that gives odds rather than a verdict. For nearly everyone, a result can shift the likelihood of dementia without settling whether it will happen. A small number of families carry genes that do decide the outcome, and testing for those follows a careful, separate path through specialists.1Journal of Genetic Counseling. What to Know About Genetic Testing for Dementia What a result is good for is also changing fast, because it now feeds into decisions about newer Alzheimer’s drugs.
Risk Genes Versus Genes That Cause Dementia
A risk gene raises the chance of developing a disease but doesn’t guarantee it will happen.2Alzheimer’s Association. Is Alzheimer’s Hereditary / Genetic? A deterministic gene works differently: it causes the disease, so nearly everyone who inherits one develops it.3Alzheimer’s Association. Is Alzheimer’s Hereditary / Genetic? Those genes are rare.4Mayo Clinic. Alzheimer’s genes: Are you at risk? For Alzheimer’s, the best-known risk gene is APOE4, a version of the APOE gene linked to late-onset Alzheimer’s.5PubMed Central. Comparing test-specific distress of susceptibility versus deterministic genetic testing for Alzheimer’s disease Changes in genes such as the presenilins, by contrast, almost inevitably cause early-onset familial Alzheimer’s.6PubMed Central. Comparing test-specific distress of susceptibility versus deterministic genetic testing for Alzheimer’s disease
The split matters because a risk-gene result is a probability, while a causing-gene result is close to a diagnosis of the future. Most people who get dementia don’t have a causing gene.
Inheritance works differently for each. Everyone has two copies of APOE, one from each parent. Causing genes pass on with a 50% chance to each child of a carrier. A parent’s dementia in late life usually doesn’t point to a causing gene. Risk genes are common and can turn up without any obvious pattern in a family, so no family history doesn’t mean no genetic risk.
What an APOE4 Result Means for You
The number of copies matters. One copy of APOE4 raises Alzheimer’s risk about 2 to 3 times.7PubMed Central. Apolipoprotein E and Alzheimer’s disease Two copies raise it about 10 to 15 times.8PubMed Central. Apolipoprotein E and Alzheimer’s disease
“Times higher” multiplies your baseline risk, and that baseline depends heavily on your age. Ask what your absolute lifetime risk is, not just the multiplier.
Results appear as two letters, such as e3/e4 or e4/e4. The pairing e3/e3 is the most common. The pairing e4/e4 is the highest-risk result and the one with the biggest implications.
About 42% of people with Alzheimer’s have no APOE4 copy at all.9PubMed Central. Informed Choice in Direct-to-Consumer Genetic Testing for Alzheimer and Other Diseases: Lessons from Two Cases So a result without APOE4 doesn’t rule Alzheimer’s out.
Ancestry changes the size of the effect. APOE4’s effect is about half as strong in people of African ancestry as in people of European ancestry.10VA Research Communications. What to Know About Genetic Testing for Dementia Hispanic and Latino people often show lower APOE4-linked risk than non-Hispanic white people, though it varies by background.11PubMed Central. Impact of APOE4‐related dementia risk in underrepresented groups from the All of Us research program A kit’s risk figure may therefore not fit you if it was built on mostly European data.
Some services also add a score that combines many small genetic effects into one number. These scores add only limited prediction on their own, and they are never a stand-alone answer.
When Testing Is Recommended
Doctors separate diagnostic testing, for people who already have symptoms, from predictive testing for healthy relatives.12Journal of Genetic Counseling. What to Know About Genetic Testing for Dementia They may suggest testing when a young or middle-aged person has symptoms and a family history of early-onset Alzheimer’s.13Journal of Genetic Counseling. What to Know About Genetic Testing for Dementia Certain signs suggest a family may carry a causing gene: several relatives affected across generations, onset in the 40s or 50s, and a pattern where about half of a parent’s children are affected. PSEN1 and APP changes account for 71% of families with inherited early-onset Alzheimer’s.14PubMed Central. Early-onset autosomal dominant Alzheimer disease: prevalence, genetic heterogeneity, and mutation spectrum Most carriers of these changes develop symptoms by about age 61.15PubMed Central. Early-onset autosomal dominant Alzheimer disease: prevalence, genetic heterogeneity, and mutation spectrum
The process usually starts with a referral to a neurologist or memory clinic. You meet a genetic counselor before and after the test. A blood or saliva sample goes to a clinical lab, and the results are discussed in person, not sent by email.
Order matters for relatives. Once a family’s specific mutation is known, relatives can learn whether they inherited it.16Journal of Genetic Counseling. What to Know About Genetic Testing for Dementia Testing a healthy relative first, before a family mutation is known, gives a negative result that is inconclusive.17Journal of Genetic Counseling. What to Know About Genetic Testing for Dementia So where possible, testing starts with an affected family member.
General screening isn’t advised. For late-life dementia, a result rarely changes medical care outside decisions about the newer Alzheimer’s drugs, and counselors weigh the emotional cost of testing healthy people. Most guidelines don’t support routine APOE testing for curious adults.
Genetic Testing for Other Types of Dementia
Changes in MAPT, GRN and C9ORF72 are the major known genetic causes of frontotemporal dementia.18PubMed Central. Mutation Frequency of the Major Frontotemporal Dementia Genes, MAPT, GRN and C9ORF72 in a Turkish Cohort of Dementia Patients C9ORF72 carriers may also develop ALS, a disease of the nerves controlling muscles, and more often show psychotic symptoms.19PubMed. Distinct clinical and pathological phenotypes in frontotemporal dementia associated with MAPT, PGRN and C9orf72 mutations Many frontotemporal cases, though, have no identified genetic cause.20PubMed Central. The Missing Heritability of Sporadic Frontotemporal Dementia: New Insights from Rare Variants in Neurodegenerative Candidate Genes Lab panels can now test many genes at once.21PubMed Central. An update on genetic frontotemporal dementia It’s worth raising with a doctor if you notice personality or behavior changes in midlife, if a relative had ALS alongside dementia, or if movement problems appear together with memory trouble or hallucinations.
In frontotemporal families, predictive testing is usually offered only after a relative’s mutation is confirmed. Age of onset is hard to predict even in carriers, and support services lag behind those for other conditions.
For Lewy body dementia, changes in the GBA1 gene raise the risk of Parkinson’s and dementia with Lewy bodies.22PubMed Central. A severe neurodegenerative disease with Lewy bodies and a mutation in the glucocerebrosidase gene Carriers are roughly 8 times more likely to have Lewy body dementia.23PubMed. Exploring the link between GBA1 mutations and Dementia with Lewy bodies, A mini-review
Consumer DNA Kits and Third-Party Reports
People who uploaded raw kit data to outside interpretation services have been told of disease variants that clinical testing later showed weren’t there.24PubMed. Direct-to-consumer genetic testing with third party interpretation: beware of spurious results Consumer tests can give false negatives as well as false positives.25PubMed. Direct-to-consumer genetic testing with third party interpretation: beware of spurious results Any worrying result should be confirmed with a clinical-grade test before you act on it.
Most kits check a few specific points, like APOE, rather than reading whole genes. A clear kit result therefore doesn’t exclude rare causing genes. Seeing your APOE result is usually a separate opt-in choice.
If you do see an alarming result, don’t make major decisions on it yet. Book a genetic counselor rather than only your GP, bring the actual report rather than a screenshot, and expect a confirmatory lab test.
Before opening the APOE report, think about how you’d react to e4/e4. Consider who else in the family the result reveals information about, and check insurance timing first.
Uncertain and Unexpected Results
Panel tests often turn up gene changes nobody can interpret. In one series, more than a quarter of samples showed a gene change whose effect isn’t known, which your report may list as a variant of uncertain significance.26PubMed. Validation of next-generation sequencing technologies in genetic diagnosis of dementia Some changes once reported as harmful are probably harmless or only weakly harmful.27PubMed Central. Predictors for a dementia gene mutation based on gene-panel next-generation sequencing of a large dementia referral series Even some known causing changes skip people: one APP change has appeared in people healthy at 88.28PubMed Central. APP, PSEN1, and PSEN2 mutations in early-onset Alzheimer disease: A genetic screening study of familial and sporadic cases
Labs may reclassify an uncertain variant as evidence grows, so keep your contact details current with the lab so they can update you. Don’t use an uncertain result alone for family or reproductive decisions.
Onset age varies too. Some changes show symptoms anywhere from the 50s to the late 70s, so counselors give ranges rather than exact ages. Other genes and health factors likely shift the timing.
How Learning Your Result Can Feel
People told their APOE results showed no more anxiety or depression overall than those not told.29PubMed Central. Disclosure of APOE Genotype for Risk of Alzheimer’s Disease Those with APOE4 did feel more test-related distress than those without.30PubMed Central. Disclosure of APOE Genotype for Risk of Alzheimer’s Disease People with two copies had the most distress in the first week, easing by six weeks.31PubMed Central. Impact of learning APOE genotype on cognitively unimpaired adults: a pre-screening cohort study of the Alzheimer’s Prevention Initiative Generation Study 1 Depression didn’t rise at any point after disclosure.32PubMed Central. Impact of learning APOE genotype on cognitively unimpaired adults: a pre-screening cohort study of the Alzheimer’s Prevention Initiative Generation Study 1 How anxious you are before testing strongly predicts how you’ll feel after.33PubMed Central. Disclosure of APOE Genotype for Risk of Alzheimer’s Disease
To prepare, bring someone to the results appointment and think through what each possible result would mean beforehand. A history of anxiety or depression is worth raising with the counselor.
Telling family takes thought as well. Your result can reveal a sibling’s or child’s risk, and some relatives won’t want to know. Counselors can help plan the conversation.
How a Result Can Change Your Choices
Before starting lecanemab, a newer Alzheimer’s drug, experts recommend APOE testing.34PubMed Central. Lecanemab: Appropriate Use Recommendations The reason is that APOE4 carriers, especially with two copies, face higher risk of a side effect called ARIA, meaning brain swelling or small bleeds seen on scans.35PubMed Central. Lecanemab: Appropriate Use Recommendations Brain swelling occurred in about 35% of two-copy carriers versus about 7% of non-carriers.36PubMed Central. Amyloid-related imaging abnormalities (ARIA) in Alzheimer’s disease: from pathophysiology to individualized risk assessment Symptomatic cases occurred in up to about 11% of two-copy carriers.37PubMed Central. Amyloid-related imaging abnormalities (ARIA) in Alzheimer’s disease: from pathophysiology to individualized risk assessment Non-carriers still get the brain swelling or bleeding side effect, so scans stay necessary for everyone.38PubMed Central. Utility of APOE testing for reducing ARIA under probabilistic stopping rates to treat with anti-amyloid therapy for ε4-homozygote patients: A simulation study
Useful questions before starting: Has my APOE status been checked? How often will I need MRI scans? Which symptoms, like headache or confusion, should I report?
Results can also guide planning. APOE4 carriers who learned their status more often made decisions about future care directives.39PubMed. APOE Genotype Disclosure Influences Decisions About Future Planning but not Adoption of Healthy Lifestyle Changes in Cognitively Unimpaired Individuals Learning a higher genetic risk led to modest lasting diet improvements in one long-term follow-up.40Lifestyle Genomics. A Long-Term Follow-Up Study on Disclosing Genetic Risk Information (APOE) to Promote Healthy Lifestyles in Finland A result can be a prompt to address exercise, blood pressure and cholesterol.
Blood markers like p-tau217, combined with APOE, may help estimate timing. Among those with high p-tau217, APOE4 carriers developed symptoms in about 3 to 4 years, versus 5 to 6 for non-carriers. These are mostly used in research for now.
Family planning is one of the most common reasons people at risk see a genetic counselor.41PubMed Central. New Approaches to Genetic Counseling and Testing for Alzheimer’s Disease and Frontotemporal Degeneration Embryos made through IVF can be tested so a causing gene isn’t passed on.42PubMed Central. New Approaches to Genetic Counseling and Testing for Alzheimer’s Disease and Frontotemporal Degeneration This has led to the birth of a child free of a family’s early-onset Alzheimer’s gene.43PubMed. Preimplantation diagnosis for early-onset Alzheimer disease caused by V717L mutation
Insurance and Privacy Before You Test
In the US, GINA bars health insurers and employers from using genetic results against you.44PubMed Central. Genetic Testing For Alzheimer’s And Long-Term Care Insurance It doesn’t cover life, disability or long-term care insurance.45PubMed Central. Beyond the Genetic Information Nondiscrimination Act: ethical and economic implications of the exclusion of disability, long-term care and life insurance Consider buying those policies before testing. Long-term care insurance matters most for dementia planning, and a counselor can explain timing.
Applications may ask about known genetic results. Consumer kit data can be shared depending on your settings, so read the kit’s privacy policy and delete raw data if you’re unsure. Laws differ outside the US, so check locally.