Doctors use the APP gene test during an Alzheimer's workup to look for an inherited mutation that causes early-onset disease. They order it only for selected patients, not as a routine dementia test.
The APP gene provides instructions for amyloid precursor protein. Enzymes cut that protein into amyloid-beta fragments that can accumulate as plaques, and many APP variants cause Alzheimer's beginning before age 65, according to MedlinePlus Genetics from the U.S. National Library of Medicine.
Medical information disclaimer: This article is for general educational purposes only and does not provide medical advice, diagnosis, or treatment. Always consult a physician or other qualified health professional about symptoms, medications, tests, or treatment decisions.
Table of Contents
- What the test looks for
- Who doctors test
- How testing proceeds in families
- Counseling and next steps after a result
- What a negative result does not mean
What the test looks for
APP sits on chromosome 21, while PSEN1 and PSEN2 sit on chromosomes 14 and 1. Mutations in all three are deterministic causes of dominantly inherited Alzheimer's with virtually full penetrance, according to the Dominantly Inherited Alzheimer Network review.
A child of a parent carrying an APP, PSEN1, or PSEN2 mutation has a 50% chance of inheriting it. In these families, disease often appears at ages 30 to 60, according to the joint guidelines from the American College of Medical Genetics and National Society of Genetic Counselors.
Who doctors test
Doctors reserve APP testing for dementia diagnosed at age 65 or younger. They also consider it with strong autosomal-dominant family history.
Other candidates include people with a known APP, PSEN1, or PSEN2 mutation in a first- or second-degree relative. These limits come from the eviCore Healthcare Lab Management Guidelines for early-onset familial Alzheimer testing.
- Memory loss or dementia at age 65 or younger
- Several relatives affected across generations in an early-onset pattern
- A known familial APP, PSEN1, or PSEN2 mutation in a close relative
How testing proceeds in families
Best practice is to test an affected family member first. That step identifies the exact familial variant when one exists.
Doctors then offer targeted predictive testing to at-risk adult relatives. They provide pre- and post-test genetic counseling around that testing, following the ACMG and NSGC joint practice guidelines.
Counseling and next steps after a result
Predictive APP testing in healthy adults must include counseling. Topics include the purpose of testing, the meaning of positive or negative results, family implications, benefits, risks, and confidentiality.
Counseling also covers the option to decline without loss of care. A positive result guides reproductive planning, cascade testing of relatives, and referral to early-onset registries or prevention trials, according to the Alzheimer's Association genetics page.
What a negative result does not mean
APP accounts for under 10% of early-onset cases and under 1% of all Alzheimer's. Other genes, APOE risk, and sporadic disease can still cause dementia.
So a negative APP result does not rule out Alzheimer's. Families with a strong early-onset pattern may still need review for PSEN1, PSEN2, and other causes.
