Down syndrome, chromosome 21, and Alzheimer's disease

Why chromosome 21 matters

Down syndrome usually results from having a third copy of chromosome 21, also called trisomy 21. Chromosome 21 includes the APP gene, which provides instructions for amyloid precursor protein.

An additional copy of APP increases production of amyloid-related material over a lifetime. Most adults with Down syndrome develop amyloid plaques and tau tangles in the brain by middle age. These biological changes help explain the high risk of Alzheimer's disease, although other genes and biological processes on chromosome 21 and elsewhere also contribute.

Brain pathology and clinical dementia are not the same thing. Many, but not all, people with Down syndrome develop dementia. The timing and first signs vary from person to person.

Look for change from the person's own baseline

Standard cognitive tests may not capture the most meaningful changes in a person with lifelong intellectual disability. Family members, direct-support professionals, and clinicians who know the person well often provide essential information.

Possible early changes may include:

• needing more help with familiar daily tasks;
• losing skills that were previously stable;
• changes in communication, attention, or learning;
• new withdrawal, irritability, anxiety, or loss of interest;
• changes in walking, coordination, or continence;
• new seizures or a change in seizure pattern;
• disruption of sleep or daytime alertness.

None of these changes is specific to Alzheimer's disease. The pattern over time and the full medical evaluation matter.

Do not assume every change is dementia

Several treatable or manageable problems can affect thinking and function in an adult with Down syndrome. These can include thyroid disease, sleep apnea, hearing or vision loss, depression, medication effects, pain, infection, seizures, and major changes in living situation or support.

A careful evaluation asks what changed, when it changed, whether the change is progressive, and what else was happening at the same time. It also reviews medical conditions, medications, sleep, mood, sensory function, and safety.

The possibility of Alzheimer's disease should be taken seriously without making it the automatic explanation for every new difficulty.

Is this the same as an APP mutation?

No. In trisomy 21, there is an additional copy of chromosome 21 and therefore an additional copy of the APP gene. In rare autosomal dominant Alzheimer's families, a pathogenic variant changes the sequence or dosage of APP itself.

Both situations affect amyloid biology, but they are different genetic conditions with different family implications. The Alzheimer's risk associated with Down syndrome is generally not passed from an adult with Down syndrome to other relatives in the same way as a familial APP pathogenic variant.

Some less common forms of Down syndrome involve a chromosome translocation and can have distinct reproductive implications. A genetics professional can review the person's original chromosome report if that question is relevant.

What role do biomarkers have?

Amyloid PET, tau PET, blood biomarkers, spinal-fluid testing, and other measures are being studied extensively in adults with Down syndrome. These tools can provide evidence about Alzheimer's biology, but their interpretation and availability may differ from testing in the broader adult population.

A biomarker does not replace evidence of clinical change. The result should be connected to the person's baseline, current function, medical evaluation, and goals of care.

APOE can modify Alzheimer's risk in people with Down syndrome, but it is not the main explanation for the increased risk created by trisomy 21 and is not a stand-alone diagnostic test.

Planning an evaluation

Whenever possible, document abilities before a major decline is suspected. A baseline can include communication, work or day-program function, self-care, social engagement, movement, sleep, and behavior.

If changes emerge, bring the following to the visit:

• examples of what the person could do previously and what is different now;
• a timeline of the changes;
• an updated medication list;
• information about sleep, hearing, vision, mood, seizures, and recent medical illness;
• the original chromosome or genetic report if available;
• observations from more than one setting when possible.

The goal is not simply to assign a label. It is to identify contributors that can be treated, understand whether a progressive pattern is present, and support the person's function and quality of life.