Which genetic mutations cause Alzheimer’s disease and in which patients should genetic testing be offered?

Alzheimer's Genetic Mutations and Testing Recommendations

Genetic testing for Alzheimer's disease should be offered to symptomatic patients with early-onset AD (EOAD, onset <65 years) who have a family history of dementia, or to those with an autosomal dominant family pattern of dementia, testing specifically for mutations in PSEN1, PSEN2, and APP genes—and this testing must occur only within the context of formal genetic counseling. 1

Causative Genetic Mutations

The deterministic (disease-causing) mutations for Alzheimer's disease include:

• PSEN1 (Presenilin 1): Most common cause of autosomal dominant early-onset AD
• PSEN2 (Presenilin 2): Less frequent cause
• APP (Amyloid Precursor Protein): Less frequent cause

These mutations follow autosomal dominant inheritance with 50% transmission risk to offspring and typically cause symptoms before age 65. 1, 2, 3

Who Should Be Offered Genetic Testing

Offer testing for PSEN1/2 and APP mutations in these specific situations: 1

1. Symptomatic individuals with EOAD who have:

   • A family history of dementia, OR
   • Unknown family history (e.g., adoption)

2. Families with autosomal dominant dementia pattern showing:

   • One or more cases of EOAD
   • Multiple generations affected
   • Approximately 50% of family members developing dementia

3. Any relative of a known mutation carrier (regardless of symptoms)

The likelihood of finding a mutation decreases with:

• Fewer affected family members in the pedigree
• Older ages of onset in affected relatives 1

Who Should NOT Be Tested

Do not offer genetic testing in these situations: 1

• APOE genotyping for diagnostic purposes: APOE ε4 is a susceptibility gene, not deterministic. Testing has limited clinical utility, poor predictive value, and should not be used for diagnosis or in asymptomatic individuals 1, 2
• Pediatric testing: Never appropriate 1
• Prenatal testing: Not advised if pregnancy will be continued 1
• Late-onset AD (LOAD) without autosomal dominant family pattern
• Sporadic or familial cases without clear autosomal dominant inheritance

Critical Testing Protocol Requirements

All genetic testing must follow this framework: 1

Before Testing:

• Obtain ≥3-generation family history documenting:
  • Ages of onset of neurologic/psychiatric symptoms
  • Types of dementia and diagnostic methods
  • Current ages or ages at death
  • Causes of death
• Confirm AD diagnoses with medical records when possible
• Perform pedigree analysis to distinguish EOAD vs LOAD and inheritance pattern
• Test an affected family member first whenever possible—if unavailable and asymptomatic person still wants testing, counsel about low likelihood of informative results 1

Mandatory Genetic Counseling:

• Must be conducted by someone with expertise in AD genetics
• For symptomatic patients: Include legal guardian or family member
• For asymptomatic patients: Follow International Huntington Association protocol 1
• Explore motivations, personal values, and impact on quality of life
• Walk through scenarios of both positive and negative results

Key Counseling Points:

• General population lifetime AD risk: 10-12% by age 75-80 1
• Currently no proven interventions to reduce AD risk or stop progression 1
• Potential for genetic discrimination despite legislative protections
• Results should not be used to predict specific phenotype 1
• Consult Alzheimer Disease & Frontotemporal Dementia Mutation Database before result disclosure 1

Common Pitfalls to Avoid

1. Never order APOE testing for diagnosis or prediction in asymptomatic individuals—this violates consensus guidelines and has poor clinical utility 1, 2

2. Do not bypass genetic counseling—direct-to-consumer testing is explicitly not advised 1

3. Do not test asymptomatic individuals first—always attempt to test affected family members initially to establish whether a mutation exists in the family 1

4. Do not assume mutation presence predicts specific symptoms—significant phenotypic variability exists even within families carrying the same mutation 3

5. Recognize phenotypic overlap—AD and frontotemporal dementia can have overlapping presentations, complicating genetic interpretation 3

Special Considerations

For families without clear autosomal dominant pattern:

• Inform them their history suggests familial or sporadic AD
• Explain that genetic susceptibility may still play a role
• Risk estimates only available for first-degree relatives
• Genetic testing for susceptibility loci not clinically recommended 1

If patient insists on APOE testing despite counseling:

• May be considered at clinician's discretion
• Must follow Huntington disease genetic testing guidelines
• Requires extensive genetic counseling with qualified clinician
• Direct-to-consumer testing remains inadvisable 1

DNA banking option:

• Discuss when affected relatives are unavailable or uninterested in testing
• Preserves future testing options for family 1

The 2025 Alzheimer's Association guidelines 2 reinforce that deterministic genetic testing increases diagnostic confidence to "definite" category when mutations are identified in appropriate clinical contexts, but emphasize this requires specialized expertise for proper interpretation and disclosure.