More Than a Risk Factor
Overall, study findings provide evidence that APOE4 homozygotes represent another form of genetically determined Alzheimer’s disease, similar to autosomal-dominant Alzheimer’s disease and down syndrome-associated Alzheimer’s disease, investigators said.
“Our work showed that APOE4 homozygotes meet the three main characteristics of genetically determined Alzheimer’s disease, namely near-full penetrance, symptom onset predictability and a predictable sequence of biomarker and clinical changes,” they wrote.
Based on the results, investigators recommend that future clinical trials avoid combining single and double APOE4 carriers into one study group.
Because the global average proportion of APOE4 homozygotes is estimated to be approximately 2%, APOE4-homozygous Alzheimer’s disease may represent one of the most frequently occurring Mendelian diseases worldwide. This could have implications for genetic counseling and genetic screening recommendations, they said.
“We may need to start treating these homozygotes as a separate group in our research so we can really understand the relation between amyloid and tau and symptoms in E4 homozygotes in a way that we have not been able to because of our practice in the field of thinking that APOE4 is this unitary risk effect,” co-investigator Sterling Johnson, PhD, professor of geriatrics and dementia, University of Wisconsin-Madison, said at a press briefing.
The findings may also have implications for Alzheimer’s disease prevention, investigators added.
“What’s particularly important is the promise that perhaps we could treat people before symptoms, particularly in people who already have the disease in their brain such as APOE4 homozygotes, which reliably predicts that they will have impairment and try to treat them beforehand,” co-investigator Reisa Sperling, MD, director of the Center for Alzheimer Research and Treatment at Brigham and Women›s Hospital and Massachusetts General Hospital, Boston, said at a press briefing.
“This is important for preventing Alzheimer’s-related dementia and a real movement forward in defining the disease on the basis of genetics and biomarkers,” she added.
Experts Offer Mixed Reactions
Commenting on the findings, Paul Mathews, MD, DPhil, group leader of the UK Dementia Research Institute Centre at Imperial College, said that the data point to a need to look at APOE4 differently.
“One implication of this work is that testing for APOE4 gene homozygosity should be assessed for use clinically, when late middle-aged people present to their doctors with symptoms of dementia,” Dr. Mathews, who was not part of the study, said in a statement.
In an accompany editorial, Yadong Huang, MD, PhD, Departments of Neurology and Pathology, University of California, San Francisco, and co-authors noted that the findings also have implications for clinical drug trials.
“So far, APOE4 homozygotes have not been treated as a separate predefined treatment group in clinical trials,” they wrote. “Following this study, APOE4 status must be recognized as a crucial parameter in trial design, patient recruitment and data analysis, with APOE4 homozygotes and heterozygotes being clearly separated. Such an approach may enhance the treatment efficacy and help tailor therapeutic interventions more effectively towards genetically defined patient populations.”
Other experts urge caution when interpreting the findings.
“It is clear that APOE4 homozygosity is tightly linked to the appearance of Alzheimer’s-related pathology, but even at age 80, 12% of people with APOE4/E4 did not have amyloid/tau biomarkers,” said Yuko Hara, PhD, director of aging and Alzheimer’s disease prevention at the Alzheimer’s Drug Discovery Foundation. “Also, having two copies of APOE4 does not mean you will definitely develop symptoms of Alzheimer’s disease in your lifetime,” Dr. Hara added.
Researchers have long known that APOE4 is a strong risk factor for Alzheimer’s disease and that people with two copies of the gene are at especially high risk, David Curtis, MD, PhD, Genetics Institute at University of College London, England, said in a statement.
“I do not see anything in this paper to justify the claim that carrying two copies of APOE4 represents some ‘distinct genetic form’ of Alzheimer’s disease,” Dr. Curtis said. “No matter how many alleles of APOE4 one carries, the underlying disease processes seem similar across cases of Alzheimer’s disease, suggesting that any effective treatment and prevention strategies, which have yet to be developed would have broad applicability.”
Study funders included Fondo de Investigaciones Sanitario, Carlos III Health Institute, Fondo Europeo de Desarrollo Regional, Unión Europea, National Institutes of Health, the Department de Salut de la Generalitat de Catalunya, Horizon 2020–Research and Innovation Framework Programme from the European Union, La Caixa Foundation, EIT Digital, and the Alzheimer Association. Dr. Fortea reported receiving personal fees for service on the advisory boards, adjudication committees or speaker honoraria from AC Immune, Adamed, Alzheon, Biogen, Eisai, Esteve, Fujirebio, Ionis, Laboratorios Carnot, Life Molecular Imaging, Lilly, Lundbeck, Perha, Roche, and outside the submitted work. Dr. Johnson has served at scientific advisory boards for ALZPath, Enigma and Roche Diagnostics. Dr. Sperling has received personal consulting fees from AbbVie, AC Immune, Acumen, Alector, Bristol Myers Squibb, Janssen, Genentech, Ionis and Vaxxinity outside the submitted work. Dr. Huang is a co-founder and scientific advisory board member of GABAeron, Inc. Dr. Mathews reports consultancies with Sudo Biosciences, Nimbus, Redburn. Dr. Hara and Dr. Curtis reported no conflicts. Complete funding sources and disclosures are included in the original articles.
A version of this article appeared on Medscape.com.