Apolipoprotein E Genetic Testing in a New Age of Alzheimer Disease Clinical Practice

Abstract

The recent FDA approval of amyloid-lowering drugs is changing the landscape of Alzheimer disease (AD) clinical practice. Previously, apolipoprotein E (APOE) genetic testing was not recommended in the care of people with AD because of limited clinical utility. With the advent of amyloid-lowering drugs, APOE genotype will play an important role in guiding treatment recommendations. Recent clinical trials have reported strong associations between APOE genotype and the safety and possibly the efficacy of amyloid-lowering drugs. Therefore, a clinical workflow that includes biomarker and genetic testing should be implemented to provide patients with the opportunity to make informed decisions and instruct safety monitoring for clinicians. Pretest consent, education, and counseling will be an essential aspect of this process for patients and their family members to understand the implications of these tests and their results. Given that the approved amyloid-lowering drugs are indicated for patients with mild cognitive impairment or mild dementia with biomarker evidence of AD, biomarker testing should be performed before genetic testing and genetic testing should only be performed in patients interested in treatment with amyloid-lowering drugs. It is also important to consider other implications of genetic testing, including burden on and need for additional training for clinicians, the role of additional providers, and the potential challenges for patients and families.

Introduction

The recent FDA approvals of 2 amyloid-lowering drugs for Alzheimer disease (AD) are leading to a paradigm shift. Apolipoprotein E (APOE) genotypes are clearly associated with treatment risks and possibly with the efficacy of treatment. Although APOE testing was not previously recommended in the care of people with AD, it will be an important element of care when these treatments are prescribed. We summarize the available evidence related to the role of APOE testing when prescribing recently approved AD treatments and consider the implications of incorporating APOE genetic testing in the clinical care of people with AD.

APOE and AD

In 1993, 3 seminal studies first reported the association between genetic variation in APOE and the risk of developing sporadic AD.^1-3 APOE has 3 allelic variants (ε2, ε3, and ε4), which encode for the isoforms apoE2, apoE3, and apoE4, respectively. APOE ε3 is the most common allele and therefore the benchmark for comparison. APOE ε2 is protective against AD, with heterozygotes having approximately 61% and homozygotes having 87% reduced risk of disease, relative to APOE ε3 homozygotes.^1,4 By contrast, APOE ε4 is the strongest genetic risk factor for AD, increasing the risk 2- to 4-fold for heterozygotes and 8- to 12-fold for homozygotes.⁵ APOE ε4 carriage has been shown to lower the age of AD onset in a gene-dose–dependent manner.¹ The mean age of onset is 84 years in noncarriers, 76 years in ε4 heterozygotes, and 68 years in homozygotes.¹ Although ε4 increases AD risk, it is not deterministic. Similarly, ε4 noncarriers and ε2 carriers may develop AD because other genetic and environmental factors contribute to disease risk.⁶

The mechanisms by which APOE ε4 increases the risk of AD are not fully understood. The primary function of apoE is to transport cholesterol and other lipids in the CNS, but there is evidence to suggest that APOE ε4 plays a key role in AD neuropathologic changes (ADNC).⁷ APOE genotypes strongly affect the deposition of Aβ to form senile plaques.⁷ APOE ε4 has also been implicated in pathways that are independent of amyloid, for example, increasing levels of tau in the medial temporal lobe, even in the absence of amyloid plaques.⁸ APOE ε4 may also dysregulate the immunomodulatory function of microglia and astrocytes, which can lead to neuroinflammation and neuronal damage.⁹

APOE has a strong association with the development of cerebral amyloid angiopathy (CAA),⁷ the deposition of amyloid in the walls of the cerebral microvessels. CAA is present in approximately 80%–90% of individuals with AD,¹⁰ can weaken blood vessel integrity, leads to microvascular dysfunction, and increases risk for stroke or other cerebrovascular injuries. Although CAA is strongly associated with ADNC, the association between APOE ε4 and CAA remains even after controlling for AD.¹¹ Despite APOE ε2's protective role against AD, it has been reported that APOE ε2 is a risk factor for CAA-related lobar hemorrhage.¹²

The role of APOE in AD clinical presentation and progression remains unclear. One study found that ε4 carriers were 3 times more likely to present with memory deficits rather than nonamnestic disorders such as agnosia and apraxia.¹³ The APOE ε4 effect on clinical symptoms might be mediated by changes in ADNC and earlier age of onset. Alternatively, it is possible that APOE genotype affects other amyloid-independent and tau-independent mechanisms. APOE genotype may also contribute to heterogeneity in disease progression.^14-16 Whether APOE ε4 accelerates the progression of cognitive decline compared with noncarriers,¹⁴ has no effect,¹⁵ or slows decline¹⁶ remains uncertain. The effects of APOE ε4 on progression may vary depending on other confounding factors, such as age, sex, gene-environment interaction, the type of AD (typical vs atypical),¹⁶ and the increasingly clear role of non-APOE risk genes.¹⁷

Clinical Practice Before the Advent of Antiamyloid Therapies

Given the uncertainty around the role of APOE ε4 in disease progression and prognosis, current clinical practice guidelines do not recommend APOE genetic testing in patients with suspected AD.¹⁸ Until recently, the only FDA-approved treatments for AD were symptomatic agents that target neurotransmitter levels or activity, including cholinesterase inhibitors (donepezil, galantamine, and rivastigmine) and an NMDA antagonist (memantine). The FDA-approved labels for these agents do not comment on potential effect modification by APOE genotypes for either treatment efficacy or safety. Although one trial suggested a potential benefit of donepezil in APOE ε4 carriers in slowing progression from mild cognitive impairment (MCI) to dementia, the overall primary outcome for the trial was negative.¹⁹

Beyond the limited clinical value, genetic testing carries risks. Although some protections against discrimination based on genetic results are in place, gaps remain. For example, legal protections do not include long-term care, disability, or life insurance, for patients with AD or their offspring.^18,20

Clinical Practice After the Advent of Antiamyloid Therapies

Data from recent clinical trials of amyloid-lowering drugs suggest that clinical practice will need to evolve to include APOE testing as an important component guiding treatment recommendations. The primary rationale for this change is a clear relationship between APOE genotype and the main safety concern with amyloid-lowering therapies, amyloid-related imaging abnormalities (ARIA). ARIA was coined based on observations of MRI signal changes that represent vasogenic edema (ARIA-E) as well as cerebral microhemorrhages and superficial siderosis (ARIA-H).²¹ Radiographic severity of ARIA-E is evaluated by fluid-attenuated inversion recovery (FLAIR) hyperintensities and is classified as mild (single region with <5 cm), moderate (single region with 5–10 cm or multiple regions all <10 cm), or severe (any region >10 cm). The radiographic severity of new incident microhemorrhages and superficial siderosis is also classified as mild (≤4 microhemorrhages; 1 area of superficial siderosis), moderate (5–9 microhemorrhages; 2 areas of superficial siderosis), or severe (≥10 microhemorrhages; >2 areas of superficial siderosis). Most ARIA cases are asymptomatic. When symptoms do occur, they commonly include headaches, visual disturbances, confusion, dizziness, and nausea.^22,23

ARIA was first reported in a phase II trial of the monoclonal antibody bapineuzumab, and even in this relatively small trial, APOE risk was evident.²⁴ Subsequent trials of bapineuzumab and other amyloid-lowering drugs have confirmed a gene-dose risk for ARIA, with homozygotes at increased risk relative to heterozygotes and heterozygotes at greater risk than noncarriers.^23,25

Two drugs have received FDA approval under the accelerated pathway based on a surrogate endpoint (a reduction in amyloid PET burden), aducanumab in 2021 and lecanemab in 2023. In 2023, lecanemab also received full approval. The labels for both drugs indicate the effect of APOE on treatment safety.^26,27 In 2 phase III trials of aducanumab, EMERGE and ENGAGE, ARIA-E was the most frequently reported adverse event (AE) and was observed in 65% of homozygotes and 35% of ε4 heterozygotes (compared with 18% and 23% of noncarriers in the 2 trials).²⁵ Most ARIA-E cases were reported within the first 8 infusions. APOE ε4 genotype did not increase the risk of ARIA-H, but later studies found that ARIA-H incidence was higher in individuals who experienced ARIA-E compared with those without.²² In the phase III Clarity AD trial of lecanemab, 32.6% of ε4 homozygote and 10.9% of ε4 heterozygote participants experienced ARIA-E, compared with 5.4% in noncarriers.²³ Symptomatic ARIA-E was observed in 9.2% of ε4 homozygotes and 1.7% of heterozygotes, compared with 1.4% of noncarriers.²³ ε4 homozygote participants had a higher incidence of ARIA-H (39.0%) than ε4 heterozygote participants (14.0%) and noncarriers (11.9%).²³ A small number of fatal cases observed during open label extension studies of lecanemab have been reported.²⁸ These cases are critically important, but complicated by the occurrence during uncontrolled study phases and mixed presentations of APOE carriage and the use of anticoagulant therapies.²⁹ They nonetheless provide important cautions for prescribing physicians.

Based on the findings from EMERGE and ENGAGE, the current prescribing information for aducanumab is the same for APOE ε4 carriers and noncarriers and recommends a titration schedule over 6 months, ramping up to the reported therapeutic dose of 10 mg/kg thereafter.²⁷ For safety monitoring, MRI brain scans are recommended before the fifth, seventh, and twelfth infusions; however, expert recommendations acknowledge that some clinicians may wish to implement additional safety imaging, particularly for ε4 carriers.³⁰ For lecanemab, the Clarity AD trial implemented biweekly 10 mg/kg dosing (with no dose ramping) to all APOE genotypes and this approach is indicated in the current prescribing information.²⁶ To monitor for ARIA, MRI scans should be obtained before the first treatment and before the fifth, seventh, and fourteenth infusions. Expert appropriate use recommendations advise an additional MRI scan before the twenty-sixth infusion for ε4 carriers.³¹

The EMERGE and ENGAGE trials of aducanumab showed conflicting results on clinical efficacy; the EMERGE trial suggested disease slowing based on the Clinical Dementia Rating Scale Sum of Boxes (CDR-SB) of approximately 22% at 78 weeks, compared with placebo.²⁵ Sensitivity analyses showed no clear differences between ε4 carriers and noncarriers on clinical outcomes. The Clarity AD trial met its primary endpoint (CDR-SB) at week 79; lecanemab showed about a 27% reduction in decline compared with placebo, as well as consistent benefits across secondary outcomes.²³ Results from subgroup analyses suggested potentially reduced clinical benefit in APOE ε4 carriers but should be interpreted with caution because the analyses were underpowered and were not adjusted for potential confounding variables.

Role of Genetic Testing in a Practice That Includes Amyloid-Lowering Therapies

The clinical evaluation to determine whether a patient is an appropriate candidate for amyloid-lowering drugs begins with a thorough assessment for potential contributors to cognitive impairment and the severity of symptoms (Figure).³² For patients with MCI or mild dementia in whom AD is a potential contributor and are considering amyloid-lowering therapies, this work-up should include biomarker testing. Biomarker testing, on its own, brings important opportunities to help patients understand the cause of cognitive impairment, prognosis, and for taking action.^33,34 This action now will potentially include treatment with amyloid-lowering drugs. Clinicians will likely need to devote considerable time to educating patients about biomarker testing and setting appropriate expectations for test results and next steps.³⁴

Figure. Flowchart Illustrating the Clinical Evaluation for Patients With Early Alzheimer Disease Interested in Treatment With Amyloid-Lowering Drugs.

At some point in this clinical workflow, discussions with patients and their families about APOE genetic testing will also need to occur. Discussion of genetic testing will ideally be included in early conversations about therapies because the information is so closely tied to treatment risk. Indeed, some working protocols for clinical implementation of antiamyloid therapies delineate the need for clinical informed consent, which will inherently require describing the genotype-specific drug risks. Some clinicians may feel comfortable discussing the risks and benefits of APOE genetic testing as well as the risks and benefits of amyloid-lowering drugs, but many will not. Clinicians may wish to involve geneticists and/or genetic counselors in this clinical workflow to facilitate complete coverage of critical issues and to ensure that patients and families have the opportunity to have all of their questions answered before proceeding with genetic testing.

We suggest that APOE testing should be performed after AD biomarker testing and only in patients interested in treatment with amyloid-lowering drugs. In individuals who do not wish to pursue treatment or in whom biomarker tests do not support treatment, APOE testing offers little clinical value and previous recommendations against genetic testing remain valid.

Regardless of when in the workflow genetic testing is performed, pretest consent, education, and counseling will be essential.¹⁸ Amyloid-lowering drugs are indicated for early AD, when many patients retain capacity to make treatment decisions, but the role of caregivers in this decision and in monitoring patients who are ultimately treated will be critical. Family members can also provide support, assist in conveying results, and facilitate retention of important information and clinician recommendations.³⁴ Discussing the implications of test results to family members is particularly important for genetic testing because the results are relevant to more than just the patient with cognitive impairment.^18,20 For patients who are ε4 homozygotes, this guarantees that their biological children are ε4 carriers.¹⁸ Learning this result can affect the patient's and family members' emotional well-being, lifestyle choices, future planning, and decisions around insurance. Clinicians should, however, be aware of potential conflicts of interest when genetic testing is requested by a family member concerned about their own risk.¹⁸ Genetic testing should not be performed against the will or without the knowledge of the patient.

Available research indicates that APOE testing can be performed safely. A randomized study assessing the safety of disclosing APOE genetic risk to MCI participants found that anxiety and depression scores did not significantly increase for those who learned their genetic results, compared with those who did not.³⁵ Although most research on disclosure has been performed through in-person protocols, practice needs may dictate more feasible approaches. In a randomized controlled trial of telephone compared with in-person APOE disclosure among cognitively unimpaired individuals, noninferiority was observed for anxiety, depression, and distress, in the overall sample and in ε4 noncarriers.³⁶ In APOE ε4 carriers, however, the authors reported noninferiority only for the outcome of anxiety. Although the subanalysis results were inconclusive, levels of depression and distress trended higher among ε4 carriers learning through telephone. Notably, the study was conducted before the COVID-19 pandemic; telehealth has seen important strides including the increased access to nontelephone remote approaches to delivering care, such as Zoom and other video platforms. Given the increased familiarity with remote consultations during the COVID-19 pandemic, there may be opportunity for further research on remote APOE disclosure practices.

CMS has announced their intention to cover amyloid-lowering therapies fully approved by the FDA.³⁷ This is likely to amount to 80% coverage, leaving significant out-of-pocket costs for those without supplemental insurance. Although CMS also intends to cover biomarker tests to assess appropriateness for therapies (cms.gov/medicare-coverage-database/view/ncacal-decision-memo.aspx?proposed=Y&NCAId=308#:∼:text=There%2C%20one%20amyloid%20PET%20scan, ncdid%3D356%26ncdver%3D1%20), it is unclear whether APOE testing will be included. Such costs, if born by patients and families, could dissuade patients from undergoing genetic testing. There is significant variation in the cost of such tests by different companies. Clinicians should help their patients by providing information on these out-of-pocket costs and minimizing financial burden for patients.

Potential Challenges in Clinical Practice

The rapid growth of the aging population has led to a shortage of neurologists and other specialists skilled in diagnosing cognitive disorders.³⁸ Similar shortages exist for genetic counselors,³⁹ and the requirements of genetic testing for new AD treatments could exacerbate this shortage. When considering patients for amyloid-lowering drugs, healthcare specialists will be required to spend their already limited time on education and counseling, including discussions of treatments, biomarkers, and genetic testing.³⁸ Each of these requires substantial knowledge, expertise, and experience. Clinicians may, therefore, require additional training to feel equipped and comfortable answering patients' and family members' questions and guiding their decisions. Furthermore, the shortage of resources may be exacerbated by the fact that the patient's biological family members (i.e., siblings and children) may seek genetic testing and disclosure after learning that their relative is an APOE ε4 carrier.

The 21st Century Cures Act mandates immediate availability of test results to patients, which may limit the ability of clinicians to deliver results in a safe and sensitive way. Another reality is that direct-to-consumer (DTC) genetic testing is already accessible to the public. Although current clinical practice guidelines do not advise APOE testing through these companies,¹⁸ some patients may arrive to the clinic with APOE results in hand. These patients often do not have access to education and counseling before DTC testing, so providing them with this opportunity will likely be beneficial. DTC tests may or may not be from FDA-approved sources, leading to questionable integrity of results and the potential need for repeat testing.

The lecanemab Clarity AD trial included a relatively diverse population of participants,²³ while the ENGAGE and EMERGE trials were far from representative.⁴⁰ In both cases, essentially no data are available to instruct conversations with patients from diverse backgrounds about whether APOE effects are modified by race and ethnicity. This adds to the overall challenges of fewer data to understand performance of clinical assessment tools, biomarker tests, and genetic underpinnings of disease in diverse communities, particularly patients of minority races or ethnicities.

Collectively, these changes in practice will dramatically increase the time requirements for diagnosing and treating clinicians. This could further exacerbate shortages in the availability of expert clinicians, delaying diagnoses and treatment initiation, perhaps even to the point that patients become no longer eligible for treatment. This could also exacerbate current healthcare disparities in the care of people with cognitive disorders because diagnoses may already be delayed in some communities. Changes in reimbursement will be essential to compensate the teams of clinicians required to provide this new model of care but also to incentivize clinical trainees to choose these specialties.

Conclusions

The recent FDA approvals of aducanumab and lecanemab, 2 amyloid-lowering drugs for AD, will have immediate and long-term ramifications on clinical practice. These treatments carry a differential risk of ARIA based on APOE genotype. As a result, genetic testing should be discussed with patients with early AD interested in receiving amyloid-lowering drugs. The inclusion of genetic testing is a marked change from current clinical guidelines. Genetic testing in the clinical decision-making process for these drugs will give patients and their families the opportunity to make informed decisions and instruct safety monitoring for clinicians.

Appendix. Authors

Name	Location	Contribution
Marina Ritchie, MA	UC Irvine Institute for Memory Impairments and Neurological Disorders, University of California, Irvine; Department of Neurobiology and Behavior, University of California, Irvine	Drafting/revision of the manuscript for content, including medical writing for content; study concept or design
Seyed Ahmad Sajjadi, MD, PhD	UC Irvine Institute for Memory Impairments and Neurological Disorders, University of California, Irvine; Department of Neurology, University of California, Irvine	Drafting/revision of the manuscript for content, including medical writing for content
Joshua D. Grill, PhD	UC Irvine Institute for Memory Impairments and Neurological Disorders, University of California, Irvine; Department of Neurobiology and Behavior; Department of Psychiatry and Human Behavior, University of California, Irvine	Drafting/revision of the manuscript for content, including medical writing for content; study concept or design

Study Funding

This work was supported by NIA P30 AG066519.

Disclosure

M.R. reports no disclosures relevant to the manuscript; J.D.G. reports research support from Eli Lilly, Genentech, Biogen, Eisai, NIA, the Alzheimer's Association, and BrightFocus Foundation; he has provided consulting to SiteRx, Cogniciti, and Flint Rehab; S.A.S. has provided consulting to Eisai and Genentech. Full disclosure form information provided by the authors is available with the full text of this article at Neurology.org/cp.