Biomarkers in Alzheimer’s disease: Emerging trends and clinical implications

Alzheimer’s disease (AD) is the leading cause of dementia and its incidence continues to increase, thereby placing a heavy burden on caregivers as well as society in general. The primary pathological features of AD include extracellular amyloid-beta (Aβ) plaques and intracellular neurofibrillary tangles (NFTs), as well as synaptic and neuronal loss, activated microglia, and reactive astrocytes.^[1] These pathological changes generally progress from an asymptomatic phase to clinically detectable cognitive impairment, and this process typically occurs over several years. Treatment with monoclonal antibodies that lower the amyloid plaque burden has recently been approved for the treatment of patients with early-stage AD, thus underscoring the importance of biomarkers that accurately reflect AD pathology in clinical practice.^[2] Biomarkers can indicate early signs of AD and enable early intervention in the disease process. Supplementary Figure 1, http://links.lww.com/CM9/C404 illustrates the neuropathological features of the AD brain that can be detected using fluid or imaging biomarkers.

Advances in positron emission tomography (PET) multitarget probes have provided more powerful diagnostic tools for AD. In the area of fluid biomarkers, significant breakthroughs in blood-based biomarkers (BBMs) have been shown to have significant potential for non-invasive diagnostic applications. This article provides a comprehensive review of recent advances in AD imaging and fluid biomarkers and discusses their respective strengths, limitations, and potential future developments. The aim of this article is to assist clinical and research professionals with the construction of an integrated biomarker system for AD, thereby promoting early diagnosis and treatment within the framework of precision medicine.

Imaging biomarkers of AD: The principal imaging biomarkers in AD are Aβ-PET, which visualizes Aβ deposition; tau-PET, which detects tau protein pathology; alongside ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG)-PET and structural magnetic resonance imaging (MRI), both of which serve as indicators of neurodegeneration. Aβ-PET can reflect the burden of Aβ in the brain and has a high negative predictive value, meaning that patients with a negative Aβ-PET are unlikely to have AD. However, even with a negative Aβ-PET, it is still not possible to completely rule out the possibility that some patients will eventually progress to AD. Tau-PET can reflect the overall accumulation and spatial distribution of tau proteins in the brain, and it exhibits good consistency with the clinical manifestations of AD and Braak staging. Compared with Aβ-PET, tau-PET exhibits a stronger association with cognitive decline, demonstrates superior prognostic accuracy across the AD spectrum, and provides enhanced utility in predicting disease progression, particularly in preclinical and prodromal stages.^[3] FDG-PET can be used to assess glucose uptake in neurons and glial cells, thus reflecting the cellular functional status and serving as a basis for evaluating neurodegenerative changes. The distribution of hypometabolic regions in the brain helps distinguish AD from other neurodegenerative diseases, with hypometabolism in the parietotemporal association area, posterior cingulate, and precuneus highly suggestive of AD.^[4] Structural MRI typically reveals atrophy in the medial temporal lobe and the hippocampus, and it is used as an auxiliary diagnostic tool for AD.^[5]

In addition to the pathophysiological features of AD indicated by these imaging biomarkers, characteristics such as neuroinflammation and synaptic loss also occur during the progression of AD, and the corresponding PET imaging techniques are developing rapidly. Neuroinflammation plays a significant role in the pathological progression of AD, and microglia are the primary cells involved in this process. Currently, the most common target in neuroinflammation PET imaging is mitochondrial translocator protein (TSPO), which provides evidence of microglial involvement in AD pathogenesis.^[6] Synaptic loss is a prominent early feature of AD. Synaptic vesicle glycoprotein 2A (SV2A), a glycoprotein located on the membrane of synaptic vesicles, is commonly regarded as a biomarker of synaptic density. Currently, 11C-UCB-J is considered an ideal tracer with high specificity for SV2A.^[7] Although these novel tracers are still in the research stage, they can offer valuable insights into the pathophysiology of AD and may significantly contribute to the development of interventional strategies targeting inflammatory pathways.

Fluid biomarkers of AD: The development of AD fluid biomarkers commences with the cerebrospinal fluid (CSF) as a substrate, given its advantageous proximity to the brain parenchyma. Aβ and tau proteins can be released directly from neurons or released extracellularly as a result of neuronal injury. As a portion of the released proteins can enter the CSF with the interstitial fluid, CSF biomarkers are reliable indicators of the pathological changes in the brain. Aβ42, the Aβ42/Aβ40 ratio (reflecting Aβ deposition), total tau (t-tau, reflecting the degree of neurodegeneration), and phosphorylated tau protein (p-tau, reflecting the extent of NFTs changes) are widely recognized as core diagnostic biomarkers for AD. CSF Aβ42/Aβ40 exhibits strong concordance with Aβ-PET, thus supporting its validity as a cost-effective and reliable biomarker of AD pathology.^[8] For p-tau, CSF levels of p-tau181, p-tau217, and p-tau231 correlate well with AD pathology. Of these, CSF p-tau217 appears to have the best diagnostic performance, irrespective of whether Aβ-PET or tau-PET is used as the diagnostic gold standard.^[9] A number of novel CSF biomarkers specific for insoluble tau aggregates have been identified, such as the microtubule-binding region (MTBR) tau variant, MTBR-tau243, which exhibits a strong association with tau-PET imaging and longitudinal increases in insoluble tau.^[10] Combined assessment of these CSF biomarkers can be expected to further aid in differential diagnosis. For instance, post-mortem validation has confirmed that CSF p-tau/Aβ42 achieves a sensitivity and a specificity of 100% in distinguishing AD from other dementias.^[11]

Neurofilament light chain (NfL), neurogranin, synaptotagmin-1, growth-associated protein 43, and synaptosomal-associated protein 25, which reflect axonal damage and synaptic dysfunction, are upregulated in patients with AD and are potential diagnostic biomarkers. Glial fibrillary acidic protein (GFAP), a marker of astrocytic reactivity, and soluble triggering receptor expressed on myeloid cell 2 (sTREM2),^[12] which reflect microglial reactivity, are key inflammatory and immune biomarkers of AD. GFAP is associated with early Aβ-PET positivity, increased dementia risk, and accelerated cognitive decline, whereas TREM2 has garnered attention for its role in AD-related immune processes. TREM2 reduces AD progression by mitigating Aβ-dependent tau accumulation, spreading, and associated neurodegeneration.^[13] The levels of sTREM2 in the CSF are increased in the early stages of AD and are also associated with higher CSF t-tau and p-tau levels.^[14] These are important but non-specific biomarkers in the pathogenesis of AD and may help indicate disease severity. Establishing standardized measurement protocols for biomarkers, exploring reliable clinical cutoff values, and reducing heterogeneity across studies are key current issues that need to be addressed.

Most biomarkers of AD neuropathological changes can cross the blood–brain barrier from the CSF to the peripheral blood, although their concentrations in the peripheral blood are typically very low and thus difficult to detect. With the development and spread of highly ensitive technologies such as single-molecule arrays, immunoprecipitation mass spectrometry, and electrochemiluminescence, accurate detection of small amounts of BBMs has become feasible. Compared to CSF, BBMs are easier to obtain and more acceptable, allowing for widespread application in population screening.^[15] The currently assessed BBMs with good diagnostic efficacy include Aβ42/40,^[16] p-tau181, p-tau217, p-tau231, NfL,^[17] and GFAP.^[18] The diagnostic performance of plasma Aβ42/40 is inferior to that of CSF. However, several studies have shown that highly sensitive blood testing methods can be comparable to, or even surpass, CSF testing for AD pathology, such as p-tau217 or the ratio of p-tau217 to non-phosphorylated tau tested by plasma, with an area under the curve (AUC) of 0.95–0.98.^[19] The plasma NfL concentration correlates well with the CSF NfL concentration, thus making it a promising alternative for monitoring neurodegeneration, predicting disease progression, and assessing the risk of AD in symptomatic as well as in cognitively unimpaired individuals.^[17] The magnitude of changes in plasma GFAP surpasses that of CSF GFAP, and it exhibits a stronger correlation with Aβ pathology and a higher accuracy in discriminating between Aβ-positive and Aβ-negative individuals compared to CSF GFAP.^[20] In recent years, techniques for detecting peripheral blood biomarkers have significantly improved, thus increasing the likelihood of their clinical application. It is important to note that large-scale application of blood biomarkers requires further validation through real-world clinical studies and the establishment of standardized cut-off values. The specific pathogenic processes of the various biomarkers are summarized in Supplementary Table 1, http://links.lww.com/CM9/C404.

Research into the use of saliva, tears, and urine as potential biomarkers of AD is gaining increasing attention. Lactoferrin and exosomal microRNAs (miRNAs) in saliva may serve as biomarkers of AD.^[21] In tears, eukaryotic translation initiation factor 4E and miRNAs are also potential biomarkers of AD; however, their diagnostic value remains to be further validated owing to the limited volume of available tear fluid.^[22] In addition, the Alzheimer’s disease-associated neuronal thread protein (AD7c-NTP), which is overexpressed in the brains of patients with AD, exhibits a positive correlation with the disease course in the urine, although its specific cut-off values require further investigation.^[23] The collection of these bodily fluids is more straightforward and non-invasive compared to that of CSF and blood, thus offering potential advantages for large-scale screening, although related research is still in the early exploratory stages.

Clinical applications of AD biomarkers: The diagnosis of AD has evolved in recent years, transitioning from a syndromal framework to a biological basis. In 2024, the National Institute on Aging and the Alzheimer’s Association (NIA-AA) workgroup published updated research guidelines for the diagnosis and staging of AD,^[24] underscoring the critical role of characteristic biomarkers in establishing a definitive diagnosis. Biomarkers have been categorized based on their relevance to AD pathophysiology. Core biomarkers are those molecules that are associated with specific pathogenic mechanisms of AD (Aβ proteopathy, phosphorylated and secreted AD tau, or AD tau proteopathy). In addition, biomarkers of non-specific processes involved in AD pathophysiology (neuropil dysfunction or neuroinflammation) can provide valuable support for auxiliary diagnosis and staging. Finally, biomarkers of non-AD co-pathology (vascular brain injury or α-synuclein) can help identify coexisting pathologies that frequently occur alongside AD in older adults.

According to the guidelines, abnormalities in Core 1 biomarkers (i.e., amyloid PET, CSF Aβ 42/40, CSF p-tau 181/Aβ 42, CSF t-tau/Aβ 42, and accurate plasma biomarkers) can confirm the diagnosis of AD. Unlike previous guidelines, which focused on the clinical presentation, CSF biomarkers, and neuroimaging, the 2024 update introduced BBMs into the diagnostic framework. Accurate plasma biomarkers need to meet the requirement of a minimum accuracy of 90% in detecting abnormal amyloid PET within the intended population.^[25] However, standardized cut-offs for these biomarkers and their association with clinical prognosis have not yet been formally established for BBMs. For patients with abnormal BBMs, further confirmation of the diagnosis through CSF tests or PET imaging is required, which is equivalent to the accuracy of the approved CSF test. Core 2 biomarkers (tau-PET and certain soluble tau fragments associated with tau proteinopathy) can be used for staging, prognosis, and as indicators of biological treatment effects, but they cannot be used as a stand-alone diagnostic test for AD in most cases. When Core 1 biomarker results are normal, any abnormalities in Core 2 biomarker results should be interpreted with caution.

Currently, the diagnosis and staging of AD rely on the aforementioned biomarkers; however, their practical application may encounter several challenges. PET imaging exhibits high specificity and sensitivity, effectively detecting the distribution of Aβ and tau pathological protein deposits, thus enabling differentiation from other neurodegenerative disorders. However, adverse reactions to contrast agents and the high cost of PET limit its use in large-scale screening. CSF testing is less expensive than PET and has high consistency and reliability. Several CSF biomarkers have been approved for clinical use. However, the procedure is invasive and needs to be performed by specialized clinicians, taking into account specific indications and contraindications. Large-scale clinical cohorts have been shown to exhibit high concordance between a number of precise BBMs and CSF or PET biomarkers for AD. With advantages such as lower cost, ease of use, and minimally invasive sampling, BBMs have significant potential for broader clinical application. BBMs may serve as a primary screening tool or as an option for patients who are not suitable candidates for lumbar puncture or PET imaging. Clinicians should judiciously select the most appropriate biomarker(s) based on individual clinical needs and practical considerations.

Future perspectives for biomarkers: Neuroimaging and CSF biomarkers are currently well-established methods for accurate diagnosis of AD. Simple BBMs offer significant promise for advancing AD research, drug development, and clinical practice, particularly because of their cost-effectiveness and potential for broad clinical accessibility and population screening during the early stages of AD. However, the complexity of AD pathology poses challenges for single biomarkers to fully capture the intracerebral pathology, predict disease progression, and meet other clinical demands. The combined use of fluid and imaging biomarkers can provide comprehensive biological insights at multiple levels, thereby offering new opportunities for personalized medicine and effective disease management. In the future, multidimensional data, including multimodal PET/MRI, fluid biomarkers, and clinical information, may be integrated using artificial intelligence methods to achieve optimal combinations, thereby enabling accurate diagnosis and personalized treatment of AD.

Funding

This work was supported by grants from the Science and Technology Innovation 2030 Major Projects (No. 2022ZD0211600) and the Shanghai Municipal Health Commission Emerging Interdisciplinary Research Project (No. 2022JC014).

Conflicts of interest

None.

Supplementary Material

cm9-138-1009-s001.pdf ^{(130.6KB, pdf)}