Blood-based peripheral biomarkers for dementia: are we any closer to their use in the clinical setting?

Dementia is one of the leading causes of disability in old age and places a huge burden on society. The growing prevalence of dementia calls for accurate, more accessible biomarkers to facilitate clinical diagnosis and prognosis. Peripheral mediums, such as blood and blood derivates (i.e. plasma and platelets), are currently being investigated for their potential as biomarkers of dementia subtypes. There is a lack of reproducibility in dementia biomarker studies, likely because of unaccounted factors such as age, ethnicity and gender, which is stalling their translation from research to the clinical setting. However, several blood-based biomarkers have been consistently reported from plasma and blood cells, including amyloid and tau protein, clusterin and immunoglobulins, as well as α-synuclein. This review highlights the need for further validation of the current blood-based dementia biomarkers for their routine clinical use.

by Oluwatomi E. S. Akingbade and Prof. Elizabeta B. Mukaetova-Ladinska

Introduction
The increasing incidence of neurodegenerative diseases, such as dementia, legitimizes the search for readily accessible biological markers. At present, there are 50 million people worldwide living with dementia, with 10 million new cases reported each year [1]. Dementia care costs are as high as £26 billion a year [1], with the likelihood of increasing further owing to higher numbers of people living with dementia. The increasing incidence of dementia calls for more efficient diagnosis. Currently, dementia diagnosis relies on extensive clinical assessments, facilitated by invasive (i.e. lumbar puncture) and technical (i.e. MRI) testing, all costly and inefficient in keeping up with the growing number of dementia patients.

The definitive diagnosis of dementia is done neuropathologically, and besides the clinical evidence of dementia, is based on the characteristic hallmarks of plaques and tangles (for Alzheimer’s disease (AD), the most common form of dementia [2]), Lewy bodies (for dementia with Lewy bodies (DLB)), and vascular changes (for vascular dementia). Some of the molecular substrates of the characteristic neuropathological dementia features have been taken forward both in neuroradiological (i.e. β-amyloid radiotracers) and biochemical assessments [i.e. amyloid-β (Aβ₄₂), total tau and phosphorylated tau₁₈₁ measurements in the cerebrospinal fluid (CSF)] [3]. However, their costs, limited access and invasive approach, as well as involvement in secondary inflammatory processes in dementia [4], are restricting their wider clinical utility. Thus, there is a need for the development of less invasive and more cost-effective peripheral biomarkers to facilitate the clinical diagnosis of dementia.

Unlike CSF, blood and blood derivates (platelets and plasma) are easily accessible in the clinical setting and the potential of using them to corroborate dementia diagnosis will likely lead to earlier, and more accurate dementia diagnoses. Although the blood–central nervous system barrier provides a physiological and physical barrier, changes in peripheral fluids and organs have been identified in people with dementia. Notably, erythrocyte [5] and platelet [6] physiology and function are largely effected in dementia. Proteins in peripheral organs are also being explored: i.e. α-synuclein, p53 protein, tau and amyloid in the skin, kidneys and liver; tau protein in the testes; Aβ_1–42 and acetylcholinesterase in serous fluid, as well as amyloid and tau proteins in the gastrointestinal tract. Explorative studies continue to report that peripheral components are influenced in dementia disease. Reoccurring peripheral proteins of interest in the search for dementia biomarkers include α-synuclein, Immunoglobulin G (IgG), Aβ precursor protein (AβPP), clusterin (all found in both platelets and plasma) and myeloperoxidase (MPO, present in plasma only).

Clinical relevance of potential blood-based dementia biomarkers
Peripheral Aβ and AβPP
Aβ plaques in the brain are key pathological hallmarks of AD, and lower Aβ CSF level is a known marker in autopsy-confirmed AD subjects [22]. Aβ is also reported in the periphery, i.e. blood and skin. In a healthy population, plasma Aβ₄₀, Aβ₄₂ and Aβ_40/42 ratio levels were significantly higher in older participants than in the younger ones [23]. In the Rotterdam Study Cohort [21], lower levels of plasma Aβ_1–28 and Aβ_40–42 were linked to higher risks of dementia in older age [7]. In platelets, the AβPP ratio [upper (130kDa) to lower (106–110kDa) immunoreactive bands] has been investigated (Table 1). Thus, there are differences in platelet AβPP ratios of those with cognitive deficits pertaining to dementia (i.e. very mild AD [10]; mild AD [10]; and AD [9–11]) and control patients. However, there are conflicting reports on the differences between the AβPP ratio in patients with varying severity of cognitive deficit with some studies [9,10]. Additionally, some studies have reported that increased AβPP levels in platelets correlated with higher Mini-Mental State Examination (MMSE) [11, 12] and Cambridge Cognition Examination (CAMCOG) scores [11]. Whether or not peripheral measurements of Aβ can be used as a pre-symptomatic marker of dementia (and/or the associated cognitive decline) remains uncertain. However, the studies mentioned in this review provide a consistent foundation for future studies to standardize measurement techniques that can be used in the clinical setting.

Peripheral α-synuclein
Studies have consistently reported the potential of α-synuclein as a biomarker of dementia – particularly in DLB as α-synuclein is reported to be a key constituent of Lewy bodies [24]. About 95 % of the α-synuclein in blood is predominantly found in erythrocytes, with the levels in plasma being extremely low. It is only recently that an assay was developed that is sensitive enough to detect the low levels of α-synuclein in plasma – reported in the range of 2.1–19.4 µg/L [25]. However, platelet α-synuclein levels in dementia have been reported to be similar among control and AD subjects [12] (Table 1). The utility of α-synuclein as a dementia biomarker is somewhat questionable, largely owing to its unknown physiological function in the human brain, as well as its involvement in Lewy body formation, which can be present in both normal aging as well as in a number of synucleinopathies, including Parkinson’s disease, multiple system atrophy, DLB, amyotrophic lateral sclerosis, etc, [26].

Blood clusterin
Increased plasma clusterin levels were reported to be indicative of both brain atrophy in AD patients [27] and an increased risk of dementia in older people [13]. Indeed. elevated levels of plasma clusterin have been reported as a risk factor for dementia [13, 28]. One study identified that lower MMSE cognitive scores in an AD population were associated with higher levels of plasma clusterin, whereas AD patients with higher cognitive scores had lower plasma clusterin levels [15]. Furthermore, longitudinal assessments identified that increased plasma clusterin concentrations are related to cognitive decline in mild cognitive impairment [28]. Interestingly, platelet levels of clusterin appear to be similar in both control and AD subjects [12, 16], but the ratio of platelet and plasma clusterin is positively correlated to specific neuropsychiatric inventory sub-categories, in particular agitation, apathy, motor aberrant behaviour and irritability seen in AD subjects [12] (Table 1).

Peripheral immunoglobulins
Increased IgG levels in plasma of AD subjects has been reported [17] with no link to disease progression. In an independent study, plasma IgG levels remained unaltered but increased in the platelet samples of the same AD subjects when compared to IgG levels in their control counterparts [12]. Inconsistent results found in IgG in plasma may be due to experimental constraints and, therefore, the potential of IgGs to form part of dementia diagnosis should not be ruled out. A recent study has reported that electrochemical techniques can be used to detect immunoglobulin in the plasma in the pg/mL range [29] – such sensitivity will likely aid in more consistent findings in plasma IgG studies in dementia.

Peripheral tau protein
CSF tau (and its ratio to Aβ) has been extensively investigated in relation to dementia. A recent literature review has addressed the limited clinical value of CSF tau biomarker studies [30], attesting to the need to address the uncertainty behind CSF tau as a suitable peripheral biomarker for dementia. More recently studies have focused on the presence of tau in the periphery. In plasma, high levels of tau were weakly associated with AD and were longitudinally associated with increased brain atrophy and poor cognition [18]. Platelet tau protein showed a more complex relationship than the reported plasma tau protein levels; studies have also reported a higher ratio between high to low molecular weight tau ratio in AD patients when compared to controls [19, 31] – with acknowledgement of no significant differences found between low or high molecular weight tau protein levels in the platelets of control and AD patients [19] (Table 1). Another study, reported a negative correlation between total tau and phosphorylated tau in the platelets of AD participants [20]. All in all, investigation into peripheral tau as a biomarker of dementia (and even specific dementia subtypes) is still in the early stages and requires further investigation before it can be considered for use in clinical diagnosis.

Summary and conclusions
Historically, the search for peripheral, blood-based dementia biomarkers has focused largely on the protein profiles of plasma and serum in dementia patients. This review, based on 13 dementia biomarker studies, has shown that proteins in the periphery are influenced by neurodegeneration in dementia. Namely, increased concentrations of Aβ₄₀, Aβ₄₂ and clusterin in plasma were all shown to be indicative of an increased risk of developing dementia in the elderly. Most recently, increased levels of plasma neurofilament light chain were reported to be closely related to amyloid processing in both mild cognitive impairment and AD, and to correlate with poor cognitive performance and AD-related brain atrophy and hypometabolism [32]. Changes in proteins in platelets were shown to coincide with cognitive decline in dementia: lower levels of AβPP and higher levels of clusterin were present in those with poorer performance in cognitive tests. Interestingly, increased plasma levels of tau protein were associated with brain atrophy while total and phosphorylated tau levels in platelets were negatively correlated. These findings provide encouraging evidence for the measurable presence of blood-based proteins that are closely linked to AD hallmarks that have the potential to be used in routine clinical setting not only for diagnosis, but also for severity and progression of the dementia process. The reproducibility and causes of possible heterogeneity, i.e. age, ethnicity, co-morbidity and genetics, that may influence protein expression at the periphery will need to be explored further before these biomarkers can be used routinely in the clinic, and their accuracy for distinct dementia subtypes will also need to be determined.

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The authors
Oluwatomi E. S. Akingbade^1,2 and Elizabeta B. Mukaetova-Ladinska*^2,3 MD, PhD, MRCPsych
¹School of Life Sciences, Queen’s Medical Centre, University of Nottingham, Nottingham, UK
²Department of Neuroscience, Psychology and Behaviour, University of Leicester, Leicester, UK
³Evington Centre, Leicester General Hospital, Leicester, UK

*Corresponding author
E-mail: eml12@le.ac.uk