Translating stroke biomarkers for patient benefit

Stroke biomarkers provide much insight into stroke biology that could be translated for patient benefit. When carefully harnessed, these biomarkers could guide decision-making in challenging clinical scenarios. This article offers an overview on current notable brain biomarkers that could aid clinicians in acute stroke management.

by Geelyn J.L. Ng and Dr Raymond C.S. Seet

Introduction
Stroke is a leading cause of permanent disability and the second most important cause of death globally [1]. Against a backdrop of a rapidly ageing society, there are concerns that a silent epidemic of stroke looms over our population.

Ischemic stroke, a subset that affects 87% of stroke population, results from atherosclerosis that affects predominantly the cerebral vasculature. Atherosclerosis of the blood vessels can lead to a cessation or depletion of blood flow to the brain, triggering cerebral ischemia when brain tissues are no longer viable. Blood clots can also be formed in blood vessels and in the heart, subsequently dislodging into the brain (‘embolic stroke’). Presently, there are only two clinically adopted methods of acute reperfusion treatment – intravenous recombinant tissue plasminogen activator (TPA) [2] and endovascular treatment through device-driven retrieval or aspiration of blood clots [3]. Although good functional recovery is five times more likely to occur with early reperfusion [3], the use of acute reperfusion treatment is restricted to a small group of patients where the benefits of treatment are weighed against the persisting risk of hemorrhagic transformation [4].

Sieving out stroke patients who are at risk of recurrent attacks is the first step to enable accurate triaging of patients to specialized units for in-depth observation and individualized treatment for complications arising from stroke. Presently, such identification is highly reliant on a clinician’s intuition and knowledge of neurologic deficits, as well as neuroimaging results. Tapping into the use of cerebral ischemia biomarkers could shed light on the complex pathological consequences following ischemic stroke and bring forth an unbiased system to weigh risks and benefits of treatments for clinicians and researchers alike.

Biomarkers are biological indicators of physiology that are objectively measured for use in risk stratification and development of therapeutic strategies. Having high sensitivity and specificity for the outcome it is expected to diagnose is generally a trait of a good biomarker, especially when targeting a complex and heterogenous disease such as stroke. Using a multi-biomarker platform could aim at different pathways of this multifaceted disease, thereby allowing for a more comprehensive treatment. Due to the presence of the blood-brain barrier (BBB) that holds a tight control over the inflow and outflow of particles, human brain tissues are typically difficult to access, making it impracticable to measure a biomarker within the brain. During cerebral ischemia, the BBB is broken down, causing brain-derived biomarkers to be released into the blood circulation, making it possible for a closer examination of the pathologic processes that take place following stroke onset. Although many biomarkers exist that could aid in stroke research, we have previously focused on notable blood-based stroke biomarkers that may play a bigger part in supporting the difficult clinical decision-making process [5]. This article will be providing an overview of several well-researched blood-based biomarkers, with much potential in aiding the clinical assessment of stroke patients.

Stroke biomarkers in the clinical scene
Studies in ischemic stroke have investigated the usefulness of blood-based biomarkers in identifying stroke mimics, establishing stroke etiology and prognosticating stroke severity and outcomes, such as vascular events and functional recovery [6, 7]. Presently, use of biomarkers in routine clinical practice remains uncommon, as stroke severity is still determined mainly through a thorough clinical neurological assessment and subjective interpretation of neuroimaging findings by a skilled physician. Nevertheless, having an objective means to prognosticate an outcome via a blood sample retrieved from a patient upon stroke presentation could add value to clinical decision-making, especially during times when neuroimaging results and clinical interpretations are unable to yield conclusive results. As stroke is a heterogeneous condition, investigating biomarkers that target different stroke pathways could be promising in establishing a multi-biomarker platform, especially for outcomes such as hemorrhagic transformation (HT), early neurologic deterioration (END) and malignant cerebral infarction.

Matrix metalloproteinase-9
Although administrating TPA could potentially achieve the benefit of arterial recanalization, the risk of symptomatic intracranial secondary hemorrhage within the infarcted brain tissues must not be forgotten. Matrix metalloproteinase-9 (MMP-9) is an enzyme that degrades the basal lamina and breaks down the extracellular matrix when activated during TPA treatment. The function of the BBB is crippled in this process and an inflammatory cascade is initiated, resulting in edema and the dreaded HT [8]. Apart from its involvement in HT, MMP-9 could also be used to identify high-risk END patients and plays a part in malignant cerebral infarction.

C-reactive protein
C-reactive protein (CRP) is a sensitive systemic marker of inflammation and a well-researched biomarker of ischemic stroke found in the blood plasma. CRP has been associated with END and noted to be predictive of adverse outcome, where ischemic stroke patients with higher CRP levels tend to suffer from a significantly worse outcome and mortality [9, 10].
S100β
S100β is a biomarker of ischemic stroke expressed by neuronal cells that can be released into the bloodstream when the BBB is compromised. Its concentration needs to be carefully balanced, as it may be protective in low concentrations, but at high levels has been shown to predict cerebral malignant infarction and correlate with infarct size [11, 12]. However, trial data on the use of biomarkers to guide clinical decisions leading to early decompressive surgery are currently lacking. Several studies have also uncovered an increase in S100β in ischemic stroke patients who present 1 to 7 days from symptom onset [13, 14]. In acute stroke patients, elevated S100β serum levels before thrombolytic therapy have also been demonstrated as a risk factor for HT [15].

N-terminal pro-brain natriuretic peptide
The brain natriuretic peptide (BNP) and its precursor, N-terminal proBNP (NT-proBNP), have been extensively studied as useful biomarkers for the prognosis and diagnosis of heart failure [16]. In recent years, BNP is gradually gaining recognition as a marker of atrial fibrillation (AF) and, therefore, as a biomarker to diagnose and predict stroke of cardioembolic origin [17, 18]. Plasma BNP levels have also been demonstrated to have significant correlations with infarct volume and National Institutes of Health stroke scale (NIHSS), making it a potentially powerful clinical biomarker for acute ischemic stroke [19].

Uric acid
Although uric acid has been adopted clinically for metabolic diseases, it is slowly garnering interest in the field of cardiovascular diseases due to its antioxidant properties. Despite data to suggest a strong association between uric acid levels and positive stroke outcomes [20, 21], several studies have observed an adverse relationship where higher uric acid levels were found to predict poor functional outcome and increased mortality [22, 23]. This disparity could highlight a dual role of uric acid in stroke, where both high and low levels of uric acid could adversely affect stroke outcomes [24]. Much remains to be explored for this biomarker before it could be roled out for use in stroke prognosis or diagnosis.
F2-isoprostanes
The product of arachidonic acid peroxidation generated by free radicals, F2-isoprostanes is well-established as a reliable biomarker for oxidative damage. Even though stroke is widely known as partly the result of oxidative damage, the relationship between F2-isoprostanes and human stroke remains poorly understood. Studies have demonstrated its importance in ischemic stroke as elevated levels of F2-isoprostanes could be observed in patients during the early course of stroke onset, with one even as early as three hours after [25–27].

Conclusion
Cerebral ischemia biomarkers have the potential to bridge translational gaps in medicine by shedding light on the pathological events leading to cerebral infarction and the ischemic cascade, aiding in clinical assessment during the critical time-sensitive decision-making process. Results in this area are still emerging, and more efforts could focus on ensuring the feasibility of incorporating stroke biomarkers for patient benefit. The translation of stroke biomarkers to clinical practice is challenging but can be extremely rewarding, especially when such concerted efforts of researchers, clinicians, industry partners and regulatory authorities result in a positive outcome for stroke patients.

Acknowledgements
We would like to thank the National Medical Research Council, Singapore (NMRC/CSA-SI/0003/2015, NMRC/CNIG/1115/2014 and NMRC/MOHIAFCat1/0015/2014) for their generous support.

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The authors
Geelyn J.L. Ng^1,2 BSc, Raymond C.S. Seet*^1,2 MBBS, MRCP (UK), MMed (Int Med), FRCP (UK)
¹Department of Medicine, Yong Loo Lin School of Medicine, National University of Singapore, Singapore
²Division of Neurology, Department of Medicine, National
University Health System, Singapore

*Corresponding author
E-mail: raymond_seet@nuhs.edu.sg