S100 Beta Protein as a Marker of Hepatic Encephalopathy: A Breakthrough in Diagnostics or a False Trail? Review of the Literature.

Hepatic encephalopathy (HE) is a complication of acute and chronic liver diseases as well as portal-systemic bypass surgery [1]. It is defined as a complex of psychological disorders related to the nervous and musculoskeletal systems. These disorders are potentially reversible. Hepatic encephalopathy is associated with several symptoms that have a significant impact on health-related quality of life [2]. These symptoms include the following: deterioration of cognitive functions, disturbed sleep-wake rhythm, muscle tremors, memory disorders, and an extended reaction time to stimuli.

According to the recommendations of the European Association for the Study of the Liver (EASL) and the American Association for the Study of Liver Diseases (AASLD) [1], the qualification of hepatic encephalopathy should include four factors. These are: the relationship with the underlying disease, the severity of clinical manifestation, the duration of the course, and the presence of triggering factors. Due to the relationship with the underlying disease, hepatic encephalopathy is divided into type A – in the course of acute hepatic failure; type B – in the course of the dominant portal-systemic bypass flow or leak; and type C – in the course of cirrhosis. The classification of encephalopathy according to the severity of clinical manifestation takes into account the West Haven (WHC) classification and the International Society for Hepatic Encephalopathy (ISHEN) classification. Minimal hepatic encephalopathy (MHE) is the most challenging for clinicians, due to the lack of overt symptoms. Depending on its duration, HE can be divided into episodic, recurrent, and chronic form. HE can also be divided on the basis of presence of triggering factors, such as gastrointestinal bleeding, infections, electrolyte disturbances, constipation, and overdose of diuretics. All four factors should be used to fully assess a patient with HE.

Due to the increasing clinical importance of HE, it has become very important to search for a biomarker that would allow easier identification of patients with hepatic encephalopathy, especially in its minimal form. The most commonly used biomarker of HE in clinical practice is ammonia. This is due to the participation of ammonia in the pathogenesis of hepatic encephalopathy (it causes damage to the blood-brain barrier, which enables the penetration of other toxins into the central nervous system [CNS], and is also converted into glutamine in astrocytes, which leads to astrocyte edema). Its availability in hospital laboratories is also a factor. However, the role of ammonia is limited. Elevated serum ammonia levels are also observed in patients with cirrhosis without encephalopathy [3], and no correlation between ammonia concentration and hepatic encephalopathy has been proven [4]. Ammonia level in serum is also influenced by the patient’s age, physical activity, and smoking [5]. The technical aspect of preparing the material for the study also poses significant difficulties. False neurotransmitters (phenylethanolamine, octopamine, synephrine) also seem to play important roles in the pathogenesis of HE. However, their determination is currently not possible in clinical practice.

The search for a biomarker that can accurately and easily identify patients with hepatic encephalopathy, including minimal encephalopathy, is ongoing. It would also be beneficial to correlate its level with the severity of the disease and its applicability to monitoring responses to therapy. As HE mainly concerns the nervous system, the search for the desired biomarker is related to the function of this system.

The pathomechanism of hepatic encephalopathy is not yet fully understood. It has been proven that it is characterized by damage to the blood-brain barrier, which increases the concentration of ammonia and false neurotransmitters in the brain. False neurotransmitters displace real neurotransmitters (dopamine, noradrenaline) from synaptic connections, causing neuronal dysfunction. The increase in the concentration of ammonia in the brain leads, in addition to the increased production of false neurotransmitters, to an increase in the concentration of glutamine and glutamate in neurons and astrocytes [6]. This results in an increase in the osmotic pressure and the inflow of water to these cells, and leads to their swelling. The relationship of hepatic encephalopathy with edema of astrocytes contributes to research on the possibility of using molecules previously used in neurological diagnostics to diagnose and monitor hepatic encephalopathy.

One of the markers of astrocyte damage, known from research in neurology and neurosurgery, is the protein S100B. The S100B protein belongs to the S100 family of proteins: small, acidic, calcium-ion-binding proteins that are found in many tissues of the body. They were first identified in high concentrations in the brain structures of various mammalian species in 1965 [7]. It has a dimeric structure, consisting of two subunits (alpha and beta). The S100B protein is found mainly in astroglia and Schwann cells [8], but is not only specific to the brain. It is produced in a smaller amount by adipose tissue, skin, and T lymphocytes [9]. Its concentration in the brain is by far the highest of any system. This protein is metabolized in the kidneys and excreted in the urine [10]. There were no differences in protein expression between ethnic groups, gender, or the daily cycle [11].

The serum concentration of S100B protein under normal conditions does not exceed 01 micg/l. The increase in its concentration is observed in the case of damage and activation of astrocytes. The kinetics of the increase in serum S100B protein levels in trauma-related brain injuries [12] and in ischemic strokes [13] have been most accurately investigated. Research has also been conducted on the use of S100B as a marker of epilepsy, schizophrenia, and Creutzfeldt-Jakob disease [14].

Studies on the use of the S100B protein level in EW are scarce. In 1999, Wiltfang et al. [15] investigated the level of S100B in 36 patients with portal-systemic encephalopathy confirmed by psychometric tests and clinical examination. The group consisted of patients with overt and latent HE. High specificity and sensitivity (56.5% and 100%, respectively) of the S100BP level in the detection of HE on the substrate of portal systemic leak were found. The study also showed the advantage of S100BP level determination over ammonia level in detecting this type of hepatic encephalopathy.

Vaquero et al. [16], using material from the US Acute Liver Failure Study serum bank, tested S100B protein levels in 54 random samples taken on days one and three of the study. The randomly selected patients were divided into groups according to the advancement of HE. S100B protein levels were elevated in all samples but did not correlate with the severity of encephalopathy and liver function.

In a study by Saleh et al. conducted in 2007 [17], 52 patients were divided into 3 groups: patients with liver cirrhosis without encephalopathy, patients with liver cirrhosis and hepatic encephalopathy, and healthy controls. A significant correlation was found between the level of S100B protein in patients with HE (stages I and II), compared to healthy and cirrhotic patients without HE. Based on the obtained results, the authors concluded that S100B may be a valuable marker signaling the initial stages of HE.

In a study published in the next year, Isobe-Harima et al. [18] investigated the levels of S100B in patients with fulminant hepatitis. Nine patients with fulminant hepatitis of various etiologies (HBV, drug-induced, and indeterminate) were included in the study. All patients presented symptoms of overt hepatic encephalopathy. A significantly higher level of S100B in HE patients has been proven, but its influence on prognosis has been noted.

Duarte Rojo et al. [19] analyzed the level of S100B in 61 patients (46 with cirrhosis and a 15-person control group) divided into 4 homogeneous groups: healthy, with cirrhosis without encephalopathy, with cirrhosis with overt hepatic encephalopathy and with cirrhosis of the liver with latent hepatic encephalopathy. The explicit HE was determined on the basis of WHC, while the latent HE was determined on the basis of the psychometric hepatic encephalopathy score (PHES) and Critical Flicker Frequency (CFF). S100B serum levels were found to be significantly higher in patients with cirrhosis than in healthy subjects and it was elevated in patients with HE. The obtained data confirmed the possibility of using the serum level of S100B as a marker of the initial stages of HE.

In turn, Strebel et al. [20] analyzed the studies of 30 patients divided into groups with overt and with minimal hepatic encephalopathy (based on PSE and CFF), assessing, among other things, S100B protein level before treatment initiation and after 6 days of ornithine aspartate (LOLA) therapy. Elevated S100B levels were observed in 63% of patients, 46% of MHE patients, and 80% of OHE patients, respectively. There was no correlation between the level of the protein tested and the severity of HE. There was also no change in the level of S100B after 6 days of LOLA therapy.

Studies on the use of the S100B protein level as a biomarker of hepatic encephalopathy were also conducted in children with acute childhood hepatic insufficiency (PALF). Toney et al. [21] analyzed data from a multicenter study conducted in the USA, which enrolled 82 children with PALF, finding a significant increase in the level of the S100B protein in the serum correlating with the presence of HE. The authors suggest that the level of S100B can be used as a biomarker in the assessment of the neurological status of PALF patients.

The studies discussed here indicate that the assessment of the level of the S100B protein may be a promising marker in the diagnosis of hepatic encephalopathy. However, the results so far do not indicate the possibility of using the level of this protein to assess the effectiveness of therapy or to predict the course of the disease. In this respect, there is still a lack of multicenter studies in large groups of patients. Such studies would allow for final conclusions regarding the role and importance of the S100B protein level in the diagnosis and monitoring of ECT treatment.