Peripheral Uric Acid as a Biomarker in Individuals with Bipolar Disorder

Background: At present, no well-established biomarkers were ever found to distinguish unipolar disorder (UD) and bipolar disorder (BD). This study aimed to explore whether uric acid (UA) could be a biomarker to distinguish UD and BD. Methods: Peripheral UA of 119 patients with BD in acute stage (AS) and 77 in remission stage (RS), and 95 patients with UD in AS and 61 in RS were measured, so were 180 healthy controls. Results: UA levels in BD group were higher than UD and HC groups regardless of the AS or RS, while differences in UA levels between UD group and HC group were not signicant. Differences of UA levels between BD-M and BD-D subgroups were not signicant, and UA levels of BD-M and BD-D subgroups were higher than UD and HC groups. Only in UD group, UA levels of drug-use subgroup were higher than drug-naïve/free subgroup, but differences disappeared when analyzed stratied by sex; whether in drug-use or drug-naïve/free subgroup, differences of UA levels between BD-M and BD-D groups were not signicant. Conclusion: The study suggests UA levels may be a biomarker of BD to distinguish from UD. BD-D, bipolar depression; UD, unipolar disorder; HC, healthy control; AS, acute stage; RS, remission stage; DSM-5, Diagnostic and Statistical Manual of Mental Disorders, fth edition; MECT, Modied electroconvulsive therapy.


Background
Bipolar disorder (BD) is a serious mental disorder with a low diagnosis rate result from the onset of BD is often characterized by a depressive episode, which is similar in presentation to unipolar depression (UD) [1]. Due to misdiagnosis, inappropriate treatment with antidepressants without concomitant mood stabilizers results in switching to mania or hypomania and repeated attacks of depression [2]. A recent study showed that family history of BD, early age at onset of the rst depressive episode ( 25 years), postpartum depressive episodes, rapid onset of depressive episodes, worse response to antidepressants and the presence of psychotic symptoms or atypical depressive symptoms might be the most consistent clinical predictors of BD [3]. However, no laboratory or imaging marker is identi ed to allow for a diagnosis of BD or distinguishing between BD and UD.
The purinergic system is a critical neurotransmitter system with the end product of Uric acid (UA), which involves the occurrence and development of mental illness [4]. It has been proved that increased levels of UA are associated with the accelerated purinergic transformation [5]. UA acts on presynaptic and postsynaptic neurons and speci c receptors in the glial cell membrane that can affect other neurotransmitters' activities involved in the pathophysiological process of mood disorders, including dopamine, gamma-aminobutyric acid, glutamate and serotonin [6].
In the late 19th century, researchers found that some patients with gout and hyperuricemia suffered from mood disorders and were relieved after receiving lithium treatment. Since then, the relation between UA and mood disorders has raised the hypothesis of purinergic system dysfunction [7]. Recent studies showed that the highest UA levels were observed in patients with BD compared with other mental disorders and healthy controls (HC) [8][9][10][11], and elevated UA levels were associated with impulsivity, excitatory behavior, irritability, hyperthymia temperament and severe manic symptoms [6,12]. While the lowest UA levels were observed in patients with UD, suggesting that UA may be a potential biomarker for distinguishing between BD and UD. Besides, patients with BD have an increased risk of gout [13], while allopurinol, an inhibitor of xanthine oxidase used to treat and prevent gout, can be used as an add-on therapy for patients with BD to reduce manic symptoms [14]. Some studies also implied that compared with bipolar depression and remission, the highest UA levels were observed in the manic episode, indicating that UA may be a status marker of manic episodes rather than a trait marker [15][16][17]. However, similar results were not detected in similar studies. Studies by Salvadore G et al. and Gültekin BK et al. showed that UA levels were higher in patients with BD than in healthy controls but not associated with the severity of mania. Furthermore, some studies showed there were no statistically signi cant differences in UA levels between BD and UD, neither did to healthy controls [18][19][20].
Previous studies on UA of patients with BD and UD are limited and con icting. Therefore, whether UA could be a biomarker of BD remains to be further studied. The present study aimed to evaluate whether UA could be a potential biomarker to distinguish between BD and UD.

Subjects and participants
The study protocol was approved by the Clinical Research Ethics Committee of Shandong Mental Health Center and is compliant with the Code of Ethics of the World Medical Association (Declaration of Helsinki). Informed written consent was obtained from all participants or their legal guardians after a complete and extensive description.
The study was conducted at Shandong Mental Health Center, form May 2018 to May 2019, inpatients and outpatients aged from 18-60 years with the Diagnostic and Statistical Manual of Mental Disorders, fth edition (DSM-5) diagnosis of BD or UD in the acute stage (AS) or remission stage (RS) were recruited. Furthermore, healthy individuals with no family history of psychiatric disorders were enrolled in the study as the control group.
The exclusion criteria were as follows: (1) Combined with organic brain diseases or brain trauma. (2) Hypertension, diabetes, gout or liver, kidney, biliary, and other physical diseases or abnormal renal and liver function. Measurements of serum UA levels Serum UA levels test as part of routine blood checks was performed during the inpatient stays and the regular return visit of outpatients, while serum UA levels test of healthy individuals in this study was performed after enrollment. The assay was prepared as follows: ve milliliters of fasting venous blood samples were drawn from all participants. According to the manufacturer's instructions, serum levels of UA were detected by Roche Cobas C702 automatic biochemical analyzer (Swiss Roche Diagnostics Co., Ltd.). In Shandong Mental Health Center, the normal range of serum UA values has been standardized as 208-428 µmol/L in males and 155-357 µmol/L in females.

Statistical analysis
The results were expressed as mean ± standard deviation (Mean ± SD). Differences in continuous variables among groups were assessed by the independent samples t-test and one-way analysis of variance. The chi-square test was applied to categorical data such as gender. The paired samples test was adopted to evaluate changes in UA levels before and after treatment. P-values <0.05 were considered statistically signi cant.

Results
Demographic and clinical data  Table 1. Differences in UA levels among BD, UD, and HC group in AS There were signi cant differences in UA levels among three groups. Post-hoc analysis showed that UA levels of the BD group were higher than UD (P < 0.001) and HC (P < 0.001) group, while differences in UA levels between UD and HC group were not signi cant (P = 0.453). Similar results were detected when analyzing separately by sex. (Table 2, Fig. 1) Afterward, the BD group was divided into bipolar mania/hypomania (BD-M, n = 64) subgroup and bipolar depression (BD-D, n = 55) subgroup to be compared with the UD group and HC group. There were signi cant differences among the four groups. The post-hoc test showed that differences of UA levels between BD-M subgroup and BD-D subgroup were not signi cant (P = 0.213), as well as between UD group and HC group (P = 0.895); UA levels of BD-M (UD, P < 0.001; HC, P < 0.001) and BD-D (UD, P < 0.001; HC, P = 0.003) subgroups were higher than UD group and HC group. When analyzed strati ed by sex, similar results were detected in female patients, there was no signi cant difference on UA levels between subgroup BD-M and BD-D subgroup, so did to UD group and HC group; UA levels of BD-M (UD, P < 0.001; HC, P < 0.001) and BD-D (UD, P = 0.039; HC, P = 0.048) subgroups were higher than UD group and HC group. In male patients, the differences of UA levels between BD-D group and HC group were not signi cant, so did to UD group and HC group; UA levels of BD-M subgroup were higher than the other groups (BD-D, P < 0.001; UD, P < 0.001; HC, P < 0.001), UA levels of BD-D subgroup were higher than UD group (P = 0.028). (Table 2, Fig. 1) Differences in UA levels among BD, UD and HC group in RS Signi cant differences in UA levels were spotted among three groups. The post-hoc test showed that UA levels of the BD group were higher than UD (P < 0.001) and HC (P < 0.001) group, while differences in UA levels between UD and HC group were not signi cant. The similar results were detected when analyzed strati ed by sex. (Table 3, Fig. 1)

Effects of treatment on UA levels
Drug-use subgroup vs. drug-naïve/free subgroup Patients were divided into drug-use subgroup and drug-naïve/free subgroup (mania and depression unmedicated rst episode or no treatment was used within eight weeks). Only in the UD group, there were signi cant differences in UA levels between drug-use subgroup and drug-naïve/free subgroup, and the differences disappeared when analyzed strati ed by sex. (Table 4)

Discussion
The purinergic system is involved in neurodevelopment and pathophysiological processes of psychotic disorders, such as the process of genesis, differentiation on neurocyte and in ammation of neuro-glial cell, and so on [21][22][23][24]. Purinergic receptors can be divided into P1 and P2 receptors according to their biochemical and pharmacological properties [25]. P1 receptors can regular plasticity of synapse and the release of neurotransmitters [23,24,26,27], while P2 receptors are closely related to embryonic neural development [28]. The dysfunction of the purinergic system result from any causes may lead to psychotic disorders. UA, as the end product of the purinergic system, is in connection with some physiological functions, including sleep, motor, cognitive function, appetite, and social activities, as well as the pathophysiology of mood disorders [6,12]. Additionally, UA is also related to speci c traits, including driving and disinhibition, which is very common in BD. It is also noticed that the peripheral UA levels are consistent with that in the central nervous system [29,30].
In the study, UA levels in the BD group were higher than UD and HC groups, whether in AS or RS, which suggested that UA levels may be a potential biomarker to distinguish between BD and UD. Nevertheless, a recent study indicated that UA levels in UD were lower than HC; a possible reason was the heterogeneity of subjects in the UD group because the UA diagnosis is only based on clinical symptoms at present while some patients with BD often begin with depression. It was further con rmed by a recent study that the higher UA levels might be a predictor of BD [31]. The previous study showed that sex was an important factor that could affect UA levels [19], but the study analyzed separately by sex and got similar results. Beyond that, UA is also a selective antioxidant whose level is considered as a marker of oxidative stress, and results in this study indicated that patients with BD might have a higher oxidative stress level. Moreover, the study divided acute patients with BD into BD-M and BD-D subgroups, with results showing that differences of UA levels between BD-M subgroup and BD-D subgroup were not signi cant, and UA levels of both subgroups were higher than UD group. When analyzed strati ed by sex, signi cant differences in UA levels between male patients of BD-M and BD-D subgroups were spotted, but the differences were not signi cant in female patients, which suggested that male patients might be more susceptible to different stages of illness.
In order to detect the effects of treatment on UA levels, the study divided the acute patients into drug-use and drug-naïve/free subgroups. It was observed that UA levels of the drug-use subgroup were higher than drug-naïve/free subgroup only in the UD group, and the differences were not signi cant when analyzing strati ed by sex, which suggested that UA might be a steady biomarker to distinguish BD and UD. Moreover, UA levels in male patients of the BD-M subgroup