目的
先前的研究表明，菸鹼酸紅腫減弱反應是思覺失調症的生物標記之一。並有研究指出，菸鹼酸紅腫反應的減弱可能與菸鹼酸受體GPR109A和前列腺素受體DP1的訊號傳導異常存在關聯。較低的菸鹼酸受體GPR109A和前列腺素受體DP1在白血球上的表現量可能會影響難治型思覺失調症患者的神經保護功能及抗發炎反應能力，進而影響疾病的嚴重程度與神經認知功能。因此，本研究分析了難治型思覺失調症患者與非難治型思覺失調症患者和健康對照組的菸鹼酸紅腫反應與菸鹼酸受體GPR109A和前列腺素受體DP1的表現，以更了解思覺失調症的發病機制，並且建立多變量的難治型思覺失調症預測模型，提供患者可能為難治型思覺失調症的風險程度。

方法
本研究共招募了84位思覺失調症患者（包括42位難治型思覺失調症患者和42位非難治型思覺失調症患者），21位難治型思覺失調症患者的一等親家屬和82位健康對照者。我們利用菸鹼酸皮膚紅腫測試與流式細胞儀分析測量了菸鹼酸紅腫反應和GPR109A、DP1受體的表達。並利用接收者操作特徵曲線分析(ROC)、線性判別分析(LDA)和支持向量機分類（SVM）評估利用菸鹼酸紅腫反應和GPR109A和DP1受體的表達區分難治型思覺失調症患者與非難治型思覺失調症患者的鑑別力。

結果
在菸鹼酸紅腫減弱反應中，難治型思覺失調症患者與非難治型思覺失調症患者的紅腫反應較健康對照組的反應弱。而菸鹼酸受體GPR109A與前列腺素受體DP1的在健康受試者的表現量最高，在非難治型思覺失調症患者次之，在難治型思覺失調症患者最低。並且我們使用機器學習演算法之支持向量機分析菸鹼酸紅腫反應及GPR109A表現量、DP1表現量分類難治型思覺失調症患者與非難治型思覺失調症患者能力，結果顯示合併使用GPR109A和DP1的表現量可以有最好的鑑別能力。另外，在重複測量的結果中，菸鹼酸紅腫反應、菸鹼酸受體GPR109A與前列腺素受體DP1的表現在第一次測量與第二次測量無顯著差異。同時，也發現菸鹼酸紅腫分數與GPR109A、DP1表現量較低的思覺失調症患者，其正負性症狀也有較低的趨勢。而菸鹼酸紅腫反應的男女差異在非難治型思覺失調症與健康受試者中顯示有顯著的差異，且女性的得分較高，而難治型思覺失調症則有相反的結果。

結論
菸鹼酸紅腫減弱反應可能有助於從健康對照中辨認出思覺失調症患者，而較低的GPR109A受體和DP1受體的表現量可能可以作為TRS患者的潛在生物標誌。難治型思覺失調患者和非思覺失調患者的菸鹼酸紅腫減弱反應、菸鹼酸受體 GPR109A和前列腺素受體DP1不同的表現量可能反應菸鹼酸紅腫反應的分子機制，有助於區分難治型和非難治型思覺失調症患者，並進一步預測難治型思覺失調症的風險。

Purpose
Previous studies showed that attenuated niacin flush response is one of the biomarkers in schizophrenia. Previous studies pointed out an association between attenuated niacin flush response and its abnormal signal transduction of niacin receptor GPR109A and prostaglandin receptor DP1. The low expression of niacin receptor GPR109A and prostaglandin receptor DP1 on white blood cells may affect the neuroprotective function and anti-inflammatory response ability of patients with TRS, thereby affecting the severity of the disease and neurocognitive function. Therefore, this study analyzed the expression of attenuated niacin flush response, niacin receptor GPR109A and prostaglandin receptor DP1 in treatment-resistant schizophrenia patients (TRS), non-TRS patients and healthy controls to more understand the pathogenesis of schizophrenia and establish a multivariate prediction model, providing the potential risk of TRS.

Methods
This study recruited 84 schizophrenia patients (42 TRS patients and 42 non-TRS patients), 21 first-degree relatives of schizophrenia patients and 82 healthy controls. We measured the level of niacin flush response and GPR109A, DP1 receptor expression by niacin skin patches and flow cytometry analysis. Receiver operating characteristic (ROC) analysis, linear discriminant analysis, and support vector machine classification (SVM) were used to evaluate the discrimination performance of niacin flush response, GPR109A expression and DP1 expression in schizophrenia patients and healthy controls.

Results
In the attenuated niacin flush response, the flush response of TRS patients and non-TRS patients was weaker than healthy controls. The niacin receptor GPR109A and prostaglandin receptor DP1 had the highest expression levels in healthy controls, followed by non-TRS patients, and the lowest was the TRS patients. Moreover, we used machine learning algorithm support vector machine to analyze the niacin flush response and GPR109A expression, DP1 expression classifying the ability of TRS patients and non-TRS patients. The results showed that the combined use of GPR109A expression and DP1 expression can have the best discriminating ability. In addition, the results of repeated measurements, there were no significant difference between the niacin flush response, GPR109A expression and DP1 expression in the first measurement and the second measurement. Meanwhile, we also found that schizophrenia patients who had lower niacin flush scores and lower expression levels of GPR109A and DP1 tended to have severer positive and negative symptoms. The gender differences in niacin flush response showed significant differences in non-TRS patients and healthy controls, the scores were higher in women, while TRS patients had the opposite results.

Conclusion
The results demonstrated that attenuated niacin flush response may be useful for screening schizophrenia patients from healthy control, the lower expressions of GPR109A and DP1 may as potential biomarkers for TRS patients. The molecular mechanism of the niacin flush pathway may be manifested through different expression level of attenuated niacin response, niacin receptor GPR109A, and prostaglandin receptor DP1 in TRS patients and non-TRS patients, helping to distinguish the TRS patients from non-TRS patients and further predict the risk of treatment-resistant schizophrenia.

Alacam H, Akgun S, Akca H, et al. miR-181b-5p, miR-195-5p and miR-301a-3p are related with treatment resistance in schizophrenia. Psychiatry research, 2016;245:200-6.
Andersson RG, Åberg G, Brattsand R, et al. Studies on the mechanism of flush induced by nicotinic acid. Acta pharmacologica et toxicologica, 1977;41(1):1-10.
Benyó Z, Gille A, Kero J, et al. GPR109A (PUMA-G/HM74A) mediates nicotinic acid–induced flushing. The Journal of clinical investigation, 2005;115(12):3634-40.
Chang S-S, Liu C-M, Lin S-H, et al. Impaired flush response to niacin skin patch among schizophrenia patients and their nonpsychotic relatives: the effect of genetic loading. Schizophrenia bulletin, 2009;35(1):213-21.
de Bartolomeis A, Balletta R, Giordano S, et al. Differential cognitive performances between schizophrenic responders and non-responders to antipsychotics: correlation with course of the illness, psychopathology, attitude to the treatment and antipsychotics doses. Psychiatry research, 2013;210(2):387-95.
Digby JE, Martinez F, Jefferson A, et al. Anti-inflammatory effects of nicotinic acid in human monocytes are mediated by GPR109A dependent mechanisms. Arteriosclerosis, thrombosis, and vascular biology, 2012;32(3):669-76.
Elkis H. Treatment-resistant schizophrenia. Psychiatric Clinics of North America, 2007;30(3):511-33.
Fleischhacker WW, Arango C, Arteel P, et al. Schizophrenia—time to commit to policy change. Schizophrenia bulletin, 2014;40(Suppl_3):S165-S94.
Gasperi V, Sibilano M, Savini I, et al. Niacin in the central nervous system: an update of biological aspects and clinical applications. International journal of molecular sciences, 2019;20(4):974.
Gille A, Bodor ET, Ahmed K, et al. Nicotinic acid: pharmacological effects and mechanisms of action. Annu Rev Pharmacol Toxicol, 2008;48:79-106.
Graff EC, Fang H, Wanders D, et al. Anti-inflammatory effects of the hydroxycarboxylic acid receptor 2. Metabolism, 2016;65(2):102-13.
Guy W, Cleary P, Switalski R. A comparison of drug-trial schizophrenics with their clinical counterparts. Psychopharmacology bulletin, 1977;13(3):44-5.
Huang S, Cai N, Pacheco PP, et al. Applications of support vector machine (SVM) learning in cancer genomics. Cancer Genomics-Proteomics, 2018;15(1):41-51.
Iasevoli F, D'Ambrosio L, Francesco DN, et al. Clinical evaluation of functional capacity in treatment resistant schizophrenia patients: comparison and differences with non-resistant schizophrenia patients. Schizophrenia research, 2018;202:217-25.
Kamanna V, Ganji S, Kashyap M. The mechanism and mitigation of niacin‐induced flushing. International journal of clinical practice, 2009;63(9):1369-77.
Kay SR, Fiszbein A, Opler LA. The positive and negative syndrome scale (PANSS) for schizophrenia. Schizophrenia bulletin, 1987;13(2):261.
Khandaker GM, Cousins L, Deakin J, et al. Inflammation and immunity in schizophrenia: implications for pathophysiology and treatment. The Lancet Psychiatry, 2015;2(3):258-70.
Koga M, Serritella AV, Sawa A, et al. Implications for reactive oxygen species in schizophrenia pathogenesis. Schizophrenia research, 2016;176(1):52-71.
Lagranha CJ, Silva TLA, Silva SCA, et al. Protective effects of estrogen against cardiovascular disease mediated via oxidative stress in the brain. Life sciences, 2018;192:190-8.
Langbein K, Schmidt U, Schack S, et al. State marker properties of niacin skin sensitivity in ultra-high risk groups for psychosis-An optical reflection spectroscopy study. Schizophrenia research, 2018;192:377-84.
Liang X, Wu L, Hand T, et al. Prostaglandin D2 mediates neuronal protection via the DP1 receptor. Journal of neurochemistry, 2005;92(3):477-86.
Lien Y-J, Huang S-S, Liu C-M, et al. A genome-wide quantitative linkage scan of niacin skin flush response in families with schizophrenia. Schizophrenia bulletin, 2013;39(1):68-76.
Lin A-S, Chan H-Y, Peng Y-C, et al. Severity in sustained attention impairment and clozapine-resistant schizophrenia: a retrospective study. BMC psychiatry, 2019;19(1):1-9.
Lin S-H, Liu C-M, Chang S-S, et al. Familial aggregation in skin flush response to niacin patch among schizophrenic patients and their nonpsychotic relatives. Schizophrenia bulletin, 2007;33(1):174-82.
Maciejewski-Lenoir D, Richman JG, Hakak Y, et al. Langerhans cells release prostaglandin D2 in response to nicotinic acid. Journal of Investigative Dermatology, 2006;126(12):2637-46.
Messamore E. The niacin response biomarker as a schizophrenia endophenotype: A status update. Prostaglandins, Leukotrienes and Essential Fatty Acids, 2018;136:95-7.
Messamore E. Niacin subsensitivity is associated with functional impairment in schizophrenia. Schizophrenia research, 2012;137(1-3):180-4.
Messamore E, Hoffman WF, Janowsky A. The niacin skin flush abnormality in schizophrenia: a quantitative dose–response study. Schizophrenia research, 2003;62(3):251-8.
Messamore E, Hoffman WF, Yao JK. Niacin sensitivity and the arachidonic acid pathway in schizophrenia. Schizophrenia research, 2010;122(1-3):248-56.
Miller CL, Dulay JR. The high-affinity niacin receptor HM74A is decreased in the anterior cingulate cortex of individuals with schizophrenia. Brain research bulletin, 2008;77(1):33-41.
Mustafa A, Rienow A, Saadi I, et al. Comparing support vector machines with logistic regression for calibrating cellular automata land use change models. European Journal of Remote Sensing, 2018;51(1):391-401.
Obiols JE, Serrano F, Caparrós B, et al. Neurological soft signs in adolescents with poor performance on the continuous performance test: markers of liability for schizophrenia spectrum disorders? Psychiatry Research, 1999;86(3):217-28.
Ochoa S, Usall J, Cobo J, et al. Gender differences in schizophrenia and first-episode psychosis: a comprehensive literature review. Schizophrenia research and treatment, 2012;2012.
Offermanns S, Schwaninger M. Nutritional or pharmacological activation of HCA2 ameliorates neuroinflammation. Trends in molecular medicine, 2015;21(4):245-55.
Paul-Samojedny M, Owczarek A, Kowalczyk M, et al. Association of interleukin 2 (IL-2), interleukin 6 (IL-6), and TNF-alpha (TNFα) gene polymorphisms with paranoid schizophrenia in a Polish population. The Journal of neuropsychiatry and clinical neurosciences, 2013;25(1):72-82.
Peinhaupt M, Roula D, Theiler A, et al. DP1 receptor signaling prevents the onset of intrinsic apoptosis in eosinophils and functions as a transcriptional modulator. Journal of leukocyte biology, 2018;104(1):159-71.
Peinhaupt M, Sturm EM, Heinemann A. Prostaglandins and their receptors in eosinophil function and as therapeutic targets. Frontiers in medicine, 2017;4:104.
Potkin SG, Kane JM, Correll CU, et al. The neurobiology of treatment-resistant schizophrenia: paths to antipsychotic resistance and a roadmap for future research. NPJ schizophrenia, 2020;6(1):1-10.
Prestwood TR, Asgariroozbehani R, Wu S, et al. Roles of inflammation in intrinsic pathophysiology and antipsychotic drug-induced metabolic disturbances of schizophrenia. Behavioural brain research, 2021;402:113101.
Puri B, Richardson A, Counsell SJ, et al. Negative correlation between cerebral inorganic phosphate and the volumetric niacin response in male patients with schizophrenia who have seriously and dangerously violently offended: A 31P magnetic resonance spectroscopy study. Prostaglandins, leukotrienes and essential fatty acids, 2007;77(2):97-9.
Shah S, Vankar G, Peet M, et al. Unmedicated schizophrenic patients have a reduced skin flush in response to topical niacin. Schizophrenia research, 2000;43(2-3):163-4.
Smesny S, Rosburg T, Klemm S, et al. The influence of age and gender on niacin skin test results–implications for the use as a biochemical marker in schizophrenia. Journal of psychiatric research, 2004;38(5):537-43.
Sun L, Yang X, Jiang J, et al. Identification of the niacin-blunted subgroup of schizophrenia patients from mood disorders and healthy individuals in Chinese population. Schizophrenia bulletin, 2018;44(4):896-907.
Taniguchi H, Mohri I, Okabe-Arahori H, et al. Prostaglandin D2 protects neonatal mouse brain from hypoxic ischemic injury. Journal of Neuroscience, 2007;27(16):4303-12.
Tunaru S, Lättig J, Kero J, et al. Characterization of determinants of ligand binding to the nicotinic acid receptor GPR109A (HM74A/PUMA-G). Molecular pharmacology, 2005;68(5):1271-80.
Wakade C, Giri B, Malik A, et al. Niacin modulates macrophage polarization in Parkinson's disease. Journal of neuroimmunology, 2018;320:76-9.
Ward P, Sutherland J, Glen E, et al. Niacin skin flush in schizophrenia: a preliminary report. Schizophrenia Research, 1998;29(3):269-74.
Wise A, Foord SM, Fraser NJ, et al. Molecular identification of high and low affinity receptors for nicotinic acid. Journal of Biological Chemistry, 2003;278(11):9869-74.