Exploration of the NRG-ErbB genetic pathway for biomarkers of Clozapine mediated symptom remission and symptom severity in treatment-resistant schizophrenia

Abstract

© 2017 Dr Md Shaki MostaidSchizophrenia is a disabling mental health disorder that is characterized by positive symptoms (delusions, hallucinations etc.), negative symptoms (apathy, social withdrawal, emotional blunting etc.) and cognitive deficits (impaired memory, lack of attention etc.). Current pharmacological treatment includes typical and atypical antipsychotics but 20-30% of patients do not adequately respond to these treatments and are thus defined as treatment-resistant. Clozapine is indicated for the treatment of treatment-resistant schizophrenia (TRS). However, biomarkers of clozapine mediated symptom remission and symptom severity in TRS have yet to be identified. One promising biomarker is neuregulin 1 (NRG1), a growth factors that activates ErbB receptor tyrosine kinases and initiates the NRG-ErbB signalling pathway, which plays a key role in neurodevelopment. Genomic, transcriptomic, and proteomic abnormalities in NRG-ErbB pathway have been linked to schizophrenia and clozapine has been shown to modulate NRG1 gene and protein expression. Thus, NRG-ErbB pathway gene and protein expression profiles, as well as genetic variation, may serve as biomarkers for clozapine mediated symptom remission and symptom severity.  In this thesis, we will present our investigation of the peripheral gene and protein expression levels of NRG-ErbB pathway genes in TRS patients and healthy controls and how they relate to clozapine mediated symptom remission as well as symptom severity. In addition, we will discuss the role genetic polymorphisms in NRG1 play in regulating its gene and protein expression. Finally, we will present results from healthy peripheral blood mononuclear cells exposed in vitro to clozapine for 24 hours and seven days and discuss the effects of clozapine on NRG-ErbB pathway gene and protein expression. Chapter 1 contains systematic review of scientific literatures and justifies the main 3 goals of the thesis. Chapter 2 of this thesis aimed at investigation of the candidate SNPs and microsatellites within the NRG1 gene among 16,720 patients, 20,449 controls, and 2,157 family trios via a meta-analytic procedure. We found significant association for three polymorphisms at the 5’ end (rs62510682, rs35753505, and 478B14-848) and two (rs2954041 and rs10503929) at the 3’ end of the NRG1 with schizophrenia. We could not find association for haplotypes. Chapter 3 aimed to assess the peripheral expression pattern of major NRG1 mRNA isoforms in whole blood and NRG1-β1 protein in serum in patients with TRS to find clinically useful biomarkers of clozapine mediated symptom severity and symptom remission. Using RT-qPCR we found upregulation of three NRG1 mRNA isoforms (NRG1 EGFα, NRG1 EGFβ, NRG1 typeI(Ig2)) in whole blood in TRS patients. However, protein assay via ELISA showed lower level of serum NRG1-β1 in TRS patients but it was confounded by smoking. Expression of NRG1 EGFα, NRG1 EGFβ was also negatively correlated with age of illness onset. In Chapter 4, we continued to examine the peripheral mRNA expression pattern of the major NRG-ErbB pathway downstream signaling genes in TRS patients and controls to see if increased expression in ligands leads to overexpression of receptors and subsequent upregulation of the full pathway in treatment-resistant schizophrenia. We found that five mRNA transcripts (ErbB3, PIK3CD, AKT1, P70S6K, eIF4EBP1) were upregulated in TRS patients, although only one (P70S6K) survived after correction for multiple comparisons. Moreover, investigation of the clinical factors revealed that expression of ErbB2, PIK3CD, PIK3R3, AKT1, mTOR, and P70S6K were negatively correlated with duration of illness. Chapter 5 summarises the main findings of the thesis, its relevance to previous literature, advancement of knowledge, implications and future steps in investigation of the NRG-ErbB genetic pathway for suitable biomarkers in schizophrenia, more specifically treatment-resistant schizophrenia