Loss of the protein disrupted in schizophrenia 1 ( DISC 1 ) impairs cardiac resistance against ischemia . Running title : The protein DISC 1 a novel player in ischemic heart disease ?

Treatable targets that hinder heart failure development following myocardial infarction remain limited. Through an unbiased transcriptional regulation study for ischemic heart disease, we identified the protein disrupted in schizophrenia 1 (DISC1), which has been almost solely characterized in the brain. Here, we show that loss of DISC1 is a major driver of heart disease and ischemic damage. Silencing of DISC1 sensitizes human cardiomyocyte cell lines to hypoxia, whereas DISC1 overexpression is cardioprotective. Mechanistically, we provide evidence that reduced DISC1 protein levels interrupt multiple signaling processes vital for cardiomyocyte survival whereas overexpression of DISC1 enhances pro survival signalling. The present study has implications for understanding scenarios where cardiomyocytes are unable to increase the levels of DISC1 during ischemia, e.g., in individuals with DISC1 genetic defects. Indeed, patients with severe mental illness display 20 years earlier mortality, with IHD as the greatest cause of death. The present study can thus shed light on this long-term enigma in epidemiology. Introduction Ischemic heart disease (IHD) is the leading cause of death globally (Organization, 2014). Despite substantial improvements in clinical outcome, a significant share of patients still suffers from premature death after myocardial infarction (MI) (Hausenloy et al., 2017). To address this challenge, we established a new model using human inducible pluripotent stem cell-derived cardiomyocytes (hIPS-CMs) and subjected these cells to pathological hypoxic stress. Exploring the global transcriptional landscape through next-generation RNA sequencing allowed identification of key transcripts involved in the CM response to hypoxia. To our surprise, disrupted schizophrenia 1 (DISC1) was the most strikingly responsive gene in cardiomyocytes exposed to hypoxic conditions. DISC1 was originally identified from a gene directly disrupted by a balanced chromosomal translocation t(1;11) (q42;q14.3) in a large Scottish family with a high load of major psychiatric illness (Millar et al., 2000). Today, DISC1 is viewed as an important regulator of neurodevelopment, and its loss of function can be a driver of severe mental illness, such as major depressive disorder and schizophrenia (SZ)(Brandon and Sawa, 2011; Niwa et al., 2016; Yerabham et al., 2013). At the cellular level, DISC1 is a scaffolding protein that binds to and regulates a range of signaling cascades and protein networks in neuronal cells (Brandon and Sawa, 2011; Yerabham et al., 2013). In the heart, however, no data exist on the role of DISC1, beyond which DISC1 expression is as high as in neural tissue (Ozeki et al., 2003). Interestingly, epidemiological data from patients with severe mental illness, in particular SZ, show a 20-year lower life expectancy than the general population, with IHD as the leading cause of death (Nielsen et al., 2015). Furthermore, recent reports display deteriorated cardiac function measured by echocardiography (Korkmaz et al., 2016) and MRI (Pillinger et al., 2019) in SZ patients. Myocardial T1 mapping (proton spin-lattice relaxation time), which enables detection of myocardial edema, infarction, ischemia, cardiomyopathies and diffuse fibrosis (Liu et al., 2017), further suggests that an early diffuse fibroinflammatory myocardial process in SZ patients is independent of established CVD-risk factors and could contribute to excess cardiovascular mortality (Pillinger et al., 2019). Although we cannot completely exclude the secondary influence of medication, severe mental illness comorbidities frequently include cardiovascular deficits (Pillinger et al., 2019; Sun et al., 2019). Therefore, we hypothesized that DISC1 is not only important for the development of SZ but that loss of function or depletion of DISC1 directly renders the heart and cardiac cells more vulnerable to and prone to IHD damage. Furthermore, we hypothesize that DISC1 is a new and hitherto unknown multifunctional protein, a potential master regulator, that orchestrates intracellular processes in the heart that are vital to resist cardiac damage caused by ischemic stress, such as myocardial infarction. Results Discovery of the cardiac role of DISC1 by transcriptomics A major limitation in establishing novel treatment strategies is that most experimental studies target only single intracellular signalling pathways. The lack of success in such strategies calls for novel targets directed at multiple signalling pathways, which remains a gap in existing knowledge (Hausenloy et al., 2017). To answer this challenge, we established a new model using human inducible pluripotent stem cell-derived cardiomyocytes (hIPS-CMs) with pathological hypoxic stress stimulation. Exploring the global transcriptional landscape through next-generation RNA sequencing allowed identification of key transcripts involved in the CM response to hypoxia. Our first indication of a role for DISC1 in CMs was revealed by RNA-seq of hIPS-CMs exposed to pathological hypoxic stress for 48 hours. In hIPSCMs, we detected 1232 significantly regulated transcripts (cut-off value adjusted P <0.05) between hypoxia and normoxia. Here, we found that DISC1 scored among the most regulated coding transcripts, displaying downregulation by Log2FC= -2,67 because of hypoxia (P-value = 3,41E-51) (Fig. 1A-B and Fig S1). To further explore the global transcriptional response to hypoxia in CMs, we expanded our bioinformatics analyses by generating regulatory networks that depicted negative correlations between overexpressed microRNAs (miRs) and their putative downregulated mRNA targets from our RNA-seq data. In our analyses, we found a total of 774 mir-mRNA pairs, and of these, DISC1 stood out in these analyses, targeted by as many as 26 specific miRs (Fig. 1C). Genome-wide association study associated DISC1 with heart disease Given the unknown role of DISC1 in the heart, we were enthusiastic to discover that several signalling pathways and interacting protein partners of DISC1 identified in a neuronal setting are similar to earlier reported candidates for cardioprotection following myocardial ischemia (Hashimoto et al., 2006; Hausenloy et al., 2017; Ming and Song, 2009; Sciarretta et al., 2018; Zheng et al., 2012). To determine the association between DISC1 and cardiac disease, we therefore performed GWAS enrichment analysis and found a significant overrepresentation of risk genes for heart diseases among DISC1 protein interactors (P-value = 2.8E-17) (Data S1, Table S1). With these associations, we hypothesized that DISC1 is not only important for neurodevelopment and mental disorders but that loss of function or depletion of DISC1 directly renders the heart and CMs more vulnerable and prone