Myocardial infarction is a complex disease where the genetic makeup may contribute to distinct cellular processes and to the development of disease phenotypes [10, 11]. MI is associated with systemic inflammatory dysregulated response including elevated levels of circulating inflammatory mediators, and activation of peripheral leukocytes and platelets.
In concordance with previous studies, our work highlights a possible immune change in MI pathology. The crucial role of immune system in myocardial infarction is well established [12, 13]. Previous studies suggested an intense immune and inflammatory responses trigger several distinct cardiac pathological conditions including early onset MI [14–16]. The immune signals may indirectly contribute to the reverse remodeling in dilated cardiomyopathy, dilative remodeling, infarcted size increase and heart failure [17, 18].
We found that the highest ranked genes identified in the whole PPI network, beside their role in the immune, inflammatory responses and interleukins signaling pathways, were strongly associated with MI development including CYBB [19, 20], CXCL2 [21, 22], NRG1[23, 24], VWF  and MMP9 . Moreover, TLR family genes (TLR2, TLR4 and TLR10) were also correlated with immune and inflammatory response functions, which suggests their association with MI development [27, 28].
Identification of key genes in major pathways might be an attractive therapeutic target for MI. MCODE analysis found four tightly connected sub-networks (modules) containing most of the topologically significant nodes of the whole networks. The transcriptional profiling presented in identified modules demonstrating the relationship variability in the functional biological pathways of MI. Several identified hub genes were significantly associated with MI. Thrombomodulin (THBD) is an endothelial anticoagulant protein, involved in anti-inflammatory responses, including innate immunity, fibrinolysis and complement activity [29, 30]. Genetic studies linked several polymorphisms within THBD to be associated with higher risk of coronary artery disease and myocardial infarction [31–35].
Another identified gene is Superoxide Dismutase 2 (SOD2). SOD2 is one of the major mitochondrial antioxidant enzymes. Homozygous mice for SOD2 have a severe phenotype, heart and liver complications, metabolic acidosis, and early neonatal death . Recent study showed that plasmatic concentration of SOD2 was higher in CAD than in healthy control [37, 38]. Dubois-Deruy et al. showed a direct interaction between circulatory MicroRNAs and SOD2 in heart failure post MI rat model . These data indicate that SOD2 might serve as a potential biomarker candidate for post-myocardial infarction.
Endothelin 1 (EDN1) is a potent endogenous vasoconstrictor and a crucial modulator of neutrophil function [40, 41]. High plasmatic levels of EDN1 have been linked to poor clinical outcomes after ST-segment elevation myocardial infarction (STEMI), microvascular obstruction presence and were found to be a strong predictor of 1-year survival post-MI [42–44].
TLR2 a member of TLR family was also identified as DEGs in MI and an important node in MCODE modules. TLR2 is a key player in innate immunity, inflammatory response and pathogenesis of heart failure after MI . TLR2 modulates ventricular remodeling after myocardial infarction and its expression is negatively correlated with the inflammatory response which may influence myocardial ischemia-reperfusion injury therapy. Monoclonal anti-TLR2 antibody causes a pronounced reduction of leukocyte influx, cytokine production and preserves cardiac function and geometry in vivo . TLR2 deficiency has also been shown to have a protective effect in a mouse [47, 48]. These experimental evidences increase the potential of TLR2 as a therapeutic target.
Hubs and bottlenecks are crucial components in signaling networks as they are hypothesized to be encoded by essential genes and more relevant to biologically significant process with respect to the disease [49–51]. We filtered out 32 key genes by using Cytohubba. We utilized the gene expression data to infer the disease related gene expression to identify candidate regulatory genes and to infer signaling pathway networks directly from candidate genes expression. According to DisGeNET database, role of 17 hub and bottleneck genes in MI is supported by many experimental work across the globe [52–55, 55–63] including VEGFA, which limits myocardial damage in MI animal models , ICAM1, implicated in pathophysiologic responses and neutrophil infiltration [55, 61], TLR4 that mediates maladaptive left ventricular remodeling and impairs cardiac function . Interleukins family members such as IL1A and IL1B as well as PTGS2 which will be discussed further below.
Detection of these key MI related genes indicated that our framework of integrated bioinformatics analysis pipeline is sensitive enough to rapidly identify potential target gene biomarkers or novel drug targets for any complex disease. Remaining genes in hubs and bottlenecks network were tightly connected with known MI functions. Prior to the present study, few studies have addressed the gaps in identifying potential biomarkers in MI early stage[64–66]. Our strategy in identifying distinct biomarkers that correlate with MI and its development were based on comparing predicted physical interaction and genetic association of MI-gene targets using Open Target Platform. Twelve genes from the hubs and bottlenecks were target for other diseases (Additional file 2: Table S5) and seven hubs and one bottleneck gene in total were linked to MI (Additional file 1: Fig. S2). Among identified targets IL1A and IL1B, member of the interleukin-1 gene family, which are proinflammatory factors produced by different cell types, including endothelial cells, in response to various stimuli [67–69]. The intense inflammatory response is a key process is which play a central role in many cardiovascular diseases including Myocardial infarction [70–72]. Targeting inflammatory pathways key genes such as Interleukin-1 members in cardiovascular disease in order to neutralization elimination of specific activated inflammatory mediators may contribute to protective effect in the infarcted heart without disturbing the reparative response . Variants in IL1A and IL1B are associated with acute MI as well as heart failure [74–76], Interleukin-1 blockers are used to improve clinical outcome in patients with prior MI. There are currently four clinically available IL-1 blockers, anakinra, rilonacept, canakinumab and gevokizumab each with a different efficacy and safety profile . For instance, IL-1 blocker, was originally approved for the treatment of rheumatoid arthritis (anakinra) . However, It has also been successfully studied in small clinical trials to test its effectiveness in patients with myocardial infarction and favorably affect cardiac remodeling and exhibit a protective effect from post-infarction heart failure development [79–81]. Nowadays, only canakinumab has an indication for cardiovascular disease .
Our study revealed that PTGS2 is one of the key players in MI development. PTGS2, also known as COX-2, is inducible enzyme that plays an important role in several pathophysiological processes, including inflammation, angiogenesis, and tumorigenesis [83, 84]. GO analysis showed that PTGS2 was enriched in TNF and Interleukins signaling pathways. PTGS2 has previously been associated with MI, ischemic heart disease and stroke risk [56, 85, 86]. However, the role of selective COX-2 inhibitors in medical practice remain controversial. Saito el al., have shown that COX-2 induction during MI appears to contribute to myocardial injury, and treatment with the specific inhibitor of the enzyme ameliorated the course of the disease . Similarly, COX-2 inhibition was shown to be associated with improved hemodynamics and lower cardiomyocyte apoptotic rates in a genetically modified mouse model of non-ischaemic heart failure and in a canine model of myocardial ischemia-reperfusion [88, 89]. Others have shown that COX-2 inhibitors may be detrimental for myocardial infarction [90, 91].