Hypertrophic cardiomyopathy (HCM) is the most common genetic heart disease and associates with mutations in sarcomeric genes. Nearly half of HCM patients with positive genetic tests would carry mutations in genes encoding sarcomeric proteins, which were important in regulating cardiomyocyte contraction and cardiac function[24, 25]. However, variable penetrance and expressivity of HCM patients (even in the same family) implies that specific mechanism behind HCM is complicated and many factors such as epigenetic modifiers or environmental influences may affect the phenotype, as witnessed by previous researches.[26, 27]. At present, no treatments have been found to completely stop the progression and cure the disease. Therefore, it is of great significance to understand etiological explanations for HCM and explore new effective therapeutic strategies.
In this study, we identified co‐expression genes and possible biological pathways of HCM, aimed to detect hub genes and facilitate new therapy in the future. We acquired 2 datasets from GEO database and identified 899 and 4,579 DEGs with R package, respectively. To further research the association between genes and external clinical features, we selected top 5000 variable genes of GSE36961 to carry out a WGCNA study. 15 different modules were identified and genes clusterd into the red and brown module were mostly associated with HCM. Functional enrichment analysis indentified cellular divalent inorganic cation homeostasis, neutrophil mediated immunity, and neutrophil degranulation to be highly engaged in the status of HCM. With a threshold of |MM| ≥ 0.8 and |GS| ≥ 0.2, 44 and 86 key genes were chosen from the red and brown modules, respectively. We later overlapped the DEGs (dataset1 & dataset2) and key genes. After choosing genes with high connectivity using the Cytoscape plug-in software ‘cytoHubba’ based on calculation of degree and researching entensive literature, 4 genes including VSIG4, CD163, FCER1G, and LAPTM5 were finally identified as hub genes of HCM.
The V-set and immunoglobulin domaincontaining protein 4 (VSIG4) is a member of complement receptor of the immunoglobulin superfamily (CRIg), mainly expressed on macrophages. Association between VSIG4 and macrophage was largely researched. Jialin Li et al demonstrated that VSIG4 could inhibit macrophage activation through PI3K/Akt–STAT3 pathway. Xiaoyong Huang et al also verified that VSIG4 is specifically expressed in non-activated macrophages and mediates inhibitory signals to suppress the transcription of Nlrp3 (Nod-like receptor protein 3), an important inflammasome engaged in pathogenesis of various inflammatory syndromes including cardiovascular disease. Cardiac remodeling is a major pathological process in the progression of HCM and the role of macrophages in cardiac remodeling has been largely studied. Sarah Kitz et al compared 18 feline HCM cases to 18 cats without HCM and suggested that HCM is the consequence of cardiac remodeling processes in which macrophages were involved by maintaining an inflammatory and profibrotic environment. Specifically, CCR2+ macrophages are able to produce and secrete substantial proinflammatory cytokines, including NLPR3 and IL-1β, to facilitate adverse cardiac remodeling such as hypertrophy, fibrosis, and LV dilation. However, the role of macrophages in cardiac remodeling is not absolute. Take the role of monocytes and macrophages in fibrotic response as an example. ‘Pro-fibrotic’ macrophage may alter the extracellular matrix by producing cytokines (TNF-α, IL-1β, IL-10), chemokines (MCP-1) and growth factors including TGFβ and PDGF (platelet-derived growth factor) to activate transformation of cardiac fibroblasts into myofibroblasts and therefore promote the fibrotic response[33, 34]. On the other hand, some macrophages, we call them ‘anti-fibrotic’ macrophages, could restrain fibrosis by removing apoptotic myofibroblasts and suppressing fibroblast activation[35, 36]. In our study, we observed a decreased expression of VSIG4 in HCM tissue compared to normal heart.
CD163, also known as the hemoglobin (Hb) scavenger receptor, is a 130-kDa membrane protein with a short cyto-plasmic tail, a single transmembrane segment, and a large ectodomain. Being a macrophage-specific protein, CD163 is restricted to the monocytic-macrophage linage such as red pulp macrophages, bone marrow macrophages, and liver macro-phages (Kupffer cells). . CD163 has been verified to indirectly contribute to atheroprotective and anti-inflammatory response through scavenging of the oxidative Hb, stimulation of the heme-oxygenase-1 and production of anti-inflammatory heme metabolites. However, apart from its overall anti-inflammatory function, CD163 could also produce inflammatory cytokines, for example TNF-a and IL-4, in inflammation response. The upregulated expression of this receptor reflects the switch of macrophage to activated phenotypes in inflammation and thus, a high CD163 expression in macrophages reflects the activated response of tissues to inflammation. Enhanced expression of CD163+ or soluble CD163+ macrophages were detected in human inflammatory diseases including chronic cardiovascular disease like atherosclerosis, atrial fibrillation[42, 43], and chronic heart failure[44, 45]. Therefore, novel evidence indicated CD163 a potential inflammation biomarker and a therapeutic target in a large spectrum of acute and chronic inflammatory disorders[46, 47]. Accumulating evidence has suggested the existence of low-grade systemic and local inflammation in HCM. Chronic inflammatory cell infiltration was observed in the myocardium of patients with HCM[48, 49]. Lu Fang et al  observed an increasing expression of tumor necrosis factor (TNF)-α, interleukin (IL)-6 and serum amyloid P (SAP) in HCM patients compared to controls and that systemic inflammation is related to the severity of HCM patients, especially regional and diffuse fibrosis. They also suggested that mechanic stress could upregulate NF-κB nuclear positivity in cardiac cells, fueling the inflammatory process and transdifferentiation of fibroblast to active myofibroblast, the main contributor of cardiac collagen. To conclude, modifying inflammation may reduce myocardial fibrosis in HCM patients.
FCER1G is localized on chromosome 1q23, which encodes the γ subunit of the fragment crystallizable (Fc) region (Fc R) of immunoglobulin E (IgE). In normal immune fuction of human body, Fc binds to Fc R of immune cells to recognize and eliminate extraneous antigens via the antigen-antibody binding reaction. However, such combination may generate adverse effects as it may trigger destructive inflammation, immune cell activation, phagocytosis, oxidative burst, and cytokine release under pathological circumstance. Sandra B. Haudek reported that serum amyloid P (SAP) could prevent cardiomyopathy in the heart by binding to Fc receptors (FcRs) expressed on monocytes and inhibiting monocyte-to-fibroblast transition. Mice lacking the FcRγ, a common membrane-signaling component of activating FcRs developed fibrosis and cardiac dysfunction when subjected to a murine fibrotic cardiomyopathy model. It was reported that FcRγ-Chain deficiency reduces the development of diet-induced obesity and possible mechanism may be antibody-mediated immune responses involved in the development of insulin resistance. The normal physiological activity of heart requires a large amount of energy and there is increasing evidence of the link between metabolism and cardiac hypertrophy, especially oxidation of fatty acid and glucose. Liling Zheng et al found that the ability of energe utilization was impaired in hypertrophic hearts manifested as reduced fatty acid oxidation rates and myocardial insulin resistance. Mitochondrial function and cardiac systolic function in hypertrophic hearts were also disrupted. Owing to possible correlation between metabolic dysfuction and cardiac hypertrophy, FCER1G may be a target for further research.
Lysosomal-associated protein multispanning transmembrane 5 (LAPTM5), a membrane protein highly expressed in lymphoid lineage cells, plays a fundamental role in mediating vesicle trafficking . LAPTM5 was associated with inhibition of T cell receptor (TCR), B cell receptor (BCR), and pre-B cell receptor (pre-BCR) expression on the cell surface via promotion of lysosomal degradation of these proteins, indicating its negative regulator in immunization. Moreover, association between LAPTM5 and inflammation in macrophages was reported by Glowacka WK. LAPTM5 was a positive regulator of multiple inflammatory cascades in the activation of macrophages through NF-κB and MAPK pathway and downstream proinflammatory cytokine release (IL6, TNF, and IL12). A large number of studies have been carried on LAPTM5 and human cancer[61, 62], but little on LAPTM5 and cardiovascular disease. In the future, exploration of LAPTM5 may be targeted as a new direction for research on HCM.