Identification of DEGs in atherosclerosis
A total of 2,037 DEGs (atherosclerosis vs. control) were identified with the threshold at adj. p-value < 0.05, including 850 up-regulated genes and 1,187 down-regulated genes (Fig. 1 A). The expressions of the top 15 up-regulated genes and top 15 down-regulated genes (sorted by adj. p-value) were shown in Fig. 1B.
Identification and enrichment analysis of DEIRGs
199 overlapping genes from DEGs and IRGs were retained as DEIRGs for subsequent analysis (Fig. 2 A). Besides, the DEIRGs were enriched in different GO terms, such as “positive regulation of cytokine production”, “regulation of cell development”, “negative regulation of response to external stimulus”, “reproductive structure development” and “positive regulation of cell development” (Fig. 2B, C). The result of KEGG pathway enrichment [14] indicated that DEIRGs were mainly enriched in “cytokine-cytokine receptor interaction”, “JAK-STAT signaling pathway”, “Ras signaling pathway”, “human cytomegalovirus infection” and “axon guidance” and so on (Fig. 2D, E).
Identification of hub genes by PPI network
To better understand the interplay among the identified DEIRGs, we used the STRING online server to construct a PPI network (Fig. 3A). Then, the core module was obtained from PPI network via the MCODE plug-in. There were 8 genes selected as hub genes, which were IFIH1, IFIT1, IFIT2, IFIT3, IRF7, ISG15, OAS3 and RNASEL (Fig. 3B).
Validation of hub genes on IS
In GSE100927, IFIH1, IFIT1, IFIT2, IFIT3, IRF7, ISG15, OAS3 and RNASEL were all expressed higher in the atherosclerosis group compared to the control group (Fig. 4 A). Figure 4B showed that all the hub genes had good diagnostic ability. Next, the hub genes were further tested in the validation set GSE43292. IFIH1, IFIT1, IFIT2, IFIT3, ISG15 and OAS3 had similar expression differences to the training set (Fig. 4 C) and had good diagnostic abilities for atherosclerosis (Fig. 4D).
Infiltrating immune cell analysis
Figure 5 A showed the distribution of 22 infiltrating immune cells in the atherosclerosis and normal samples. The result of the Wilcoxon test presented there were 3 types of immune cells with adjusted P-value < 0.05, which were memory B cells, resting dendritic cells and neutrophils (Fig. 5B). Moreover, the result of the correlations between hub genes and differentially infiltrating immune cells indicated that the memory B cells and resting dendritic cells were positively correlated with IFIH1, IFIT1, IFIT2, IFIT3, ISG15 and OAS3, and neutrophils were positively correlated with IFIH1 and OAS3 (Fig. 5 C).
Functional similarity analysis and GSEA of hub genes
Moreover, we ranked the 6 hub genes based on the average functional similarity (Fig. 6 A). IFIT3, IFIT2 and IFIT1 were the top three proteins potentially playing key roles in atherosclerosis. Through GSEA of hub genes, we found that “KEGG_GRAFT_VERSUS_HOST_DISEASE”, “KEGG_LYSOSOME” and “KEGG_ALLOGRAFT_REJECTION” were enriched in highly expressed samples of all 6 hub genes, and “KEGG_RIBOSOME” was enriched in lowly expressed samples of all 6 hub genes (Fig. 6B).
Construction of miRNA-gene-TF regulatory network
Subsequently, we predicted miRNAs and TFs of the 6 hub genes using the miRNet database. A total of 171 miRNAs and 36 TFs were identified that could potentially regulate the expressions of hub genes. Then, a miRNA-gene-TF regulatory network was constructed using Cytoscape to understand the possible regulation mechanism of hub genes (Fig. 7). hsa-mir-26a-5p, hsa-mir-212-3p, hsa-mir-449a, hsa-mir-129-2-3p, hsa-mir-21-3p, hsa-mir-130a-3p, hsa-mir-27a-5p, hsa-mir-133a-3p, hsa-mir-449b-5p, hsa-mir-210-3p and hsa-mir-16-5p could regulate all hub genes. And IFIT3 was the hub gene that was regulated by the largest number of miRNAs. For TFs, FOXC1 and GATA2 was the TF that regulated the largest number of hub genes. And IFIT1, IFIT3 and ISG15 were the top three hub genes regulated by the largest number of TFs.
Verification of hub gene expression in carotid artery tissue samples
The mRNA expression of hub gene was evaluated in carotid artery patients’ tissue by qRT-PCR. As shown in Fig. 8, All hub genes were found to be upregulated in tissues of AS patients compared with the adjacent noncancerous tissues(p < 0.05), which is consistent with the results of the bioinformatics analysis.