Screening for differential genes
The GSE121211 dataset contains five FSGS samples and five normal samples, while the GSE15779 dataset contains eight FSGS and eight corresponding normal specimens (Figure 1). After normalization of the expression matrices of the two datasets, the box plot distribution trend revealed nearly straight lines (Figure 2A-2B). To assess the in-group data reproducibility, UMAP analysis was performed on the two datasets, which showed good reproducibility (Figure 2C-2D). Through the “limma” package of the R language, the analysis of variance between the two datasets (screening criteria were P < 0.05 and |logFC| ≥ 1) revealed 340 upregulated and 160 downregulated DEGs from the GSE121211 dataset (Figure 2E) and 39 upregulated, and 63 downregulated DEGs from the GSE125779 dataset (Figure 2F). A total of 82 differentially expressed genes were screened using the Venn diagram, and one gene with an inconsistent expression trend was excluded, resulting in 81 genes. A total of 35 upregulated and 46 downregulated DEGs were obtained (Figure 3A-3B).
KEGG and GO enrichment analysis of differential genes
To analyze the DEG-related biological classification, GO and KEGG analyses of the obtained DEGs were conducted. The KEGG analysis indicated that the DEGs were mostly associated with focal adhesion, malaria, age-range pathway, diabetic complications, complement and coagulation cascades, fluid shear stress, atherosclerosis, microRNAs in hormone synthesis, secretion and action of cancer, parathyroid hormone, proteoglycans in cancer, and regulation of the actin cytoskeleton.
GO enrichment analysis was conducted from the biological process (BP), molecular function (MF), and cell component (CC). Our enrichment results indicated that the DEGs in FSGS were mainly associated with the cellular response to organonitrogen compounds, cellular response to growth factor stimulus, development of vasculature, negative regulation of response to external stimulus, regulation of epithelial cell proliferation, Ritz response, transport of nitric oxide, regulation of peptidase activity, response to extracellular stimulus, multicellular organismal homeostasis, and other BPs. The products of differential genes were mainly distributed in the platelet alpha granule, collagen-containing extracellular matrix, endocytic vesicular lumen, endoplasmic reticular lumen, transcription factor AP-1 complex, tertiary granular lumen, vacuolar lumen, microvillus, and sarcolemma, among others. The molecular functions involved include binding of growth factor, extracellular matrix structural constituent, binding of tissue-derived growth factor, heme binding, integrin binding, peptidase regulator activity, glucocorticoid receptor binding, proteoglycan binding, calcium ion binding, mRNA 3'-UTR binding, enzyme activator activity, and growth factor receptor binding, among others (Figure 4A-4D).
Ferroptosis-related genes in FSGS
A total of 259 FRGs were obtained from the ferroptosis database, including 108 genes with driver function, 69 genes with suppressor function, 111 genes with marker function, and one gene with three different roles (HMOX1) (Figure 5A). When the two-cube variance analysis of the datasets GSE121211 and GSE125779 was set to the standard P < 0.05 and |logFC| ≥ 0.5, 466 different genes and 16 ferroptosis-related genes were obtained with the combination of the related database (Figure 5B). The GO functional enrichment analysis of 16 ferroptosis-related genes and KEGG pathway enrichment analysis results in Figure 5C-5D. The results of the PPI network analysis are presented in Figure 5E. Interaction maps of important proteins were obtained by the software Cytoscape, with a total of 19 edges and 12 nodes, with one edge representing protein interaction and one node representing one protein. The node size was adjusted according to the degree. PPI results were imported into Cytoscape and opened with the CytoHubba plugin. The top five genes of MCC, MNC, and degree topology algorithms were selected to obtain common genes as ferroptosis-related hub genes (Figure 5F-5H). A total of five hub genes were obtained, which were JUN, ALB, ATF3, HIF1A, and DUSP1 (Table 1).
Gene network establishment and GO/KEGG analysis of FRGs
The program NetworkAnalyst 3.0 was used to predict the target miRNAs for key genes. Finally, 240 target miRNAs for five specific FRHGs were obtained, and 333 mRNA-miRNA pairs were determined (Figure 6C). A total of 118 miRNAs regulated JUN, 101 regulated HIF1A, 79 regulated DUSP1, nine modulated ALB, and 26 modulated ATF3. All four miRNAs hsa-mir-155-5p, hsa-mir-124-3p, hsa-mir-27a-5p, and hsa-mir-1-3p were correlated with the five specifically expressed FRHGs, which is highly significant. Each of the four miRNAs, hsa-mir-6-5p, hsa-mir-30a-5p, hsa-mir-107, and hsa-mir-10b-5p had four genes associated with them. Seven circRNAs corresponding to miRNAs were screened in the ENCORI database (no corresponding circRNA was predicted in the ENCORI database for hsa-mir-27a-5p), and a total of 54 common associated circRNAs were obtained (Figure 7).
The five ferroptosis-related DEGs were subjected to GO and KEGG analysis. The most significantly enriched GO terms included transcription regulation of RNA polymerase II promoter upon stress, regulation of DNA-templated transcription upon stress, response to starvation, nuclear transcription factor complex, RNA polymerase II transcription factor complex, the activity of DNA-binding transcription activator, transcription factor complex, and RNA polymerase II-specific and nuclear chromatin. The results of KEGG indicated that these DEGs were mostly associated with mitophagy-animal, renal cell carcinoma, choline metabolism, Th17 cell differentiation, PD-L1, expression, and PD-1 checkpoint pathway in cancer (Figure 6A-6B).
GSE108112 confirmed the expression and diagnostic value of FRHGs
The dataset GSE108112 was used to check the levels of selected targets. It was observed that three key FRGs (ALB, ATF3, and DUSP1), which were differentially expressed between renal tubular tissues with and without FSGS in patients receiving tumor nephrectomy, conformed to the predicted results (Figure 8A-8E).
Then, the GSEA approach was used for functional enrichment between FSGS and normal subjects, and the obtained results were compared with the five above-mentioned genes. It was observed that the genes were mainly present in the REACTOME_M_PHASE (NES =-1.599; P.adjust = 0.017; FDR = 0.012) and REACTOME_NEUTROPHIL_DEGRANULATION (NES =-2.312; P.adjust = 0.017; FDR = 0.012) signal pathway enrichment (Figure 9A-9C).
The ROC curves were plotted based on the obtained data in renal tubular tissues with and without FSGS. The above five genes were identified as the diagnostic genes for FSGS. The AUC value of the variable DUSP1 was 0.962 (95% CI: 0.926–0.999), while the AUC values for ATF3, ALB, JUN, and HIF1A were 0.908 (95% CI: 0.0838–0.978), 0.834 (95% CI: 0.730–0.938), 0.730 (95% CI: 0.589–0.0.871), and 0.650 (95% CI: 0.519–0.781), respectively (Figure 8F-8J).
Targeted drug prediction
We used the DSigDB database to predict the potential targeted agents that were associated with FRGs, which might treat FSGS by modulating ferroptosis. A total of 75 drugs were predicted, among which, nitroglycerin was identified as a targeted inhibitor of HIF1A (Table 2).