Identification of potential candidate genes for hypertensive nephropathy based on gene expression profile

Background This study was aimed to explore the molecular mechanisms of hypertensive nephropathy (HTN). Methods Gene expression profile of GSE37460, which based on 27 healthy living donor samples (HTN group) and 15 hypertensive nephropathy samples (control group), were downloaded from Gene Expression Omnibus (GEO) database. The differentially expressed genes (DEGs) between two groups were identified. STRING database was used to reveal protein-protein interaction (PPI) network of DEGs, followed by the functional enrichment analysis of the PPI network. Additionally, miRNA-DEG regulatory network was constructed to reveal the validated miRNAs targeting the DEGs. Results In total, 51 up-regulated genes and 140 down-regulated genes were obtained. In the PPI network, cytochrome P450 3A4 (CYP3A4) and angiotensin II receptor type 1 (AGTR1) had a higher degree, and CYP3A4 interacted with CYP4A11. The DEGs in the network were significantly enriched in drug metabolism, focal adhesion and arachidonic acid metabolism. Furthermore, in the miRNA-DEG regulatory network, hsa-miR-335-5p and hsa-miR-26b-5p were the two most outstanding miRNAs. AGTR1, CYP3A4 and CYP4A11 were predicted to be regulated by hsa-miR-26b-5p. Conclusion The DEGs, such as AGTR1, CYP3A4 and CYP4A11 may play critical roles in the development of HTN likely via the regulation by hsa-miR-26b-5p and taking part in some pathways.

Recently, the studies based on gene or protein investigation are successfully used to reveal the potential mechanisms of HTN. For instance, using distinct lines of the spontaneously hypertensive rat, Dmitrieva et al. have shown a major change in transcriptional control by hepatocyte nuclear factor 1 that affects pathways like redox and other genes, which further lead to the hypertensive renal injury [7]. Periostin, also called osteoblast-specific factor 2, strongly associated with plasma creatinine, proteinuria and renal blood flow, has been identified as a critical marker of progression and regression in HTN [8]. Moreover, SMAD family member 7 has also been discovered to inhibit AngIImediated HTN through the Sp1/SMAD family member 3/nuclear factor kappa B (NF-κB)/miR-29b regulatory network, and it is identified as a therapeutic biomarker for AngII-mediated HTN [9]. Furthermore, the role of micro-RNAs (miRNAs) in HTN has also been investigated in recent years. A set of miRNAs (e.g. miR-429, miR-200a, miR-205, miR-200b, miR-141, and miR-192) have been found to be highly expressed in hypertensive nephrosclerosis, and the degree of upregulation is closely related to disease severity [10]. Hsa-miR-181a has confirmed to regulate REN (renin) and apoptosis-inducing factor, mitochondrion-associated, 1 mRNA, and modulate REN expression in HTN [11]. Using a mRNA expression profiling dataset GSE37460, Berthier et al. have discovered a series of pathways, such as endothelial cell activation/injury, immune cell infiltration/activation, and tissue remodeling/fibrosis, with macrophage/dendritic cell activation in both murine models and human lupus nephritis, and they have also found that nuclear factor κB1 and peroxisome proliferator-activated receptor γare major regulatory nodes in the tubulointerstitial and glomerular networks [12]. However, the differences between human HTN and healthy controls remain unclear, and more genes and pathways associated with HTN have not been found.
In the present study, based on the expression profile data of healthy living donor samples and HTN samples deposited by Berthier et al. [12], a bioinformatics analysis was performed. After identification of differentially expressed genes (DEGs) and functional enrichment analysis of the DEGs, protein-protein interactions (PPIs) of the DEGs were analyzed. Furthermore, miRNAs that regulate DEGs were further investigated. These results may contribute to a better understanding of the molecular mechanisms of HTN pathogenesis, and provide valid biological information for further investigation of HTN.

Methods
As the study did not involve any human or animals, the ethical approval was not required.

Affymetrix microarray data
The mRNA expression profile of GSE37460 [12] was downloaded from a public functional genomics data repository GEO (Gene Expression Omnibus, http:// www.ncbi.nlm.nih.gov/geo/), which was based on two platforms, including Affymetrix Human Genome U133 Plus 2.0 Array (GPL11670) and Affymetrix GeneChip Human Genome HG-U133A Custom Array (GPL14663) (Affymetrix, California, USA). Glomeruli from kidney biopsy samples from 27 healthy living donors (control group), 15 hypertensive nephropathy (HTN group) and 25 IgA nephropathy (IgAN group) participants were included in this profile. To explore the abnormal transcription of HTN, the samples from HTN group and healthy control group were specially enrolled for the following investigation.
The data in the CEL source files were normalized by using the R package CONOR [13], including background correction, quantile normalization and probe summarization.

Identification of DEGs
The DEGs between control group and HTN group were analyzed by using the Linear Models for Microarray Data (LIMMA, http://www.bioconductor.org/packages/ release/bioc/html/limma.html) package in Bioconductor software [14]. The raw p-value of each gene was adjusted into the false discovery rate (FDR) by using Bonferroni correction [15]. Only the genes with FDR-value < 0.05 and |log 2 FC (fold change)| ≥ 1.5 were identified as DEGs.

Construction of PPI network
The Search Tool for the Retrieval of Interacting Genes/ Proteins (STRING) database includes known and predicted PPIs [16]. The interactions of proteins encoded by DEGs were selected as the background network according to STRING v9.1 database with combined score > 0.9. Degree (the number of interactions linked to one target) was used to describe the frequency of interactions. Subsequently, the DEGs and related first neighbors were further extracted to construct the PPI network, which was visualized by Cytoscape (http://www.cytoscape.org/) [17].

Functional enrichment analysis of pathways for genes in the PPI network
The plugin in Cytoscape software, ClueGO, can integrate Gene Ontology (GO) terms and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways to create a functionally organized network [18]. By calculating the kappa coefficient [19], the functional pathways can be divided into several function groups in the network. To reveal the biological functions of genes in the PPI network, the KEGG pathway enrichment analysis for DEGs in the PPI network was performed based on ClueGO. The raw p-value of each pathway term was adjusted into the FDR by using Bonferroni correction [15]. FDR-value < 0.05 was considered as the cutoff criterion of significant difference, and pathway groups were generated based on kappa = 0.4.

Identification of DEGs
A large number of calculations were performed, and the original data were analyzed and filtered. A total of 51 up-regulated DEGs and 140 down-regulated DEGs were obtained with thresholds of FDR < 0.05 and |log 2 FC| ≥ 1.5. The heat map of DEGs showed that the HTN samples were distinguished clearly from the healthy samples by the identified DEGs (Fig. 1).

PPI network investigation and functional enrichment analysis
With the combined score > 0.9, a total of 1220 nodes (36 up-regulated, 88 down-regulated and 1096 nonsignificant ones) were included in the PPI network (Fig. 2). The average degree for all enrolled DEGs was 14.5. The genes with nodes degree value > 50 [e.g. cytochrome P450 family 3 subfamily A member 4 (CYP3A4) and angiotensin II receptor type 1 (AGTR1)] were listed in Table 1.

The miRNA-DEG regulatory network investigation
To study the validated miRNAs of DEGs, the miRNA-DEG regulatory network was constructed based on mul-tiMiR software. A total of 217 nodes (103 miRNAs, 34 up-regulated DEGs and 80 down-regulated DEGs) and 295 interactions were included in this network (Fig. 4). Among the miRNAs, hsa-miR-335-5p and hsa-miR-26b-5p modulated the majority of DEGs in this network. For example, hsa-miR-26b-5p regulated COL4A1, CYP4A11 and AGTR1. Fig. 1 Heat map for the differentially expressed genes. Each row represents a single gene; each column represents a tissue sample. Green blocks represent the downregulated differentially expressed genes; red blocks represent the upregulated differentially expressed genes; black blocks represent non-significant genes; light blue represents the control group, while pink represents hypertensive nephropathy group Discussion HTN is an increasingly common kidney disease in patients with hypertension recently [6]. However, the potential mechanisms of the progress of HTN is still unclear. In this study, a bioinformatics analysis of gene expression profile for healthy living donor samples and HTN samples was performed to explore the mechanisms of HTN. In total, 51 up-regulated DEGs and 140 downregulated DEGs were identified in the HTN samples compared with the healthy controls. The DEGs were significantly enriched in pathways like drug metabolism, focal adhesion and arachidonic acid metabolism.
In the present study, CYP3A4 and AGTR1 were the outstanding down-and up-regulated DEGs with the highest degree in the PPI network, respectively. CYP3A4 encodes an enzyme belonging to the cytochrome P450 (CYP P450) superfamily, which is a group of hemethiolate monooxygenases and participate in a variety of oxidation reactions [21]. CYP P450 expression can be altered by inflammation [22], which is involved in renal injury [23]. The expression of CYP3A4 is induced by Fig. 2 Protein-protein interaction network consisting of differentially expressed genes and non-significant genes. White diamonds represent the down-regulated genes; white round nodes represent the up-regulated genes; gray round nodes represent the non-significant genes. The node size is proportional to the degree value glucocorticoids and involved in the metabolism of multiple drugs [24]. A previous study has reported that CYP3A4 T16090C SNP responses to amlodipine among African-Americans with early HTN [25]. Moreover, the expression of CYP3A4 is elevated in patients with endstage renal disease [26]. CYP4A11, a homologue of CYP3A4, had a higher degree in the PPI network and interacted with CYP4A11. In this study, CYP4A11 was significantly enriched in the pathway of arachidonic acid metabolism. CYP P450 metabolites of arachidonic acid play an important role in the control of blood pressure, chronic kidney disease through the maintenance of the glomerular permeability barrier to albumin [27,28]. Furthermore, 20-hydroxyeicosatetraenoic acid (20-HETE) has renoprotective actions in hypertension, and mutations Fig. 3 KEGG pathway enrichment analysis for differentially expressed genes in protein-protein interaction network. Each node is a KEGG pathway item, and node size is proportional to the pathway significance. Edge between nodes mean that they share common genes, and the width of the edge is proportional to the number of common genes. Pathways are classified into several functional groups (different node color) accordance with the kappa value. The most significant pathway of each group labels with a highlighted color. KEGG, Kyoto Encyclopedia Of Genes And Genomes   Fig. 4 The microRNA-differentially expressed gene regulatory network. White triangles represent microRNAs; white ellipses represent the up-regulated genes; grey ellipses represent the down-regulated genes in CYP4A11 that produces 20-HETE have been linked to elevated blood pressure in humans [29,30], indicating the important role of CYP4A11 in HTN. In the present study, CYP4A11 was predicted to be regulated by hsa-miR-26b-5p. An recent study has demonstrated that expression of miR-26b-5p is significantly decreased in basal serum samples from those patients with acute kidney injury, and it is a diagnostic biomarker of acute kidney injury [31]. Currently, there is no any other evidence to prove the associations of CYP3A4, CYP4A11 and hsa-miR-26b-5p with HTN. Given the above studies, we speculated that CYP3A4 and CYP4A11, as well as hsa-miR-26b-5p may play pivotal roles in the progress of HTN. AGTR1 also had a higher degree in the PPI network, and it was modulated by hsa-miR-26b-5p. AGTR1 encodes of angiotensin II type 1 receptor, which is an important effector in the control of blood pressure [32]. Previous studies have shown that variants on genes including AGTR1 are associated with hypertension [33,34]. The (−535) T allele of AGTR1 is believed to increase hypertension risk among African Americans [35]. Moreover, AGTR1 polymorphisms are believed to be associated with the renal function [36,37]. Durvasula et al. have reported that intrarenal production of angiotensin II plays an important role in mediating HTN through inducing podocyte injury and promoting the development of glomerulosclerosis [38]. Furthermore, a previous study has found that angiotensin II-induced arterial hypertension and vascular dysfunction are mediated by lysozyme M-positive monocytes [39], which participate in renal injury [40]. Although there is no direct evidence to prove the association of AGTR1 and HTN, we speculate that AGTR1 may exert critical functions in the progress of HTN.
However, this study has several limitations. The major limitation is that the aforementioned results should be validated by other microarray data or experimental studies, which will be conducted and reported later. Furthermore, more patients with HTN should be included for the analysis. Additionally, the clinical data of the patients are not available, thus the patients may be heterogenous. In the further study, more samples from patients with HTN will be used for the verification experiments to confirm our results.

Conclusion
In conclusion, 51 up-regulated DEGs and 140 downregulated DEGs were identified in the HTN samples compared with the healthy controls. The DEGs such as CYP3A4, CYP4A11 and AGTR1, may be crucial in the progress of HTN, via the regulation by miRNAs (e.g. hsa-miR-26b-5p) and participation in the biological pathways (e.g. arachidonic acid metabolism). Notably, the above discussed genes and miRNA are new-found to be correlated with HTN in this study, and they are worth further investigation. These findings provide new information for further experimental studies. If these genes and miRNAs are confirmed by experiments, they will be promising to be used in the diagnosis or clinical therapy of HTN.