Background
Although last two decades have witnessed a great success in prevention, diagnosis, and therapy, cancer is still one of the leading cause of death worldwide with nearly 1.6 million new cases reported only in the United States in 2016 with almost one third estimated to die of the disease [
1]. Hepatocellular carcinoma (HCC) has become the most common primary hepatic malignancy, with average survival rates between 6 and 20 months [
2]. Incidence rate of HCC worldwide has increased, likely due to the rising incidence of chronic hepatitis B and C infections [
3]. It now ranks sixth in the world among all the malignancies, contributing to the third leading cause of mortality attributed to cancer [
4]. Although recent developments in the field of functional genomics have improved our knowledge greatly, the molecular pathogenesis of HCC is not fully understood [
5]. Numerous levels of transcriptional regulation are initiated during the process of tumor development and promote disease onset and progression. Hence, a better understanding of these transcriptional regulations and molecular mechanisms behind HCC onset and progression, and identification of potential biomarkers and targets are essential for effective diagnosis and therapeutic treatment.
Gene expression is regulated in many ways including promoter/gene methylation, altered transcriptional regulation, post-transcriptional modifications, mRNA transport from nucleus to cells, mRNA degradation by microRNAs (miRNAs) and post-translational modifications [
6]. DNA methylation of promoter regions of tumor-suppressor genes is known to inhibit transcriptional initiation, thereby, silencing the gene expression [
7]. Individual studies have shown that aberrant DNA methylation of different tumor-suppressor genes including APC [
8], CDKN2A [
9], RASSF1A [
10] and GSTP1 [
11] is correlated with clinical outcome of HCC. miRNAs are 20–22 nucleotide long, small non-coding RNAs which negatively regulate gene expression post-transcriptionally by preferentially binding to the seed sequence in the 3′-UTR of target mRNAs leading to either mRNA destabilization or degradation [
12]. miRNAs are known to regulate diverse biological processes and their aberrant expression has been reported to contribute to tumorigenesis and cancer progression [
13]. Aberrant expression of several miRNAs has been shown associated with disease onset, progression, therapy resistance, and metastasis in HCC [
14]. Recently, whole genome profiling techniques have come up as powerful tools to identify novel molecular mechanisms and biomarkers of disease onset and progression in HCC [
15], therefore, we aimed (1) to identify the pathways dysregulated in HCC due to changes in DNA methylation pattern or due to miRNA mediated post-transcriptional inhibition of genes involved by using methylation profiling, and miRNA and gene expression profiling data and (2) to construct regulatory network of disease onset and progression.
In this study, we identified differentially expressed mRNAs, miRNAs, and differentially methylated genes in HCC compared to normal adjacent tissues by analyzing gene expression, miRNA expression, and methylation profiling data of HCC patients from the National Center for Biotechnology Information Gene Expression Omnibus (NCBI GEO) and integrated top miRNAs along with mRNA targets and their methylation profile into a regulatory methylation-miRNA–mRNA (meth-miRNA–mRNA) network. Flowchart of developing this network is shown in Additional file
1: Fig. S1. Pathway enrichment analyses of identified genes revealed suppressed metabolic signaling and hyperactive cell cycle signaling as key features of HCC onset and progression which we validated in 10 different HCC patients’ datasets. Next, we confirmed the inverse correlation between gene methylation and its expression, and between miRNA and its targets’ expression in various datasets. Furthermore, we validated the clinical significance of identified methylation, miRNA, and mRNA signatures by checking their association with clinical features and survival in HCC patients.
Discussion
In this study, we systematically identified differentially expressed mRNAs, miRNAs, and differentially methylated genes in HCC compared to normal adjacent tissues by analyzing gene expression, miRNA expression, and methylation profiling data of HCC patients from NCBI GEO database and integrated top miRNAs along with mRNA targets and their methylation profile into a regulatory meth-miRNA–mRNA network (Figs.
1 and
2). Changes in DNA and/or promoter methylation patterns of individual or set of genes have been implicated in HCC onset and progression by modulating gene expression patterns of individual cells and/or tissue as a whole [
8‐
11,
18], but a systematic methylation signature based study was lacking in the field. To the best of our knowledge, this is the first instance where altered methylation profile between tumor and adjacent normal tissues has been integrated into a network along with expression profiling data in HCC. We found inverse correlation between gene methylation and its expression for most of the identified hypo- and hyper-methylated genes (Fig.
4a–c) which needs to be validated in vitro and in vivo to certify the identified meth-GS as prognostic factor for clinical outcome and survival in HCC patients (Fig.
4d, e, Additional file
7: Fig. S3).
Similar to changes in DNA methylation pattern, altered miRNA expression has also been shown to regulate gene expression, but at post-transcriptional level [
13,
40]. Considerable amount of research has identified differentially expressed miRNAs between HCC and adjacent normal tissues; thus, regulating cellular processes like metabolism, cell cycle, proliferation and apoptosis by suppressing the expression of their target mRNAs [
41]. Most of these studies were based on single miRNA/single target approach and very few studies have endeavored to identify the combinatorial effect of differentially expressed miRNAs in HCC by miRNA-gene interaction networks [
42,
43]. In this study, we have reported five oncogenic miRNAs (miR-330-3p, miR-671-3p, miR-671-5p, miR-877-5p and miR-939-5p) which are upregulated in HCC, and five tumor suppressor miRNAs (miR-105-5p, miR-149-3p, miR-194-3p, miR-200b-3p and miR-340-5p) which tend to be downregulated in HCC (Figs.
1 and
2). Notably, some of these miRNAs have already been proposed as oncogenic or tumor suppressor miRNAs in HCC or other cancer types confirming the significance of our findings. For instance, miR-330-3p has been reported as oncogenic miRNA in non-small-cell cancer promoting cell proliferation, cell invasion and metastasis [
44,
45] whereas miR-105-5p was found downregulated in HCC and correlated with good prognosis and survival in HCC [
46]. In addition, miR-149-3p has been shown inhibiting proliferation, migration and invasion in bladder cancer [
47] and miR-200b-3p was found to inhibit growth and metastasis in breast and prostate cancer cells by regulating different targets [
48,
49]. As aberrant miRNA expression go hand in hand with altered methylation pattern to modulate gene expression patterns and regulating specific oncogenic phenotype, we integrated both parameters along with gene expression patterns into our regulatory meth-miRNA–mRNA network to better understand the molecular basis of HCC onset and progression. We found inverse correlations between expression of miRNAs and their targets (Fig.
5a, b) which needs to be validated in vitro and in vivo to certify the identified miRNA-GS as diagnostic factor for clinical outcome and survival in HCC patients (Fig.
5c–f).
Pathway enrichment analyses of identified tumor suppressors and oncogenes revealed suppression of metabolic pathways like fatty acid degradation and amino acid metabolism, and hyperactive cell cycle signaling assisted with aberrant DNA replication and, purine and pyrimidine metabolism as key features of HCC onset and progression (Figs.
1 and
2) which we validated in 10 different HCC patients’ datasets (Fig.
3) and found correlated with clinical characteristics and survival in HCC patients (Fig.
6d–i, Additional file
8: Fig. S4). Liver is metabolic hub of the body and plays crucial part in lipid and fatty acid metabolism. Increasing evidence has shown that aberrant activation of lipogenesis due to deregulated lipid metabolism play key role in regulating tumor growth and metastasis in HCC [
50‐
52]. Progression through cell cycle is tightly controlled by cyclins and cyclin-dependent kinases (CDKs) and aberrant expression of these genes can trigger cell cycle programming to run at a faster pace [
53]. We observed upregulation of different cyclins (CCNA2, CCNB1, CCNB2), cyclin dependent kinases (CDK2), cell cycle associated transcription factors (E2F6, E2F7, E2F8) and phosphatases (CDC20, CDC25A, CDC25C) in HCC, which agrees with the existing literature reporting the overexpression of cell cycle related genes in different cancer types including HCC [
54‐
56]. DNA replication during S-phase of cell cycle is one of the crucial step in the completion of cell cycle which also needs supplementation of purines and pyrimidines to build the macromolecules [
57]. Similarly, availability of excessive amino acid pool due to decreased catabolic degradation go hand in hand to fulfil the production of protein products in cells undergoing uncontrolled cell division [
58]. These existing literature are also in line with our findings that catabolic enzymes involved in amino acid metabolism and, purine and pyrimidine metabolism are downregulated whereas anabolic enzymes are upregulated (Additional file
3: Fig. S2) in HCC as compared to adjacent normal tissues.
Although independent studies have implicated the inhibited metabolic processes and the aberrant cell cycle progression in HCC [
59‐
61], we have shown that both events happen side by side and collectively contribute to the onset and progression of the disease. But we still need to identify the causal relationship between these events; whether suppressed metabolic signaling triggers cell proliferation to efficiently fulfil metabolic tasks resulting in onset and progression of HCC, or it is hyperactive cell cycle signaling which keeps cell’s focus on proliferation rather than performing metabolic processes resulting in onset and progression of HCC.
Authors’ contributions
KRZ downloaded the data and did functional annotation and correlation analyses, and drafted the manuscript. MS helped in miRNA target prediction analysis and creating networks, improved the discussion and critically reviewed the manuscript. ARRK helped in downloading data and Z-score calculation. SH helped in miRNA target prediction analysis and creating networks. GD conceived the study, participated in its design and coordination, and critically reviewed the manuscript. UR downloaded the data, did differential expression, miRNA-target prediction and survival analyses, conceived the study, participated in its design and coordination, and drafted the manuscript. All authors read and approved the final manuscript.