This article is mentioned in:

Introduction

Esophageal cancer is one of the most common types of malignant tumor encountered in the clinic and is the 6th most frequent cause of cancer-associated mortality. Esophageal adenocarcinoma (EAC) is the predominant pathological subtype of esophageal cancer in Europe and the USA (1). Due to a lack of reliable early diagnostic methods and efficient intervention approaches, EAC has had a poor five-year survival rate of <20% (2). Although significant progress has been made in the medical field over the past few years, the median survival time of EAC has only slightly increased (3). Therefore, more research effort is required to explore potential prognostic markers and therapeutic strategies for EAC.

In recent years, molecular biomarkers with an early diagnostic and prognostic value for EC have been investigated. Chen et al (4) performed a systematic review of the literature and summarized that cyclooxygenase-2 and HER-2 are valuable prognostic markers for EAC. A study by Darlavoix et al (5) on EAC reported that the decrease in E-cadherin expression and the altered expression of CD44 and dickkopf WNT signaling pathway inhibitor 1 may indicate an increased risk of EAC progression. A large number of microRNA (miR) biomarkers have been identified; for instance, the loss of miR-31 is considered as an early molecular marker for the metaplasia-to-dysplasia transition of EAC, while miR-194, miR-196a and miR-375 are considered as oncogenic factors (6,7). In spite of the large number of miR biomarkers identified, only few of them are suitable for clinical use due to a lack of sensitivity and specificity. An increased knowledge of EAC-associated biomarkers may provide further mechanistic insight and lay a foundation for novel approaches to prevent, diagnose and treat EAC.

High-throughput sequencing technology is a widely used tool to explore genetic alterations during tumorigenesis. In the present study, a high-throughput sequencing dataset was analysed using bioinformatics tools to identify differentially expressed genes (DEGs). The DEGs were subjected to functional enrichment, co-expression network analysis and survival analysis to identify key genes in EAC, while the validation of key gene expression levels in EAC and normal epithelial tissues was performed using data from The Cancer Genome Atlas (TCGA) database and another microarray dataset.

Materials and methods

High-throughput sequencing data

The GSE94869 gene expression profile obtained through high-throughput sequencing was downloaded from the Gene Expression Omnibus database (https://www.ncbi.nlm.nih.gov/geo/) (8,9). The data]]set was based on the Illumina MiSeq [Homo sapiens (hsa)] platform (Illumina, Inc., San Diego, CA, USA). The GSE94869 dataset included 40 pairs of EAC and matched normal esophageal squamous epithelium tissues.

DEG analysis

The DEGs in EAC tissues compared with normal tissues were identified using the Linear Models for Microarray Analysis (Limma) package in R software (10). The Benjamini-Hochberg false discovery rate (FDR) was used to correct the P-values. An adjusted P-value of <0.05 and a fold change of ≥2 were used as the cut-off criteria for the DEG analysis.

Functional enrichment analysis of DEGs

Gene Ontology (GO) (11) and the Kyoto Encyclopedia of Genes and Genomes (KEGG) (12) analyses are widely used to identify the most correlative GO terms in the category biological process (BP) and relevant pathway information, and were performed through the online tool Database for Annotation Visualization and Integrated Discovery (13). A corrected P-value (P-value adjusted using Benjamini-Hochberg) of <0.05 was used as the cut-off criterion.

Co-expression module detection and functional enrichment analysis for each module

The weighted gene co-expression network analysis (WGCNA) (14) was restricted to the DEGs by using the WGCNA package in R software. An unsupervised co-expression association was constructed on the basis of the adjacency matrix of connection strengths using Pearson's correlation coefficients. Gene modules that had a high topologic overlap were identified as gene sets and a minimum gene module size of 30 was used to cut branches. The top 10 genes with the highest network connectivity in each module were identified as hub genes. Subsequently, the co-expression network of each module was presented using Cytoscape 3.4.0 (15). GO and KEGG analyses for each module were then performed as specified above. A corrected P-value (P-value adjusted using Benjamini-Hochberg) of <0.05 was used as the cut-off criterion.

Survival analysis depending on hub genes

The mRNA transcript per million of 104 EAC tissue samples and corresponding follow-up information for these patients were downloaded from the Xena browser from the University of California Santa Cruz (https://xenabrowser.net/datapages/?host=https://tcga.xenahubs.net). The survival R package (https://CRAN.R-project.org/package=survival) (16) was used to explore the prognostic value of hub genes. Kaplan-Meier survival curves were plotted. Relapse-free survival (RFS) was used as the survival endpoint and only the 62 patients with RFS data were included. Patients with EAC were divided into low and high expression groups according to the median expression value of each hub gene. A Log-rank P-value of <0.05 was considered to indicate statistical significance.

Cox proportional hazards (CPH) analysis

The association of clinical parameters, including age at diagnosis, history of alcohol use or Barrett's esophagus, pathologic tumor-nodes-metastasis stage with survival was assessed using the univariate CPH model. The results of the CPH analysis were presented as hazard ratios, along with parameters of significance and 95% confidence intervals. All prognosis-associated genes identified by the Kaplan-Meier analysis were converted into ordinal or unordered categorical variables based on quartiles (Q) in order to determine the risk of recurrence of EAC for each quartile expression level of the prognosis-associated genes. A P-value for the trend referred to different quartiles was obtained from the CPH model when the expression of prognosis-associated genes was categorized into ordinal categorical variables. As for unordered categorical variables, the CPH analysis was applied with the lowest quartiles as a reference. Model 1 remained unadjusted and Model 2 was adjusted for risk factors that were indicated to have an effect (P<0.2) on survival in the univariate analysis. P<0.05 was considered to indicate statistical significance.

Validation of differential expression of prognosis-associated genes

Validation of the expression levels of all prognosis-associated genes identified through Kaplan-Meier analysis was performed using a box plot to visualize the difference in expression between EAC and normal tissues based on data from TCGA and the dataset GSE26886 (17). The data of 8 EAC tissues and paired paracancerous lesions were obtained from the TCGA database. The GSE26886 dataset was based on the Affymetrix Human Genome U133 Plus 2.0 Array platform (Thermo Fisher Scientific, Inc., Waltham, MA, USA) and included the data of 21 EAC tissues and 19 normal esophageal squamous epithelial tissues. The expression levels were presented as the mean ± standard deviation. A P-value of <0.05 was considered to indicate a statistically significant difference.

Results

DEG and functional enrichment analysis

In total, 130 DEGs, including 82 upregulated and 48 downregulated genes, were identified using the Limma package. The GO BP analysis demonstrates that the upregulated DEGs were significantly associated with extracellular matrix organization, endodermal cell differentiation and extracellular matrix disassembly, while the downregulated DEGs were significantly associated with cell fate commitment, neuron differentiation and the canonical Wnt signaling pathway. The KEGG pathway analysis demonstrated that the upregulated genes were significantly enriched in focal adhesion, the phosphoinositide-3 kinase (PI3K)-Akt signaling pathway and the Rap1 and Ras signaling pathways, while the downregulated genes were significantly associated with signaling pathways regulating pluripotency of stem cells and the Wnt signaling pathway. These results are presented in Table I.

Table I. GO terms in the category biological process and Kyoto Encyclopedia of Genes and Genomes pathways significantly enriched by the differentially expressed genes in esophageal adenocarcinoma.

Table I.

GO terms in the category biological process and Kyoto Encyclopedia of Genes and Genomes pathways significantly enriched by the differentially expressed genes in esophageal adenocarcinoma.

Term/pathway	Description	Enriched genes (n)	Adjusted P-value
Upregulated
GO:0030198	Extracellular matrix organization	12	9.43×10−7
GO:0035987	Endodermal cell differentiation	5	2.92×10−3
GO:0030168	Platelet activation	7	3.95×10−3
GO:0007596	Blood coagulation	8	4.23×10−3
GO:0022617	Extracellular matrix disassembly	6	4.33×10−3
GO:0048013	Ephrin receptor signaling pathway	6	6.57×10−3
hsa04510	Focal adhesion	13	7.58×10−6
hsa04151	PI3K-Akt signaling pathway	14	1.53×10−4
hsa04611	Platelet activation	8	1.69×10−3
hsa05200	Pathways in cancer	13	1.70×10−3
hsa04015	Rap1 signaling pathway	10	1.74×10−3
hsa04014	Ras signaling pathway	10	1.86×10−3
hsa05146	Amoebiasis	7	3.78×10−3
hsa04071	Sphingolipid signaling pathway	7	6.50×10−3
hsa04512	ECM-receptor interaction	6	9.85×10−3
Downregulated
GO:0045165	Cell fate commitment	5	1.69×10−3
GO:0030182	Neuron differentiation	6	2.03×10−3
GO:0060070	Canonical Wnt signaling pathway	6	2.09×10−3
GO:0000122	Negative regulation of transcription from RNA polymerase II promoter	11	3.38×10−3
hsa04550	Signaling pathways regulating pluripotency of stem cells	9	1.39×10−5
hsa04310	Wnt signaling pathway	6	9.79×10−3

[i] GO, Gene Ontology; hsa, Homo sapiens; ECM, extracellular matrix; PI3K, phosphoinositide-3 kinase.

Co-expression module detection and functional enrichment analysis

WGCNA was performed to construct gene co-expression modules from the expression profiles of the 130 DEG. A total of 2 modules comprising of 36 and 50 genes each were identified. The co-expression networks of the 2 modules including their hub genes are visualized in Figs. 1 and 2. The genes involved in the blue module were associated with the Rap1 signalling pathway, melanogenesis and holinergic synapses according to KEGG analysis, while the genes in of the turquoise module were associated with positive regulation of osteoblast differentiation and the ephrin receptor signaling pathway according to GO BP analysis, and the Ras signaling pathway, pathways in cancer and signaling pathways regulating the pluripotency of stem cells according to KEGG analysis. Theseresults are presented in Table II.

Figure 1. Co-expression network of the blue module identified by WGCNA. The colour spectrum illustrates the degree of connectivity of the nodes, with red indicating the highest degree of connectivity and turquoise the lowest degree of connectivity. The nodes with a larger size represent hub genes among the top 10 with the highest network connectivity degree. WGCNA, Weighted Gene Co-Expression Network Analysis.

Figure 2. Co-expression network of the turquoise module identified by WGCNA. The colour spectrum illustrates the degree of connectivity of the nodes, with red indicating the highest degree of connectivity and turquoise the lowest degree of connectivity. The nodes with a larger size represent hub genes among the top 10 with the highest network connectivity degree. WGCNA, Weighted Gene Co-Expression Network Analysis.

Table II. Significantly enriched GO terms in the category biological process and Kyoto Encyclopedia of Genes and Genomes pathways in the different co-expression modules.

Table II.

Significantly enriched GO terms in the category biological process and Kyoto Encyclopedia of Genes and Genomes pathways in the different co-expression modules.

Term or pathway	Description	Enriched genes (n)	Adjusted P-value
Module blue
hsa04015	Rap1 signaling pathway	5	4.10×10−2
hsa04916	Melanogenesis	4	4.28×10−2
hsa04725	Cholinergic synapse	4	4.31×10−2
hsa04540	Gap junction	4	4.46×10−2
hsa05143	African trypanosomiasis	3	4.54×10−2
hsa04071	Sphingolipid signaling pathway	4	4.59×10−2
hsa04918	Thyroid hormone synthesis	4	4.61×10−2
hsa04915	Estrogen signaling pathway	4	4.67×10−2
hsa04270	Vascular smooth muscle contraction	4	4.82×10−2
hsa04911	Insulin secretion	4	4.84×10−2
Module turquoise
GO:0045669	Positive regulation of osteoblast differentiation	5	1.48×10−2
GO:0048013	ephrin receptor signaling pathway	5	3.04×10−2
hsa04014	Ras signaling pathway	9	3.09×10−4
hsa05200	Pathways in cancer	10	7.90×10−4
hsa04550	Signaling pathways regulating pluripotency of stem cells	7	1.05×10−3
hsa05205	Proteoglycans in cancer	7	3.84×10−3
hsa04810	Regulation of actin cytoskeleton	7	4.12×10−3
hsa05166	HTLV–I infection	7	9.65×10−3
hsa04360	Axon guidance	5	1.93×10−2
hsa04015	Rap1 signaling pathway	6	1.99×10−2
hsa04510	Focal adhesion	6	2.09×10−2
hsa04310	Wnt signaling pathway	5	2.35×10−2
hsa04151	PI3K-Akt signaling pathway	7	2.43×10−2

[i] GO, Gene Ontology; hsa, Homo sapiens; PI3K, phosphoinositide-3 kinase; HTLV, human T-lymphotropic virus.

Survival analysis depending on the hub genes

In the dataset from TCGA database, the follow-up information for only 62 cases included the RFS. The prognostic value of 20 hub genes was assessed using the survival package in R software. Kaplan-Meier analysis indicated that a higher expression of methyltransferase like 7B (METTL7B) was associated with a poorer RFS of EAC patients (P=0.006; Fig. 3).

Figure 3. Prognostic value of METTL7B for relapse-free survival of EAC patients. EAC patients were divided into low and high expression groups according to the median expression level. EAC, esophageal adenocarcinoma; METTL7B, methyltransferase like 7B.

CPH analysis

In the univariate CPH model, only the pathological TNM stage was marginally significantly associated with a higher risk of recurrence, while the pathological TNM stage (P=0.056) and the pathological N stage (P=0.140) were used to adjust the prognostic gene, METTL7B, in the multivariate CPH model (Table III). As an ordinal categorical variable, overexpressed METTL7B was significantly associated with a higher risk of recurrence. However, the pathological TNM stage and N stage weakened the association between METTL7B expression levels and RFS (P=0.092). As unordered categorical variables, Q3 and Q4 of METTL7B were significantly associated with a higher risk of recurrence in the univariate CPH model, while only Q3 of METTL7B was significantly associated with a higher risk of recurrence compared with that of the lowest quartile adjusted by the pathologic TNM stage and N stage (Table IV).

Table III. Univariate Cox proportional hazard regression analysis of parameters affecting relapse-free survival.

Table III.

Univariate Cox proportional hazard regression analysis of parameters affecting relapse-free survival.

Parameter	n	HR (95% CI)	P-value
Age (≤65 vs. >65 years)	29:32	0.93 (0.47–1.83)	0.825
History of alcohol use (yes vs. no)	10:50	0.59 (0.25–1.36)	0.215
History of Barrett's esophagus (yes vs. no)	37:17	1.03 (0.48–2.22)	0.936
Location of lesion (mid vs. distal)	5:56	0.73 (0.21–2.45)	0.605
Pathologic N-stage (0/1 vs. 2/3)	38:9	2.07 (0.79–5.43)	0.140
Pathologic T-stage (1/2 vs. 3/4)	26:21	1.41 (0.59–3.37)	0.435
Pathologic TNM stage (1/2 vs. 3/4)	26:18	2.37 (0.98–5.72)	0.056

[i] HR, hazard ratio; CI, confidence interval.

Table IV. Risk of recurrence for each quartile of methyltransferase like 7B expression levels.

Table IV.

Risk of recurrence for each quartile of methyltransferase like 7B expression levels.

Calculation	Q1	Q2	Q3	Q4	P-value for trend
Range	6.39–8.17	8.18–9.01	9.04–9.49	9.5–10.62
Patients, n	16	15	14	16
Model 1, HR (95% CI)	Ref	2.78 (0.74–10.53)	6.46 (1.78–23.38)	4.16 (1.16–14.96)
P-value		0.131	0.005	0.029	0.013
(No. of patients)	(n=13)	(n=11)	(n=7)	(n=12)
Model 2 (HR (95% CI))	Ref	2.18 (0.52–9.12)	4.62 (1.08–19.75)	2.9 (0.76–11.08)
P-value		0.287	0.039	0.12	0.092

[i] Range, expression level range in each quartile; Model 1, crude data with no adjustment; Model 2, adjustment for pathological TNM stage and N stage; Q, quartile. P-value for trend, P-value of cox proportional hazard regression analysis of METTL7B expression level transformed into ordered categorical variable affecting relapse-free survival. HR, hazard ratio; CI, confidence interval. HR (95% CI), HR and CI compared with Q1 group as METTL7B expression level transformed into unordered categorical variable. Ref, reference.

Validation of differential expression of prognostic genes

As displayed in Fig. 4, the METTL7B expression levels were significantly increased in EAC tissues compared with those in paracancerous lesions in the two additional datasets used for validation. The relative METTL7B expression levels in EAC vs. paired paracancerous lesions were 8.95±0.55 vs. 4.58±0.42 (P<0.0001) based on the data from the TCGA database and 8.66±0.25 vs. 5.99±0.07 (P<0.0001) based on data from the GSE26886 dataset.

Figure 4. Validation of differential expression of METTL7B in EAC. (A) Expression level of METTL7B in EAC (n=8) and normal epithelium tissues (n=8) based on data from The Cancer Genome Atlas. (B) Expression level of METTL7B in EAC (n=21) and normal epithelium tissues (n=19) based on data from the microarray dataset GSE26886. In the box plot, the boxes indicate the median and interquartile range of data, while the error bars represent the minimum and maximum values. The data points display the exact numerical expression level for each sample. EAC, esophageal adenocarcinoma; METTL7B, methyltransferase like 7B.

Discussion

In the present study, a total of 130 DEGs associated with EAC were identified, including 82 upregulated and 48 downregulated genes. The upregulated genes were associated with extracellular matrix organization, disassembly, PI3K-Akt, and the Rap1 and Ras signaling pathways, while the downregulated genes were associated with the Wnt signaling pathway. Among them, 2 co-expression modules and 20 hub genes were identified. The blue module was associated with the Rap1 signaling pathway, while the turquoise module was associated with the Ras and Rap1 signaling pathways. Subsequently, Kaplan-Meier analysis of hub genes revealed that higher expression of METTL7B was significantly associated with poorer RFS. The CPH model also confirmed this result. Finally, the overexpression of METTL7B in EAC tissues was also validated using data from TCGA and another microarray dataset, GSE26886. Therefore, METTL7B may be regarded as a key gene, while the Ras and Rap1 signaling pathways may be crucial mechanisms of genesis of EAC. In addition, all enriched GO BPs and KEGG pathways may participate in mechanisms underlying EAC progression and require to be considered in future studies.

METTL7B is a protein-coding gene located at chromosome 12q13.2. It was reported to be a major component of hepatic lipid droplets and a prognostic marker of non-alcoholic steatohepatitis (18). Although certain studies have revealed that METTL7B is associated with common lipid metabolism and functional organelle production, its precise functions have been rarely reported (19,20). A recent study focusing on breast cancer cell lines identified that downregulation of METTL7B, which may be caused by a reduction in Rho-related BTB domain-containing protein 1, leads to a profound fragmentation of the Golgi apparatus; this then contributes to a loss of normal epithelial polarity and finally converts mammary epithelial cell phenotypes into an invasive one (21). However, to the best of our knowledge, no previous study has assessed the role of METTL7B in esophageal neoplasms. Therefore, the potential existence of a mechanism of this specific gene in EAC analogous to that in breast cancer is worthy of investigation.

The Ras signaling pathway is one of the most thoroughly studied pathways in various types of human cancer, including EAC. The mitogen-activated protein kinase (MAPK) signaling pathway, a key downstream effector of Ras signaling, is shared by four distinct cascades named as extracellular signal-regulated kinase (ERK)1/2, c-Jun N-terminal kinase (JNK), p38MAPK and ERK5. Upregulation of the Ras signaling pathway activates the MAPK pathway, and this or other dysregulations in downstream signaling probably lead to carcinogenesis. The ERK1/2 pathway has a crucial role in regulating cellular growth and differentiation. Accordingly, Fan et al (22) demonstrated that inhibition of ERK activation prevented proliferation of human esophageal cancer cells by inducing cell cycle arrest in G0/G1 phase. The p38MAPK and JNK pathways are closely linked to stress, including inflammation and apoptosis. It was observed that Ras-JNK signaling significantly enhances the invasive behaviour of EAC cell lines (23), while Connor et al (24) indicated that JNK activation leads to the induction of apoptosis of EAC cells. A similar phenomenon was reported for Ras-p38MAPK signalling (25,26), and it may therefore be hypothesized that these two pathways have a tumor suppressor or oncogenic role depending on certain conditions, e.g. the stage of EAC. From the above information, it is apparent that the Ras signaling pathway has a definite role in EAC, and therefore, the proteins associated with this signal transduction pathway that were identified in the present bioinformatics analysis are worthy of further investigation.

The Rap1 pathway is another ‘hot spot’ of cancer research. Rap1 is a member of the Ras-like small GTPase family with two subtypes, Rap1a and Rap1b. The Rap1 signaling pathway is known to be over-activated in various tumor types. Furthermore, it has been reported to promote tumor cell migration, invasion and metastasis in the development of cancer, including, prostate cancer (27), head and neck cancer (28) and esophageal squamous cell carcinoma (29). Conversely, active Rap1 also prevents tumor invasion and metastasis in the bladder, lung and brain (29). However, to the best of our knowledge, the role of the Rap1 signaling pathway in EAC has remained unreported. Further experiments are required to verify the role of the Rap1 signaling pathway and EAC.

In conclusion, in the present study, a total of 130 DEGs were identified in EAC, from which 2 co-expression modules were established. The enriched pathways, including the Ras and Rap1 signaling pathways, may be closely linked to EAC progression. Furthermore, METTL7B may be a key gene associated with the prognosis of EAC. However, no additional experiments or clinical research were performed to verify these results. Thus, further studies are required, which should focus on a more detailed clinical application of this key gene and associated pathways.

Acknowledgements

Not applicable.

Funding

The present study was supported by National Natural Science Foundation of China (grant nos. 81570484, 81600424 and 81502041).

Availability of data and materials

Not applicable.

Authors' contributions

ZD conducted the research, analysed the data and wrote this manuscript. SX designed the study, performed the research and revised the paper. JW performed the research and wrote this manuscript. HZ, TZ and YC performed the research and acquired the data.

Ethical approval and consent to participate

Not applicable.

Patient consent for publication

Not applicable.

Competing interests

The authors declare that they have no competing interests.

Glossary

Abbreviations

Abbreviations:

References