In this study, for the first time, the expression and significance of the E2F family members between the two main histological subtypes of EC (ESCC and EAC) were systemically analyzed. The novel findings of the present study were as follows: (1) The mRNA expression levels of the E2F family members were significantly elevated in EC compared with the normal control tissues. (2) The patterns of the E2F gene expression alterations were different between the two EC forms: ESCC and EAC. (3) The genetic mutations (e.g., mutation frequency, CNVs) and epigenetic changes (e.g., methylation and histone modifications) in some E2F family member genes were different between ESCC and EAC. (4) The coexpression profiles of E2F family member genes and their incorporation with protein-protein interactions revealed interactions between the E2F family members and interplay with proteins involved in m6A and histone modifications in EC. (5) Kaplan–Meier analysis showed a significant correlation of lower levels of E2F1, E2F3, E2F5, E2F7, and E2F8 mRNA expression with a better prognosis for EAC; and lower E2F1, E2F3, E2F6, E2F7, and E2F8 mRNA levels indicated a poorer outcome for ESCC. Thus, these E2Fs have potential as biomarkers in the differential diagnosis of EAC and ESCC as well as in the prognosis of EC.
The abnormal expression of E2F family member genes has been detected in a range of malignancies. For instance, overexpression of E2F1 has been shown to contribute to the development of EC, and this effect is enhanced by dysregulated pRb-p53-MDM2 circuitry .In addition, amplification of E2F1 is frequently found in ESCC . In our study, TCGA datasets revealed that the expression of E2F1 was higher in human EC than in normal tissues. However, E2F1 mRNA expression was not significantly altered between the patients with ESCC versus those with EAC. Using the Kaplan–Meier Plotter, we found that the upregulation of E2F1 mRNA expression was significantly associated with a good prognosis (overall survival, OS) in ESCC, while downregulation of E2F2 mRNA expression predicted a good prognosis (OS) in EAC.
E2F2 is a tumor activator in many types of cancer. Using a high-frequency subpathway mining approach, Li et al. identified a six-gene model containing E2F2 that predicted clinical outcomes in patients with ESCC . In the present study, the mRNA expression of E2F2 was not significantly different between ESCC and EAC. Furthermore, the mRNA expression of E2F2 was not significantly associated with the OS in either ESCC or EAC.
The abnormal overexpression of E2F3 is thought to be an oncogenic event in patients with EC. A novel four-gene signature including E2F3 was identified, externally validated, and shown to provide excellent distinction between EAC and the condition of Barrett’s esophagus . In this study, we demonstrated that the expression of E2F3 in EC tissues was higher than that in normal tissues. Additionally, the mRNA expression of E2F3 was significantly different between ESCC and EAC. Surprisingly, greater levels of E2F3 mRNA expression were significantly associated with a higher OS in the patients with ESCC, while lower levels of E2F3 mRNA expression predicted a higher OS in the patients with EAC.
E2F4 has been reported to be associated with the occurrence and progression of malignant tumors . In general, it has been thought that E2F4 may play an important role in inhibiting proliferation-related genes. For example, Bultmann et al. have identified E2F4 as a transcription repressor that strongly regulates the expression of latent transforming growth factor-beta-binding protein 4S to inhibit the migration of esophageal cancer cells . In the present study, the expression of E2F4 was demonstrated to be upregulated in EC tissues compared with normal tissues. Interestingly, the expression of E2F4 mRNA was significantly different between ESCC and EAC. Furthermore, greater levels of E2F4 mRNA expression predicted a poorer OS in the patients with ESCC using the Kaplan–Meier Plotter. Nevertheless, it must be pointed out that this finding seems to be inconsistent with the role of E2F4 as a tumor suppressor.
E2F5 has been found to be highly expressed in several malignancies, including glioblastoma and prostate cancer. However, the expression and prognostic effects of E2F5 on EC have been rarely reported. Sun et al. have found that microRNA-544 inhibited ESCC cell proliferation and enhanced the sensitivity to cisplatin by repressing E2F5 . In this study, it was shown that the expression of E2F5 in EC tissues was higher than that in normal tissues. In addition, higher E2F5 expression was significantly correlated with a better OS in the patients with ESCC.
The expression and prognostic effects of E2F6 in ESCA have not been reported previously. Li et al.have found that miR-424 can inhibit the proliferation and migration abilities of A549 cells by negatively regulating the expression of E2F6 in non-small cell lung cancer (NSCLC) . E2F6 is also upregulated in NSCLC blood samples, indicating that E2F6 possesses an oncogenic feature in lung cancer . In this study, it was demonstrated that the expression of E2F6 in ESCC tissues was higher than that in normal tissues. Moreover, a higher E2F6 expression level was significantly correlated with a poorer OS in the patients with EAC.
E2F7 and E2F8 are recently identified, novel members of the E2F family. Considering that a difference in the tumor histological subtypes could influence the measured tumor gene expression, they also likely serve as activators. E2F7 and E2F8 have been shown to be activators of transcription, while E2F8 binds to and activates the cyclin D1 promoter in a dominantly negative manner by blocking other E2Fs. E2F7 and E2F8 directly bind to and stimulate the promoter of vascular endothelial growth factor A by cooperating with hypoxia-inducible factor-1. To date, the roles of E2F7 and E2F8 in the development and progression of EC remain unclear. E2F8 has been reported to be upregulated in ESCC, thus promoting cell proliferation and affecting the expression of cyclin D1/p21 . In the present study, E2F7 and E2F8 were significantly overexpressed in ESCC and EAC tissues, and their expression levels were markedly different between ESCC and EAC. Interestingly, higher E2F7 and E2F8 expression levels were significantly correlated with a better OS in the patients with ESCC, while lower levels of E2F7 and E2F8 mRNA expression predicted a better OS in the patients with EAC.
Taken together, these novel findings reveal distinct landscapes of genetic and epigenetic alterations in the E2F family genes between ESCC and EAC, which may contribute to different molecular mechanisms for the two histological forms of EC through interactions among the E2F family members or with other functional proteins. This study may have important clinical implications that certain E2Fs hold promise as biomarkers for the differential diagnosis of EAC and ESCC, or in the prognosis of patients with EC.