So far, many studies have reported that CBXs are related to the occurrence and development of a variety of tumors. However, the relationship between CBXs and the occurrence and development of ESCC urgently needs further elucidation. In this study, we analyzed the multiple levels of CBXs in ESCC including mRNA expression, mutation, prognostic value and Immune Cell Infiltration through a variety of online databases.
High expression of CBX1 was related to poor differentiation of breast cancer and led to poor OS22. The expression of CBX1 was closely related to the staging of gastric cancer and lymph node metastasis23. In addition, CBX1 interacted with the transcription factor HMGA2, activates the Wnt/βCatenin signaling pathway and affected the prognosis of patients with liver cancer24. In this study, the expression of CBX1 in the tissues of patients with ESCA was significantly higher than that in normal tissues, which was basically consistent with the results of other researchers. However, our research showed that the expression of CBX1 was not significantly correlated with the OS of patients with ESCA. This might be because the sample size was too small and need further research with a larger sample size to confirm. Further, the mRNA expression of CBX1 was significantly correlated with individual cancer stage and nodal metastatic status.
Recent studies have shown that CBX2, a member of the CBXs family, is overexpressed in several tumors. Loss of CBX2 impaired the proliferation of leukemia cells25. In ovarian, breast and lung tumors, the total rate of CBX2 amplification exceeded 30% and CBX2 was significantly associated with HER-2 positive status in breast cancer26. In addition, CBX2 was a critical regulator of the spread of ovarian cancer and chemoresistance27. In this study, the expression of CBX2 in esophageal cancer tissues was significantly increased, while it was also significantly correlated with individual cancer stage and nodal metastatic status. Although patients with high expression of CBX2 had lower OS in esophageal adenocarcinoma, the difference was not significant.
Up-regulation of CBX3 has been found in a variety of cancers, such as LUAD, colorectal cancer (CRC), gastric cancer and ESCC6,28−30. CBX3 directly inhibited the expression of transcription repressors NCOR2 and ZBTB7A, thereby affecting the proliferation, colony formation and migration of LUAD cells28. In addition, CBX3 controlled the development of CRC by directly regulating CDKN1A (p21Waf1/Cip1)29. CBX3 affected the prognosis of gastric cancer by regulating the cell cycle, mismatch repair and immune-related pathways30. Down-regulation of miR-377 could promote the self-renewal of ESCC stem cells by promoting the expression of CBX3 and inhibiting the activation of the P53/P21 pathway6. In the present study, CBX3 was highly expressed in esophageal cancer tissues. The highly expressed CBX3 was significantly related to the poor OS of esophageal adenocarcinoma patients, which implied that CBX3 as an oncogene might take a significant part in the prognosis of EAC. Furthermore, CBX3 was also significantly correlated with individual cancer stage and nodal metastatic status, which further suggested that CBX3 might serve as a prognostic marker for EAC.
In HCC cells, after knocking out CBX4, proliferating cell nuclear antigen and cyclin E2 were down-regulated, and p16 was up-regulated, resulting in decreased cell proliferation and impaired cell cycle progression31. CBX4 promoted the migration and invasion of breast cancer cells32. Moreover, CBX4 increased the expression and activity of P53, CDK2, Cyclin E, MMP2, MMP9 and CXCR4 by up-regulating the expression of BMI-1, and promoted the proliferation and metastasis of lung cancer in vitro33. In this study, the expression of CBX4 in the tissues of patients with ESCA was significantly higher than that in normal tissues. Furthermore, the mRNA expression was significantly correlated with individual cancer stage and nodal metastatic status, while higher CBX4 mRNA expression was correlated with worse OS and the differences were significant in ESCC. In summary, CBX4 might be a potential prognostic marker for ESCC.
Although the expression of CBX5 in esophageal cancer tissues had no significant difference compared with normal tissues in our study, the role of CBX5 in other malignant tumors deserved our attention. CBX5 was highly expressed in NSCLC and affected the growth of tumor cells34. The high expression of CBX5 was related to the poor prognosis of breast cancer patients and promoted tumor metastasis35 ,which was different from our research results. Therefore, whether CBX5 is related to the occurrence and development of cancer still needs further research to confirm.
Similar to CBX5, CBX6 was confirmed to be upregulated in a variety of cancers including HCC34, glioblastoma multiforme36and breast cancer37. However, our report did not find a significant correlation between CBX5 and the occurrence and development of ESCA. So the role of CBX5 in ESCA is still vague.
Surprisingly, the role of CBX7 in tumors seemed to be inconsistent with other CBXs members. CBX7 was found to be an important tumor suppressor in pancreatic cancer and inhibited the PTEN/Akt signaling pathway by increasing the level of PTEN transcription38. Moreover, CBX7 inhibited breast cancer tumorigenicity through epigenetic induction of DKK-1 mediated39. CBX7 expression in CRC tissue was significantly reduced or absent compared with normal colonic mucosa40. In our study, the expression of CBX7 in esophageal cancer tissues was significantly decreased. In EAC, patients with high expression of CBX7 had a higher OS, while patients with high expression of CBX7 had lower cancer stages and better lymph node metastasis status. Therefore, the data reported here suggested that the expression of CBX7 might reflect the prognosis of EAC and CBX7 might be an important tumor suppressor.
As an important member in the CBX family, CBX8 was defined as a cancer-promoting factor in a variety of malignant tumors, such as colorectal cancer13, metastatic prostate cancer41, ESCC42.High CBX8 expression was associated with a low distant metastasis rate and good prognosis in patients with colorectal cancer13 .CBX8 knockout inhibited cell proliferation, colony- forming ability, DNA repair and promoted apoptosis in ESCC42.In this study, the expression of CBX8 was significantly increased in ESCA and the mRNA expression was significantly correlated with individual cancer stage and nodal metastatic status. Moreover, high CBX8 expression was associated with poor OS in ESCC. The above results all indicated that CBX8 might have prognostic value in ESCC.
Then, GO analysis of CBXs and their co-expressed genes that may have prognostic value showed that the functions of these genes mainly involved DNA replication, methylation and mitosis. Changes in KEGG were mainly enriched in mismatch repair, DNA replication, pyrimidine metabolism, cell adhesion molecules (CAMs). These functions and pathways were all related to the occurrence and development of cancer.
Many studies had confirmed that immune cells in the tumor microenvironment played a great role in the occurrence and development of tumors43–45. Our research showed that CBXs were related to the infiltration of a variety of immune cells including B cells, CD4+T cells, CD8+T cells, myeloid dendritic cells, macrophages and neutrophils. This suggested that CBXs might become a new target for immunotherapy.
Our research has certain limitations. First, our data are all derived from online databases, lacking further verification by cell experiments, animal experiments and clinical experiments. Second, the number of samples in the online database was small, so a larger sample size is needed to verify our results. Finally, the specific molecular mechanism of CBXs needs further research to verify our research and findings.