mRNA expression levels of CBX3 in various human cancer types
To evaluate the expression levels of CBX3 in different types of cancer and normal tissues, we first used the Oncomine database for preliminary exploration. Of the 20 evaluated tumor types, CBX3 was found to be highly expressed in 16 cancer tissues (Fig. 1A). Compared to normal tissues, expression levels of CBX3 in gastric cancer, bowel cancer, breast cancer and other tumor types were elevated. Therefore, CBX3 may be involved in the oncogenesis of many cancer types. However, this database does not show the expression levels of CBX3 in normal liver and liver cancer tissues.
Expression levels of CBX3 in HCC
We used the UALCAN database (http://ualcan.path.uab.edu/) to analyze the expression levels of CBX3 at the transcriptome level. We found that, compared to adjacent normal tissues, expression levels of CBX3 in HCC tissues were significantly elevated (Fig. 2A). Simultaneously, representative data for CBX3 protein level analysis (Fig. 2B) were obtained from the Human Protein Atlas database. The degree of immunohistochemical staining in HCC tissues (Patient id: 983) was stronger than in the normal liver tissue. These findings suggest that CBX3 is highly expressed in liver cancer.
Correlation between CBX3 expression levels and survival outcomes
Using the GEPIA database, it was found that, compared to patients with suppressed expression levels of CBX3, patients with elevated expression levels of CBX3 exhibited significantly reduced overall survival outcomes (p = 0.00046; Fig. 3A). Disease-free survival outcome analysis revealed comparable results to those of overall survival outcomes (Fig. 3B).
In addition, we verified the correlation between expression levels of CBX3 and patient’s survival outcomes using other databases. Using the Kaplan-Meier Plotter database, it was found that survival time for patients with elevated CBX3 expression levels was significantly low than that of patients with suppressed CBX3 expression levels (p = 6.9e − 07; Fig. 3C). Disease-free survival time (Fig. 3D) for patients with elevated CBX3 expression levels was significantly low compared to that of patients with suppressed CBX3 expression levels (p = 2e − 05).
Diagnostic values of CBX3 in HCC
Diagnostic values of CBX3 were evaluated through the TCGA database. First, we used dichotomy to allocate patients into two groups; the CBX3 high expression group and the CBX3 low expression group (Fig. 4A). Figure 4B shows that the median survival time for patients in the CBX3 low-expression CBX3 group (5.8 years) was significantly more than that of the CBX3 high-expression group (3.1 years; p = 0.000142). The predicted diagnostic efficacy of CBX3 is shown in Fig. 4C. With 1-year survival time as the cut-off point, the AUC value of CBX3 as a diagnostic marker was 0.724 (95% CI, 0.665 − 0.783); With 3-year survival time as the cut-off point, the AUC value of CBX3 as a diagnostic marker was 0.658 (95% CI, 0.593 − 0.723); With 5-year survival time as the cut-off point, the AUC value of CBX3 as a diagnostic marker was 0.648 (95%CI, 0.571 − 0.725). These findings show that CBX3 is a potential good prognostic marker for liver cancer.
CBX3 expression is correlated with immune infiltration levels in HCC
We further evaluated whether the expression level of CBX3 is correlated with prognosis and immune infiltration. Using the QUANTISEQ algorithm, the results (Fig. 5A) revealed that immune infiltrating cells that are significantly correlated with CBX3 expression levels are T regulatory cells (Tregs), CD8 + T cell, CD4 + T cells, monocytes, macrophage M2, macrophage M1 and B cells. Through the TIMER algorithm (Fig. 5B), the associated immune infiltrating cells were found to be CD4 + T cells, neutrophils, myeloid dendritic cells, macrophages and B cells. Based on these findings, we postulated that infiltration levels of CD4 + T cells, macrophages and B cells are positively correlated with expression levels of CBX3 in HCC.
Correlation between CBX3 and immune checkpoints
Immune checkpoint inhibitors play an increasingly important role in cancer treatment. To evaluate the correlation between expression levels of CBX3 and immune checkpoints, we made an online prediction using onlin website The results showed Immune checkpoints, including CD274, CTLA4, HAVCR2, LAG3, PDCD1, SIGLEC15, and TIGIT, which were associated with CBX3, were differentially expressed (Fig. 6).
Among these immune checkpoints, the most closely related and statistically significant is HAVCR2. Therefore, it could be a good therapeutic target for HCC patients. More studiesshould determine whether CBX3 can be used as a marker for the application of immune checkpoint inhibitors in cancer treatment.