The occurrence and development of malignant tumors involve multiple signaling molecular pathways. In recent years, the genomic changes that are key to the progression of NPC have been revealed as the multiple functional loss mutations in the negative regulator of NF-κB; recurring gene damage, including deletion of the CDKN2A/CDKN2B locus, amplification of CCND1, TP53 mutation, and changes in the PI3K/MAPK signaling pathway; chromatin modification; and DNA repair (24–27). In addition, some studies have explored molecular markers related to nasopharyngeal carcinoma. For example (28–30), CKS1, P27, and CENP-F have been found to be involved in the regulation of an abnormal cell cycle. Some oncoproteins and tumor suppressor genes, such as RAP2A, PTP4A2, and ECRG4 (31–33), are closely related to cell proliferation. Although many oncogenes and carcinogenic factors have been found to affect the occurrence and development of NPC, the exact mechanism is still unknown.
Many studies recently reported that ZIC2 might regulate the development of various tumors, including prostate cancer (23, 34–36), lung adenocarcinoma (17), breast cancer, clear cell renal cell carcinoma (15, 37, 38), colorectal cancer (14, 39–41), hepatocellular carcinoma (13, 42–44), and cervical cancer (10, 45). All these tumors, except breast cancer, were upregulated. In this study, a total of 7 microarrays, 3 TMEs, and in-house tissues were incorporated, including 120 control samples and 323 NPC samples. ZIC2 was excessively expressed in NPC than in the normal tissues according to the gene chip, tissue chip data, and IHC results of the tissue samples. These results are consistent with those of related studies, and certainly add to our understanding of the expression of ZIC2 in NPC at the human tissue level, which no relevant study had previously investigated. Moreover, by mining data from the microarrays data as mentioned above, we observed that ATAD2 expression is positively correlated to ZIC2 expression. Interestingly, when we searched for the binding position of the ChIP-seq data in the Cistrome DB, we found that ZIC2 and ATAD2 could combine and showed peaks. ATAD2—or ANCCA, CT137, or PRO2000, as it is also called—is a member of the ATPase family, which is associated with various cellular activities by regulating protein complexes and is responsible for ATP binding and hydrolysis (46). Members of the ATPase family of proteins participate in diverse cellular processes that include cell cycle regulation, protein proteolysis and decomposition, organelle biogenesis, and intracellular trafficking (47). Being deficient in normal regulation, either due to the ATAD2 locus amplification or to specific changes in the core members of the transcription mechanism, may cause ATAD2 abnormal activation, which will eventually lead to oncogenesis (48, 49). Moreover, ATAD2 showed expression disturbance in hepatocellular carcinoma, ovarian cancer, stomach cancer, and other cancers (50–52). The discovery of Liu et al., through bioinformatics analysis, that ATAD2 may take part in the tumorigenesis of NPC is noteworthy (53). This also agrees with our findings. Besides, our GO and KEGG enrichment analysis showed that ZIC2 positive relation genes are mainly enriched in cell division and in the cell cycle. Likewise, our GSEA analysis revealed that the upregulation of ZIC2 is associated with the proliferation and division of cells and DNA synthesis, which is consistent with the results of our GO and KEGG analysis. Another crucial finding is that highly expressed genes are also enriched in DNA-dependent ATPase activity in GO-MF, because their putative target gene, ATAD2, functions similarly as it belongs to the ATPase family. In general, the ATPase family may play a role in the preceding cell functional activities, among which is ATAD2.Evidence from function analysis supports this result.
Nevertheless, this study had some limitations. First, it tested only the expression of ZIC2 protein using IHC and TME. Further research should investigate the clinical significance of ZIC2 mRNA expression in NPC; and to confirm the results, in vivo and in vitro experiments are needed. Another limitation of this study is that none of its experiments verified the transcriptional regulation between ZIC2 and ATAD2. Therefore, further analysis is needed. Finally, the results of the WGCNA analysis suggested that the modules in GSE12452 have little significant correlation with clinical traits. However, this was possibly due to the limited number of samples. More clinical data should bfirste included in the follow-up study.