Breast cancer is the most frequent malignant tumor in females. It is still easy to recur even after combined therapy. In the era of precise medicine, microarray has been widely used to analyze the expression changes of mRNA in breast cancer and predict the potential therapeutic targets. However, better biomarkers for cancer specific prognosis and progression were still required.
In the present study, we performed WGCNA to explore gene co-expression modules associated with progression of breast cancer. A total of 3668 DEGs were used to construct co-expression network and 5 modules were identified. Turquoise module was found to have the highest association with tumor grade, ER status and triple-negative tumor. Thirty-three genes with high connectivity were screened from the module. Among them, CASC5 and RAD51 were negatively associated with the prognosis of patients.
CASC5, also known as D40, encodes a protein that functions as a scaffold for proteins influencing the spindle assembly checkpoint during the eukaryotic cell cycle. It is required for creation of kinetochore-microtubule attachments and chromosome segregation. CASC5 is widely expressed in various cultured human cancer cell lines and primary tumors of different origins. Yuri N et al. characterized CASC5 as a member of the cancer/testis gene family, and CASC5 knockdown significantly inhibited the growth of human cancer cell lines both in vitro and in vivo(16). In poorly differentiated primary lung cancer, CASC5 expression level was significantly higher. It was the first gene in cancer/testis gene family for which expression is associated with smoking habits of lung cancer patients(17). In patients with malignancies, some of the cancer/testis genes could encode antigens on tumor cells and cause immune responses by cytotoxic T cells which were called cancer/testis antigens. However, it was not clear whether CASC5 protein elicits immune responses in breast cancer patients(18, 19). A sequence homology search of public database revealed that the CASC5 sequence was identical to a gene on human chromosome 15, AF15q14. AF15q14 is a partner that fuses to mixed-lineage leukemia genes, which were involved in the development of acute leukemias(20–22). In dataset GSE37751, CASC5 was highly expressed in breast cancer tissues compared with normal breast tissues. In addition, ROC curve indicated that CASC5 exhibited excellent diagnostic efficiency for normal and tumor tissues. One-way ANOVA and t test demonstrated that the expression level of CASC5 was higher in triple-negative tumor and associated with tumor progression. Besides, the Oncomine database also showed that the expression level of CASC5 was significantly higher in breast cancer samples. To obtain further insight of translational level of CASC5, we used the Human Protein Atlas database to examine the immunohistochemistry staining of CASC5 in both normal breast and breast cancer, and discovered that the protein level of CASC5 was significantly up-regulated in breast cancer tissues compared with normal breast tissues. Survival analysis revealed that high expression of CASC5 was associated with the worse overall survival and relapse free survival. CASC5 has the potential to be a prognostic biomarker. GSEA demonstrated that high expression of CASC5 was associated with the pathway of proteasome, spliceosome, RNA polymerase and oxidative phosphorylation. The ubiquitin-proteasome system (UPS) plays an important role in the degradation of proteins. Increasing evidence indicated that the UPS involved in development cancer development(23, 24). Proteasomes participate in many cellular degradation processes, including inflammation, cell cycle, responses to oxidative stress, and regulation of gene expression(24, 25). The proteasome inhibitor bortezomib (N-acyl-dipeptidyl boronicacid) was approved by the FDA for the treatment of mantle cell lymphoma and relapsed multiple myeloma (MM). Alternative splicing generates different, even antagonistic products from a single gene locus. This prevalent process greatly expands the coding capacity of complex genomes(26, 27). Spliceosome plays an important role in cancer progression including the control of cell proliferation and programmed cell death, angiogenesis, metabolism of cancer cells and metastasis. Misregulation of alternative splicing patterns often cause or modify human disease, including cancer(28). RNA polymerase is required for tumorigenesis, it was reported that MYC's tumorigenic potential is regulated by polymerase I-III(29). Some studies demonstrated a reduction of oxidative phosphorylation capacity in different types of cancer cells, while other investigations revealed contradictory modifications with the upregulation of oxidative phosphorylation components and a larger dependency of cancer cells on oxidative energy substrates for anabolism and energy production. Guppy and colleagues stated that breast cancer cells generated 80% of their ATP by the mitochondrion, introducing the concept of oxidative tumors(30). It was reported that the typical “glycolytic” type of cancer cells includes enhanced glycolytic machinery confronted to a low efficiency oxidative phosphorylation system, while the “oxidative phosphorylation” type of cancer cells relies mainly on mitochondrial respiration to produce ATP from glucose and glutamine oxidation(31–33).
The protein encoded by RAD51 is known to be involved in the repair and homologous recombination of DNA. RAD51 interact with BRCA1 and BRCA2, which regulate both the intracellular localization and DNA-binding ability of this protein. Loss of these controls may be a key event causing genomic instability and tumorigenesis(34, 35). In breast cancer, RAD51 expression correlates with high-grade metastatic breast tumor and poor prognosis. Cells with RAS51 overexpress exhibit disruption of cell cycle, resistance to apoptotic signals and associated resistance to radiotherapy and chemotherapy(36, 37). In dataset GSE37751, RAD51 was highly expressed in breast cancer tissues compared with normal breast tissues. In addition, ROC curve indicated that RAD51 exhibited excellent diagnostic efficiency for normal and tumor tissues. One-way ANOVA and t test demonstrated that RAD51 was highly expressed in triple-negative tumor and associated with high-grade tumor. Oncomine database showed that the expression level of RAD51 was significantly higher in breast cancer samples. Human Protein Atlas database with the immunohistochemistry staining of RAD51 was used to further investigate the translational level of RAD51. The protein level of RAD51 was significantly up-regulated in breast cancer tissues compared with normal breast tissues. Survival analysis revealed that high expression level of RAD51 was associated with the worse overall survival and relapse free survival. GSEA demonstrated that high expression level of RAD51 was associated with the pathway of pyrimidine metabolism, spliceosome, oxidative phosphorylation, RNA polymerase, DNA replication, and basal transcription factors.