BC is one of the most common malignancies among women and most BC patients with advanced stage disease have a poor prognosis. Thus, it is imperative to seek encouraging strategies against the disease. With the rapid progress of high-throughput techniques and the availability of public genetic databases, more disease-related gene-based analysis of data derived from microarray studies can be analyzed or screened, which may facilitate early diagnosis and treatment.
The NCBI GEO database features a striking volume of microarray profiling data and next-generation sequencing results for a variety of human cancers. In the present study, three GEO datasets which include cases of BC with metastasis in which gene profiles were obtained were downloaded for further analysis. A total of 295 genes were identified as metastasis-related genes, including 151 up-regulated and 144 down-regulated genes. For a more in-depth understanding of function regarding these genes, we performed a functional enrichment analysis, which indicated that these hub genes played a role in the progression of metastasis through certain pathways.
The results demonstrated that the up-regulated genes were mainly involved in the type I interferon signaling pathway (BP), apoptotic processed (BP), focal adhesion (CC), protein homodimerization activity (MF), the Rap1 signaling pathway,[19] the MAPK signaling pathway[20–22], antigen processing and presentation (KEGG pathway), cell adhesion molecules (CAMs) (KEGG pathway), While down-regulated genes were mainly enriched in the regulation of signal transduction by p53 class mediators (BP), cell division (BP), the innate immune response (BP), the estrogen signaling pathway (KEGG pathway), and the chemokine signaling[23] pathway (KEGG pathway). These findings are highly consistent with reported studies indicating the essential role of apoptosis, cell migration and adhesion during the progression of BC. Previous studies have shown the importance of protein homodimerization activity in predicting invasive potential and clinical outcome of BC, which is supportive of our current results[24]. Further PPI analysis was carried out to construct a protein interaction network to yield a holistic map of these important genes. From the PPI map, 20 genes with higher connectivity were selected as hub genes. GO analysis of hub genes using Cytoscape was consistent with the results from the differential gene expression analysis (Fig. 3B) indicating that hub genes mainly focused on regulation of cell division, cell death, cell apoptosis (BP), nucleotide binding, protein binding, catalytic activity and molecular transducer activity (MF), and with membrane-bounded organelles (CC).
Twenty genes were selected as hub genes using the maximal clique centrality (MCC) method. Kaplan-Meier plots stratified for differential expression of these genes suggested that low expression of TYMS, SKA1, or ADCY7 was associated with worse OS whereas MX1 manifested an opposite trend. Low expression of POLR3H and high expression of CDCA8, ASE1, KIF14, MX1 contributed to worse DMFS. Previous studies have provided considerable evidence about the function of these genes in the development of BC cancer or other cancers. For instance, TYMS has been reported to be a target for chemotherapeutic agents (i.e., 5-fluorouracil) and is relevant for tumor growth due to its function as thymidylate synthase [25, 26]. Its expression was associated with lymph node metastasis in low grade glioma[27] and serves as a prognostic indicator of lymph node metastasis in patients with colorectal cancer[28], some studies have shown that it is overexpressed only in tumors that are generally refractory to the drug pemetrexed[29] and was related with the resistance and sensitivity to chemotherapy in patients with advanced BC[30]. Studies of the relevance of TYMS with BC metastasis are rare and whether it could be a prognostic factor for BC still needs further investigation. CDCA8 encodes a component of the chromosomal passenger complex and is an essential regulator of mitosis and cell division, while SKA1 serves as a key factor for spindle and kinetochore associated complex subunit 1, which is also associated with the mitotic cell cycle, both have been identified as hub genes for BC distant metastasis[5, 31]. KIF14 has been reported to promote BC progression through the negative regulation of the Rap1a-Radil signaling pathway[32]. Interestingly, up-regulated MX1, which is involved in the cellular antiviral response mechanism, has been associated with BC aggressiveness and lymph node invasion[33, 34].
PRC1(ASE1) encodes a protein that is involved in cytokinesis and is upregulated during the S and G2/M phases of mitosis, Of note, although PRC1 has been reported to contribute to tumorigenesis in lung adenocarcinoma and early recurrence of hepatocellular carcinoma by the Wnt/beta-catenin signaling pathway[35, 36], and has a strong link with the metastasis of nasopharyngeal carcinoma[37]. The mechanism for its association with BC metastasis has rarely been reported. Brynychova et al.[38] studied the gene expression profile of BC patients and found poor DFS in the PRC1 overexpression group, yet could not explain paclitaxel-induced PRC1 expression (Paclitaxel toxicity) by functional analyses. Uribe et al.[39] successfully inhibits mammary tumors metastasis in mice by using TSHZ2, which binds to the cytokinesis regulator PRC1. A recent study has revealed the potential role of phosphorylating PRC1 in the progression of triple-negative breast cancer[40], Whether PRC1 plays a role as an indirect marker of BC metastasis still needs further validation.
POLR3H has been reported to be associated with pro-tumoral effects in various kinds of cancers[41, 42]. Although together with MX1, there is no evidence of differences associated with clinical stages, they were associated with poor DMFS and is worthy further in-depth study to determine the exact functions or mechanisms involved in BC tumor metastasis.
Our Study has several limitations. Firstly, the data included in our article has unavoidable heterogeneity. Though they were mainly focusing on western people (Canada, Australia and America) and targeting the same gene type (total RNA) and using the same gene chip platforms (illumina HumanHT). Several characteristics including origin, family history, age of onset, Genetics information were unavailable from each datasets. Secondly, in our study, the gene expression is only confirmed by protein expression at single cell level, several of the hub genes we found need further experimental validation, such as trans-well analysis or gene knock-out analysis, among which, we found two novel metastasis-related genes (PRC1 and POLR3H). One experimental study related to PRC1 illustrated its association of poor survival outcome of breast cancer patients, which might due to its role in tumor metastasis according to our study, Therefore, Further research will include experimental verification of these two genes to confirm the possible effects and mechnisms.