Due to its poor prognosis and survival, TNBC has brought serious troubles to patients and society. Many researchers have studied the mechanisms and treatment strategies of TNBC from the molecular, genetic, and cellular levels, and have made many achievements. However, research using different methods and from different aspects of TNBC leads to different results. Although the understanding of TNBC has been greatly enriched, the treatment for TNBC has become more complex. This study used bioinformatics algorithms to study TNBC from two aspects, expounded the potential biological significance, and compared and integrated the two research methods in an attempt to more clearly reveal the similarities and differences of TNBC at the two levels.
Firstly, this study used bioinformatics algorithms to expound on the potential biological significance of TNBC at the transcriptome level. The DEGs modules of TNBC were initially screened at the transcriptome level, which included 475 DEGs, followed by functional enrichment analysis and submodule screening. The DEGs were mainly enriched in function and pathway, mainly including microtubule binding, chromosome segregation, response to xenobiotic stimulus, cancer pathway, tyrosine metabolism pathway, Mucin type O-glycan biosynthesis pathway and p53 signaling pathway. The key submodule in the TNBC procession was then identified. Moreover, ASPM is a specific gene of cell cycle regulator, which is related to the survival rate of TNBC [11] and can be used as a new molecular target for TNBC treatment [12]. NCAPG is a component of the condensin complex and acts as a major molecular effector of chromosome condensation and segregation during mitosis, which is also closely associated with prognostic survival [13]. CDCA8 and KIF2C are key genes in TNBC. The expression of CDCA8 gene is significantly correlated with TNBC [14]. KIF4A is a circular RNA that may act as a prognostic factor and progression mediator of breast cancer [15]. The main biological function of CDK1 gene is to regulate the centrosome cycle and control the eukaryotic cell cycle, which can be used as an effective therapeutic target in cancer treatment. The submodule was mainly enriched in cell cycle, microtubule motor activity, histone kinase activity, and DNA replication origin binding.
Afterward, the single-cell sequencing data of TNBC were collected to reveal the TNBC progression at the single-cell level. The proportion of NK T cells, luminal epithelial cells, B cells, and basal cells was significantly decreased in the TNBC group, while the proportion of T cells, monocytes, and neural progenitor cells was significantly increased in the TNBC group compared with the non-TNBC group. We combined the changes in cell subsets with a decreased or an increased proportion and the DEGs identified at the transcriptome level to determine the common DEGs - TNFAIP6 and TM4SF1, respectively. TNFAIP6 encodes inflammatory response factors and regulates anti-inflammatory responses and immune mechanisms, while cancer cells can generate an inflammatory microenvironment to enhance tumor metastasis, indicating that TNFAIP6 is a prognostic cytokine for breast cancer [16]. TM4SF1 gene can be used as a strong mediator of breast cancer metastasis and reactivation, involved in the migration and invasion of TNBC [17]. HSPA6 gene has inhibitory effects on the growth, migration and invasion of TNBC cells [18]. SOD2 gene is associated with the prognosis and survival of TNBC [19]. PLCG2 gene is involved in immune response [20]. IFI44L gene is a type I interferon-stimulated gene involved in congenital immune process. PLIN2 gene is overexpressed in tumors [21]. CXCL1 gene is a chemokine. IL7R gene is associated with prognosis and treatment, involved in the progression of TNBC. MUCL1 gene is an attractive tumor-associated antigen and potential therapeutic target [22]. SRGN gene can interact with TGFβ2 which regulates the metastasis of TNBC through autocrine and paracrine pathways [23]. CCR7 gene promotes the metastasis of TNBC and may act as a target for breast cancer diagnosis and treatment [24].
Finally, we combined transcriptome and single-cell sequencing results to integrate the key prognostic genes of TNBC, including RRM2, TPX2, CENPF, and TOP2A, which are closely related to the prognosis and treatment of TNBC. Ribonucleotide reductase (RR) is a rate-limiting enzyme used to induce 2′-deoxyribonucleoside 5′-diphosphates that is essential for DNA replication and repair. RRM2 is a critical RR subunit and has received significant attention in carcinoma research because its expression is dysregulated in multiple cancer types, including breast cancer [25]. The high expression of RRM2 has a worse prognosis in patients with breast cancer with specific features [26]. Pathway-centric integrative analysis identifies RRM2 as a prognostic marker in breast cancer associated with poor survival and tamoxifen resistance [27]. Expression RRM2 and its correlation with clinicopathological parameters could help in evaluating outcome in breast cancer [28]. In addition, RRM2 is significantly associated with the prognostic survival of TNBC and can be used as a biomarker for the prognosis of TNBC [29, 30]. Targeting protein for Xenopus kinesin-like protein 2 (TPX2) plays a critical role in chromosome segregation machinery during mitosis [31]. TPX2 silencing negatively regulates the PI3K/AKT and activates p53 signaling pathway by which breast cancer cells proliferation were inhibited whereas cellulars apoptosis were accelerated, suggesting that TPX2 may be a potential target for anticancer therapy in breast cancer[32]. TPX2 is a key gene that is closely related to survival time of TNBC patients [33, 34]. Significantly upregulated TPX2 expression is observed in breast cancer tissue and cells, and contributes to promote the proliferation, migration and invasion of breast cancer cells [35]. CENP-F is a cell cycle-regulated protein associated with kinetochores, the site at which chromosome-microtubule interactions are monitored and the source of checkpoint signals [36]. CENPF interacts with microtubules and participates in cell cycle development, which is a reliable indicator of poor survival for breast cancer [37, 38]. According to the CENP-F expression level, some investigators have reported that CENP-F is immunohistochemically correlated with highly proliferative cancer cells and poorer prognosis [39]. Topoisomerase 2 alpha (TOP2A) is a key enzyme in DNA replication and a target of various cytotoxic agents such as anthracyclines. As such it has been widely investigated for potential applications in breast cancer detection and management [40]. The TOP2A expression was an independent prognostic indicator of 5-DFS in TNBC [41]. TOP2A gene mainly helps DNA replication and transcription, controls and changes the topological state of DNA, which is closely related to tumor proliferation and invasion [42]. The expression of these prognostic genes was significantly increased in B cells, endothelial cells, and luminal epithelial cells. Combined with the changes of cells, it can be seen that B cells can generate antibodies and play a crucial role in the process of tumor immunity [43]. Meanwhile, endothelial cells play an active role in the growth and metastasis of solid tumors [44], and luminal epithelial cells are the tumor-originating cells that define the subtype of breast cancer. In neural progenitor cells, the expression of prognostic genes was significantly lowered. Among them, the reduction in the proportion of neural progenitor cells indicates that the infiltration of nerve fibers during tumorigenesis will release some nerve signals to promote tumor growth and metastasis [45, 46]. Therefore, the changes in these prognostic genes not only at the transcriptome levels but also at the single-cell level should be considered in the research.