3.1. Cell-Cycle-Related Genes Are Extensively Overexpressed in Various Types of Cancers
The expression of 57,035 genes in 12 types of cancer and their paring normal tissue from TCGA were analyzed. The 12 analyzed types of cancers were breast invasive carcinoma (BRCA) [13], kidney renal clear cell carcinoma (KIRC) [14], lung adenocarcinoma (LUAD) [15], stomach adenocarcinoma (STAD) [16], colon adenocarcinoma (COAD) [17], kidney renal papillary cell carcinoma (KIRP) [18], lung squamous cell carcinoma (LUSC) [19], thyroid carcinoma (THCA) [20], head and neck squamous cell carcinoma (HNSC) [21], liver hepatocellular carcinoma (LIHC) [22], prostate adenocarcinoma (PRAD) [23], and uterine corpus endometrial carcinoma (UCEC) [24]. The sample number for each cancer type is listed (Table 2).
For statistical analysis, only those reaching genome-wide significance were included and defined as DEGs p value less than 5 × 10− 8) [25]. About three-quarters of the genes did not show elevated expression in any cancer type. For the remaining genes, about 60% of them (n = 8434) had elevated expression in only one cancer type. Thus, only 9% of all the analyzed genes (n = 5343) showed elevated expression in more than one type of cancer. Among them, 28 genes were found to be overexpressed in no less than 10 types of cancer (Table 3). It was noted that, in stomach adenocarcinoma and thyroid carcinoma, only six and thirteen of the 28 pan-cancer DEGs were overexpressed, respectively, while in ten other cancer types, at least 20 DEGs were overexpressed. This difference was not caused by sample size, as thyroid carcinoma had the second largest sample size. Therefore, this indicates the existence of an intrinsic difference in stomach adenocarcinoma and thyroid carcinoma compared with other cancer types. The GO molecular pathway analysis showed that the 28 pan-cancer DEGs were enriched in 116 biological processes, most of which were cell-cycle-related processes (Table S1). This is no surprise, since cancer is essentially a disease of uncontrolled cell proliferation. However, among the 1263 genes that participate in the cell cycle, only those 28 DEGs appear to be extensively altered in various cancer types, indicating that those genes may be key for cell-cycle regulation in cancer. The 28 selected DEGs also have strong protein–protein interactions, as plotted using STRING (Fig. 1).
3.2. KIF4A and STIL Overexpression Can Serve as Diagnostic and Prognostic Markers in Various Cancers
Among all pan-cancer DEGs, two genes, KIF4A and STIL, showed elevated expression in eleven types of cancer (Fig. 2A,Fig. 2B). In a previous pan-cancer transcriptome study, KIF4A was also found to be upregulated in multiple cancer types, while STIL was not identified, probably due to the different p-values and fold-change thresholds applied in screening [26].
KIF4A encodes the chromokinesin protein KIF4A, an ATP-dependent molecular motor that promotes mitotic chromosome condensation and segregation [27]. KIF4A can also directly bind to chromatin and participate in DNA damage by associating with BRCA2 [28, 29]. Its roles in cancer are complicated. On the one hand, KIF4 knockout in mouse embryonic stem cells leads to the formation of tumors in nude mice [30]. On the other hand, many other studies have proven the upregulated expression of KIF4A in various cancer tissues and its positive correlation with poor prognosis [31–34]. Its overexpression can promote lung cancer resistance to cisplatin, while for breast cancer cells, its overexpression promotes cell apoptosis during treatment with doxorubicin [35, 36]. Our results strongly support the oncogenic role of KIF4A in various cancers. STIL encodes for SCL-interrupting locus protein (STIL), a centrosome protein located in the pericentriolar material. STIL is necessary for centriole duplication, as its absence could lead to centriole loss [37]. Meanwhile, its overexpression could generate extra copies of centrioles and promote chromosome missegregation [38, 39]. The deletion of most STIL exons and the formation of a TIA-1 and STIL recombinant is thought to be a cause of T-cell acute lymphoblastic leukemia; thus, the STIL gene was identified as an oncogene in NCG 6.0 [40, 41]. For solid tumors, upregulated expression of STIL has been found in lung cancers when compared with paring normal tissue, and its high expression was positively related to the mitotic index and metastatic activity [42]. Its mRNA expression amount was also positively correlated with the histological grade in ovarian cancers [43]. Our results prove the overexpression of STIL in multiple solid tumors and indicate that it can serve as a pan-cancer diagnostic marker.
The elevated expression of the KIF4A protein has also been verified in five types of cancers using immunohistochemistry data from the Human Protein Atlas (HPA; Fig. 2C). For all of the five cancer types, KIF4A showed moderate protein expression in normal tissue, while in some tumor samples, its expression was strong.
We next studied the roles of KIF4A and STIL expression in cancer prognosis. High expression of either gene was significantly correlated with poor prognosis in four kinds of cancer, KIRC, KIRP, LUAD, and LIHC, as shown by Kaplan–Meier survival analysis (Fig. 3). In other cancer types, the correlation was insignificant (p-value > 0.05). It is noteworthy that the cancer types in which KIF4A and STIL showed prognostic values are perfectly overlaid with each other, implying a shared mechanism for cancer progress among these cancers.
3.3. KIF4A and STIL Coexpression and Interaction in Various Cancers
The similar characteristics between KIF4A and STIL encouraged us to explore the relations between them. In all 12 types of cancers, the mRNA expression of KIF4A and STIL was highly correlated, with a Pearson’s correlation coefficient (r) of 0.61–0.88 (Fig. 4A). The GeneMANIA interaction network between KIF4A and STIL was plotted (Fig. 4B). Although no physical interaction between KIF4A and STIL was found, they are connected by eight other genes (KIF2C, KIF11, KIF14, KIF23, MELK, AURKB, CCNA2, and PRC1) and form a strong interaction hub. In the hub composed of ten genes, interactions exist among all pairs of genes. Moreover, all of them showed increased expression in multiple types of cancer tissue (Figure S1, Table 4). Five of them, KIF4A, KIF2C, KIF11, KIF14, and KIF23, belong to the kinesin family. KIF4A, KIF23, KIF2C, MELK, and CCNA2 have previously been identified as hub genes that have high prognostic value in kidney renal cell cancer [44]. The protein regulator of cytokinesis 1 (PRC1) was reported to directly interact with KIF4 and form a PRC1–KIF4 complex that tags the plus end of microtubule in a microtubule-length-dependent way. This tagging process is critical for cells to differentiate among microtubules of different lengths and is important for subsequent spindle formation [45, 46]. Aurora kinase B (AURKB) directly binds and activates KIF4A to promote binding between KIF4A and PRC1, subsequently promoting spindle organization [47]. Maternal embryonic leucine zipper kinase (MELK) has previously been identified, together with KIF4A, to be associated with poor prognosis in obese luminal A breast cancer patients [48]. Moreover, it was identified in another study as the top gene associated with elevated pan-cancer expression compared with normal tissue [26]. Pathway-enrichment analysis using GO was conducted on all ten hub genes based on GeneMANIA network analysis (Fig. 4C). As shown, all genes are strongly enriched in mitotic chromosome-segregation-related pathways, including mitotic nuclear division, mitotic spindle organization, and mitotic spindle elongation. The regulation of these processes is crucial for cell fate, as increased centrosome amplification could directly induce tumorigenesis [49].
4.1.The expression and clinical significance of KIF4A an STIL in osteosarcoma tissue and normal paracancerous tissue.
The expression of KIF4A and STIL were detected osteosarcoma tissue and normal paracancerous tissue tissue by immunohistochemistry. We found that expression rate of KIF4A and STIL in osteosarcoma tissues were 96.4% (55/57) and 96.4% (55/57) (Fig. 5A, Fig. 5B, Fig. 5D, Fig. 5E). Compared with 1.75% (1/57) and 3.50%(2/57)in normal paracancerous tissues(Fig. 5C, Fig. 5F), KIF4A and STIL were significantly over-expressed in osteosarcoma tissues (P༜0.001). A significant correlation was indicated between KIF4A and STIL expression in osteosarcoma tissues(Table 5).
4.2. KIF4A and STIL were correlated with poor overall survival of osteosarcoma patients
The results of univariate Cox hazard analysis of overall survival of OS patients showed that Enneking stage and expression of KIF4A and STIL were significantly correlated with survival rate(Table 6, Fig. 6A, Fig. 6B). The results of multivariate Cox hazard analysis showed that KIF4A and STIL identified as independent prognostic factors of OS patients(Table 7).