Metastasis, drug resistance and recurrence are the main causes of the low survival rate of patients with GC. Self-renewal of GCSCs is considered to be the underlying cause of the metastasis, drug resistance and recurrence of GC [3–4]. After conventional surgical treatment or adjuvant chemotherapy, the number of GCSCs does not decrease but stemness is enriched, leading to metastasis, drug resistance and recurrence of GC . Moreover, Bekaii-Saab et al.  indicated that identification and targeted therapy of cancer stem cells had important significance in the treatment of GC. Therefore, targeted therapy of GCSCs is crucial. This study used WGCNA based on the mRNAsi score to identify key genes related to GCSC characteristics. This study found that the mRNAsi of gastric tumor tissue was significantly higher than that of normal gastric tissue. In addition, we used the WGCNA package to construct a gene coexpression network. Finally, we obtained a total of 16 modules; the brown module was the most positively correlated with mRNAsi, and the turquoise module showed a high negative correlation with mRNAsi. Therefore, we selected the brown module as the module of most interest and identified 16 key genes that are associated with the characteristics of GCSCs: BUB1, BUB1B, NCAPH, KIF14, RACGAP1, RAD54L, TPX2, KIF15, KIF18B, CENPF, TTK, KIF4A, SGOL2, PLK4, XRCC2 and C1orf112. These genes were upregulated in GC and had a certain correlation. The most significantly enriched GO terms of these key genes were the spindle cellular component, the sister chromatid segregation biological process and the motor activity molecular function. The significantly enriched KEGG pathways were cell cycle and homologous recombination. Except for KIF14 and KIF18B, the expression of all key genes was related to the prognosis of patients with GC.
BUB1 encodes a protein that is essential for the function of the mitotic spindle check-point . Grabsch et al.  showed that BUB1 was overexpressed in up to 80% of GCs and was associated with tumor cell proliferation. Meanwhile, related studies indicated that high expression of BUB1B was related to the invasion, lymph node metastasis, liver metastasis and recurrence of GC . Kinesins are a family of molecular motors that play important roles in intracellular transport and cell division . Previous studies demonstrated that KIF14 can enhance tumor adhesion, invasion and chemical resistance during tumor development . Tong et al.  also revealed that KIF14 was overexpressed in GC and that this was related to tumor progression, invasion and metastasis. Spheroid colony formation is an effective model for the characterization of cancer stem cells . Oue et al.  showed that KIF15 and KIF4A were upregulated in GC spheroid cells. A previous study also demonstrated that patients with high expression of KIF14 mRNA had a higher risk of malignancy, recurrence and metastasis than did those without high expression of KIF14 mRNA . Overexpression of KIF18B increases the proliferation of hepatocellular carcinoma cells, and downregulation of KIF18B in vitro can inhibit the migration and invasion of cervical cancer cells . KIF4A is considered to be an oncogene, indicating its involvement in malignancy, and its expression was indeed related to the occurrence, development and metastasis of GC . Rac GTPase accelerating protein1 (RacGAP1), which belongs to the GTPase activation protein family, not only induces cytokinesis but also interferes with mitotic spindle assembly, and thus participating in the regulation of cell proliferation . Studies have indicated that RacGAP1 is involved in cell transformation, movement, migration and metastasis  and is positively correlated with the proliferation marker Ki67 . In addition, RacGAP1 is highly expressed in GC and significantly associated with tumor progression and poor prognosis . CENPF is a member of the mitochondrial family and regulates the proliferation of various tumor cells . Chen et al.  showed that CENPF was overexpressed in GC and promoted its proliferation and metastasis. TTK is also known as a mitotic kinase , which is highly expressed in many types of tumors and promotes the proliferation of cancer cells [31–32]. Park et al.  found that TTK plays an indispensable role in the development and maintenance of tumor stem cells. PLK4 is a major regulator of centromeric repeats, and its overexpression in somatic cells can disrupt spindle formation, while its depletion can inhibit cell proliferation . Shinmura et al.  revealed that PLK4 is overexpressed in GC and plays an important role in cell proliferation, tumorigenesis, invasion and drug resistance. C1ORF112 is overexpressed in various tumors, including GC, and plays an important role in the growth of cancer cells, but it is still unclear whether C1ORF112 is particularly necessary for the proliferation of cancer cells . Condensins are multiprotein complexes that play a central role in chromosome assembly and segregation during mitosis and meiosis . Non-SMC condensin I complex subunit H (NCAPH) is one of the three non-SMC subunits of condensation I. Studies have indicated that non-SMC condensin I complex subunit G (NCAPG) is overexpressed in colon cancer and hepatocellular carcinoma and promotes the proliferation and migration of tumor cells .
In addition, survival analysis and Oncomine analysis revealed that the prognosis of patients with GC and the expression of three genes (RAD54L, TPX2 and XRCC2) were consistently related. XRCC2 plays a crucial role in DNA repair and chromosome arrangement, and its dysfunction may lead to tumor development and progression [39–40]. Wang et al.  found that XRCC2 is overexpressed in GC. RAD54L and XRCC2 are homologous recombination factors that are overexpressed in lung cancer and GC, and there is a significant correlation between the expression levels of these two proteins . The inactivation of these factors leads to an obvious deficiency in homologous recombination and DNA repair in all eukaryotes . In addition, related studies indicated that abnormal function of homologous recombination leads to genomic instability, resulting in the occurrence of cancer [44–45]. In this study, KEGG enrichment analysis indicated that RAD54L and XRCC2 were significantly enriched in homologous recombination. TPX2 is a microtubule-associated protein that activates the cell cycle kinase protein Aurora-A and then plays an important role in the formation of spindles in mitosis . Liang et al.  showed that TPX2 was overexpressed in GC and was related to poor progression and prognosis of GC.
In conclusion, this study found that the 16 key genes identified play an important role in the maintenance of GCSCs. In addition, RAD54L, TPX2 and XRCC2 are most likely to be therapeutic targets for inhibiting the stemness characteristics of GC, providing a new idea for the treatment of GC. However, further research is needed to verify this hypothesis.