mRNA Expression of SMC members in HCC
To explore the mRNA expression level of SMC family members (SMC1A, SMC2, SMC3, SMC4, SMC5, SMC6) in HCC, Perl software was used to obtain the expression data of SMCs in 375 HCC cases and 50 normal tissues derived from TCGA. Pearson’s correlation of SMCs was evaluated using corrplot package to determine whether there is a correlation between these genes. Interestingly, as shown in Fig. 1, there is a significant correlation between SMC family members.
As shown in Fig. 2A, limma package was used to show the differentially expressed SMCs and pheatmap package was used to visualize the results. As shown in Fig. 2B, SMC1A, SMC2, SMC3, SMC4 and SMC6 were significantly upregulated in HCC tissues, whereas no difference was found in SMC5 expression between HCC and normal tissues. Moreover, the Human Protein Atlas database was used to explore the protein levels of SMCs in HCC. Next, in order to study the Protein levels of the SMC gene family in liver cancer tissues, we demonstrated the differences in the expression of SMC family proteins in HCC and normal liver tissues through the Human Protein Atlas database. As shown in Fig.2C, the expressions of SMC1A, SMC2, SMC3, SMC4 and SMC6 in HCC tissues were higher than those in normal liver tissues, while there was no significant difference between SMC5 in liver cancer tissues and normal tissues.
Prognostic Values of SMC members in HCC
Subsequently, the prognostic values of SMC members for HCC were further assessed. Univariate Cox proportional hazards regression analysis was utilized to analyze the predictive ability and clinical characteristics of differentially expressed SMC members (SMC1A, SMC2, SMC3, SMC4, SMC5, SMC6). It was found that four SMC members (SMC2, SMC3, SMC4, SMC6) and one clinical feature (stage) were closely associated with poor outcome of HCC patients (hazard ratio [HR] for SMC2: 1.137 (1.051–1.230); HR for SMC3: 1.041 (1.009–1.075); HR for SMC4: 1.094 (1.038–1.152; SMC6: 1.119 (1.001-1.250)); HR for stage: 1.679 (1.368– 2.061) (Table 1). In order to further explore the correlation between the expression level of SMC members and the overall survival (OS) and progression-free survival (PFS) of HCC patients, we analyzed the expression profile and clinical data of HCC patients from TCGA database. The results showed that elevated expression of SMC2, SMC3, SMC4, and SMC6 in HCC patients were negatively correlated with patients' OS, while higher expression levels were associated with shorter OS (P values of 0.002, 0.001, < 0.002, and 0.014), as shown in Fig. 3A. Meanwhile, high expression of SMC2, SMC3, SMC4, and SMC6 was also negatively correlated with PFS, with higher expression levels associated with shorter PFS (P values of < 0.001, 0.001, < 0.001, and 0.01), as shown in Fig 3B. Next, multivariate Cox proportional hazards regression analysis was used to evaluate the independent prognostic values of SMC2, SMC3, SMC4 to control the influence of clinical features on the prognosis. The results, which are exhibited in forest plots (Fig. 4), confirmed that the expression of SMC2, SMC3, and SMC4 and a clinical parameter (stage) were independent biomarkers for the prognosis of HCC patients.
Correlations Between SMC Members and TIICs in HCC
TIICs, an important part of the complex tumor microenvironment, have been reported to be associated with the progression of many cancers. Hence, it would be meaningful to explore the association between immune infiltration and SMC members. We calculated the coefficient of SMCs and the level of immune infiltration in HCC patients to determine whether they were related. The first column in Fig. 5 showed significant correlations between SMCs expression and tumor purity. In addition, SMC expression was notably correlated with the infiltration levels of most immune cell types in HCC. Specifically, the level of SMC2 expression positively correlated with the infiltration levels of B cells (r=0.372, p=1.03e-12), CD8+ T cells (r=0.353, P=1.67e-11), CD4+T cells (r=0.332, P=2.59e-10), macrophages (r=0.451, P=1.70e-18), neutrophils (r=-0.484, P=1.12e-12), and DCs (r=0.468, P=7.03e-20). The same thing with SMC3 and SMC4. These results indicated that 3 members of SMC gene family were related with tumor purity and immune infiltration levels in HCC.
Mechanism Underlying the Prognostic role of SMC Members in HCC
A GSEA of differentially expressed SMC members with statistical prognostic value was conducted to evaluate the potential biological mechanism by which differential expression of SMC2, SMC3, and SMC4 affects the carcinogenesis of HCC. The GSEA indicated that high expression of SMC2 was related to “cell cycle”, “oocyte meiosis”, and “ubiquitin mediated proteolysis” (Fig. 6A), high expression of SMC3 was related to “cell cycle”, “ERBB signaling pathway”, “oocyte meiosis” and “ubiquitin mediated proteolysis” (Fig. 6B), high expression of SMC4 was related to “chronic myeloid leukemia” (Fig. 6C).