Characteristics of the included patients.
We screened 863 patients with breast cancer treated in 2014 at our hospital. According to our eligibility criteria, 202 patients were included in the final analysis. The workflow for the screening processes is illustrated in Figure 2.
The mean age of the included patients was 52.41±10.97 (mean± SD). Most of the tumors (191/202, 94.6%) were ≤ 5 cm. Forty-one patients (20.3%) had TNM stage III disease. The details of the baseline pathological characteristics are listed in Table 1.
The expression of PBK/TOPK protein
Among 202 breast cancer tissues, PBK/TOPK protein was expressed ('+' and '++') in 182 (90.1%). According to our criteria, high expression and low expression of PBK/TOPK were found in 85 (42.1%) and 117 (57.9%) samples, respectively.
Correlation between PBK/TOPK protein expression and clinicopathological characteristics
According to the Spearman correlation analysis, histological grade (P=0.015), TNM stages (P=0.013), lymph node metastasis (P=0.002), ER (P=0.005), PR (P=0.027), HER-2 (P=0.037), and Ki-67 (P<0.001) were associated with PBK/TOPK protein expression. However, the patients' age (P=0.431), menopausal status (P=0.163), tumor size (P=0.163), and vascular tumor thrombus (P=0.110) had no significant correlation with PBK/TOPK protein expression. The detailed data for the correlation analysis are presented in Table 1.
In terms of the molecular types, high expression of PBK/TOPK was observed in 9.10% (3/33), 41.88% (31/74), 55.1% (27/49), and 60.00% (18/30) of patients with Luminal A-type, Luminal B type, HER-2 (+) type, and triple negative breast cancer (TNBC), respectively (P<0.001, Table 1). According to the box plots (Figure 3), the median expression level of PBK/TOPK in Luminal A samples was lower than that in Luminal B samples. Additionally, the median expression levels of PBK/TOPK in Luminal A and Luminal B samples were lower than those of HER-2(+) and TNBC.
PBK/TOPK protein expression level and prognosis
The follow-up for all included patients ended on 31 December 2020 and 21 patients died due to breast cancer during this period. The median follow-up was 78 months (ranging from 72 to 83 months). The median OS and DFS were not calculated during the study period.
Kaplan-Meier analysis revealed that the expression level of PBK/TOPK was negatively correlated with the OS of breast cancer patients. The 5-year survival rate was 81.2% and 95.7% for the high and low PBK/TOPK expression groups (Figure 4A). According to the Log-rank test, breast cancer patients with high PBK/TOPK expression had a poor prognosis (P=0.001). Similarly, the expression level of PBK/TOPK was negatively correlated with the DFS of the patients. The 5-year DFS rates were 78.8% and 92.3% for the high and low PBK/TOPK expression groups (Figure 4B). The Log-rank test showed that breast cancer patients with high PBK/TOPK expression had poor prognoses (P=0.006).
Subgroup analysis was performed for OS according to the TNM stages using Kaplan-Meier analysis. For patients with stages I-II breast cancer, the expression level of PBK/TOPK was negatively correlated with OS. The 5-year OS rates were 86.7%and 95.8% for the high and low PBK/TOPK expression groups (Figure 4C). According to the Log-rank test, breast cancer patients with high PBK/TOPK expression had poor prognoses (P=0.042). For patients with stage III breast cancer, the expression level of PBK/TOPK was negatively correlated with OS as well. The 5-year OS rates were 65.2% and 95.2%for the high and low PBK/TOPK expression groups (Figure 4D). According to the Log-rank test, breast cancer patients with high PBK/TOPK expression had poor prognoses (P=0.018).
Influential factors related to the prognosis
Univariate Cox regression analysis revealed that tumor size (OS: hazard ratio (HR)=4.148, P=0.010; DFS: HR=3.452, P=0.007), lymph node metastasis (OS: HR=3.390, P=0.012; DFS: HR=2.351, P=0.032), HER-2 (OS:HR=2.016, P=0.002; DFS:HR=1.977, P=0.001), and PBK/TOPK (OS: HR=4.590, P=0.003; DFS: HR=2.890, P=0.009) were risk factors for both OS and DFS among the included patients. On the contrary, ER expression (OS: HR=0.342, P=0.014; DFS: HR=0.401, P=0.018) was a protective factor for the OS and DFS of the patients. Further, the TNM stage (OS: HR=2.658, P=0.030) was a risk factor for the OS of the patients. The results of the univariate analysis are listed in Table 2.
The meaningful indicators generated from the univariate analysis (P<0.05) were included in the multivariate Cox regression, and the results are presented in Table 3 and Figure 5. High expression of PBK/TOPK was an independent influential factor on OS in breast cancer (P=0.034) and HER-2 expression was an independent influential factor on DFS (P=0.014). Analysis of the ROC curve in Figure 6. revealed that the area under the curve was 0.690 (95% confidence interval (CI): 0.576-0.805) for the expression of PBK/TOPK in the prediction of 5-year OS (P=0.004).