Background. Studies have suggested that HOXB9 expression in breast cancer cells promotes cellular invasiveness, metastatic ability, and tumor neovascularization in the surrounding tissue in in vitro and in vivo assays. These findings imply that HOXB9 overexpression may alter tumor-specific cell fates and the tumor stromal microenvironment, contributing to breast cancer progression. The objective of this study was to analyze whether these results could be applied to clinical practice. Methods. A total of 141 consecutive, invasive ductal carcinoma patients who underwent surgical treatment were examined. Immunohistochemical staining was performed to evaluate the expression of HOXB9, Ki-67, CD31, and CD34, and the association of tumor proliferation and angiogenesis with HOXB9 expression was analyzed. Results. Of the 141 tumor specimens immunostained for HOXB9, 69 (48.9%) stained positive. Larger primary tumor size, hormone receptor negativity, HER2 positivity, higher nuclear grade, and number of pathologic nodal metastases were significant variables associated with HOXB9 expression. Notably, 12 (92.3%) of 13 triple-negative breast cancer cases showed HOXB9 expression. Disease-free survival and overall survival were significantly different between the HOXB9-positive and HOXB9-negative groups (hazard ratio 20.714, P = 0.001; and hazard ratio 9.206, P = 0.003, respectively). Multivariate analysis indicated that HOXB9 expression was the only independent prognostic factor for disease-free survival (hazard ratio 15.532, P = 0.009). HOXB9-positive tumors showed a significant increase in the number of vasculature and the Ki-67 ratio compared with HOXB9-negative tumors. Conclusions. HOXB9 expression, which promotes tumor proliferation and angiogenesis, is a significant prognostic factor in breast cancer.
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