Background/Aim: p62 (also known as sequestosome 1) is involved in cancer progression, and high expression of p62 indicates poor clinical outcome in several cancer types. However, the association between p62 gene expression and cancer stem cells (CSCs) in breast cancer subtypes remains unclear. Materials and Methods: In the present study, genomic datasets of primary breast cancer (The Cancer Genome Atlas, n=593; and Molecular Taxonomy of Breast Cancer International Consortium, n=2,509) were downloaded. p62 Expression was then examined in normal and breast cancer tissues derived from the same patients. Kaplan-Meier and multivariate Cox regression analyses were employed to evaluate disease-specific survival. Next, the effect on cell viability and in vitro tumor-sphere formation of p62 knockdown using targeted small interfering RNA was assessed by using cells with high activity of aldehyde dehydrogenase 1 (ALDH1 high ). Results: Patients with normal-like, luminal A or luminal B breast cancer with p62 high had poor prognosis. Furthermore, patients with p62 high ALDH1A3 high luminal B type also exhibited poor prognoses. Knockdown of p62 suppressed viability and tumor-sphere formation by ALDH1 high cells of the luminal B-type cell lines BT-474 and MDA-MB-
These results suggest that p62 is essential for cancerous progression of ALDH1-positive luminal B breast CSCs, and contributes to poor prognosis of luminal B breast cancer. Conclusion: p62 is potentially a prognostic marker and therapeutic target for ALDH1-positive luminal B breast CSCs.Breast cancer has the highest prevalence among cancers of women worldwide, with 2.26 million new cases (24.5% of all cancer cases in women) and 685,000 cancer-associated mortalities (15.5% of all cancer-associated mortalities among women) annually (1). Breast cancer is classified using two parameters: Immunohistochemistry and gene-expression patterns [prediction analysis of microarray 50 (PAM50)] (2-8). Based on its PAM50, breast cancer is classified into at least six subtypes: Normal-like, luminal A, luminal B, human epidermal growth factor receptor type 2 (HER2)-enriched, claudin-low and basal-like (5, 7, 8). Among these, the luminal B type expresses estrogen receptor, and certain luminal B tumors express HER2 and highly express proliferation-related genes such as marker of proliferation Ki-67 (MKI67). In addition, the luminal B type has poorer prognosis (7,(9)(10)(11)(12)(13)(14)(15). Breast cancer treatment mainly entails surgery, radiotherapy and drug therapy, including chemotherapy, endocrine therapy and molecular targeted therapy. However, there are still numerous 3299