Expression of estrogen receptor-α and Ki67 in relation to pathological and molecular features in early-onset infiltrating ductal carcinoma

Estrogen causes breast cancer by triggering proliferation via an estrogen receptor (ER)-mediated mechanism. However, paradoxically, ERalpha, one of the two known ER subtypes, and the proliferation marker, Ki67, are not usually expressed in the same breast tumor. To explore whether ERalpha-positive tumors and proliferating (Ki67-positive) tumors have different tumorigenic characteristics, we performed an immunohistochemical study on 74 early-onset infiltrating ductal carcinomas of the breast. To test this hypothesis, we examined whether ERalpha-positive and Ki67-positive tumors showed differences in (i) pathological grade, (ii) three indices of tumor grade (tubule formation, nuclear pleomorphism, and mitotic number), and (iii) expression of important proteins implicated in breast tumorigenesis (cyclin D1, ErbB2, ATM, BRCA1, Rb, p53, and p21). The results of the multigenic analysis showed that ERalpha and Ki67 were the only two important markers significantly and independently associated with tumor grade, consistent with the above hypothesis. ERalpha-positive, Ki67-negative tumors frequently displayed a low tumor grade (i.e. being well differentiated), whereas Ki67-positive, ERalpha-negative tumors were more likely to exhibit a high tumor grade. In addition, positive ERalpha expression (46 of 74 cases, 62%) correlated well with positive cyclin D1 expression (p < 0.005), less nuclear pleomorphism (p < 0.001), and a low mitotic count (p < 0.005), whereas positive Ki67 expression (36 of 74 cases, 49%) correlated with reduced BRCA1 expression (p < 0.01) and high mitotic activity (p < 0.01). These findings suggest that the expressions of ER alpha and Ki67 might be involved in distinct pathological and molecular features during breast cancer development. Copyright (C) 2004 National Science Council, ROC and S. Karger AG, Basel.