Associations between the Expression of Mucins (MUC1, MUC2, MUC5AC, and MUC6) and Clinicopathologic Parameters of Human Breast Ductal Carcinomas

Breast cancer is the most common malignancy in women, and therefore, much effort is devoted to identifying factors of prognosis and therapeutic significance. Mucins are high molecular weight glycoproteins with abundant O-linked carbohydrate chains. Their common domains are attached to serine or threonine residues that are O-glycosylated [1, 2]. To date, several mucins have been identified: MUC1, MUC3, MUC4, MUC12, MUC13, and MUC17 are membrane bound types, and MUC2, MUC5AC, MUC5B, and MUC6 are secreted or gel forming types [3]. Alterations in mucin expression or glycosylation accompany the development of cancer and influence cellular growth, differentiation, transformation, adhesion, invasion, and immune surveillance [4]. The expressions of mucins in malignant tumors have been investigated most prominently in adenocarcinomas of organs, such as the pancreas, ovary, and stomach [5-9]. MUC1 is detected on the apical surfaces of most normal glandular epithelial tissues [10]. In contrast, MUC1 demonstrates variable localization in malignant tissues, including cytoplasmic and membrane expression [11]. Moreover, some studies suggest that the overexpression of MUC1 is associated with an increased metastasis rate [12, 13]. MUC2 is a major gel-forming secretory mucin, which is expressed primarily in intestinal goblet cells and airway epithelium [14, 15]. MUC2 demonstrates higher expression levels in mucinous carcinomas than in ductal carcinomas from several organs, including pancreas, breast, and prostate [16-19]. MUC5AC and MUC6 are gastric-type secretory mucins. MUC5AC is located mainly in the gastric mucosa of the cardia, fundus, and antrum, where as MUC6 is located in pyloric glands [20, 21]. Interestingly, prior studies demonstrated a correlation between increased expression of MUC6 and less aggressive biological behavior in mucinous carcinoma of the breast [18].

The aims of the current study were to evaluate the expression patterns of various mucins in invasive ductal carcinoma of the breast and to determine the relationships between mucin expression and clinicopathologic parameters.

In the current study, we investigated different mucin expression patterns and their associations with clinicopathologic parameters in human breast cancer. In a previous study, even though benign breast showed a different expression pattern, DCIS and invasive ductal carcinoma showed a similar expression pattern [23]; moreover we also observed a similar expression pattern between DCIS and invasive ductal carcinoma. Hence, we evaluated the expression pattern of DCIS and invasive ductal carcinoma with the same criteria. However, there was no statistical correlation between the expression of mucins with DCIS and invasive ductal carcinoma. Consistent with the previous study demonstrating about 90% of MUC1 expression [23, 24], we observed MUC1 expression in the majority of invasive ductal carcinoma cases (93.6%). Rakha et al. [24] and van der Vegt et al. [25] found that apical MUC1 expression was associated with superior outcomes. Different from the study of Rakha et al. [24] showing apical staining in 4.5% of cases, another study of MUC1 in Korean breast cancer patients showed only one case of apical expression in 244 cases (0.4%); further the evaluation of prognostic correlation was not conducted [23]. Even though we observed in 4.1% of apical expression, the current study did not demonstrate any correlations between MUC1 expression sites and clinicopathologic parameters. The differences between the results of previous studies and the present study are controvertible; thus further study might be required. Regardless of subcellular localization, MUC1 expression was associated with ER and with PR positivity and nontriple negativity. In the cases of MUC2 and MUC5AC, any expression in malignant cells was regarded as positive due to their low expression level, and they showed a similar positivity rate with the previous study [18]. MUC2 and MUC5 expressions were not correlated with any prognostic factors. A previous study of mucinous carcinoma demonstrated an association between better prognosis and MUC2 expression. However, the expression rate was significantly higher in mucinous carcinoma, as it plays a role in barrier formation in mucinous carcinoma: however, its role in invasive ductal carcinoma is not clear. In the current study, MUC6 expression was observed in 12.4% of invasive ductal carcinoma cases, and there were correlations between MUC6 expression and poor prognostic factors, such as a higher histologic grade, lymphatic invasion, and higher N stage. Previous studies of MUC6 expression in breast cancer by Pereira et al. [26] and Rakha et al. [24] detected MUC6 expression in approximately 20% to 23% of cases. The differences in MUC6 expression proportions between the current study and the previous two studies may be due to different criteria for positivity. We defined positivity as when more than 30% of cancer cells expressed MUC6, where as the prior studies used a cutoff value of more than 5%. Because we failed to attain a valuable receiver operating characteristic (ROC) curve to evaluate the relationship between the overall survival and MUC6, for the determination of the optimal cutoff value, we have evaluated a 5% interval and found that the value of 30% shows a relationship with some clinicopathologic parameters. Additionally, the previous studies did not demonstrate any correlations between MUC6 expression and clinicopathologic parameters. These differences may also be partially explained by the different criteria for positivity. In addition, because all cases of this study are from Asian women and all previous studies were from Western women, genetic differences should be considered and a molecular level study might be needed. Moreover, a previous study investigating MUC6 expression in mucinous carcinoma of the breast by Matsukita et al. [18] found higher MUC6 expression in mucinous carcinomas (71%) and suggested that MUC6 expression is associated with superior outcomes. These authors explained that, as a gel-forming secretory mucin, MUC6 may act as a barrier to cancer cell extension, resulting in a better prognosis. However, as the number of cases in their study was only 17 cases and relatively limited, larger studies would be helpful to confirm this result. Even though MUC6 expression was observed in the current study, we did not observe a mucin barrier in invasive ductal carcinoma. The role of MUC6 may differ between invasive ductal carcinoma and mucinous carcinoma of the breast. We note that our study has some limitations. First, the representative tumor areas may not have been evaluated, because mucins evaluation was carried out only on one cut surface of each tumor specimen without an analysis of whole mount section. Second, the result of HER2 has a limitation because the evaluation was performed via immunohistochemistry staining without the evaluation of fluorescence in situ hybridization or silver in situ hybridization.

In summary, MUC1 is expressed in most cases of invasive ductal carcinoma in the breast and is associated with hormonal receptor expression; however, it is not associated with clinicopathologic parameters. Among the various mucins studied, MUC6 expression was associated with parameters known as poor prognostic factors such as grade, lymphatic invasion and N stage, when more restrictive criteria were applied. Further studies, including studies at the genetic level, are needed in order to clarify the differences in the results between the current and previous studies.

Supplementary Table 1