Background: MUC1 has been used in clinical practice as a serum tumor marker (CA15-3) for monitoring recurrence and response to the treatment of breast cancers. However, it is not so practical to use the conventional anti-MUC1 antibody (C-Ab) for the differential diagnoses among malignant and benign tumors, because of its limited specificity to cancer-associated sugar chain structures. Recently, a novel epitope-defined (ED) antibody, which specifically recognizes MUC1 with cancer-associated carbohydrate antigens including Tn and sialyl-T antigens, was developed by our collaborators. In the present work, we focused on this Tn antigen-recognizing ED antibody (Tn-ED Ab) for MUC1, and examined the potential utility of this novel antibody as an immunohistochemical diagnostic marker of the breast.Design: Tissue microarray (TMA) consisting of invasive lesions and their non-neoplastic counterparts were constructed from 169 lesions of 167 individuals with invasive breast carcinomas (including 2 bilateral cases) resected and diagnosed at Hokkaido University Hospital from 2000 to 2003. We compared MUC1 immunoreactivity of Tn-ED Ab (clone SN-102) to commercially available C-Ab (clone Ma552) using TMA to evaluate its diagnostic performance and to analyze the association with clinicopathologic factors. Results: In invasive cancer parts, the immunoreactive positivity of Tn-ED Ab was observed predominantly in the cytoplasm (146/169) rather than in the cell membrane (5/169). The positive reaction of C-Ab was found in the cytoplasm and the membrane as well (156/169 and 93/169). Tn-ED Ab showed a lower expression rate in non-neoplastic parts and a high specificity in invasive cancer parts. Sensitivity/specificity in invasive parts was 86%/92% for Tn-ED Ab and 92%/47% for C-Ab. Moreover, the invasive cancer group with Tn-ED-positivity was significantly correlated with their molecular subtypes, low Ki-67 indices, and high ages, compared to those with Tn-ED-negativity (p=0.008, p=0.010 and p=0.027, respectively). Conclusion: Compared to C-Ab, the newly-developed Tn-ED Ab for MUC 1 showed a higher specificity for invasive breast cancer with a low reactivity to non-neoplastic components. Future study plans are ongoing to show that this cancer-specific Tn-ED MUC1 Ab could be useful for differentiating malignant from benign intraductal breast lesions as well.

Citation Format: Ai Shimizu, Kanako Hatanaka, Yutaka Hatanaka, Kentaro Naruchi, Masaharu Sato, Hiroshi Kase, Tomoko Mitsuhashi, Hiroko Yamashita, Yoshihiro Matsuno. Cancer-associated MUC1 epitope-recognizing antibody as a novel immunohistochemical marker for breast carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4616.