Abstract P3-09-04: Possible role of p53/Mieap-regulated mitochondrial quality control as a tumor suppressor in human breast cancer

Abstract

Abstract [Background] In sporadic breast cancers, the most important gene is p53, given that it is a tumor suppressor gene and mutated in greater than 50% of human cancers. According to previous reports, p53 is also mutated in approximately 20–40% of breast cancers. Recent data from the cancer genome atlas (TCGA) revealed that 37% of breast cancer specimens had alterations in p53-particularly, 72% in (human epidermal growth factor) HER2-rich and 80% of basal-like breast cancer cases—indicating that it is a critical driver of tumor development even in breast cancer. p53 is clinically very important not only because of its high mutation rate but also because mutation is associated with more aggressive disease and worse overall survival. Mitochondria-eating protein (Mieap) is a p53-target gene that plays an important role in mitochondrial quality control. Mieap has been reported to have a critical role in tumorsuppression of colorectal cancer. Here, we investigated the role of Mieap as a tumorsuppressor in breast cancer. [Marerial and methods] We overexpressed Mieap using the constructed adenovirus in breast cancer cell lines such as MCF-7, SK-BR-3, and MDA-MB-231 cells. The percentages of cells in different cell cycle phases (subG1, G1, S, and G2/M) were determined using FACS analysis and also caspase activities (Caspase, 3/7, 9) were measured. Cleaved PARP, which is a marker of cells undergoing apoptosis, was detected by western blot. For in vivo experiments, we examined the expression of Mieap using surgical specimens (invasive ductal carcinomas (IDCs): 75, ductal carcinoma in situ (DCIS): 27, fibroadenomas (FAs): 18) by immunohistochemistry. Next, we performed methylation-specific PCR (MSP) for Mieap, NIX, and BNIP3 promoters and p53-mutation search using 46 samples that were cryopreserved, among 75 IDC cases used for immunohistochemistry. These studies were approved by the central ethics committee of Gifu University.[Results] The enforced-expression of exogenous Mieap in breast cancer cells induced caspase-dependent apoptosis, with activation of both caspase-3/7 and caspase-9. Immunohistochemistry revealed endogenous Mieap in the cytoplasm in 24/75 (32%) invasive ductal carcinomas (IDCs), 15/27 (55.6%) cases of ductal carcinoma in situ (DCIS), and 16/18 (88.9%) fibroadenomas (FAs) (IDC vs DCIS; p = 0.0389, DCIS vs FA; p = 0.0234, IDC vs FA; p < 0.0001). In IDCs, the Mieap promoter was methylated in 6/46 (13%) cases whereas p53 was mutated in 6/46 (13%) cases. Therefore, the p53/Mieap-regulated mitochondrial quality control pathway was inactivated in 12/46 IDCs (26.1%). Interestingly, all of the tumors derived from the 12 patients with the Mieap-promoter methylation or p53 mutation pathologically exhibited more aggressive and malignant phenotype of breast cancers, resulting in significantly shorter disease-free survival (DFS) (p = 0.021). [Conclusion]These results indicate that p53/Mieap-regulated mitochondrial quality control has a critical role in tumor suppression of breast cancer, possibly in part, through mitochondrial apoptotic pathway. Citation Format: Futamura M, Gaowa S, Arakawa H, Yoshida K. Possible role of p53/Mieap-regulated mitochondrial quality control as a tumor suppressor in human breast cancer [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P3-09-04.