Abstract 162: DNA methylation activates TP73 expression in human breast cancer tumorigenesis

Abstract

Abstract Epigenetic events that cause changes in gene expression are common in human cancers. However, the connection between TP73 expression and its epigenetic functionality in tumorigenesis has not been widely explored. As a transcription factor, p73 is activated in a similar manner to p53 in response to DNA damage and regulates the expression of downstream genes involved in cell cycle arrest and apoptosis. However, there are other compounding functions of this gene that reflect its non-tumor-related characters, thus making it very difficult to assess its specific role in tumorigenesis, which results from an accumulation of mutational and epigenetic changes that alter normal cell growth and survival pathways. TP73 is the homologue of the master tumor suppressor TP53 involved in cellular responses to stress and development. The TP73 gene encodes two different proteins TAp73 and ΔNp73 and maps to a region on chromosome 1p36 that is frequently deleted in neuroblastoma and other tumors and is thought to contain multiple tumor suppressor genes. However, the analysis of p73-knockout mice yielded conflicting results with respect to tumor suppression. World-wide efforts in sequencing the TP73 gene in patient tumor samples have not provided evidence for genetic alterations as a common cause of TP73 inactivation in human cancers. Therefore, the role of TP73 in tumorigenesis has remained elusive to date. In this study, we determined TP73 expression in normal human mammary epithelial cells (HMEC), non-cancerous Li-Fraumeni Syndrome (LFS) skin fibroblasts, human breast cancer cell lines (MCF7 & MDA321), neuroblastoma cell lines (IMR32 & SK-N-SH) and normal skin fibroblast cell line (HS27). We also conducted an immunohistochemistry analysis of TP73 expression in human normal breast tissues, cancerous breast tissues and non-cancerous (cancer-adjacent) breast tissues. The results show that TP73 expression is evident only in cancer cell lines and breast cancer tissues. Moreover, methylation-specific PCR and bisulfite sequencing studies revealed that TP73 promoter is activated only in cancer cell lines by DNA methylation. Furthermore, ChIP assay results demonstrated that CTCF (a chromosomal networking protein CCCTC binding factor) and tumor protein p53 (TP53) bind to TP73 promoter and regulate TP73 expression. These observations demonstrate the existence of a positive correlation at the promoter site of TP73 between its expression and DNA methylation. However, TP73 expression is not involved in cell proliferation, which may prove significant for the development of future diagnostic and therapeutic applications. Citation Format: Zhixing Yao, David R. Yao, William Yu, Zaki A. Sherif. DNA methylation activates TP73 expression in human breast cancer tumorigenesis [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 162.