Abstract CT012: Novel effect of tamoxifen therapy: disruption of ER-p53 interaction leading to altered gene expression profile in human breast tumors

Introduction: We have reported that ERα binds to wild type (wt) p53 and inactivates its tumor suppressor activity. This interaction is augmented by estradiol and disrupted by tamoxifen (TAM). Moreover, retrospective clinical studies have shown that ER+ breast cancer (BC) expressing wt p53 is more responsive to TAM therapy compared to those expressing mutant p53. We conducted a window of opportunity trial in women with ER+ BC expressing wt p53 to test our hypothesis that TAM relieves the functional suppression of p53 by ERα. Methods: Women with ER+ invasive BC were randomized to 20mg TAM daily for 4 weeks prior to surgery or standard of care (SOC.) Wt p53 status in tumors was confirmed by massively parallel sequencing. Only ER+ wt p53 tumors were analyzed. ERα-p53 interaction was determined by Proximity Ligation Assay (PLA). Global transcriptome analysis in surgically resected tumors was conducted by RNA-seq. The multiplexed libraries were sequenced on HiSeq 2500 using 100 cycle paired end sequencing and an average 50 million paired end reads were generated for each sample. Immunohistochemistry was performed on FFPE tissue from diagnostic biopsies and surgically resected tumors to compare expression of ERα and p53 selected downstream targets.17β-estradiol and TAM metabolites were measured in the plasma, tumor and surrounding normal tissue using LC-MS/MS. Steady-state levels of TAM were confirmed. Polymorphisms affecting TAM metabolism enzymes were determined by genotyping. Findings: 53 women completed the study and 36 had adequate tissue for analysis (16 TAM and 20 SOC). PLA demonstrated that ERα-p53 interaction was disrupted in 75% of TAM treated patients. Differential gene expression (DEG) analysis using DESEq2 R package followed by GSEA pathway analysis showed that 300 genes were upregulated and 272 genes were downregulated (p