Abstract 2494: Identification of novel oncogenic targets of mutant p53 in esophageal squamous cell carcinoma
Authors: George Sara A. , Kotapalli Viswakalyan , Adduri Raju S. , Kumar Raju , Sarkar Sanjana , Pandilla Ramaswamy , Bashyam Murali D. 
Published: July 2021

Abstract
Background: Mutations in the TP53 gene are observed at a high frequency in Esophageal Squamous Cell Carcinoma (ESCC) with a majority of alterations being of the missense type affecting residues located in the region encoding the DNA binding domain. These mutations are associated with poor patient survival. However, characterization of the transcriptional networks regulated by p53 mutant proteins is limited to only a few frequent ‘hotspot' mutants, while comparatively rarer mutants have not been characterized.
Objectives: Identification and characterization of differentially expressed genes associated with p53 mutation status in ESCC tumor samples. Elucidation of the mechanism of transcriptional regulation of mutant p53 target genes.
Methods: A defined set of ESCC tumor samples with known p53 mutation status was selected for this study. The ability of each 'rare' p53 mutation to modulate tumor-related phenotypes was evaluated by performing phenotypic assays such as proliferation, colony formation and migration, in head and neck cancer cell-lines. Gene expression microarray analysis was performed on 36 ESCC tumors to determine putative target genes of 'rare' p53 mutant forms. Following dimension reduction, differentially expressed (putative mutant p53 target) genes were identified using Significance Analysis of Microarrays (SAM) and validated using reverse transcription quantitative PCR (RT-qPCR). Transcription induction of the putative target genes by ectopically expressed wild-type and mutant p53, in p53-null H1299 cell-line, were also evaluated by RT-qPCR. Chromatin immuno-precipitation (ChIP) assay was used to determine localization of mutant p53 proteins to chromosomal loci harboring the putative target genes. Similarly, ability to activate target promoters by mutant p53 was validated through promoter-luciferase assay.
Results: Genome-wide analysis revealed 27 differentially expressed genes, of which 9 exhibited up-regulated transcript levels in p53 mutant tumors. Three putative mutant p53 target genes - C1QBP, ARF6 and TRIM23 - were selected for further analysis due to previous reports of their association with cancer. TP53 transcript levels positively correlated with transcript levels of the identified potential mutant targets. The ectopically-expressed p53 mutants - P190T and P278L, induced the increased expression of the target genes, were localized to and activated the promoters of the target genes. Further characterization of the target genes is currently underway.
Conclusions: Three novel potential oncogenic targets of mutant p53 proteins were identified. The study further strengthened the heterogeneity in the functioning of different p53 mutants.
Citation Format: Sara A. George, Viswakalyan Kotapalli, Raju S. Adduri, Raju Kumar, Sanjana Sarkar, Ramaswamy Pandilla, Murali D. Bashyam. Identification of novel oncogenic targets of mutant p53 in esophageal squamous cell carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2494.