Abstract Introduction: The tumor suppressor protein p53 (p53) functions as a transcription factor that activates downstream target genes such as p21, PUMA, NOXA, BAX, and GADD45α to induce cell cycle arrest, senescence, apoptosis, and other biological processes. However, p53 activity is frequently compromised during tumorigenesis, with approximately half of human cancers harboring TP53 mutations. Even in tumors retaining wild-type p53 (wtp53), it gets degraded or inhibited through various mechanisms, including the overexpression of its E3 ubiquitin ligase MDM2. MDM2 antagonists such as Nutlin-3a are promising strategies to restore p53 function and inhibit tumor growth. Yet, factors/biomarkers regulating the efficacy of MDM2 antagonists and p53-mediated tumor suppression remain poorly understood. Identifying such factors and understanding their functions are essential for optimizing p53-targeted therapies. Experimental Procedures: Using a human whole-genome shRNA library screen, we sought to identify factors regulating p53-dependent tumor suppression in osteosarcoma U2OS cells treated with Nutlin-3a. This screening and subsequent validation revealed APOBEC4 as a critical factor for p53-mediated colony suppression following Nutlin-3a treatment. Deletion of APOBEC4 attenuated Nutlin-3a-induced p53 transcriptional activation and reduced p53-dependent cell cycle arrest and apoptosis across multiple p53-proficient cancer cell lines. To investigate the underlying mechanism, we performed co-immunoprecipitation (Co-IP) and proximity ligation assays (PLA), which revealed APOBEC4 binds to p53. Furthermore, cellular senescence induced by H2O2 and oncogenic HRASG12V, was also reduced upon APOBEC4 knockdown. Importantly, APOBEC4 deletion significantly attenuated Idasanutlin-mediated tumor suppression in vivo. Conclusion: Our findings identify APOBEC4 as a novel regulator of p53-dependent cell cycle arrest, senescence, apoptosis, and tumor suppression, highlighting its potential as a biomarker for p53-targeted therapies. Citation Format: Dipesh Thapa, Atul Ranjan, Alejandro Parrales Briones, Elizabeth Thoenen, Shigeto Nishikawa, Jay Vivian, Tomoo Iwakuma. APOBEC4, a novel regulator of p53-dependent tumor suppression abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 5958.
Thapa et al. (Fri,) studied this question.