Abstract Background: In breast cancer (BC), the presence of TP53 mutation is a negative prognostic factor, also predicting endocrine resistance. Still, the clinical usefulness of determining TP53 mutation status in BC is arguable. In uterine cancer, where pathologists utilize immunohistochemistry (IHC) identifying p53 protein as a surrogate for TP53 mutation status, treatment is impacted; mutated tumors are treated more aggressively. Utilizing the well-established DO-7 p53 antibody and interpretative guidelines from uterine cancers, we sought to prove the validity of IHC testing for p53 in BC. Methods: DO-7 antibody validity in BC was performed in both laboratory and clinical samples. In the laboratory, 4 BC cell lines were utilized; 2 TP53 mutated (TP53m) (MDA-231, T47D) and 2 wild-type (MCF7, ZR75). DO-7 antibody was utilized in immunofluorescence and also western blots separated into total lysate, nuclear, and cytoplasmic compartments utilizing standard techniques. In clinical studies, 70 samples, paraffin-embedded, from 57 patients, all of whom had tissue Next Generation Sequencing (NGS), were reviewed for H minimal nuclear staining in wild-type. Expectation for p53 staining in wild-type would be weak or no staining in the nucleus. Western blots showed protein expression in areas expected with increased intensity both in the nucleus and cytoplasm in TP53m. Clinical data: 57 high-risk patients, 16 neoadjuvant, 41 metastatic with 24 TP53m. Of the 49 ER+ specimens, 37% were TP53m. NGS, 31/70 tissue samples were TP53m. Concordance between IHC and NGS was 94% (66/70) with 100% specificity, 87% sensitivity for TP53m detection (see Table). In the 31 samples with IHC positive TP53m, the most observed pattern was diffuse nuclear (14/31), null/complete absence (8/31), cytoplasmic (5/31). There were 4 false-negative results. Conclusion: Our study suggests that IHC utilizing DO-7 antibody in BC specimens reflects mutation status by NGS with high sensitivity and 100% specificity. A larger testing series is planned for validation of these preliminary findings. The utilization of IHC with high specificity may impact treatment choices for BC patients, particularly for those with ER+ breast cancer in the early stage setting where NGS is not routinely performed and knowledge of endocrine sensitivity is critical. Citation Format: J. Van Allen, A. Alvarez Soto, P. Hegde, M. Sanders, K. Claffey, B. Malowitz, S. Tannenbaum. Validity of the p53 antibody DO-7 as a surrogate for predicting TP53 mutation status in breast cancer abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS2-09-29.
Allen et al. (2026) studied this question.