Abstract 1558: Deciphering the role of ribosomal paralogs RPL39/RPL39L in metaplastic breast cancer

Abstract Metaplastic breast cancer (MpBC) is a rare, highly aggressive, and metastatic malignancy that accounts for 5% of all breast cancer diagnoses. Most MpBCs display a triple-negative breast cancer (TNBC) phenotype (ER-/PR-/HER2-). MpBCs exhibit the most dismal prognosis of all breast cancer subtypes. The standard of care for metastatic MpBC remains systemic chemotherapy, despite known resistance to most cytotoxic drugs. MpBCs are enriched with epithelial-to-mesenchymal transition (EMT) and cancer stem cell (CSC) markers. We previously described a unique molecular signature for breast cancer stem cells (BCSCs) derived from patient biopsies and identified ribosomal protein L39 (RPL39) as a critical driver of treatment resistance, BCSCs self-renewal, and lung metastases in TNBC/MpBC. Growing evidence has also demonstrated aberrant expression of the testis-specific ribosomal protein paralog 39L (RPL39L) in several cancer types. Here, we hypothesize that RPL39 and its paralog RPL39L alter ribosome composition and function to initiate a specific translational landscape that promotes tumor growth and enhanced metastatic potential in MpBC. We first examined the relative expression of RPL39 and RPL39L in TNBC/MpBC cell lines and patient-derived xenografts (PDXs) using real-time qPCR. We detected co-expression of RPL39/RPL39L transcripts, however, RPL39L mRNA levels were significantly lower than RPL39. These results suggest that both RPL39 and RPL39L contribute to ribosome heterogeneity in TNBC/MpBC. To investigate the function of RPL39/RPL39L transcripts, we designed a small interfering RNA (siRNA) -strategy targeting their differential 5' untranslated region (5’ UTR). Selective downregulation of RPL39 and RPL39L mRNA inhibited in vitro cell proliferation in TNBC/MpBC models. Notably, combined targeted silencing of both RPL39 and RPL39L further decreased cell proliferation in comparison to individual siRNAs. We next evaluated the efficacy of RPL39/RPL39L dual blockade in vivo using MpBC PDXs. MpBC PDX4664 was implanted into the mammary fat pad of NSG mice. Once tumors were established (100-150 mm3), mice received phosphate buffered saline (PBS controls) or 0. 5 mg/kg RPL39/RPL39L siRNAs encapsulated into liposome-like nanoparticles (NPs) via tail vain (i. v. ) injection. Animals were treated thrice a week for 21 days and sacrificed 48 h after the last injection. We found that siRNAs-NP formulation silenced RPL39/RPL39L expression in vivo and significantly reduced tumor growth. Overall, our findings support the notion that RPL39/RPL39L ribosome heterogeneity may confer preferential translation of oncogenic and BCSC-related genes, facilitating tumor progression and metastasis in MpBC. Ongoing studies are directed to validate ribosome heterogeneity by proteomic analysis and evaluate the role of RPL39/RPL39L on global translation by ribosome profiling. Citation Format: Maria F. Chervo, Main li, Chiara Mancino, Karina A. Ortega Martinez, Vrutant V. Shah, Wei Qian, Jianying Zhou, Liliana Guzman-Rojas, Polina Matre, Francesca Taraballi, Kristopher W. Brannan, Jenny C. Chang. Deciphering the role of ribosomal paralogs RPL39/RPL39L in metaplastic breast cancer abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 1558.