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Epitranscriptomic modifications regulate gene expression and have been implicated in cancer, including breast cancer. Using the SCAN-B cohort, we analyzed 49 messenger RNA modification regulators (mRMPs) across breast cancer subtypes. In the basal subtype, we found significant overexpression of m6A readers (IGF2BP1-3), m5C regulators (NSUN5, ALYREF, YBX1, YBX2), pseudouridine [PUS1, MARS (or MetRS), RPUSD2], and RNA editing enzymes [APOBEC3A (A3A), A3G, ADAR1], all linked to poor survival. Conversely, the m6A writer METTL14 was downregulated. Our findings highlight key mRMPs as potential biomarkers and therapeutic targets, underscoring the role of RNA modifications in breast cancer progression.

RNA modification pathways are often mis-regulated in various cancers, with N6-methyladenosine (m6A) having a pivotal role in cancer progression and metastasis. Methyltransferase-like 3 (METTL3), a core component of the m6A methyltransferase complex, functions not only as an m6A writer but also promotes tumorigenesis through m6A-independent mechanisms. Here, we show that METTL3 is mislocalized to the cytoplasm in breast cancer tumors from patients, contributing to the oncogenic phenotype. Cytoplasmic METTL3 interacts with EXOC7, a key regulator of exocytosis, promoting its stabilization. Additionally, METTL3 regulates m6A-dependent alternative splicing of EXOC7. Silencing METTL3 impairs vesicle trafficking and the breast cancer secretome – effects that do not rely on its enzymatic activity but instead involve METTL3-mediated stabilization of EXOC7 and potentially other exocyst components. Furthermore, METTL3 knockdown impairs invadopodia formation, collagen matrix invasion, and focal adhesion morphology in vitro, while inhibition of METTL3 catalytic activity does not. Our findings uncover non-catalytic roles of METTL3 in regulating exocytosis and the cancer secretome.

Fibrillarin (FBL), a core component of the C/D box small nucleolar ribonucleoprotein (snoRNP) complex, catalyzes the 2′-O-methylation (Nm) of the ribose 2′-hydroxyl moiety in ribosomal RNA (rRNA). Distinct Nm patterns contribute to ribosome heterogeneity, which is linked to selective translation of oncogenes. FBL dysregulation generates an aberrant Nm signature in triple-negative breast cancer (TNBC), the most aggressive breast cancer subtype. This study investigated the role of FBL in TNBC via translation-driven mechanisms. Our findings show that FBL knockdown impairs oncogenic traits, triggers metabolic stress, and reduces the translation efficiency of oncogenes, such as metastasis-associated protein 1 ( MTA1 ), interleukin-1 receptor-associated kinase 1 ( IRAK1 ), and thymosin beta 10 ( TMSB10 ). RiboMethSeq confirmed that the rRNA Nm sites exhibited differential sensitivity to FBL depletion. Additionally, FBL knockdown led to alterations in 18S ribosome structure confirmed by SHAPE and specifically reduced RPS28 incorporation into ribosomes. Notably, silencing RPS28 also disrupted both the oncogenic phenotype and downregulated MTA1, IRAK1, and TMSB10 expression. These findings reveal a complex interplay between FBL, rRNA Nm modifications, and RPS28 in shaping oncogenic protein pools and ribosomal composition in TNBC, offering promising insights into therapeutic approaches targeting this aggressive cancer subtype.