APOBEC3B mRNA Expression in Breast Cancer Correlates with Genomic Mutational Signatures

Despite advances in screening and treatment, breast cancer remains a leading cause of cancer-related mortality. APOBEC enzymes, particularly APOBEC3B (A3B), are upregulated in many cancers, contributing to a characteristic C-to-T mutational signature found in 30-50% of breast cancers. However, the relationship between A3B mutational signatures and A3B expression across subtypes, and the resulting potential biologic consequences, have not been fully defined. Using TCGA and ICGC datasets, we analyzed DNA and RNA expression data to assess the relationship between A3B mRNA expression and APOBEC enrichment scores. Pathway enrichment analyses (KEGG, GO, Reactome) were performed to identify biological processes associated with high A3B expression, specifically stratifying by breast cancer intrinsic subtypes (HR+/HER2-, HR+/HER2+, HR-/HER2+, and TNBC). Over 64% of tumors with enriched A3B mutational genomic signatures demonstrated above-median A3B mRNA expression (p < 0.001). High A3B-expressing tumors exhibited specific alterations in drug metabolism pathways. Notably, we observed reduced expression of CYP2D6 and CYP3A isoforms which is required for the conversion of tamoxifen to its active metabolites. Conversely, genes involved in pyrimidine metabolism, including IMPDH1, NME1, TK1, and DPYS, were downregulated in high A3B tumors. Elevated A3B expression correlates with mutational signatures and may contribute to impaired tamoxifen activation and endocrine resistance, while concurrently creating metabolic vulnerabilities to pyrimidine-based chemotherapies. Targeting A3B or exploiting these metabolic dependencies may improve therapeutic response in selected patient subsets.