Deciphering the role of APOBEC3B in t(4;14) multiple myeloma

Abstract

Multiple myeloma (MM) is a genetically heterogeneous plasma cell malignancy characterized by recurrent chromosomal abnormalities and progressive genomic instability. Among these, the t(4;14)(p16;q32) defines a high-risk molecular subtype associated with poor clinical outcomes despite advances in therapy. This translocation leads to dysregulated expression of fibroblast growth factor receptor 3 (FGFR3) and nuclear receptor-binding SET domain protein 2 (NSD2), promoting oncogenic signaling, enhanced proliferation, and aggressive disease behavior. In parallel, members of the apolipoprotein B mRNA-editing catalytic polypeptide-like (APOBEC) family of cytidine deaminases are recognized as major contributes to mutagenesis and genomic evolution across multiple cancers, including MM, where APOBEC-associated mutational processes are particularly enriched in high-risk molecular subtypes such as t(4;14). However, the precise role of APOBEC in t(4;14) MM remains incompletely understood. This thesis shows that inhibition of FGFR3 and NSD2 signaling reduced APOBEC3A (A3A) and APOBEC3B (A3B) expression and suppressed major survival pathways, including PI3K-AKT, MAPK-ERK, NF-κB, and STAT3 signaling, suggesting that A3A and A3B operate downstream of core t(4;14)-associated oncogenic networks. Analysis of two independent patient cohorts demonstrated that elevated A3B expression was consistently associated with inferior overall survival and progression-free survival in patients with t(4;14) MM. In contrast, APOBEC mutational signatures, including APOBEC-associated YTCA and RTCA signatures, were not significantly associated with patient outcome, suggesting that the prognostic impact of A3B may not be fully explained by APOBEC-mediated mutagenesis. Transcriptomic analyses revealed that high A3B expression strongly correlated with activation of cell-cycle pathways. Functional in vitro investigations using A3B knockdown and overexpression models support a role for A3B in proliferation and cell-cycle progression in t(4;14) MM cell lines. Loss of A3B reduced cell growth, impaired S-phase progression, and downregulated key regulators of G1/S transition, whereas A3B overexpression enhanced proliferative capacity. Mechanistically, A3B knockdown resulted in increased accumulation of R-loops. Elevated R-loop burden following A3B suppression was accompanied by reduced c-Myc protein stability and diminished expression of downstream c-Myc target genes involved in cell-cycle progression. These findings support a previously underappreciated mutation-independent role for A3B in contributing to R-loop homeostasis and altered c-Myc-associated signaling in cancer cells.