The c-MAF/SLP76 Axis Modulates Immune Evasion and Drives Malignancy in Multiple Myeloma Via Epigenetic Reprogramming
Authors: Dai Zhenfeng , Jia Yunlu , Zhou Jianbiao , Chng Wee-Joo , Cai Zhen 
Published: December 2024

Dysregulation of lineage-affiliated transcription factor expression represents a pivotal oncogenic mechanism, with c-MAF overexpression observed in approximately 50% of multiple myeloma (MM) cases, particularly associated with the adverse t(14;16) translocation subtype and poor prognosis. Despite c-MAF's known roles in immune regulation, its specific contribution to MM immune evasion remains unclear. This study aimed to elucidate the mechanisms underlying c-MAF-mediated immune dysregulation in MM and its therapeutic implications. Here, we found that persistent activation of c-MAF in MM is associated with T cell exhaustion in the bone marrow microenvironment. Specifically, co-culture experiments revealed that c-MAF activation in MM induced the expression of exhaustion markers PD-1, LAG-3, and TIGIT in T cells. To identify c-MAF-mediated downstream effectors modulating MM immune responses, we conducted a comparative transcriptome analysis between c-MAF-activated and c-MAF-normal MM cell lines, focusing on c-MAF-dependent transcriptional signatures. Based on correlation analysis with c-MAF expression levels in the transcriptome data of MM patients from the GEO database, we narrowed down the sets of c-MAF downstream genes and c-MAF-induced genes to 13 candidates (ITGB7, AHNAK, CCND2, SLP76, SLC37A4, TXNIP, TUBB, EI24, PRKCSH and NR2C2) and screened these to identify SLP76. SLP76 is a linker protein predominantly expressed in hematopoietic cells and plays a critical role in T cell receptor signaling pathways. Knockdown of SLP76 inhibits the expression of T cell exhaustion markers PD1, LAG3, and TIGIT, significantly enhances T cell killing capacity, inhibits cell proliferation, and induces apoptosis. Notably, in c-MAF-overexpressing cell lines, knockdown of SLP76 and c-MAF both induce degradation of PDL1, accompanied by suppression of the MAPK signaling pathway, including phosphorylated ERK and p38. The use of ERK or p38 inhibitors (STE-MEK1 or SB 202190) enhances the suppression of PDL1 by SLP76 knockdown. Given MAPK's role in PDL1 transcriptional upregulation, combined inhibition of SLP76 and MAPK signaling potentiated PDL1 suppression, suggesting a potential clinical translational direction for the combination of PDL1 immunotherapy and MAPK-targeted therapy in MM. Furthermore, clinical data suggest that SLP76 selectively promotes malignancy in c-MAF-activated multiple myeloma, strongly indicating the pathological significance of the c-MAF/SLP76 axis. Using multi-omics approaches including transcriptomic analysis, ChIP-seq and ATAC-seq to explore the upstream mechanisms of c-MAF regulation of SLP76, we found that persistent activation of c-MAF induces super-enhancer formation at the SLP76 gene locus. Direct binding of c-MAF to this super-enhancer region was observed in c-MAF-activated MM cell lines, accompanied by c-MAF-dependent accumulation of the active enhancer marker H3K27ac, BRD4 and MED1. Knockdown of c-MAF reduced H3K27ac deposition, indicating that c-MAF-mediated super-enhancer remodeling establishes the c-MAF-SLP76-PDL1 axis to modulate immune responses and drive MM malignancy. In summary, we demonstrate that c-MAF plays a non-canonical role in epigenetics, which is related to the regulation of cellular immune status, and suggest that immunotherapy combined with MAPK-targeted therapy represents a novel therapeutic direction for t(14;16) multiple myeloma.