Translation Inhibition through eIF4A1 Targeting in Multiple Myeloma Is a Promising Therapeutic Strategy

Multiple Myeloma (MM) is a plasma cell (PC) disorder without definitive treatment and associated with multiple relapses. There is therefore an urgent need to develop innovative therapeutic strategies to overcome drug resistance. mRNA translation deregulation in involved in cancer progression by promoting aberrant proliferation or cell survival, and several translation factors such as eIF4A1 are deregulated in cancer, pointing to an addiction to high protein synthesis. In this context, inhibitors of eIF4A1-mediated translation have been developed and have shown anti-tumor effects in several models of hematological malignancies. Targeting translation in MM with eIF4A1 inhibition thus represent a potential treatment strategy in MM.

To evaluate the therapeutic interest of targeting eIF4A1 in MM, we analyzed its expression in Bone Marrow Plasma Cells (BMPC, n = 22), Monoclonal Gammopathy of Unknown Significance cells (MGUS, n = 44), Smoldering MM Cells (SMMC, n = 12), MM cells (MMC, n = 345), and Human Myeloma Cell Lines (HMCLs, n = 42). We found that eIF4A1 expression increased throughout the evolution of the disease with a highest expression in HMCLs, and was significantly upregulated in the molecular subgroups Proliferation and Hyperdiploid (UAMS, TT2 cohort). Importantly, eIF4A1 high expression was associated with a poor prognosis in two independent cohorts of MM, a cohort of newly diagnosed patients treated by high dose therapy and autologous stem cell transplantation (UAMS, TT2 cohort, n = 345) and a cohort of patients at relapse treated with Proteasome Inhibitors (Mulligan, n = 188). Gene Set Enrichment Analysis (GSEA) of genes expressed in eIF4A1^high patients compared to eIF4A1^low patients revealed an enrichment in pathways related to RNA metabolism, DNA repair, epigenetic regulation and plasmablastic signature.

We investigated the impact of eIF4A1 inhibition using Rocaglamide (Roca) in HMCLs. Roca was highly cytotoxic for 13 HMCLs with an IC50 < 20nM, led to cell cycle deregulation and to the downregulation of MMSET, MYC, IKAROS and EZH2 expression. Importantly, using a co-culture of primary MM cells with normal cells from the bone marrow micro-environment, we found that Roca was cytotoxic for malignant PCs but not bone marrow cells (n = 5). To deeper characterize eIF4A1 targets in MM cells, we performed Polysome Profiling to measure translational efficiency depending on the level of ribosomes associated with individual mRNAs. Roca induced a strong increase in mRNAs with one ribosome (monosome) and a decrease in mRNAs with polysomes. Sequencing of mRNAs from the light fraction (monosomes and disomes) and mRNAs from the heavy fraction (> 2 ribosomes) identified 567 genes significantly enriched in the light fraction of Roca-treated cells compared to control cells (Fold change > 1.5; p value < 0.05). GSEA unveiled an enrichment for pathways such as MYC targets, cell cycle, DNA damage and of high interest, EZH2 targets and TP53 transcriptional regulation. More precisely, translation efficiency of genes frequently associated with genomic abnormalities in MM such as MYC, TP53, or CTNNB1 was significantly decreased. We also identified several genes related to MM pathophysiology, including RECQL involved in replication stress survival and resistance to chemotherapy in MM (Viziteu et al, 2017), and EED, which interacts with EZH2, whose targeting could overcome the resistance to IMIDs and increase anti-CD38 efficacy in MM (Herviou et al, 2018; Chemlal et al, in press).

We investigated the link between Roca sensitivity and the response of HMCLs to MM drugs, and found that higher resistance to Bortezomib was associated with higher sensitivity to Roca (n = 10, r = -0.6, p value < 0.05). Interestingly, we found a synergy between Roca and Bortezomib, suggesting that Roca could be used to overcome Bortezomib resistance. Moreover, since Roca affects IKAROS expression, a target of Immunomodulatory Drugs (IMIDs), we investigated the therapeutic interest to combine Roca with the IMID Lenalidomide. A synergy between Roca and Lenalidomide was identified in HMCLs, suggesting that the downregulation of MM oncoproteins with Roca could be used to increase the efficacy of IMIDs.

In conclusion, Rocaglamide combination with IMIDs or other targeted therapies, such as EZH2 inhibitors, could be of clinical interest and represents a promising strategy to improve the outcome of MM patients.