We have refined our understanding of genetic events in myeloma and identified clinically relevant mutations that may be used to better stratify patients at presentation.

Myeloma is heterogeneous at the molecular level with subgroups of patients characterised by features of epigenetic dysregulation. Outcomes for myeloma patients have improved over the past few decades except for molecularly defined high-risk patients who continue to do badly. Novel therapeutic approaches are, therefore, required. A growing number of epigenetic inhibitors are now available including EZH2 inhibitors that are in early-stage clinical trials for treatment of haematological and other cancers with EZH2 mutations or in which overexpression has been correlated with poor outcomes. For the first time, we have identified and validated a robust and independent deleterious effect of high EZH2 expression on outcomes in myeloma patients. Using two chemically distinct small-molecule inhibitors, we demonstrate a reduction in myeloma cell proliferation with EZH2 inhibition, which leads to cell cycle arrest followed by apoptosis. This is mediated via upregulation of cyclin-dependent kinase inhibitors associated with removal of the inhibitory H3K27me3 mark at their gene loci. Our results suggest that EZH2 inhibition may be a potential therapeutic strategy for the treatment of myeloma and should be investigated in clinical studies.

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Myeloma is a malignancy of the antibody-producing plasma cells and, as such, these cells synthesize large quantities of unfolded or misfolded immunoglobulin. The build-up of excess protein triggers a number of downstream signal transduction cascades, including endoplasmic reticulum stress and autophagy. As a result, myeloma cells are uniquely reliant on these and other protein handling pathways for their survival. Strategies aimed at targeting this vulnerability have proved successful with the proteasome inhibitor, bortezomib, already licensed for clinical use. In addition to the proteasome, various other points within the protein handling pathways are also the subject of drug discovery projects, with some already progressing into clinical trials. These include compounds directed against heat shock proteins, the unfolded protein response and pathways both upstream and downstream of the proteasome.More recently, the role of autophagy has been recognized in myeloma. In this review, we discuss the various pathways used by myeloma cells for survival, with particular emphasis on the emerging role and conundrum of autophagy, as well as highlighting pre-clinical research on novel inhibitors targeting protein handling pathways.Key words: autophagy, proteasome, ER stress, heat-shock chaperones, unfolded protein response.Citation: Aronson LI and Davies FE. DangER: protein ovERload. Targeting protein degradation to treat myeloma. Haematologica 2012;97(8):1119-1130. doi:10.3324/haematol.2012 This is an open-access paper. ABSTRACT © F e r r a t a S t o r t i F o u n d a t i o nbeen extensively reviewed and so will not be covered further here. 7,8 We aim to provide an overview of the protein handling and stress response pathways. We highlight the potential points in these pathways that can be targeted therapeutically, and review the supporting pre-clinical data. Protein handling pathwaysUnder normal conditions, the synthesis, folding and degradation of cellular proteins are balanced processes. In myeloma cells, however, the protein folding capacity of the endoplasmic reticulum (ER) is overloaded by large quantities of immunoglobulin and cells must adapt to this continual stress. 9 A close relationship with common signaling nodes therefore exists between the ubiquitin proteasome pathway, cellular stress pathways such as the unfolded protein response (UPR), heat shock response and autophagy. Ubiquitin proteasome pathwayThe main cellular pathway for the removal and destruction of proteins is the ubiquitin proteasome pathway. This involves the sequential enzymatic transfer of ubiquitin monomers onto an elongating polyubiquitin chain bound at the lysine 11 or lysine 48 residues of target proteins. 10 In the first step of this cascade, an E1 activating enzyme, typically ubiquitin activating enzyme (UAE, also known as UBA1), binds ubiquitin. A second ubiquitin monomer binds to a different site on the E1 enzyme and the first ubiquitin is transferred to an E2 ubiquitin-conjugating enzyme. The final step involves the transfer of ubiq...

Myeloma is a plasma cell malignancy characterized by the overproduction of immunoglobulin, and is therefore susceptible to therapies targeting protein homeostasis. We hypothesized that heat shock factor 1 (HSF1) was an attractive therapeutic target for myeloma due to its direct regulation of transcriptional programs implicated in both protein homeostasis and the oncogenic phenotype. Here, we interrogate HSF1 as a therapeutic target in myeloma using bioinformatic, genetic, and pharmacologic means. To assess the clinical relevance of HSF1, we analyzed publicly available patient myeloma gene expression datasets. Validation of this novel target was conducted in experiments using shRNA or inhibitors of the HSF1 pathway in human myeloma cell lines and primary cells as well as in human myeloma xenograft models. Expression of HSF1 and its target genes were associated with poorer myeloma patient survival. ShRNA-mediated knockdown or pharmacologic inhibition of the HSF1 pathway with a novel chemical probe, CCT251236, or with KRIBB11, led to caspase-mediated cell death that was associated with an increase in EIF2α phosphorylation, CHOP expression and a decrease in overall protein synthesis. Importantly, both CCT251236 and KRIBB11 induced cytotoxicity in human myeloma cell lines and patient-derived primary myeloma cells with a therapeutic window over normal cells. Pharmacologic inhibition induced tumor growth inhibition and was well-tolerated in a human myeloma xenograft murine model with evidence of pharmacodynamic biomarker modulation. Taken together, our studies demonstrate the dependence of myeloma cells on HSF1 for survival and support the clinical evaluation of pharmacologic inhibitors of the HSF1 pathway in myeloma. .