Mantle cell lymphoma (MCL) is a mature B-cell Non-Hodgkin-Lymphoma that involves the lymphoid tissues, gastrointestinal tract, blood, and bone marrow. MCL is responsive to chemoimmunotherapy but most patients relapse within a few years. Thus MCL has a relatively short median overall survival of 5-7 years, which is amongst the shortest of all B-cell lymphomas. Bortezomib (Velcade) induces responses in 30-50% of patients with relapsed disease and is equally effective in patients sensitive or refractory to prior therapy. To investigate whether the addition of bortezomib to standard chemoimmunotherapy could improve the depth of response and extend the progression free survival we initiated a single center study combining bortezomib with the EPOCH-Rituximab regimen (trial registered as NCT00131976;lead investigator Wyndham Wilson, NCI). For the first cycle bortezomib was given as single agent on day 1, 4, 8, 11 followed by 6 cycles of combination therapy with bortezomib on day 1 and 4 of 21 day cycles. Of the first 38 patients, 63% achieved a complete response, 29% a PR and 3 (8%) did not respond. Responding patients are randomized to maintenance bortezomib for up to 18 months or observation. The trial is ongoing and we continue to collect samples for further analysis correlative studies focused on the effect of bortezomib single agent. We use gene expression profiling to systematically study the effect of proteasome inhibition on the tumor biology of MCL. We hypothesized that resistant tumors preferentially upregulate homeostatic responses to survive proteasome inhibition. Surprisingly, we found the opposite: sensitive tumors strongly upregulated anti-oxidant genes, and proteasome components, while resistant cells showed minimal gene expression changes in response to bortezomib. Furthermore, we found that increased expression of anti-oxidant genes in unstressed cells correlated with decreased sensitivity to proteasome inhibition. To further investigate the mechanisms of bortezomib resistance we generated an in vitro model of bortezomib-adapted MCL cell lines that were 40-80 fold less sensitive to bortezomib than the parental cells. These bortezomib resistant sublines showed increased proteasome activity, survived at lower proteasome capacity and showed characteristics of plasmacytic differentiation. Fresh tumor cells from MCL patients with poor clinical response to bortezomib also expressed plasmacytic features. Additional studies support the conclusion that plasmacytic differentiation in the absence of an increased secretory load can enable cells to withstand the stress of proteasome inhibition and thus identify possible targets to enhance the therapeutic efficacy of proteasome inhibitors. The clinical success of bortezomib has indicated that protein homeostasis in the endoplasmic reticulum is a valid therapeutic target for cancer treatment. We found that the ERAD inhibitor Eeyarestatin I (EerI) induces an ER stress response similar to bortezomib and can synergize with bortezomib to induce apoptosis in hematologic cancer cells. We identified a nitrofuran-containing (NFC) domain in EerI as the functional group responsible for its anti-cancer activity and showed that the cytotoxic activities of EerI are caused by the binding of the NFC-domain to p97 ATPase, an essential component of the ERAD machinery. An aromatic domain in EerI, although not required for p97 interaction, can recruit EerI to the ER membrane and thus improve its target specificity and tumor selectivity. These results reveal a bifunctional agent that induces cell death in hematologic cancer cells by preferentially inhibiting membrane-bound p97. Testing in the NCI60 panel revealed that EerI is cytotoxic to a large number of cancer cell lines with a pattern of activity similar to bortezomib. Unexpectedly, EerI was also cytotoxic to bortezomib resistant MCL cells suggesting that inhibition of ER-associated p97 may be a strategy to overcome bortezomib resistance. This work has led to two patent applications. We have now expanded this work by including additional mechanistic studies that reveal components of EerI that bind to certain intracellular structures in particular to the endoplasmic reticulum and that are crucial for targeting. We have tested EerI in a mouse xenograft model. Preliminary results suggest that the poor solubility of the drug is currently limiting in vivo applications. The translocation t11;14 that leads to expression of cyclin D1 is a key event in MCL biology. We have previously described that additional changes in the mRNA of cyclin D1 increase its oncogenic potential. We are now using deep sequencing of mRNA to further study the cyclin D1 mRNA structure and to investigate whether mRNA shortening is a general mechanism for oncogene activation and to identify additional genes important for pathogenesis of MCL. We have completed sequencing of 15 tumor samples and normal B cells from two healthy individuals. First, we identified 13,057 sites for mRNA polyadenylation representing 7,115 genes mapped to the plus strand. Of these genes 25% had 2, 14% 3 and 9% 4 polyadenylation sites, respectively. Owing to the generally repressive nature of the 3UTR, shortening of this region may give rise to a gain of function. We therefore sought to identify genes in which tumor cells display shorter mRNA transcripts than normal B cells. We identified 217 transcripts where the most proximal polyadenylation site of a gene was used by tumor but not normal cells. These genes belong to a wide range of different functional groups. Analyses into the possible role of these genes in MCL pathogenesis and into the mechanism contributing to APA are ongoing.
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