AbstractsBiology & Animal Science

The proteasome inhibitor bortezomib remains a key component of high potency combination regimens for multiple myeloma (MM), whose primary site of inhibition includes proteasome subunit beta-5 (PSMB5). However, all MM patients inevitably develop resistance. We therefore investigated intrinsic and extrinsic mechanisms underlying resistance to bortezomib in vitro and in vivo. We investigated a bortezomib-resistant human cell line termed MM1.VDR-gfpluc (VDR) with a 12-fold increase in IC50 for bortezomib, compared to its isogenic parental cell line MM1.R-gfp-luc (termed MM.1R), that is resistant to dexamethasone. VDR also retained its resistance to dexamethasone, similar to parental MM.1R. In an in vivo SCID-beige mouse model, VDR also retained its decreased responsiveness to bortezomib. By whole exome sequencing we identified a previously documented mutation in the PSMB5 gene in VDR, in addition to a number of other mutations of interest. We subsequently examined both the genomic and proteomic profiles of MM.1R and VDR cells lines, and further explored target genes or proteins of interest. We examined the role the bone marrow microenvironment in bortezomib resistance in vitro. Finally we analysed bone marrow trephine samples from bortezomibrefractory multiple myeloma patients for their expression of proteasome-related subunits. In summary, we identified a number of known and potential novel biomarkers of bortezomib resistance in multiple myeloma, which firstly validated our model of bortezomib resistance, and secondly revealed a number of novel targets, some for which small molecule inhibitors are currently available. In addition we emphasized the pertinent role of the bone marrow microenvironment in the pathogenesis of drug resistance in multiple myeloma. Finally we measured the expression levels of PSMB5 and PSMB8 in clinical samples of patients with bortezomib-refractory myeloma, and suggested a role for the use of interferongamma and PSMB8 inhibitors concomitantly in the clinical setting for bortezomib-refractory multiple myeloma.