PSMB4 and PSMD4 Are Correlated with 1q21 Amplification in CD138 + Plasma Cells: New Potential Druggable Targets in Myeloma Patients

Abstract The amplification of the 1q21 (amp1q21) region is one of the most acquired cytogenetic abnormalities (CA) in multiple myeloma (MM) associated with a worse patient outcome and disease progression. Moreover, different studies have demonstrated that the number of copies (CN) 1q21 (gain1q21: three copies or amp1q21: ≥ four copies) have a different impact in the response to anti-MM therapies. Particularly, it has been proposed that in MM patients, additional copies of 1q21 may be associated with the resistance to proteasome inhibitor (PI) treatment as bortezomib. A recent study showed that newly diagnosed MM (MMD) patients carrying amp1q21 but not gain1q21 receiving carfilzomib-based treatment have an early disease progression with shorter overall survival. Previous studies underlined that the amplification of 1q21 can lead to the overexpression and/or dysregulation of several candidate genes associated with cell proliferation, apoptosis, and drug resistance. Here we aim to identify 1q21 target genes possibly correlated to the response to PI therapy. We evaluated a total cohort of 29 primary plasma cells (PCs) purified from bone marrow (BM) blood aspirates from 11 smoldering MM (SMM) and 18 MMD. The median age of our cohort was 70 years (range: 38-86). Fluorescence in situ hybridization (FISH) analysis was performed to access the presence or absence of copy number alteration (CNA) in the 1q21 region in all patients. 14 out of 29 patients carried 1q21 CNA (5 with gain1q21 and 9 with amp1q21). A score reflecting the number of 1q21 copies was calculated based on the hybridization pattern. The transcriptional profiles of the 29 BM PCs samples were generated on GeneChip ClariomD Arrays (Affymetrix Inc., Santa Clara, CA, USA). The samr package was used in R for call genes as differentially expressed between 1q21 CN-altered and wild-type samples. The correlation between the 1q21 copy number score and the gene expression levels was performed. Moreover, we have evaluated by FISH the 1q21 CNA in a panel of human myeloma cell lines (HMCLs): OCY-MY5, JJN3, RPMI-8226, NCI-H929, and OPM2. JJN3 were transfected with a control vector and PSMB4 and PSMD4 short hairpin RNA (shRNA) lentivectors. The gene and protein expression levels of PSMB4 and PSMD4 in MM cell lines were analyzed by qRT-PCR and Western Blot, respectively. Cell viability and proliferation were assessed using MTT assay and flow cytometry. Our bioinformatic analyses highlighted the overexpression of different genes (IL6R, ILF2, BCL9, MCL1, CSk1B, ADAR1, ARNT, ANP32E) in the 1q21 CNA samples with respect to the controls, as already reported in the literature. Our analysis showed a significantly higher expression of two proteasome subunits (PSMB4 and PSMD4) in patients with 1q21 CNA when compared with patients without (PSMB4 p=0.0006; PSMD4 p=<0.0001). Patients with amp1q21 showed a higher expression of PSMB4 when compared to the patients with gain1q21 (p=0,007). In our cohort, gene expression profile analysis also showed a strong positive correlation between gene expression levels and 1q21 CN for the proteasome subunits PSMB4 (p=<0.0001, r=0.5631) and PSMD4 (p=<0.0001, r=0.6391). Interestingly, we found that the PSMB4 and PSMD4 expression level was independent of the disease stage (SMM vs MM) and was only driven by 1q21 CN. We have evaluated PSMB4 and PSMD4 mRNA and protein expression levels in a 1q21 wild-type cell line (OCY-MY5) and in a panel of MM cell lines carrying different degrees of 1q21 CN (in order: JJN3, U266, RPMI-8226, OPM2, and NCI-H929). The mRNA expression level of PSMB4 and PSMD4 was higher in cell lines carrying 1q21 amp, following a 1q21 copy number fashion. Similar results were obtained when protein levels of MM cell lines were analyzed by Western Blot. To further determine the potential role of both proteasome subunits in the pathogenesis of amp1q21, we generated a PSMB4-shRNA and PSMD4-shRNA knockdown stable MM cell lines. Functional studies showed that blockade of PSMB4 and PSMD4 decreased MM cell viability. In conclusion, our study identified proteasome subunits PSMB4 and PSMD4 to be significantly upregulated in MM patients carrying amp1q21, correlated with 1q21 copy number but not with disease stage. In addition, knockdown of both, PSMB4 and PSMD4 decreased MM cell proliferation. Therefore, targeting PSMB4 and PSMD4 could be a strategy to treat MM patients with ampq21 Disclosures Giuliani: Celgene: Membership on an entity's Board of Directors or advisory committees, Other: congress, Research Funding; Bristol Mayers Squibb: Other: congress; GSK: Other: clinical studies; Takeda: Membership on an entity's Board of Directors or advisory committees; Janssen: Membership on an entity's Board of Directors or advisory committees, Other: Clinical studies, congress, Research Funding; Millenium Pharmaceutical: Other: clincial studies.