Sequence analysis of β-subunit genes of the 20S proteasome in patients with relapsed multiple myeloma treated with bortezomib or dexamethasone

Abstract

Variations within proteasome ␤ (PSMB) genes, which encode the ␤ subunits of the 20S proteasome, may affect proteasome function, assembly, and/or binding of proteasome inhibitors. To investigate the potential association between PSMB gene variants and treatment-emergent resistance to bortezomib and/or long-term outcomes, in the present study, PSMB gene sequence variation was characterized in tumor DNA samples from patients who participated in the phase 3 Assess- IntroductionThe 20S core of the 26S proteasome degrades polyubiquitinated intracellular proteins 1 and is composed of 4 stacked rings, 1-4 each with 7 ␣ and 7 ␤ subunits. Three constitutive proteasome ␤ (PSMB) subunits, ␤5, ␤2, and ␤1 (encoded by the PSMB5, PSMB7, and PSMB6 genes, respectively), 5 are responsible for chymotrypsinlike, trypsin-like, and post-glutamyl peptide hydrolyzing activities, respectively. 6-8 On IFN-␥ stimulation, constitutive ␤ subunits are replaced by the IFN-inducible subunits ␤5i, ␤2i, and ␤1i (encoded by PSMB8, PSMB10, and PSMB9 genes, respectively) 5 to form the immunoproteasome. 6,9 Variations in PSMB subunits could potentially affect proteasome structure, assembly, function, and/or binding of proteasome inhibitors.Bortezomib (trade name Velcade; Millennium Pharmaceuticals) is approved in the United States and Europe for the treatment of patients with multiple myeloma (MM), 10,11 and in the United States for patients with relapsed mantle cell lymphoma. 10 Bortezomib selectively binds to the ␤5 subunit, leading to full inhibition of ubiquitinated protein hydrolysis. 7 In addition, bortezomib interacts with the ␤1 subunit 8 and, when bound to the ␤5 subunit in the chymotryptic catalytic site, is in close proximity to the ␤6 subunit. 12 Several studies have shown that PSMB5 variants can arise in vitro when tumor cell lines are cultured with bortezomib [13][14][15][16][17][18] ; it remains unclear whether this mechanism is relevant for bortezomib resistance in the clinical setting.The present study addressed whether variations in PSMB genes affect treatment-emergent resistance in bortezomib-treated MM patients or long-term outcome in MM patients. Sequence variation was characterized in coding regions of PSMB genes in pre-and posttreatment samples from patients who participated in the phase 3 Assessment of Proteasome Inhibition for Extending Remissions (APEX) trial of single-agent bortezomib versus high-dose dexamethasone (Dex) for the treatment of relapsed MM. 19 Study designReview boards at all participating institutions approved the (APEX) study, 19 and BM aspirates were obtained from consenting patients in accordance with the Declaration of Helsinki during the APEX trial. Tumor cells were purified and frozen for nucleic acid isolation as described previously. 20 Matching germline DNA samples were not collected. DNA samples were amplified using the QIAGEN REPLI-g whole genome amplification kit and For personal use only. on May 11, 2018. by guest www.bloodjournal.org From used for PCR reactions with primers for coding regions o...

Discovery of selective small-molecule HDAC6 inhibitor for overcoming proteasome inhibitor resistance in multiple myeloma

“…The second-generation proteasome inhibitors carfilzomib (CFZ) (2), ixazomib (3), and marizomib (4) also are showing improved pharmacological properties and promising responses. Nonetheless, MM cells develop resistance to BTZ, leading to relapse of disease in most patients (5,6).…”

Profiling human protein degradome delineates cellular responses to proteasomal inhibition and reveals a feedback mechanism in regulating proteasome homeostasis

“…Proteasomal inhibition was found to activate Nrf1- [52] or Nrf2-mediated [53] transcription of some proteasome subunits, which gave rise to the so-called "proteasome bounce-back response". Overexpression of or mutations in PSMB5 gene were also reported in BTZ-resistant cancer cells [3,54,55]. It remained unclear whether these were the only mechanisms that regulate proteasome homeostasis and lead to compensatory increase in proteasome activity upon proteasome inhibition.…”

Proteasome Subunit Beta Type 1 P11A Polymorphism Is a New Prognostic Marker in Multiple Myeloma

“…14 The minor allele frequency (37.7±4.7%) in MM patients was not different from the reported allele frequency for Caucasian reference population (40.4±7.0%) in the 1000 Genome Project or (34.8-46.0%) in the National Center for Biotechnology Information (NCBI) dbSNP databases.…”

“…To the best of our knowledge the functional consequence of PSMB1 P11A polymorphism has not been tested in MM, apart from one single study with limited number of patients showing no statistically significant association between genotype and survival. 14 …”