We generated eight multiple myeloma cell lines resistant to bortezomib; five acquired PSMB5 mutations. In 1,500 patients such mutations were rare clinically. To better understand disruption of proteasomes on multiple myeloma viability and drug sensitivity, we systematically deleted the major proteasome catalytic subunits. Multiple myeloma cells without PSMB5 were viable. Drug-resistant, PSMB5-mutated cell lines were resensitized to bortezomib by PSMB5 deletion, implying PSMB5 mutation is activating in its drug resistance function. In contrast, PSMB6 knockout was lethal to multiple myeloma cell lines. Depleting PSMB6 prevented splicing of the major catalytic subunits PSMB5, PSMB7, PSMB8, and PSMB10; however, PSMB6 engineered without splicing function or catalytic activity, also restored viability, inferring the contribution of PSMB6 to proteasome structure to be more important than functional activity. Supporting this, bortezomib sensitivity was restored in drug-resistant multiple myeloma cell lines by low level expression of mutated PSMB6 lacking splicing function. Loss of PSMB8 and PSMB9 was neither lethal nor restored bortezomib sensitivity. Significant codependency of PSMB5, PSMB6, and PSMB7 expression was observed. We demonstrated elevated levels of PSMB6 and 7, but not 8 and 9, in some, but not all, serial patient samples exposed to proteasome inhibitors. In summary, we show PSMB6 and PSMB7, but not PSMB5, to be essential for multiple myeloma cell survival, this dependency is structural and that upregulation or activating mutation of PSMB5, 6, and 7 confers proteasome inhibitor resistance, while depletion confers sensitivity. Implications: These findings support modulation of PSMB5, PSMB6, or PSMB7 expression as a new therapeutic strategy.

中文翻译：

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蛋白酶体亚基差异控制骨髓瘤细胞活力和蛋白酶体抑制剂敏感性

我们产生了八种对硼替佐米耐药的多发性骨髓瘤细胞系；五个获得性 PSMB5 突变。在 1,500 名患者中，这种突变在临床上是罕见的。为了更好地了解蛋白酶体对多发性骨髓瘤生存能力和药物敏感性的破坏，我们系统地删除了主要的蛋白酶体催化亚基。没有 PSMB5 的多发性骨髓瘤细胞是有活力的。通过 PSMB5 缺失，耐药的 PSMB5 突变细胞系对硼替佐米重新敏感，这意味着 PSMB5 突变正在激活其耐药功能。相比之下，PSMB6 敲除对多发性骨髓瘤细胞系是致命的。消耗 PSMB6 阻止了主要催化亚基 PSMB5、PSMB7、PSMB8 和 PSMB10 的剪接；然而，没有剪接功能或催化活性的 PSMB6 也恢复了活力，推断 PSMB6 对蛋白酶体结构的贡献比功能活性更重要。支持这一点的是，通过缺乏剪接功能的突变 PSMB6 的低水平表达，在耐药性多发性骨髓瘤细胞系中恢复了硼替佐米敏感性。PSMB8 和 PSMB9 的丢失既不致命也不恢复硼替佐米敏感性。观察到 PSMB5、PSMB6 和 PSMB7 表达的显着共依赖性。我们在一些（但不是全部）暴露于蛋白酶体抑制剂的系列患者样本中证明了 PSMB6 和 7 的水平升高，但不是 8 和 9。总之，我们表明 PSMB6 和 PSMB7，但不是 PSMB5，对多发性骨髓瘤细胞存活至关重要，这种依赖性是结构性的，PSMB5、6 和 7 的上调或激活突变赋予蛋白酶体抑制剂抗性，而消耗赋予敏感性。