我们在前期工作中证明了蛋白酶体抑制剂确实会不可避免地导致MLL白血病细胞对其产生可逆性的耐药。肿瘤细胞对蛋白酶体抑制剂的耐药机制已有很多研究，但这些这些耐药机制只适用于多发性骨髓瘤细胞，而无法适用于MLL白血病细胞。同时，这些耐药机制也无法解释为什么肿瘤细胞会不可避免地对蛋白酶体抑制剂的产生可逆的耐药。最近的许多研究表明，肿瘤细胞群体中的持续性耐药肿瘤细胞，导致了肿瘤细胞对药物不可避免地产生可逆的耐药。在MLL白血病细胞里是否也由于存在着这一细胞亚群，从而导致了对蛋白酶体抑制剂不可避免地产生可逆性的耐药？这方面的研究目前国际上还是空白。在本项目中，我们将重点探讨蛋白酶体抑制剂为什么会不可避免地导致MLL白血病细胞对其产生可逆性的耐药，并在机制研究的基础上，研究可能的抑制耐药产生的新型治疗策略。这项工作对推进蛋白酶体抑制剂在MLL白血病治疗上的临床应用具有重要意义。

Proteasome inhibitors (PIs) significantly improve cancer outcomes, but their use is eventually followed by PI resistance and relapse. Despite the many mechanisms that have been proposed for PI resistance, how PI resistance inevitably develops is still poorly understood. Increasing evidence emphasizes the contribution of non-mutational mechanisms to the emergence of persister cancer cells which cause inevitable cancer drug resistance. Moreover, the finding that patients can regain sensitivity to PIs after a drug holiday also suggests that PI resistance could be reversible through a yet unidentified non-mutational mechanism. Here we uncovered an reversible non-mutational PI resistance mechanism that is intrinsic to MLL leukemic cells treated with PI. Upon PI treatment, MLL leukemic cells enter a slow-cycling and reversible drug-tolerant state. Mechanically, this drug-tolerant state is caused by PI-induced epigenetic reprogramming and subsequent loss of MLL function. Our findings highlight the critical role of epigenetic reprogramming in the development of PI tolerance and help to explain why resistance and relapse are inevitable after PI-based treatment and why patients can regain sensitivity to PI after a ‘drug holiday’. In this proposed project, we try to answer whether this drug tolerant state is acquired de novo by bulk cancer cells, or arising from the selection of rare preexistent drug-resistant persister cells. Through detailed mechanism investigation, we will also determine whether PI tolerance can be rescued by histone deacetylase inhibitors (HDACi). Our study will reveal whether the combination therapy using PIs and HDACi will reduce the emergence of PI-tolerant cells and therefore will be more beneficial for treating MLL leukemias than PI single-agent therapy.