Targeting the Redox System to Overcome Mechanisms of Drug Resistance in Chronic Lymphocytic Leukemia

Chronic Lymphocytic Leukemia (CLL) is the most common form of leukemia diagnosed in Western countries and is characterized by clonal expansion of B cells. The clinical course of CLL is diverse and nearly 50% of patients present with chromosomal abnormalities. Deletion of the short arm on chromosome 17 (del17p) occurs in 5-7% of cases and presents with the shortest median survival time and often respond poorly to therapy. The tumor suppressor gene, TP53 is located on this region and it is well established that the p53 protein regulates multiple functions including: mitochondria biogenesis, response to DNA damage and redox balance. The objectives of the study were to determine the status of mitochondria in the mouse TCL1/p53-/- CLL model, role of the enhancer of zeste homolog 2 (EZH2) in drug resistance, and whether use of Auranofin has any therapeutic potential in CLL. This study determined that TCL1/p53-/- had elevated expression of transcription factors which regulate mitochondria biogenesis and increase in mitochondria DNA copy number. Second, TCL1/p53-/- mice had significantly elevated expression of EZH2 protein relative to TCL1/p53+/+ cells while CLL patients had no distinct pattern of expression. CLL patients responded to two EZH2 inhibitors with increased Histone 3 Lysine 27 trimethylation, which is contrary to what has been published. Further studies to determine whether CLL patients carry EZH2 mutations which influence the response to these inhibitors are needed. Lastly, CLL cells showed significant apoptosis induction upon treatment with Auranofin despite stromal co-culture protection. Increased expression of Thioredoxin Reductase 1 (TxnR1), the target of Auranofin was seen in CLL cells upon stromal co-culture. High apoptotic induction in CLL and minimal cytotoxicity towards stromal cells warrant further examination of Auranofin for CLL therapy. The results of this study identified two relatively unexplored targets, EZH2 and TxnR1, in CLL. Identifying the mechanism for the contradictory response of the EZH2 inhibitors by CLL cells may reveal a new prognostic indicator/biomarker. Lastly, Auranofin may prove to be useful in treatment of CLL patients despite any chromosomal abnormalities or prior treatment, as majority of CLL cells co-cultured with stromal cells responded to Auranofin with significant apoptosis