Mechanisms of Glucocorticoid Resistance in Pediatric Acute Lymphoblastic Leukemia

Cancer is the leading disease-related cause of death in children in the United States. The most commonly diagnosed cancer in children is acute lymphoblastic leukemia (ALL) accounting for approximately 25% of all pediatric cancers which translates to about 3,000 new cases per year in the United States. Despite vast improvements in the treatment of newly diagnosed ALL, up to 20% of patients relapse and overall survival rates have plateaued at 25-40% even with the use of aggressive salvage therapy. Recurrent disease accounts for more cancer related death than any other childhood malignancy. Therefore, there is a need for novel therapies in the treatment of relapsed ALL. Glucocorticoid (GC) resistance is a hallmark of relapsed ALL and a major hurdle in the treatment of relapsed ALL. We have taken a two pronged approach to identify mediators of prednisolone sensitivity.;The first approach was to perform a genome-scale shRNA screen to identify mediators of prednisolone sensitivity in ALL cell lines. The integration of this data with our previously established relapsed genomic landscape allowed us to identify the mitogen activated protein kinase pathway (MAPK) as a mediator of prednisolone resistance in pediatric ALL. We show that knockdown of MAPK pathway members, MEK2 and MEK4 increased sensitivity to prednisolone through distinct mechanisms. MEK4 knockdown increased sensitivity specifically to prednisolone by increasing the levels of the glucocorticoid receptor (GR). MEK2 knockdown increased sensitivity to all agents tested by increasing the levels of p53. Furthermore, we demonstrated that inhibition of MEK1/2 with trametinib significantly increased sensitivity of leukemic cells to chemotherapy. Finally, we show MEK1/2 target ERK has increased activation via phosphorylation at relapse in primary ALL samples. Altogether, this data indicates the MAPK pathway as a therapeutic target and a driver of relapse in pediatric ALL.;The second approach was to characterize the role of recurrently deleted gene, TBL1XR1, in prednisolone resistance in relapsed ALL. TBL1XR1 encodes for an F-box like protein responsible for regulating the nuclear hormone repressor (NCoR) complex stability. We modeled TBL1XR1 deletions in B-precursor ALL cell lines and show TBL1XR1 knockdown results in reduced glucocorticoid receptor recruitment to glucocorticoid responsive genes, and ultimately decreased glucocorticoid signaling caused by increased levels of NCoR1 and HDAC3. Reduction in glucocorticoid signaling in TBL1XR1 depleted lines resulted in resistance to glucocorticoid agonists, but not to other chemotherapeutic agents. Importantly, we show that treatment with the HDAC inhibitor SAHA restores sensitivity to prednisolone in TBL1XR1 depleted cells. Altogether, these data indicates that loss of TBL1XR1 is a novel driver of glucocorticoid-resistance in ALL and that epigenetic therapy may have future application in restoring drug sensitivity at relapse. Through our two complementary approaches we have identified mechanisms of resistance to GC agonists and potential therapeutic approaches to combat GC resistance in pediatric ALL.