ABCB1 AND CHEMOTHERAPY RESISTANCE IN ACUTE MYELOID LEUKAEMIA

Abstract

Disease relapse represents the greatest barrier to the successful management of acute myeloid leukaemia (AML). It is due to the chemotherapy resistance of leukaemic cells that survive treatment and re-establish the disease. Numerous studies have shown that high levels of the drug efflux pump ABCB1 predict treatment failure in AML, but ABCB1 inhibitors have proved ineffective in clinical trials. Recent studies have identified activating ABCB1 promoter translocations in recurrent ovarian cancer, providing direct evidence for a role in clinical chemoresistance. How ABCB1 expression is established and maintained in AML is not known. In this PhD project I sought to characterise the regulation of ABCB1 in AML by identifying the enhancers that coordinate transcription and determining whether activating promoter translocations occur in relapsed disease. I also sought to define the transcriptional features of resistant blasts by comparing leukaemic cells from before and after induction chemotherapy in cases of refractory disease. Identifying and functionally validating the enhancers that control ABCB1 expression in AML revealed a stress-responsive enhancer that increased transcription in response to a variety of stressors, including daunorubicin. Targeted nanopore sequencing confirmed that the activating promoter translocations described in recurrent ovarian cancer were not a feature of ABCB1 expressing relapsed AML. Instead, ABCB1 expression appeared to be driven by its native promoters, consistent with regulation by endogenous enhancers. Prolonged stress primed enhancers for rapid increases in activity following re-exposure to daunorubicin, providing an epigenetic memory of prior drug treatment. Brief episodes of stress dramatically increased ABCB1 expression allowing leukaemic cells to escape blockade by pharmacologic doses of third-generation ABCB1 inhibitors. In primary human AML, daunorubicin exposure activated the stress-responsive enhancer and induced ABCB1 expression. Chemotherapy-induced ABCB1 upregulation could be abrogated by pharmacological inhibition of stress signalling using U0126 and ISRIB. The leukaemic blasts that survive induction chemotherapy in cases of primary refractory disease were found to share several transcriptional features; they were more proliferative than pre-treatment comparators and had upregulated a stem cell gene expression signature. FOXM1 was identified as a candidate master regulator of this resistance-associated transcriptional programme.

Date of Award	4 Feb 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Georges Lacaud (Supervisor) & Tim Somervaille (Supervisor)