Lineage-inappropriate PAX5 expression in t(8;21) acute myeloid leukemia requires signaling-mediated abrogation of polycomb repression

Abstract

Key Points• Lineage-inappropriate expression of the B-cell master regulator PAX5 in t(8;21) AML depends on aberrant MAP kinase signaling.• MAP kinase signaling by a mutated growth factor receptor leads to the dissociation of polycombrepressive complexes from PAX5 chromatin.The activation of B-cell-specific genes, such as CD19 and PAX5, is a hallmark of t(8;21) acute myeloid leukemia (AML) which expresses the translocation product RUNX1/ETO. PAX5 is an important regulator of B-lymphoid development and blocks myeloid differentiation when ectopically expressed. To understand the molecular mechanism of PAX5 deregulation, we examined its chromatin structure and regulation in t(8;21) AML cells, non-t(8;21) myeloid precursor control cells, and pre-B cells. In non-t(8;21) myeloid precursors, PAX5 is poised for transcription, but is repressed by polycomb complexes. In t(8;21) AML, PAX5 is not directly activated by RUNX1/ETO, but expression requires constitutive mitogen-activated protein (MAP) kinase signaling. Using a model of t(8;21) carrying an activating KIT mutation, we demonstrate that deregulated MAP kinase signaling in t(8;21) AML abrogates the association of polycomb complexes to PAX5 and leads to aberrant gene activation. Our findings therefore suggest a novel role of activating tyrosine kinase mutations in lineage-inappropriate gene expression in AML. (Blood. 2013;122(5):759-769) IntroductionThe t(8;21) is one of the most widely studied chromosomal translocations associated with the core-binding factor (CBF) complex in human myeloid leukemia. 1 The CBF complex consists of a member of the RUNX family of transcription factors along with its non-DNA-binding partner CBFb. 2 In t(8;21) acute myeloid leukemia (AML), the N-terminal domain of RUNX1 is fused to the transcriptional repressor ETO (also called RUNX1T1 or MTG8), 3 generating the fusion protein RUNX1/ETO. RUNX1/ ETO expression initiates extensive reprogramming of the epigenome and deregulates the normal myeloid gene expression program. 4,5 Heterozygous RUNX1/ETO knock-in mice fail to establish hematopoiesis and die at embryonic day 11.5, 6 but conditional expression of the full-length RUNX1/ETO alone in precursor cells is unable to establish AML.7-10 t(8;21) AML patients present additional mutations, such as activating mutations in growth factor receptors. This includes the KIT N822K or FLT3-length mutations [FLT3-LM]) which, in cooperation with RUNX1/ETO, cause fatal leukemia in mice. 11,12 Activating growth factor receptor mutations provide a proliferative advantage to leukemic cells; however, little is known of how aberrant signaling impacts on gene expression in t(8;21) AML.One of the defining features of t(8;21) AML is a mixed lineage phenotype indicated by the coexpression of myeloid and B-cell-specific genes such as CD19 and PAX5, an expression pattern that is only rarely seen in other types of AML. [13][14][15] Within the hematopoietic system, PAX5 is normally expressed exclusively in the B-lymphoid lineage 16 where it regulates B-cell commitment by ...

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