ASXL2 is essential for haematopoiesis and acts as a haploinsufficient tumour suppressor in leukemia

Additional sex combs-like (ASXL) proteins are mammalian homologues of additional sex combs (Asx), a regulator of trithorax and polycomb function in Drosophila. While there has been great interest in ASXL1 due to its frequent mutation in leukemia, little is known about its paralog ASXL2, which is frequently mutated in acute myeloid leukemia patients bearing the RUNX1-RUNX1T1 (AML1-ETO) fusion. Here we report that ASXL2 is required for normal haematopoiesis with distinct, non-overlapping effects from ASXL1 and acts as a haploinsufficient tumour suppressor. While Asxl2 was required for normal haematopoietic stem cell self-renewal, Asxl2 loss promoted AML1-ETO leukemogenesis. Moreover, ASXL2 target genes strongly overlapped with those of RUNX1 and AML1-ETO and ASXL2 loss was associated with increased chromatin accessibility at putative enhancers of key leukemogenic loci. These data reveal that Asxl2 is a critical regulator of haematopoiesis and mediates transcriptional effects that promote leukemogenesis driven by AML1-ETO

EZH2 in normal and malignant hematopoiesis

The histone methyltransferase Enhancer of Zeste Homologue 2 (EZH2), a component of the polycomb group complex, is vital for stem cell development, including hematopoiesis. Its primary function, to deposit the histone mark H3K27me3, promotes transcriptional repression. The activity of EZH2 influences cell fate regulation, namely the balance between self-renewal and differentiation. The contribution of aberrant EZH2 expression to tumorigenesis by directing cells toward a cancer stem cell (CSC) state is increasingly recognized. However, its role in hematological malignancies is complex. Point mutations, resulting in gain-of-function, and inactivating mutations, reported in lymphoma and leukemia, respectively, suggest that EZH2 may serve a dual purpose as an oncogene and tumor-suppressor gene. The reduction of CSC self-renewal via EZH2 inhibition offers a potentially attractive therapeutic approach to counter the aberrant activation found in lymphoma and leukemia. The discovery of small molecules that specifically inhibit EZH2 raises the exciting possibility of exploiting the oncogenic addiction of tumor cells toward this protein. However, interference with the tumor-suppressor role of wild-type EZH2 must be avoided. This review examines the role of EZH2 in normal and malignant hematopoiesis and recent developments in harnessing the therapeutic potential of EZH2 inhibition