The ASXL–BAP1 axis: new factors in myelopoiesis, cancer and epigenetics

The recent identification of germline and somatic mutations in BAP1 as well as in multiple members of the ASXL (additional sex combs-like) family of genes has highlighted the role of these proteins in a diverse array of biological functions. A diverse number of possible functions have previously been ascribed to ASXL1 in non-hematopoietic contexts, including physical co-operativity with HP1a and LSD1. Here we discuss new evidence for a BAP1-independent function of ASXL1 in regulating histone H3 lysine 27 methylation through interactions with the Polycomb-repressive complex 2 (PRC2). BAP1, a nuclear-localized deubiquitinase, has been shown to interact with a number of proteins, including ASXL1 and/or ASXL2, but the functional importance of this interaction has remained elusive. Here, we highlight recent work revealing the critical function of BAP1 in restricting myelopoiesis and in regulating hematopoietic stem cell function. These data provide evidence that BAP1 and ASXL1 function as a novel class of tumor suppressors in myeloid malignancies. BAP1 functions through effects on stability of host cell factor-1, and O-GlcNAcylation, and ASXL1 impacts histone post-translational modifications through interaction with PRC2. Future studies investigating the mechanism of transformation by loss of BAP1 and ASXL1 may result in new therapeutic approaches to treat hematological malignancies.