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Abstract Acute promyelocytic leukemia (APL) is characterised by the t(15;17)(q22;q21) leading to fusion of PML to the gene encoding the myeloid transcription factor Retinoic Acid Receptor α (RARα). Chromosomal translocations such as the t(15;17) are considered to be initiating events in leukemogenesis; however, sequencing of APL genomes has provided further evidence that the PML-RARα fusion is insufficient to induce leukemia, which depends upon the acquisition of cooperating mutations. The PML-RARα oncoprotein exerts a profound effect on nuclear architecture, disrupting multiprotein structures known as PML nuclear bodies (NBs). The function of these structures remains an enigma; however, their disruption in PML-RARα+ APL and acute lymphoblastic leukemia with the t(9;15)(p13;q24)/PAX5-PML fusion is associated with delocalisation of a number of component proteins including PML, which have been implicated in growth control and neoplastic transformation. It is now established that the PML moiety contributes to APL pathogenesis by conferring via the translocation a novel dimerisation capacity to RARα, but it has been unclear whether deregulation of PML and other NB components cooperates in leukemic transformation or impacts the response to differentiating agents. To address these questions, we generated a knock-in mouse model with targeted NB disruption achieved through mutation of key zinc-binding cysteine residues in the amino-terminal RING domain of Pml. Homozygous Pml RING mutant mice are viable, with no overt developmental defect; however, analysis of the bone marrow revealed significant expansion of the Lin(-)Sca-1(+)c-Kit(+) (LSK) population compared to wild type (WT) controls (p

The INK4/ARF locus regulates senescence and is frequently altered in cancer. In normal cells, the INK4/ARF locus is found silenced by Polycomb repressive complexes (PRCs). Which are the mechanisms responsible for the recruitment of PRCs to INK4/ARF and their other target genes remains unclear. In a genetic screen for transcription factors regulating senescence, we identified the homeodomain-containing protein HLX1 (H2.0-like homeobox 1). Expression of HLX1 extends cellular lifespan and blunts oncogene-induced senescence. Using quantitative proteomics, we identified p16INK4a as the key target mediating the effects of HLX1 in senescence. HLX1 represses p16INK4a transcription by recruiting PRCs and HDAC1. This mechanism has broader implications, as HLX1 also regulates a subset of PRC targets besides p16INK4a. Finally, sampling members of the Homeobox family, we identified multiple genes with ability to repress p16INK4a. Among them, we found HOXA9 (Homeobox A9), a putative oncogene in leukaemia, which also recruits PRCs and HDAC1 to regulate p16INK4a. Our results reveal an unexpected and conserved interplay between homeodomain-containing proteins and PRCs with implications in senescence, development and cancer. [ABSTRACT FROM AUTHOR]

Acute myeloid leukaemia (AML) is a highly heterogenous blood cancer, in which the expansion of aberrant myeloid blood cells interferes with the generation and function of normal blood cells. Although key driver mutations and their associated inhibitors have been identified in the last decade, they have not been fully translated into better survival rates for AML patients, which remain dismal. In addition to DNA mutation, studies in mouse models strongly suggest that the cell of origin, where the driver mutation (such as MLL fusions) occurs, emerges as an additional factor that determines the treatment outcome in AML. To investigate its functional relevance in human disease, we have recently reported that AML driven by MLL fusions can transform immunophenotypically and functionally distinctive human hematopoietic stem cells (HSCs) or myeloid progenitors resulting in immunophenotypically indistinguishable human AML. Intriguingly, these cells display differential treatment sensitivities to current treatments, attesting the cell of origin as an important determinant governing treatment outcome for AML. To further facilitate this line of investigation, here we describe a comprehensive disease modelling protocol using human primary haematopoietic cells, which covers all the key steps, from the isolation of immunophenotypically defined human primary haematopoietic stem and progenitor populations, to oncogene transfer via viral transduction, the in vitro liquid culture assay, and finally the xenotransplantation into immunocompromised mice., Competing Interests: Competing interestsThe authors declare no conflict of interest., (Copyright © 2021 The Authors; exclusive licensee Bio-protocol LLC.)