HOXA9 is required for survival in human MLL-rearranged acute leukemias

Abstract

Leukemias that harbor translocations involving the mixed lineage leukemia gene (MLL) possess unique biologic characteristics and often have an unfavorable prognosis. Gene expression analyses demonstrate a distinct profile for MLL-rearranged leukemias with consistent high-level expression of select Homeobox genes, including HOXA9. Here, we investigated the effects of HOXA9 suppression in MLLrearranged and MLL-germline leukemias using RNA interference. Gene expression profiling after HOXA9 suppression demonstrated co-down-regulation of a program highly expressed in human MLL-AML and murine MLL-leukemia stem cells, including HOXA10, MEIS1, PBX3, and MEF2C. We demonstrate that HOXA9 depletion in 17 human AML/ALL cell lines (7 MLL-rearranged, 10 MLL-germline) induces proliferation arrest and apoptosis specifically in MLL-rearranged cells (P ‫؍‬ . IntroductionTranslocations involving the mixed lineage leukemia locus (MLL, All-1, HRX) on chromosome 11q23 are found in a variety of hematologic malignancies, including acute myeloid leukemias (AMLs), B-precursor and T-lineage acute lymphoblastic leukemias, and myelodysplastic syndrome. MLL rearrangements are present in most infant leukemias [1][2][3][4] and in secondary leukemias after treatment with topoisomerase inhibitors. [5][6][7][8] Infants diagnosed with lymphoblastic leukemia harboring a MLL translocation respond poorly to current chemotherapy regimens and have a particularly unfavorable prognosis with an overall survival of less than 50%. [9][10][11][12][13] The pathophysiologic mechanisms by which MLL translocations cause leukemia and the genes that serve as critical downstream targets during induction and maintenance of the leukemic phenotype are incompletely characterized. Gene expression profiling in human acute myeloid and lymphoblastic leukemias demonstrated a characteristic gene expression pattern for cases with MLL rearrangements [14][15][16] that may be driven by unique histone methylation programs. [17][18][19][20] A common unifying feature in myeloid and lymphoid leukemias with MLL rearrangements is high-level expression of Homeobox (HOX) genes with a particular emphasis on the 5Ј-HOXA genes (HOXA5-11). [14][15][16]21,22 Elevated expression of certain 5Ј-HoxA cluster genes is also found in murine leukemia models after introduction of various leukemia-associated Mll-fusion proteins. [23][24][25][26][27] In a recent murine retroviral transduction/transplantation study, we determined the gene expression profile of leukemia stem cells that were initiated by expression of Mll-Af9 in committed granulocyte macrophage progenitors (GMPs). 28 5Ј-HoxA cluster genes HoxA5, HoxA10, and in particular HoxA9 were prominent members of a gene expression signature found in leukemia stem cells and were immediately induced after Mll-Af9 expression. 28 These findings support a hierarchical model of leukemia initiation by the MLL-AF9 fusion where certain HOXA cluster genes belong to a crucial subset of proximate target genes, which are immediately activated by MLL-AF9 express...

Epigenetic regulations in hematopoietic Hox code

“…For example, Hoxa9 is highly expressed in MLL-AF9 leukemia cells, and yet Hoxa9 knockout mice still develop leukemia (Kumar et al, 2004). However, in human leukemia cell lines, Hoxa9 knockout reduces proliferation and increases apoptosis and differentiation (Faber et al, 2009). A key problem to the acceptance of Hox dysregulation as a cause rather than a consequence of cancer is the lack of an obvious relationship between the transcriptional properties of Hox genes and their oncogenic potential.…”

Interaction of retinoic acid and scl controls primitive blood development

“…Hoxa9a is a homeobox gene transcription factor known to facilitate leukemic transformation and plays a role in primitive blood cell development as regulated by cdx4. 16,33 Wild-type embryos were injected with hoxa9a mRNA and treated with RA from early gastrulation (50% epiboly) or late gastrulation (90% epiboly) until 10 ss. Again, embryos treated with RA alone had very little expression of gata1 in the posterior mesoderm ( Figure 3G), and the RA-treated embryos injected with hoxa9a mRNA had a similar low level of gata1 expression ( Figure 3H) regardless of the timing of RA exposure.…”

Entinostat Prevents Leukemia Maintenance in a Collaborating Oncogene-Dependent Model of Cytogenetically Normal Acute Myeloid Leukemia

“…In particular, altered expression of HOXA9 has been observed in a significant proportion of both human acute myeloid leukemia (AML) and acute lymphoid leukemia (ALL) and reported as the most consistent indicator of poor prognosis in refractory AML [15][16][17]. The frequent co-overexpression of HOXA9 and MEIS1, particularly in leukemias harboring MLLrearrangements [18,19], suggests a vital genetic interaction between the cofactors, which may be required for leukemia maintenance in a context-dependent manner [20][21][22][23]. Notably, expression of HOXA9 and MEIS1 is also associated with cytogenetically normal AML (CN-AML) where no major genetic aberrations have been identified [24].…”