Frequent derepression of the Iroquois homeobox gene IRX3 in human acute leukemia

The Iroquois homeodomain transcription factor gene IRX3 is highly expressed in the developing nervous system, limb buds and heart. In adults, expression levels specify risk of obesity. We now report a significant functional role for IRX3 in human acute leukemia. While transcript levels are very low in normal human bone marrow cell populations, high level IRX3 expression is observed in ~30% of patients with acute myeloid leukemia (AML), ~50% of patients with T-acute lymphoblastic leukemia and ~20% of patients with B-acute lymphoblastic leukemia, typically in association with high levels of HOXA9. Expression of IRX3 alone was sufficient to immortalise murine bone marrow stem and progenitor cells, and induce T- and B-lineage leukemias in vivo with incomplete penetrance. IRX3 knockdown induced terminal differentiation of AML cells. Combined IRX3 and Hoxa9 expression in murine bone marrow stem and progenitor cells substantially enhanced the morphologic and phenotypic differentiation block of the resulting AMLs by comparison with Hoxa9-only leukemias, through suppression of a myelomonocytic program. Likewise, in cases of primary human AML, high IRX3 expression is associated with reduced myelomonocytic differentiation. Thus, tissue-inappropriate derepression of IRX3 modulates the cellular consequences of HOX gene expression to enhance differentiation block in human AML. Overall design: Murine acute myeloid leukemias - 3 samples from separate mice with AML initiated by HOXA9 and 3 samples from separate mice with AML initiated by HOXA9 and IRX3 coexpression

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Somerville TDD, Simeoni F, Chadwick JA, Williams EL, Spencer GJ, Boros K, Wirth C, Tholouli E, Byers RJ, Somervaille TCP