Constitutively Active SHP2 Cooperates with HoxA10 Overexpression to Induce Acute Myeloid Leukemia

The homeodomain transcription factor HoxA10 is maximally expressed in myeloid progenitor cells. Sustained HoxA10 expression during differentiation has been described in poor prognosis human acute myeloid leukemia (AML). Consistent with this, engineered overexpression of HoxA10 in murine bone marrow induces a myeloproliferative disorder that progresses to AML over time. This murine model suggests that HoxA10 overexpression is sufficient for myeloproliferation but that differentiation block, and therefore AML, requires acquisition of additional mutations. In myeloid progenitor cells, HoxA10 represses transcription of genes that encode phagocyte effector proteins such as gp91PHOX and p67PHOX. Tyrosine phosphorylation of HoxA10 during myelopoiesis decreases binding to these target genes. In immature myeloid cells, HoxA10 also activates transcription of the DUSP4 gene that encodes Mkp2, an anti-apoptotic protein. HoxA10 binding to the DUSP4 promoter decreases during myelopoiesis. Therefore, both myeloid-specific gene repression and DUSP4 activation by HoxA10 decrease during myelopoiesis. This results in phenotypic differentiation and facilitates apoptosis as differentiation proceeds. HoxA10 is de-phosphorylated by SHP2 protein-tyrosine phosphatase in myeloid progenitors. This mechanism maintains HoxA10 in a nonphosphorylated state in immature, but not differentiating, myeloid cells. Constitutively active SHP2 mutants have been described in human AML, which dephosphorylate HoxA10 throughout myelopoiesis. In this study, we hypothesize that constitutive SHP2 activation synergizes with HoxA10 overexpression to accelerate progression to AML. Because both HoxA10 overexpression and constitutive SHP2 activation are found in poor prognosis human AML, these studies contribute to understanding biochemical aspects of disease progression in myeloid malignancy.