Roles of the non-clustered Homeobox gene H2.0-like Homeobox (HLX) in hematopoiesis and acute myeloid leukemia

Abstract

Acute myeloid leukemia (AML) is an aggressive disease associated with poor clinical outcome. Experimental evidence suggests that AML develops from a series of aberrant molecular changes originating in immature hematopoietic stem and progenitor cells (HSPC) that promote the transition of HSC to pre-leukemic stem cells (pre-LSC) and eventually to fully transformed leukemia stem cells (LSC).;When we analyzed purified pre-leukemic HSPC in a murine AML model, we found 4-fold upregulation of H2.0-like homeobox (Hlx). We find that HLX is 2 to 16 fold overexpressed in more than 80% of patients with AML, across all major disease subtypes, and that higher levels of HLX are associated with poor overall survival in AML. Strikingly, we find that inhibition of HLX has a significant anti-leukemic and differentiation-inducing effect, suggesting HLX and its downstream targets as novel therapeutic targets in AML.;Our findings indicate HLX levels are important for maintenance of HSC as well as for myelomonocytic differentiation. We found that overexpression of HLX caused a loss of phenotypic HSC, and persistence of an aberrant progenitor population. We generated knock-in mice conditionally overexpressing Hlx in hematopoietic cells. We found that HLX overexpression in this in vivo model disrupted healthy myeloid differentiation, particularly the development of granulocytic cell types and Hlx knock-in HSC acquired the ability to serially re-plate indefinitely in methylcellulose colony assays. Using gene expression profiling, we determined that HLX regulates a network of genes important for lineage commitment and myeloid differentiation of HSPC, including PAK1, a gene downstream of both HLX overexpression in HSC and knock-down in AML cells, and part of an HLX-dependent prognostic signature in AML patients.;While overexpression of HLX promoted the differentiation block characteristic of AML, HLX overexpression alone did not appear to induce development of overt leukemia in our mouse models, indicating a requirement for cooperating factors in the development of AML. Correlative analyses suggested that HLX and mutant FLT3 functionally cooperate in AML. Strikingly, compound transgenic animals expressing both elevated HLX and FLT3-ITD developed AML after a latency of 2 months. Ongoing studies aim to further understand the basis for the cooperativity of HLX and FLT3-ITD.