HOXB9 Is Overexpressed in Blast Cells from a Subset of Acute Myeloid Leukemia Patients and Supports Proliferation of AML Cell Lines.

Emerging evidence from studies in mice and humans suggests that HOX genes integrate the myelodysplastic and leukemogenic activities of different oncogenes. Translocations involving the MLL gene, a major upstream regulator of the HOXA cluster, have been identified in AML and define a specific cytogenetic subgroup of this disease. Recently, amplification of MLL was found in AML and MDS cases, revealing a second mechanism of deregulation of MLL and its downstream targets. However, additional leukemogenic upstream pathways have to be postulated, since upregulation of specific HOX genes, particularly HOXA9, is more frequent than MLL deregulation. Given the fact that HOXA9 is targeted by several yet to be identified mechanisms, we asked whether other members of the HOX9 paralog group are also highly expressed in myeloid malignancies. Our analysis of 449 AML patients by gene expression microarray revealed that besides HOXA9 (61% of all cases), HOXB9 is upregulated in 15% of the cases, whereas the overexpression of HOXC9 and HOXD9 expression is a very rare event (>1% of all cases). High levels of HOXB9 expression did not correlate with low levels of HOXA9 expression ruling out the possibility that HOXB9 compensates for a lack of HOXA9 activity in the leukemogenesis of a HOXA9 negative AML cases. Interestingly, within the cytogenetically defined subgroups (inv(16), t(15;17), t(8;21)) that have no HOXA9 expression at all, HOXB9 was also low or absent. To test if HOXB9 has the potential to contribute to the leukemogenic phenotype we analyzed a panel of human AML cell lines. The finding that 6 out of 24 cell lines express high levels of HOXB9 further indicates the significance of HOXB9 in myeloid malignancies. We chose K-562 and HEL as model cell lines since they express high levels of HOXB9, but lack HOXA9 expression. To reveal HOXB9 dependent pathways we designed two highly efficient siRNAs to target HOXB9 and knocked down its expression by retroviral transduction. In comparison to cells expressing a control siRNA, the resulting cell lines showed <5% of the control HOXB9 protein levels and a reduction in the cellular growth rate. Consequently, HOXB9 expression is required for the rapid growth of these AML cells. To gain insight into the molecular mechanisms underlying this phenotype we are performing global gene expression analysis. Preliminary results of our nearest neighbor analysis in K-562 cells revealed the upregulation of genes associated with erythroid differentiation upon HOXB9 knockdown. Our results indicate that HOXB9 expression inhibits the differentiation of leukemic myeloid progenitor cells, consistent with a role for the aberrant expression of this major HOX protein in leukemogenesis, by maintaining cells of the leukemic clone in an undifferentiated and rapidly proliferative state.