MLL (mixed lineage leukemia) gene rearrangements account for about 10% of human acute leukemias, including ∼80% of infant acute leukemia. At present, the majority of patients die within two years of diagnosis, and a more effective therapeutic strategy is thus urgently needed. MLL fusion proteins induce aberrant expression of a group of homeobox gene super-family members, including HOXA and co-factors such as MEIS1 and PBX3. Overexpression of individual HOXA genes can induce myeloproliferation and block differentiation. Co-expression of Meis1 and Hoxa9 is sufficient to transform normal hematopoietic progenitor cells and to induce a rapidly fatal leukemia in transplanted mice, and their aberrant overexpression is required for the induction and maintenance of MLL-rearranged leukemia. Although Pbx proteins are known to interact with Hox proteins, thereby increasing the DNA-binding affinity of the latter and enhancing the transcription of downstream target genes, little is known about the interaction between Pbx proteins and Hox proteins (e.g., Hoxa9) in cell transformation and leukemogenesis.
Recently, we showed that increased expression of a four-homeobox-gene signature (HOXA7, HOXA9, HOXA11 and PBX3) was an independent predictor of shorter overall survival in patients with cytogenetically abnormal acute myeloid leukemia (CA-AML) (Li Z., et al., Blood. 2012). Our analysis of the expression profiles of three independent large-scale patient sets showed that PBX3 was the only member of the PBX family that was consistently co-expressed with HOXA9 in various subtypes of CA-AML, particularly in MLL-rearranged AML; in contrast, both PBX1 and PBX2 tended to exhibit an inverse correlation of expression with HOXA9 in CA-AML. We then investigated the role of PBX3 in CA-AML, because its function in leukemia was unclear. We found a similar pattern of co-expression of Hoxa9 and Pbx3 in MLL fusion-mediated mouse leukemia models. We then showed that depletion of Pbx3 (but not Pbx1 and Pbx2) by Pbx3 shRNA dramatically inhibited MLL-AF9 induced transformation/immortalization of mouse normal bone marrow progenitor cells (about 50% reduction in colony numbers and about 70% reduction of number of cells in each colony). Furthermore, we demonstrated that forced expression of PBX3 exhibited a significantly synergistic effect with HOXA9 in promoting cell transformation/immortalization in vitro. In mouse bone marrow reconstitution/transplantation assay, the PBX3+HOXA9 mice developed leukemia significantly faster than HOXA9 alone with overall median survival of 71 days versus 140 days (P<0.0001). Finally, we treated leukemia cell lines and cells with HXR9, a small, cell-permeable peptide, previously shown to specifically disrupt the formation of HOX/PBX heterodimers, and to be effective in treating various cancers with minimal toxicity. We found that leukemia cells with higher levels of HOXA/PBX3 expression were more sensitive to HXR9 treatment than those with lower levels. Thus, targeting the HOXA/PBX3 pathway may provide a new strategy to substantially improve outcomes of patients with MLL-rearranged leukemia and possibly, non-favorable CA-AML.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.