A Role For a Multi-Drug Resistance Protein (MRP4/ABCC4) In Pediatric Acute Myeloid Leukemia (AML)

Pediatric acute myeloid leukemia (AML) is an aggressive malignancy comprising 15-20% of childhood acute leukemias and remains a challenging disease with cure rates of only 60% using conventional therapy. However, unlike ALL de novo AML is characterized by a very low burden of genomic alterations suggesting other genetic and/or epigenetic alterations occur within hematopoietic stem cells (HSC) or progenitors to affect their normal self-renewal, proliferation and/or differentiation in this disease. Multi-drug Resistance Protein 4 (MRP4/ABCC4) is the fourth member of the ABCC subfamily of ABC transporters (de novoAML, suggesting MRP4 has a role in myeloid leukemia. Moreover, gene expression profiling from 155 pediatric AML patients showed that MRP4 was differentially expressed among AML subtypes with M7 having the highest expression.

To characterize the function of MRP4 in normal hematopoiesis and in the etiology of AML, we use a murine Mrp4 knockout model. Mrp4KO mice had a similar number of BM progenitors as WT mice did. However, hematopoietic stem cells of Mrp4KO appeared to have an intrinsic defect because short- and long-term repopulation was impaired in a competitive repopulation (CR) mouse model.

To examine the role of MRP4 in AML etiology, we generated an NMYC-overexpressing AML model because NMYC overexpression is a frequent event in pediatric AML and is highly oncogenic in mouse myeloid cells (Kawagoe et al., 2007). In addition, NMYC expression was frequently associated with elevated MRP4 expression, a recently reported target of NMYC (Porro et al., 2010; Porro et al., 2011). We showed that NMYC-overexpressing WT BM cell had an increased progenitor population, which was absent in Mrp4KO cells. Because overexpression of MRP4 in lineage-depleted bone marrow cells increased the CD34+ population which correlated with increased expression of progenitor marker such as cKit, we tested whether exogenous MRP4 could rescue the progenitor population in Mrp4KO. We demonstrated that restoring MRP4 in Mrp4KO cells indeed rescued the progenitor populations. Furthermore, Mrp4-deficiency delayed in vivo NMYC-dependent leukemogenesis in an AML mouse model, possibly by decreasing the proliferation/survival of the tumor cells. Current studies are further elucidating pathways whereby MRP4 provides a biological advantage.

In summary, Mrp4 is important for both normal and deregulated progenitor functions, especially during leukemogenesis when progenitor populations greatly increase. It is likely MRP4 alters survival and/or proliferation of progenitors. This work will further our understanding of ABC transporters in the normal biology and pathology of hematopoietic cells and has the potential to facilitate identification of new therapeutic targets for AML treatment.

This work was supported by NIH and by the American Lebanese Syrian Associated Charities (ALSAC).

† Shang Li Lian, PhD shangli.lian@stjude.org, 901-595-2782, address: Chili’s Care Center, Rm I5206, Department of Pharmaceutical Sciences, St Jude Children’s Research Hospital, 262 Danny Thomas Place, Memphis, TN 38105.