Epigenetic Therapy Inhibits NUP98-HOXA9-Mediated Myeloid Disease – Decitabine and Valproic Acid Work Synergistically to Rescue Normal Hematopoiesis in Transgenic Zebrafish.

Abstract 2391

Acute myeloid leukemia (AML) results from multiple genetic lesions that alter white blood cell development, leading to hyperproliferation and a block in differentiation. A better understanding of these molecular pathways will enable the development of therapies that selectively target the specific abnormality in a leukemic cell with improved outcome and limited toxicity.

Pertinent animal models serve as essential intermediaries between in vitro studies and the use of these new agents in clinical trials. The NUP98-HOXA9 (NHA9) fusion oncogene is found in high risk de novo AML, treatment related AML and chronic myeloid leukemia (CML) blast crisis. We previously generated a transgenic zebrafish model overexpressing human NHA 9 under the zebrafish pu.1 promoter (Forrester et al, BJH, 2012). Almost 25% of adult NHA9-transgenic fish develop a myeloproliferative neoplasm (MPN) in the kidney marrow, the site of adult hematopoiesis. Additionally, NHA9- transgenic embryos demonstrate an increase in immature myeloid cells (l-plastin1, 2.3-fold by in situ, P<0.005) at the expense of erythroid cells (gata1, 19.3-fold by qRT-PCR, P<0.00005). This embryonic phenotype in our NHA9 zebrafish provides an unprecedented opportunity to perform in vivo screens for collaborating genes in NHA9-induced leukemogenesis and evaluate the impact of molecularly targeted agents.

Microarray analysis found high expression of DNA (cytosine-5-)-methyltransferase 1 (dnmt1) in NHA9 embryos, which was confirmed by qRT-PCR (3.4-fold increase). The methylating activity of the DNMT1 enzyme is part of the epigenetic machinery that represses genes needed for terminal myeloid differentiation. Overexpression of human DNMT1 has been found in some cases of AML, but has not been previously linked with NHA9-induced disease. The overexpression of the zebrafish dnmt1 homolog in NHA9 embryos may keep cells trapped in an immature state, a hallmark of AML.

Injecting NHA9 embryos with a gene-blocking dnmt1 morpholino restored normal hematopoiesis with wild-type expression levels of both l-plastin and gata1. Similarly, a dose-dependent return to normal proportions of myeloid and erythroid cells was achieved by bathing NHA9 embryos in 50–100 μM decitabine (5-aza-2'-deoxycytidine), a demethylating agent that specifically targets the DNMT1 enzyme. However, decitabine use as a monotherapy carries the risk of genomic instability due to wide-spread DNA hypomethylation. We therefore considered treating our NHA9 zebrafish embryos with combination therapy against multiple molecular targets.

DNMT1 is part of a larger epigenetic machinery and works in parallel with histone deacetylation complexes (HDACs). Similar to decitabine, exposing NHA9 embryos to 150–250 μM of the HDAC inhibitor, valproic acid (VPA), lead to a dose-dependent return of normal hematopoiesis. In vitro studies suggest that combined treatment with a demethylating agent and an HDAC inhibitor may synergize to be more effective than either compound alone in combating myeloid disease, permitting use of lower drug doses, thus avoiding unnecessary toxicity. Indeed, we found that combination doses as low as 10 μM each of decitabine and VPA restored normal hematopoiesis as effectively as significantly higher doses of each monotherapy. This drug synergy identified in our transgenic zebrafish directly links NHA9-induced leukemia and epigenetic regulation and has set the stage for a new and exciting therapeutic approach for high risk AML. True to our goal, translating this treatment to clinical studies may ultimately improve outcome and minimize toxic side-effects, thereby increasing the long-term survival of patients with high-risk AML.