Abstract 1663: Autophagy pathways in acute promyelocytic leukemia.

Abstract A hallmark of acute myeloid leukemia (AML) is a differentiation block of hematopoietic precursors at different developmental stages. In acute promyelocytic leukemia (APL) this block can be overcome by treating the patients with retinoic acid (RA) and low dose chemotherapy allowing for neutrophil differentiation. APL is characterized by the translocation t(15;17) leading to the expression of the oncogenic PML-RARA. In contrast to the wild-type RARA protein PML-RARA acts as a transcriptional repressor causing inhibition of genes important for neutrophil differentiation. Recent data strongly suggest a role for autophagy, a self-digesting recycling mechanism, in RA-induced neutrophil differentiation by for example supporting the degradation of aggregated PML-RARA. Interestingly, the Autophagy-Linked FYVE domain-containing protein (ALFY/WDFY3) is involved in the autophagy-dependent degradation of aggregated proteins. This prompted us to investigate the role of ALFY during RA-induced neutrophil differentiation of APL cells. Firstly, we found 3-fold lower ALFY mRNA levels in a large cohort of primary AML samples as compared to normal granulocytes. Secondly, ALFY mRNA levels significantly increased following RA-induced neutrophil differentiation of NB4 and HT93 APL cells. In line with these findings, knocking down ALFY NB4 cells significantly attenuated neutrophil differentiation. Direct binding of ALFY to PML-RARA was found and is needed for the degradation of PML-RARA. Moreover, we found that RA-induced autophagy in APL cells depends on an alternative non-canonical mechanism, independent of Beclin1, but dependent on additional key autophagy genes such as ULK1, VPS34, WIPI-1, ATG5, ATG7 and MAP1S as shown by knockdown experiments. Similar to ALFY 5/6 of these key autophagy genes are significantly downregulated in primary AML patient samples as compared to healthy granulocytes. Since myeloid development is orchestrated by a panel of lineage specific transcription factors including the myeloid master regulators PU.1 and CEPBA, and since these transcription factors are often inactivated in AML, we asked if they might regulate ATG genes. Indeed, we discovered that several ATG genes are transcriptional targets of PU.1 and CEBPA. Thus, low ATG gene expression in particular AML subtypes can be attributed to aberrant expression and function of PU.1 or CEBPA in this disease. Together, our results suggest a crucial role for autophagy in APL differentiation and we identified first transcriptional regulators of ATG genes during myeloid differentiation further supporting a critical role of autophagy in this process. Lastly, our data indicate that in APL a combination of differentiation- and autophagy-inducing agents might be favorable. Citation Format: Mario P. Tschan, Anna M. Schlaefli, Daniel Brigger, Aladin Haimovici, Elena A. Federzoni, Deborah Shan, Martin F. Fey. Autophagy pathways in acute promyelocytic leukemia. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1663. doi:10.1158/1538-7445.AM2013-1663