S113 GENETICS AND MODELING OF HUMAN ACUTE ERYTHROID LEUKEMIA
Authors: Fagnan A. , Riera Piqué Borràs M. , Ignacimouttou C. , Otzen Bagger F. , Lopez C.K. , Caulier A. , Aid Z. , Thirant C. , kurtovic A. , Maciejewski J. , Dierks C. , Rambaldi A. , Pabst T. , Shimoda K. , Lapillonne H. , DeBotton S. , Micoll J.‐B. , Caroll M. , Valent P. , Kile B. , Carmichael C. , Vyas P. , Delabesse E. , Gelsi‐Boyer V. , Birnbaum D. , Anguita E. , Garcon L. , Soler E. , Schwaller J. , Mercher T. 
Published: June 2019

Background:Acute erythroid leukemia (AEL) is a subtype of acute myeloid leukemia (AML) characterized by an accumulation of variable proportions of erythroid progenitor cells and myeloblasts. Two subgroups of AEL have been proposed: 1‐pure erythroid leukemia (PEL, AML‐M6b) characterized by an accumulation of > 80% of erythroid progenitors and 2‐AML‐M6a characterized by an accumulation of both myeloid and erythroid progenitors, a group that has now been integrated into myelodysplastic syndrome (MDS) by the 2016 World Health Organization (WHO) classification. Earlier studies indicated that mutations of TP53 and epigenetic modifiers genes (e.g. TET2, DNMT3A) are prevalent alterations in AEL. However, the underlying molecular mechanisms driving the erythroid phenotype remain poorly understood.Aims:The aims of this study were to better characterize the mutational and transcriptional landscape of AEL and to model the functional consequence of these alterations.Methods:We collected AEL patient samples and performed transcriptomic (RNAseq) and genetic (exomes) analyses, on 31 and 11 samples respectively. Candidate alterations were then expressed in mouse erythroid progenitors and hematopoietic stem and progenitor cells (HSPC), in vitro and in in vivo bone marrow (BM) reconstitution assays to obtain functional insights.Results:The genetic landscape grouped patients into at least 3 molecular subgroups: 1‐TP53 mutations (32%), 2‐Epigenetic modifiers (e.g. TET2, DNMT3A) mutations (32%), 3‐Others. Some patients present combinations of mutations that could represent the basis for an erythroid bias. For example, a patient showed TP53 mutation with an out‐of‐frame fusion leading to aberrant EPO overexpression. Another patient presented with a TET2 mutation associated with a GATA1 s mutation previously associated with familial macrocytic anemia and megakaryoblastic leukemia with Down's syndrome. Gene expression signatures indicated that several human AEL samples present with drastic changes in the expression of genes related to GATA1 activity, including ERG, ETO2, SPI1 and SKI. Ectopic expression of these genes through retroviral transduction of mouse erythroblasts led to their immortalization in vitro and, when engrafted thereafter in mice, to leukemia presenting characteristics of AEL. Similarly, purified erythroblasts from a TET2

−/−

GATA1 s double transgenic mouse model showed high long‐term proliferation capacity in vitro and subsequent murine AEL. In contrast, transplantation of multipotent progenitors transduced with SKI retrovirus or purified from TET2

−/−

GATA1 s double transgenics led to the development of a more myeloid disease, respectively mimicking MDS with erythroid component or more homogeneous myeloid leukemia. Therefore, altered activity of these factors in erythroid progenitors led to pure erythroid phenotypes and to mixed erythroid and myeloid phenotypes upon expression in multipotent progenitors.Summary/Conclusion:We report that, in addition to previously described genetic alterations including TP53 and chromatin regulator mutations, human AEL is also characterized by aberrant expression of several genes interfering with GATA1 including ETS factors, ETO2 and SKI. Modeling their ectopic expression in different murine progenitors suggests that the prevalence of the erythroid phenotype is dependent on the targeted cell type. Together, alterations of the GATA1 transcriptional activity and targeting of different stages of the hematopoietic differentiation may explain the continuum of phenotype between MDS and pure erythroid in human AEL.