Leukemia-Related Transcription Factor TEL Expands Erythroid Precursors.
Authors: Mitani Kinuko , Eguchi-Ishimae Minenori , Maki Kazuhiro , Shimizu Ritsuko , Yamamoto Masayuki , Eguchi Mariko 
Published: October 2019

Abstract
TEL/ETV6 gene located at chromosome 12p13 is frequently rearranged in a variety of leukemia and solid tumor resulting in the formation of oncogenic chimeric protein product. TEL encodes a member of the ETS family of transcription factors and is also known to be essential for bone marrow hematopoiesis and vascular formation. To understand the role of TEL in erythropoiesis, we generated transgenic mice expressing TEL under the control of GATA1 promoter IE3.9int (Onodera et al, PNAS 1997) that is known to be active in erythroid-lineage hematopoiesis (GATA1-TEL transgenic mice). Although GATA1-TEL transgenic mice appeared healthy up to 18 months of age, Hb level was relatively higher in GATA1-TEL transgenic mice compared to non-transgenic littermates in 2 out of 4 transgenic lines (17.44±0.99 vs 16.52±0.97, 17.84±1.27 vs 17.06±0.75, p<0.05). There were no significant differences in white blood cell and platelet counts between GATA1-TEL transgenic mice and control littermates. FACS analysis of bone marrow cells showed no apparent differences in any hematopoietic cell population between them. Hematopoietic progenitor activities of bone marrow were assessed by means of colony formation assay, but there observed no difference in erythroid and myeloid colony-forming activity. However, CD71+/TER119+ population was more effectively expanded in GATA1-TEL transgenic mice after bone marrow cells were cultured in the presence of EPO. We further evaluated expression of endogenous and exogenous TEL transcript in erythroblasts at various differentiation stages in the controls and GATA1-TEL transgenic mice, by using FACS-sorted bone marrow cells. The amount of exogenous TEL in the transgenic mice was most abundant in CD71+/TER119+ cells having the highest induction of endogenous GATA1, while endogenous TEL showed a robust increase in CD71−/TER119+ mature erythroblast. Interestingly, in the CD71+/TER119+ cell population, expression of major-globin gene was higher in the transgenic mice than the controls, but that of erythropoietin receptor gene was comparable. The mRNA level of FLI1, one of TEL’s target genes, was relatively lower in the CD71−/TER119+ fraction of the transgenic mice compared to that of the controls. ES cells expressing TEL under the same GATA1 promoter as used in transgenic mice were established (GATA1-TEL ES) and hematopoietic differentiation capacity was examined. When ES cells were differentiated into hematopoietic cells, GATA1 began to express at day 5 of differentiation and the expression gradually increased during the differentiation. Expression kinetics of GATA1-TEL corresponded well to that of endogenous GATA1. ES cells after 7 days of differentiation were subjected to colony formation assay. Because the number of BFU-E was increased, immature erythroid precursor was considered expanded in the GATA1-TEL ES cells. Our GATA1-driven transgenic system guaranteed earlier and abundant expression of exogenous TEL during the erythroid differentiation in both mouse bone marrow and ES cells, and postulated some important roles of TEL in erythropoiesis. TEL could expand erythroid precursor at a stage of BFU-E and stimulate further differentiation afterwards. Negative transcriptional regulation of FLI1 may be one of possible underpinning mechanisms by which TEL exaggerates erythroid differentiation.