Severely Impaired Erythropoiesis in Mice Lacking Mediator Subunit Med1/TRAP220.

GATA transcription factors are essential for diverse developmental processes including erythropoiesis. Med1/TRAP220 was shown to interact with GATA factors in vitro (Crawford et al. (2002): J Biol Chem. 277(5):3585-92.). Med1/TRAP220 is part of the multiprotein Mediator complex that forms a bridge between gene-specific regulatory proteins and the RNA polymerase II (Borggrefe (2004): Research Adv. in Biol. Chem. (2: 9-20), Bourbon et al. (2004) Mol. Cell 14(5):553-7). Mediator subunit Med1/TRAP220 deficient mice die during mid-embryonic development (day 11.5dpc) due to defects in placental, cardiac and hepatic development (Ito et al. (2000): Mol Cell. 5 (4):683–93., Landles et al. (2004): Mol Endocrinol. 17(12):2418–35). In order to analyze hematopoiesis in TRAP220-deficient embryos, AGMs (aorta-gonad-mesonephros) and Yolk-sacs were isolated from day 10.5 dpc embryos and analyzed by FACS and methylcellulose assays. TRAP220-deficient embryos contain c-Kit positive cells and BFU-E (Burst forming units erythrocytes) activity. However, CFU-E (Colony-forming units erythrocytes) activity was 10-fold reduced. This correlated with severe decrease of the CD71/Ter119 double-positive cells, the CFU-E containing fraction, in FACS-analysis.

Our data shows that Mediator subunit Med1/TRAP220 is required for erythropoiesis just before the CFU-E stage. Because GATA factors affect early erythropoiesis and because MED1/TRAP220 binds to GATA factors in vitro, our data suggest that the Mediator of transcriptional regulation forms the bridge between GATA factors and the RNA polymerase II machinery.