Microarray expression analysis of wild type and Erg knockdown bone marrow hematopoietic stem and progenitor cells

Erg is an ETS family transcription factor frequently overexpressed in human leukemias and has been implicated as a key regulator of hematopoietic stem cells (HSCs). However how Erg controls normal hematopoiesis, particularly at the stem cell level, remains poorly understood. Using homologous recombination, we generated an Erg knockdown allele (Ergkd) in which Erg expression can be restored upon Cre-mediated excision of a Stopper cassette. In Ergkd/+ mice, ~40% reduction in Erg dosage perturbed both fetal liver and bone marrow hematopoiesis by reducing the numbers of Lin-Sca-1+c-Kit+ (LSK) hematopoietic stem and progenitor cells (HSPCs) and megakaryocytic progenitors. By genetic mosaic analysis, we found Erg-restored HSPCs outcompeted Ergkd/+ HSPCs for contributing to adult hematopoiesis in vivo. Intriguingly, HSC differentiation also appeared attenuated, leading to accumulation of long-term HSCs in the mutant LSK population. Accordingly, microarray analysis of Erg-restored and Ergkd/+ HSPCs from the same animal revealed enrichment of stem cell-related pathways in Ergkd/+ HSPCs. Overall, reduced Erg dosage perturbs hematopoiesis by reducing HSC numbers and impairing HSC differentiation, possibly via two Erg targets, Jun and Myc. Erg-restored (to wild type) YFP+ HSPCs and Erg-knockdown (~40% reduction) YFP- HSPCs were sorted from the same mouse [Mx1Cre;Ergkd/+;Rosa26-Stop-YFP (R26Y)]; RNA was prepared from the sorted HSPCs, amplified by Nugen V2 and applied to Affymetrix mouse genome 430 2.0 arrays.