Developmental Stage-Specific Interplay Between GATA1 and IGF Signaling in Fetal Hematopoiesis and Leukemogenesis.

Abstract 386

Oncogene-mediated transformation of hematopoietic cells has been extensively studied, but little is known about the molecular basis for restriction of oncogenes to certain target cells and differential cellular context-specific requirements for oncogenic transformation between infant and adult leukemias. Understanding cell type-specific interplay between signaling pathways and oncogenes is essential for developing targeted cancer therapies.

To study how developmental restriction is achieved in Down syndrome acute megakaryoblastic leukemia (DS-AMKL), characterized by a triad of fetal origin, mutated GATA1 (GATA1s), and trisomy 21, we generated a DS-AMKL mouse model (G1s-mAMKL) through retroviral insertional mutagenesis. Using cross-species integrated genomic analysis, we demonstrate overactivity of insulin-like growth factor (IGF) signaling in murine and authentic human DS-AMKL. ShRNA mediated IGF1R knockdown in DS-AMKL cell lines (CMK, CMY) led to a dramatic reduction of transduced cells, whereas no substantial effect was observed in “control” cell lines, including K562, CHRF288-11, and M-07. Pharmacological activation and repression of IGF1R signaling followed by in vitro culture, CFU assays and intracellular phosphoprotein staining showed that in both murine and primary human AMKL cells expressing GATA1s, the IGF/IGF1R/mTOR pathway is highly activated and is the major mitogenic pathway for growth. We observed a significant delay of leukemia onset upon IGF1R knockdown after transplantation of G1s-mAMKL cells into RAG2^-/- recipients. Gene set enrichment analysis revealed that the mitogenic activity of IGF1R is exerted at the transcriptional level by activation of E2F target genes. Using a genetic approach we demonstrate that fetal, but not adult, hematopoietic progenitor cells are dependent on IGF1R signaling, showing a developmental difference in the activity of this pathway. Gata1 restricts IGF-mediated activation of the E2F transcription network to coordinate proliferation and differentiation. In contrast, mutated Gata1s is insufficient to control the IGF1R induced proliferation in fetal megakaryocytic progenitors (MPs), which were stimulated or retrovirally transduced with IGFR ligands (IGF1 and IGF2). The hyperproliferative phenotype of Gata1s fetal MPs could be rescued by shRNA mediated repression of global E2F transcription activity.

Thus, by focusing on DS-AMKL, which is restricted to neonates and infants, we have discovered fetal stage-specific interplay between insulin-like growth factor (IGF) signaling and lineage-determining transcription factor, GATA1. Acquired mutation in GATA1 perturbs the regulatory function of GATA1 on IGF pathway. Together with overactive IGF signaling, mutant GATA1 leads to malignant transformation of DS fetal progenitors, underscoring context-dependent requirements during oncogenesis and explaining resistance to transformation of ostensibly similar adult progenitors.