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Stem Cells in Development and Disease

Research Summary

George Daley's lab focuses on stem cell biology, with an emphasis on somatic cell reprogramming, hematopoietic differentiation from human and mouse pluripotent stem cells, and common mechanisms in reprogramming and cancer. His laboratory has pioneered the study of murine and human cell culture models of leukemia and genetic bone marrow disease and the role of the Lin28/let-7 pathway in human cancer and metabolic disease. 

My laboratory seeks a better understanding of the biology, pathology, and clinical utility of hematopoietic and pluripotent stem cells and of the role of various tissue stem cells in development and disease. We focus our studies on murine and human blood development and on common mechanisms of somatic cell reprogramming and oncogenesis. Our goals are to define fundamental principles of how stem cells contribute to tissue regeneration and repair and to improve drug and transplantation therapies for patients with malignant and genetic bone marrow disease.

Pluripotent stem cells can generate any of the hundreds of distinct cell types in the body; thus, they are valuable tools for studying tissue formation and one day may provide a resource for cellular therapies. We employ generic mouse and human embryonic stem cells to study basic aspects of tissue development, and we derive customized pluripotent stem cells via reprogramming from patients with specific diseases, chiefly those affecting the blood and immune system. To reprogram adult somatic cells, we transfer genes responsible for the pluripotency of embryonic stem cells into human fibroblasts or peripheral blood and derive cell lines that exhibit immortal self-renewal, multilineage differentiation potential in vitro, and teratoma formation in immune-deficient mice—currently the gold standard definition of human stem cell pluripotency. These so-called induced pluripotent stem cells, or iPS cells, represent personalized human cell culture models that enable studies of fundamental disease mechanisms and in vitro chemical screens to identify drugs. We have worked to improve the reprogramming method, to probe its mechanisms, and to assess how closely iPS cells approximate embryo-derived stem cells. We have generated a variety of iPS cells from patients with genetic immune deficiency, hemoglobin disorders like sickle cell anemia, and bone marrow failure syndromes including Fanconi anemia, dyskeratosis congenita, Shwachman–Diamond syndrome, Diamond–Blackfan anemia, and Pearson’s syndrome.

Figure: Treatment of genetic bone marrow disease...

To fully exploit pluripotent stem cell culture models for blood disease, we first direct the differentiation of these unspecialized cells into hematopoietic lineages to observe cellular pathology. To achieve this, we have studied how hematopoietic populations develop in the vertebrate embryo and have uncovered mechanisms by which embryonic morphogens like BMP, Wnt, and Notch pattern cascades of transcriptional regulators to specify the formation of mesoderm, then hemogenic endothelium, and ultimately definitive hematopoietic stem cells. We have applied these developmental principles to the in vitro differentiation of iPS cells and observed how genetic defects responsible for human blood disease compromise blood cell formation in vitro. In certain diseases, we have begun to identify chemicals and proteins that reverse pathologic blood cell phenotypes in a Petri dish and might therefore represent a starting point for developing novel drugs.

For most hematopoietic malignancies and for many patients with genetic bone marrow conditions, hematopoietic stem cell transplantation represents the best hope for a cure. Unfortunately, the toxicity of current regimens and the shortage of suitable donors limit more widespread use. iPS cells can be generated from any patient and theoretically can provide an autologous source of cells that would preclude the need for unrelated donors and eliminate the immunologic complications of transplant. While we can readily derive and study differentiated myeloid and some lymphoid blood lineages in our in vitro iPS cell cultures, we cannot yet produce hematopoietic stem cells that might be suitable for cellular transplantation therapy. Producing clinical grade hematopoietic stem cells has therefore become an overarching goal of our research. We are using both hypothesis-driven strategies to mimic the actions of developmental morphogens and biomechanical forces that pattern embryonic blood development in our in vitro cultures as well as more unbiased, empiric screening approaches to identify small molecules and genes that might directly induce hematopoietic stem cell formation from iPS cells. In addition to applications in disease modeling and drug screening, our goal for iPS cells is clinical translation: to generate autologous iPS cells from patients with genetic bone marrow disease, repair gene defects in vitro, direct iPS cells into hematopoietic stem cells, and engraft patients in a combined platform of gene therapy and stem cell transplantation (Figure).

In another thrust of our research, which emerged from asking basic questions about the mechanisms that regulate the RAS oncoprotein in early embryos and embryonic stem cells, we stumbled upon a genetic pathway that is relevant to both cellular reprogramming and cancer. While investigating how RAS might be regulated in early embryos by the let-7 family of tumor-suppressor micro-RNAs, we discovered that the biogenesis of let-7 micro-RNAs was inhibited by the RNA-binding protein LIN28, which was originally described in Caenorhabditis elegans as a regulator of developmental timing. Fortuitously, LIN28 also functions as a reprogramming factor for human somatic cells, and we and others have shown that LIN28 is aberrantly expressed in a wide array of human tumors. Thus, our dual efforts at investigating pluripotency and malignancy have found a natural topic of intersection, reflecting the fact that somatic cell reprogramming and tumorigenesis share common pathways driving cellular dedifferentiation. While engineering murine strains that overexpress or that are deficient in LIN28, we have observed remarkable phenotypes that implicate the LIN28/let-7 axis in mammalian growth and timing of puberty, germ cell development, tissue regeneration, glucose metabolism, and predisposition to diabetes. In total, our surveys of the LIN28/let-7 axis have suggested that this primal pathway evolved to balance cell proliferation and the metabolic needs of growing tissues and organisms. Our future studies will aim at exploring modes of drug targeting of this medically important pathway for treatment of cancer and metabolic diseases.

Research in the Daley laboratory has been supported by the National Institutes of Health, NIH Director's Pioneer Award, Burroughs Wellcome Fund, Leukemia and Lymphoma Society, Alex’s Lemonade Stand Foundation, Doris Duke Charitable Foundation, Ellison Medical Foundation, Roche Foundation for Anemia Research, Manton Center for Orphan Disease Research, Harvard Catalyst, Harvard Stem Cell Institute, and Boston Children's Hospital.

As of May 28, 2013

Scientist Profile

Investigator
Boston Children's Hospital
Cell Biology, Developmental Biology