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Pre-Leukemic Hematopoietic Stem Cells and Clonal Evolution in Human AML

Ravindra Majeti

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National Institutes of Health (NIH)
Acute myeloid leukemia (AML) is an aggressive malignancy of the bone marrow characterized by the accumulation of immature myeloid cells defective in their maturation and function. AML affects 13,000 adults annually in the United States, most of them over the age of 65. Even with standard aggressive treatments with chemotherapy and/or allogeneic transplantation, five-year overall survival is between 30-40%, and much lower for those over age 65. This poor overall survival rate is primarily due to chemotherapy-resistant and/or relapsed disease that is often refractory to additional therapies. Over the last 5 years, next generation DNA sequencing has been applied to characterizing human AML genomes with great success in identifying most recurrently mutated genes. Importantly, these genomic studies have demonstrated that most cases of AML are associated with mutations in multiple genes, often occurring with different allelic frequencies, suggesting a complex clonal architecture and developmental history. These findings raise many important questions, one of which is how multiple mutations accumulate in a single clonal lineage of hematopoietic cells. Normal hematopoiesis is organized as a cellular hierarchy initiated and maintained by hematopoietic stem cells (HSC) that give rise to intermediate progenitors and eventually all the differentiated cells of the blood. In myeloid differentiation, HSC are the only long-lived self-renewing population, as all other cells and progenitors have a limited lifespan typically measured in days or weeks. From these considerations, a model has been proposed that leukemogenic mutations must serially accumulate in HSC. Therefore, HSC containing some, but not all, leukemogeneic mutations, termed pre-leukemic HSC, must persist at diagnosis and may contribute to relapsed disease. Recently, evidence supporting this model of clonal evolution of pre-leukemic HSC in human AML was reported for a small set of primary patient cases. This proposal aims to further investigate pre-leukemic HSC in human AML based on the hypothesis that leukemogenic mutations accumulate in clones of HSC and that these pre-leukemic HSC clones are critical for disease pathogenesis and relapse. The first aim is to determine the clonal evolution of pre-leukemic HSC in AML from a diversity of molecularly-defined diagnostic AML cases, and determine if there are common patterns of clonal evolution using a combination of next generation DNA sequencing, single cell genotyping, and functional xenotransplantation assays. The second aim is to determine the effect of pre-leukemic mutations on HSC self-renewal, proliferation, differentiation, and chemosensitivity using both in vitro and in vivo assays. The third aim is to identify and track pre-leukemic HSC during the course of treatment and response in individual AML patients including both clinical remission and clinical relapse, with the goal of determining if such cells contribute to relapsed disease. Ultimately, investigation of pre-leukemic HSC and the corresponding founding mutations will advance our understanding of leukemogenesis, and will be critical to the development of novel curative targeted AML therapies.

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