Pre-Leukemic Hematopoietic Stem Cells in Human AML - PROJECT SUMMARY Acute myeloid leukemia (AML) is an aggressive malignancy of the bone marrow affecting more than 20,000 adults annually in the United States. Even with aggressive chemotherapy and/or bone marrow transplantation, five-year overall survival is between 30-40%. Genomic studies have demonstrated that most cases of AML are associated with mutations in multiple genes, often occurring with a complex clonal architecture. Previous studies demonstrated that these mutations are serially acquired in clones of self-renewing hematopoietic stem cells (HSCs), identifying pre-leukemic HSCs (pHSCs) bearing pre-leukemic mutations. These pre-leukemic mutations are enriched in genes involved in regulation of the epigenome, and the pre-leukemic cells acquire additional mutations, often in genes involved in proliferation, leading to AML. Stratification of a cohort of AML patients into high or low pHSC groups demonstrated that the high group had much worse overall and relapse-free survival, indicating that the presence of pHSCs may be critical for clinical outcomes. A parallel line of genomic studies investigated the presence of these AML-associated mutations in the blood of individuals with no history of hematologic disease, a condition termed clonal hematopoiesis (CH). CH was found to be associated with an increased risk of development of myeloid malignancy. Further whole genome sequencing studies used single cell and computational methods to determine phylogenies of HSCs in individuals and to estimate time since initial mutation and clonal expansion rates. These studies led to the conclusion that these pre-leukemic/CH mutations can occur decades prior, even as far back as in utero, and that rates of clonal expansion can be predicted from a single time point assessment, adding further complexity to the study of pre-leukemia and CH. These studies provide novel and significant insights into the genetic events and cellular kinetics that occur in the development of de novo AML. However, they also raise a number of new detailed questions with significant implications for disease pathogenesis, prevention, and treatment. What is the duration of the pre-leukemic phase in individuals that eventually progress to AML? Does this duration have any impact of disease pathogenesis? The pre- leukemic HSCs give rise to a clone (clonal hematopoiesis) that produces cells of divergent phenotypes. What are the characteristics and features of the pre-leukemic clone that eventually becomes transformed into AML through the acquisition of additional mutations. As we noted in our prior studies, AML can develop in the background of varying sized pre-leukemic clones, which is associated with patient outcomes. How does the size or burden of the pre-leukemic clone impact the corresponding AML? Is this impact direct or indirect? This proposal aims to investigate key questions related to pre-leukemic HSC clonal evolution, kinetics, and cell non- autonomous effects based on the hypothesis that these features of the pre-leukemic phase and the pHSCs themselves are critical contributors to AML pathogenesis. Ultimately, we postulate the pHSCs represent a critical cellular target for the development of curative targeted therapies.
