FLT3ITD Expression Confers the Leukemogenic Properties of Growth Factor Independence and Enhanced Self-Renewal to Yolk Sac Progenitors.

Abstract 1576

Congenital leukemias are clinically and biologically distinct from those manifesting in later childhood and in adults. Derivation of the leukemic cell of origin from the primitive hematopoietic elements of the yolk sac compartment rather than definitive hematopoietic precursors may explain the unique features of this class of leukemias. We hypothesized that in the cellular context of the yolk sac precursor, common leukemogenic mutations such as the FLT3 receptor internal tandem duplication (FLT3ITD), induce characteristics allowing expansion or propagation of this developmentally self-limited cell population. Yolk sacs were dissected from pregnant C57BL/6 mice at E9.5, disaggregated and then infected with an MSCV-FLT3ITD-GFP or an MSCV-GFP control virus. To assess growth factor independence, infected yolk sac cells were plated in methylcellulose lacking growth factors. Colony numbers were scored after 7–10 days and demonstrated FLT3ITD was capable of establishing growth factor independence in yolk sac precursors, similar to results obtained using transduced fetal liver and adult bone marrow. To determine whether FLT3ITD expression in yolk sac progenitors enhanced self-renewal, the ability to serially replate was tested. Yolk sac cells were infected as above and plated in methylcellulose containing Il-3, Il-6, Scf and Epo. Colonies were scored and the cells were replated every 7–10 days. In contrast to infected fetal liver or adult bone marrow, FLT3ITD-expressing yolk sac cells were capable of replating beyond the 4^th round. Analysis of derived FLT3ITD-expressing yolk sac colonies demonstrated a CD41⁺CD34⁺c-Kit⁺ population with an undifferentiated morphology similar to primary yolk sac progenitors. In summary, aberrant expression of the FLT3ITD mutation in yolk sac cells results in the acquisition of growth factor independence and enhanced self-renewal, characteristics essential for leukemogenesis. Although FLT3ITD expression in fetal liver and bone marrow allows growth factor independence, the distinctive ability to promote enhanced self-renewal as well in yolk sac cells suggests the yolk sac is a uniquely vulnerable target for leukemic initiation during fetal development.