Title: Lin28
Description: The let-7 family of microRNAs (miRNAs) comprises multiple functionally redundant family members with roles in development and oncogenesis. Several recent reports indicate that the expression of let-7 family miRNAs may be developmentally regulated at the level of biogenesis: let-7 precursors are inefficiently processed in developmentally primitive states, but efficiently processed as development and differentiation proceed. This work in this thesis attempts to elucidate the mechanism by which let-7 processing is blocked in developmentally primitive states, and to explore the implications of this processing block in the setting of both normal development and oncogenesis. First, we demonstrate that the highly conserved, developmentally-regulated RNA-binding proteins Lin28 and Lin28b potently inhibit Drosha-mediated processing of pri- let-7 miRNAs. Lin28 is sufficient to inhibit pri- let-7 processing in an in vitro Microprocessor reaction, and knockdown of Lin28 in cell culture restores levels of mature let-7 miRNAs. The results in this chapter thus implicate Lin28 as a negative regulator of let-7 biogenesis. Subsequently, we explore the consequences of this function of Lin28 in the setting of oncogenesis. We show that blockade of l et-7 processing by Lin28 enhances cellular transformation and tumorigenesis in multiple assays, and demonstrate that aberrant activation of LIN28/LIN28B is associated with aggressive disease and poor clinical prognosis across several different human tumor types.

Finally, we explore the notion that Lin28 may regulate normal cellular differentiation. Through in vitro studies and a transgenic mouse model, we demonstrate that ectopic Lin28 expression inhibits cellular differentiation in embryonic stem cells. intestinal progenitor cells, and hematopoietic progenitor cells. We suggest that the downregulation of Lin28 and concomitant upregulation of let-7 is a general feature of progenitor cell commitment in various physiological mammalian stem cell compartments. Taken together, this work enhances our understanding of the mechanism by which let-7 expression is regulated, demonstrates that blockade of let-7 processing via ectopic expression of Lin28 can interfere with normal cellular differentiation, and indicates that aberrant activation of Lin28 in the setting of cancer can promote aggressive disease.