Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Cell & Molecular Biology

First Advisor

Sarah E. Millar

Second Advisor

Edward E. Morrisey


miRNAs make up one of the most abundant classes of regulatory molecules as each miRNA is estimated to regulate hundreds of mRNAs. The repressive effects on individual target mRNAs are typically relatively mild and the deletion of individual or even multiple related miRNAs often results in subtle phenotypes. Changes in the miRNA expression profiles of multiple tumor samples, including melenoma and epithelial skin cancers, suggest a widespread alteration in miRNA networks during tumorigenesis. Similarily, miRNA networks are essential for the development of multiple systems, including the hair follicle. Although a few epidermal miRNAs have been studied in adult skin, global miRNA loss has not been evaluated. Two key endonucleases, Dicer and Drosha, are required for miRNA biogenesis; however both enzymes have miRNA-independent functions of processing siRNA or preribosomal RNAs, respectively. To elucidate miRNA function in adult skin, we generated mice with inducible epidermal deletion of each endonuclease and assayed for shared phenotypes. Deletion of Dicer or Drosha in adult life resulted exclusively in a shared spectrum of phenotypes, indicating that their major function is in miRNA biogenesis, rather than in non-miRNA dependent roles. Interestingly neither enzyme was necessary to maintain resting hair follicles, but both were required during distinct phases of adult hair follicle growth: for the viability of the transient amplifying population; for normal hair shaft formation; and for initiation of hair follicle regression. After prolonged loss of either enzyme, follicular degradation occurred concomitantly with epidermal thickening and dermal inflammation, suggesting additional miRNA roles in hair follicle maintenance and epidermal homeostasis. Analysis of miR-205 targets, a miRNA highly expressed throughout the hair follicle growth cycle, revealed significantly increased levels of Zeb2, and E2F1 in Dicer mutant skin suggesting that deregulation of these factors may contribute to Dicer and Drosha mutant phenotypes, facilitating future studies of their regulation and function in the skin. These results demonstrate specific requirements for miRNAs in maintaining the ability of adult hair follicles to grow and regenerate, and indicate that Drosha and Dicer dependent miRNAs play multiple roles at successive time points of the hair follicle growth cycle.

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