Ectopic expression of the nude gene induces hyperproliferation and defects in differentiation : Implications for the self-renewal of cutaneous epithelia

Nude mice are characterized by the absence of visible hair, epidermal defects, and the failure to develop a thymus. This phenotype results from loss-of-function mutations in Whn (Hfh11), a winged-helix transcription factor. In murine epidermis and hair follicles, endogenous whn expression is induced as epithelial cells initiate terminal differentiation. Using the promoter for the differentiation marker involucrin, transgenic mice that ectopically express whn in stratified squamous epithelia, hair follicles, and the transitional epithelium of the urinary tract were generated. Transgenic epidermis and hair follicles displayed impaired terminal differentiation and a subset of hair defects, such as delayed growth, a waved coat, and curly whiskers, correlated with decreased transforming growth factor (TGF)-α expression. The exogenous Whn protein also stimulated epithelial cell multiplication. In the epidermis, basal keratinocytes exhibited hyperproliferation, though transgene expression was restricted to suprabasal, postmitotic cells. Hair follicles failed to enter telogen (a resting period) and remained continuously in an abnormal anagen (the growth phase of the hair cycle). Ureter epithelium developed severe hyperplasia, leading to the obstruction of urine outflow and death from hydronephrosis. Though an immune infiltrate was present occasionally in transgenic skin, the infiltrate was not the primary cause of the epithelial hyperproliferation, as the immune reaction was not observed in all affected transgenics, and the transgene induced identical skin and urinary tract abnormalities in immunodeficient Rag1-null mice. Given the effects of the transgene on cell proliferation and TGFα expression, the results suggest that Whn modulates growth factor production by differentiating epithelial cells, thereby regulating the balance between proliferative and postmitotic populations in self-renewing epithelia.