Forkhead/winged-helix transcription factor Whn regulates hair keratin geneexpression: Molecular analysis of the nude skin phenotype

The molecular characteristics of the nude phenotype (alopecia and thymic ap lasia) in humans and rodents are unknown. The nude locus encodes Whn, a tra nscription factor of the forkhead/winged-helix class. Expression of Whn in HeLa cells induced expression of human hair keratin genes Ha3-II and Hb5. C orrespondingly, in nude mice, which are homozygous for a loss-of-function m utation of Whn, expression of mouse mHa3 and mHb5 hair keratin genes is sev erely reduced. Characterization of a previously identified nude allele, nu( Y), revealed a mis-sense mutation (R320C) in the DNA binding domain of Whn. This mutant protein is unable to activate hair keratin gene expression in HeLa cells. When the Whn transcription factor was expressed in two parts, o ne containing the N-terminal DNA binding domain and the other the C-termina l activation domain, no activation of hair keratin genes in HeLa cells was observed. However, when these two proteins were noncovalently linked by mea ns of synthetic dimerizers, hair keratin gene expression was induced. This finding suggests that target gene activation by Whn depends on the structur al integrity and physical proximity of DNA binding and activation domains, providing a molecular framework to explain the loss-of-function phenotypes of all previously characterized nude mutations. Our results implicate Whn a s a transcriptional regulator of hair keratin genes and reveal the nude phe notype as the first example of an inherited skin disorder that is caused by loss of expression rather than mutation of keratin genes. (C) 2000 Wiley-L iss, Inc.