The question of how the epidermal appendages form during development and which pathways regulate the differentiation of the stratified epidermis has been central to epithelial biology. Transcription is one of the most important regulatory mechanisms controlling the stepwise program of epidermal differentiation. Our research efforts have focused in characterizing the regulation and function of Dlx3 homeobox transcription factor, a member of the murine Dlx family, with essential roles in epidermal, osteogenic and placental development. Dlx homeodomain proteins are a family of transcription factors that play crucial roles in developmental processes and differentiation of individual tissues. The Dlx3 gene is broadly expressed in the branchial arches and embryonic ectoderm, as well as in the tooth, hair follicle and interfollicular epidermis. To elucidate the functional role of the Dlx3 homeodomain protein during early development and during epidermal differentiation, we are currently in the process of determining the target genes and the Dlx3 interacting factors necessary to exert the transcription regulatory function. For the comprehensive analysis of Dlx3 gene expression throughout development, we generated a knock-in mouse carrying the reporter gene beta-galactosidase (LacZ) under the control of the endogenous Dlx3 promoter. During embryonic development and organ formation, a series of signals between epithelial cells and underlying mesenchymal cells, leads to the formation of a variety of appendages/organs. Anomalies in epithelial/mesenchymal-derived organs are characteristics of a group of about 150 human heritable pathological disorders defined as ectodermal dysplasias. DLX3 is among the few genes for which mutations have been directly linked with ectodermal dysplasias. The importance of Dlx3 in the patterning and development of ectodermal structures derived from epithelial-mesenchymal interactions during embryogenesis (i.e. tooth, hair) is corroborated by the effects of DLX3 mutations in patients with the autosomal dominant Tricho-Dento-Osseous syndrome. Recent research on the mechanistic role of Dlx3 in epidermal development and appendage formation has led to the important finding that Dlx3 is a direct target of the p63. This work presents the crucial finding that besides autosomal mutations in either DLX3 or P63 genes being associated with human ectodermal dysplasias, misregulation of DLX3 function is directly involved in the pathogenesis of ectodermal human syndromes associated to p63 molecular lesions. Furthermore, utilizing mouse models, we have determined that the selective ablation of Dlx3 in the epidermis results in complete alopecia owing to failure of the hair shaft and inner root sheath to form, which is caused by the abnormal differentiation of the cortex. Significantly, we elucidated the regulatory cascade that positions Dlx3 downstream of Wnt signaling and as an upstream regulator of other transcription factors that regulate hair follicle differentiation.Taken together with hair follicle abnormalities in humans with Tricho-Dento-Osseous (TDO) syndrome, our results establish that Dlx3 is essential for hair morphogenesis, differentiation and cycling programs. During development, Dlx3 is also expressed in teeth and bone. Thus far, the evidence that Dlx3 plays a crucial role in tooth development comes from reports showing that autosomal dominant mutations in DLX3 result in severe enamel and dentin defects leading to abscesses and infections. We have recently demonstrated that the absence of Dlx3 in the neural crest results in major impairment of odontoblast differentiation and dentin production and that Dlx3 is essential in dentin formation by directly regulating a crucial matrix protein during tooth development. We have also shown that DLX3 controls the expression of epithelial hair keratins in tooth and that these keratins are essential organic components of the mineralized tooth enamel. Missense polymorphisms in one of these hair keratins, KRT75, profoundly impact dental caries susceptibility. Impact of two missense polymorphisms in hair keratin 75, KRT75A161T and KRT75E337K, previously identified as causal in two distinct hair disorders, pseudofolliculitis barbae and loose anagen hair syndrome, was tested for the presence of dental caries in a cohort of children and adults who underwent cross-sectional assessment of their teeth to generate dental caries indices. Linear regression analysis showed that the two missense polymorphisms in the KRT75 gene significantly increased susceptibility to caries. We also demonstrated that mutations in hair keratins cause tooth enamel defects leading to altered arrangement of enamel rods and significantly reduced enamel hardness, making the defective enamel more prone to caries.
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