Molecular mechanisms at the basis of epidermal defects and skin fragility in AEC syndrome
Creators: name: Mollo, Maria Rosaria
Publisher: name: Università degli Studi di Napoli Federico II
Year: 2015

description: AEC syndrome is a rare autosomal dominant disorder, characterized by skin erosions, ectodermal dysplasia and cleft lip and/or palate. Missense mutations in the p63 gene, a key regulator of stratified epithelia, are causative of the disorder. The pathogenesis and the biological mechanisms underlying the skin erosions of AEC syndrome have been elusive. We generated a conditional knock-in mouse model carrying an inducible L514F mutation, found in AEC patients. p63+/floxL514F mice were crossed with the K14-Cre knock-in mouse line to obtain p63L514F expression soon before birth. Newborn mutant mice were indistinguishable from their wild-type littermates, however few days after birth focal skin blistering and scaling was observed accompanied by weight loss and often by death. Histological analysis and dye penetration assays revealed focal disruption of the epidermal barrier, followed by severe skin inflammation and epidermal hyperplasia. Consistently with the focal gaps in the epidermis of mutant mice, we found a strong reduction of desmosomal component Dsg1, Dsc3 and Dsp and a downregulation of two components of adherens junctions, Pvrl1 and Pvrl4 in AEC mutant keratinocytes. Similar results were obtained in human keratinocytes derived from AEC patients, indicating impaired cell-cell adhesion is AEC syndrome. In addition, we found an unbalance in the basal keratins. The intermediate filaments keratin5 (Krt5) and keratin14 (Krt14) are essential to withstand mechanical stress in the epidermis and are known p63 target genes. Both Krt5 and Krt14 expression was strongly downregulated in AEC mutant keratinocytes and in p63 knockdown, whereas in AEC mutant epidermis only Krt5 was significantly affected indicating that in vivo p63 plays a crucial function in Krt5 regulation. Importantly, we found a strong reduction in KRT5 expression also in human AEC patients, thus indicating impaired intermediate filaments network in AEC syndrome. Consistent with a reduction in Krt5, a reduced number of keratin bundles were observed by tissue-electron microscopy  (TEM) in the basal compartment of the epidermis. 
Skin fragility lead to focal failure of epidermal barrier, causing a progressive inflammation characterized by hyperplasia and hyperkeratosis in the skin of mutant mice. Clear signs of severe inflammation were preceded by strongly elevated levels of Thymic stromal lymphopoietin (Tslp), an IL-7 like cytokine, known to cause systemic defects which were observed in AEC mice and in at least one AEC patient. Strong induction of Tslp was associated with reduced Notch expression. Interestingly, reactivation of the Notch pathway could reduce Tslp expression in mutant keratinocytes.
Together these data support the hypothesis that the basal cell fragility and blistering observed in p63 mutant mice and AEC patients are due to alteration in adhesion molecules belonging to different categories. These results together with reduced Notch signaling lead to excessive production of the proinflammatory molecule Tslp.