132 Increased BMP Signalling Correlates with Increased Eczema Severity

Abstract

Filaggrin (FLG) is a key cornified envelope and epidermal barrier protein. FLG mutations are the most strongly implicated genetic risk factor for atopic eczema (AE). FLG is part of a gene complex, called the epidermal differentiation complex (EDC), the genes of which contribute to various aspects of epidermal barrier function. A number of EDC and Keratin genes were downregulated and components of the bone morphogenetic protein (BMP) signalling pathway upregulated in RNA-seq from FLG siRNA knockdown keratinocytes. This led us to speculate that BMP signalling is an important pathway in the pathogenesis of AE. Western blotting and qPCR was performed on lysates from FLG siRNA knockdown keratinocytes. Western blotting was performed from lysates extracted form tape strips taken from AE patients who were clinically phenotyped and genotyped for FLG mutation status. FLG knockdown caused significant reduction in FLG, Keratin 1, Keratin 10, Loricrin, Repetin and SPRR3 protein and RNA expression and an increase in phospho-SMAD (p-SMAD) 1/5 signaling, a marker of active BMP signaling. Western blots of tape strip samples taken from AE patients showed no correlation of EDC, Keratins or SMAD proteins with FLG expression but a statistically significant increase in p-SMAD 1/5 signalling in FLG compound heterozygote patients and a statistically significant correlation between EASI score and p-SMAD 1/5 expression in non-lesional skin. Other studies show that SMAD signalling is increased in AE and that BMP2 signalling can control FLG expression. In these studies, increasing BMP2 expression led to impaired epidermal FLG, Loricrin and Keratin expression. As our data suggests that p-SMAD 1/5 is more highly expressed in AE with a pronounced genetic FLG deficiency, we propose a positive feedback model of barrier impairment in AE in which reduced FLG expression increases BMP2 signalling, consequently reducing EDC and keratin expression, which further impairs the epidermal barrier.