Reduced Filaggrin expression induces dysregulated intracellular signalling in atopic eczema

Atopic eczema (AE) is the most common inflammatory dermatosis, affecting up to 20% of children. Loss of function mutations in the Filaggrin (FLG) gene are the most strongly implicated genetic risk factor for AE, but little is known about the signalling pathways altered in response to loss of FLG. To explore the downstream effects of loss of FLG on the cellular environment, we combined RNAseq analysis of siRNA knockdown of normal human keratinocytes and analysis of tape strip (TS) samples from AE patients who were clinically phenotyped and genotyped for FLG mutation status. RNA-seq analysis revealed an increase in BMP signalling following FLG KD, which we validated in vivo using TS samples and biopsies. Recombinant BMP2 or BMP6 increased FLG and suprabasal keratin expression in vitro. Phosphoproteomic analysis identified 237 significantly differentially phosphorylated proteins following FLG KD. Kinase enrichment analysis identified downregulation of ERK1/2 and AKT1 signalling which was confirmed in AE biopsies. cFOS was downregulated following FLG KD and correlated with FLG expression in repository datasets. cFOS was downregulated following BMP6 treatment, implying that cFOS may be an important link between FLG and BMP signalling. Finally, proteomic analysis of TS samples identified altered desmosomal expression and phosphorylation following either loss of FLG or increased BMP signalling. Therefore, we have identified a SMAD1/Filaggrin/AKT axis as a potential therapeutic avenue in AE.