133 Understanding the role of ABCA12 in the pathogenesis of Harlequin Ichthyosis

ATP-binding cassette transporter A12 (ABCA12), a lipid transporter, is known to be critical for skin barrier integrity. Mutations in this gene cause the most severe form of Autosomal Recessive Congenital Ichthyosis (ARCI): Harlequin Ichthyosis (HI). HI patients have marked hyperkeratosis at birth with fissuring, leading to life-threatening complications due to increased risk of infection, trans-epidermal water and heat loss. To understand the pathomechanisms of HI, we used siRNA knockdown of ABCA12 in primary keratinocytes with subsequent calcium-induced differentiation to model HI as well as a HPV- immortalised HI patient-derived cell line. RNA-sequencing was performed on the siABCA12 primary keratinocytes and siC controls. 136 genes were significantly down-regulated and 87 genes were significantly up-regulated (FDR < 0.01). Functional annotation clustering analysis performed using DAVID showed changes in the following biological processes: inflammatory response, endomembrane system, cytoplasmic bound vesicle/secretary granule, cell cycle, apoptosis and epithelial development. Early expression of differentiation markers was observed in the 3D organotypic model recapitulating the HI epidermis phenotype. Further experiments were performed to validate changes seen in nitric oxide pathway signaling genes. We found that Arginase-1, a regulator of nitric oxide synthase activity, was down regulated in the HI organotypic epidermis. Also, the master regulator of apoptosis and inflammation, STAT1 and STAT1-Tyr701, were strongly upregulated in the HI patient cell line compared to control. STAT1 activation is required for the production of nitric oxide from L-arginine. These data suggest that nitric oxide signaling may be dysregulated in HI skin.