The Effect of Filaggrin Mutations on Dermal Penetration of Diisocyanates

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ABSTRACT A significant portion of workers in the US could be at risk for an increase in exposure to industrial chemicals via a heritable defect resulting in a compromised epidermal barrier. The primary barrier of the skin is the epidermis, which consists of layers of differentiated keratinocytes forming a cornified structure (stratum corneum, SC). The continuity of the SC is dependent upon an integrated matrix of lipids and structural proteins, including filaggrin, that form a barrier against the external environment. The genetic determinants of barrier integrity are poorly understood. Human genetic studies suggest that the complete or partial loss of filaggrin expression results in decreased barrier function and enhanced penetration of chemicals through the skin. The importance of filaggrin to epidermal differentiation and barrier function has been made evident with the strong association of loss of function (LOF) mutations in the filaggrin gene (FLG) with the chronic human skin diseases, ichthyosis vulgaris and atopic eczema. These diseases are characterized by dry skin and a predisposition to asthma. The severity and penetrance of these heritable skin diseases are correlated with the number of FLG-mutant (LOF) alleles that may be altered by other modifier genes. We have shown that skin exposure to diisocyanates, a known respiratory sensitizer, may significantly contribute to systemic exposure. We hypothesize that partial loss (haploinsufficiency) of filaggrin content in the skin will impair barrier function resulting in increased diisocyanate penetration, dose, and toxicity. We propose to test our hypothesis by using in vitro three-dimensional (3D) human organotypic skin tissues with and without compromised barrier function. The dose-related difference in permeability and cellular toxicity to a model contact and respiratory sensitizer, 1,6-hexamethylene diisocyanate, will be measured in normal and barrier compromised human skin reconstructs. The use of a barrier-compromised human skin reconstruct model will fulfill an important need for biologically relevant methods to characterize chemical penetrance into the epidermis, to investigate the contribution of increased skin exposure to skin and systemic toxicity, and to identify individuals (susceptible subpopulation) at increased risk for adverse health effects, including sensitization, allergy, and asthma. The proposed research represents a cutting-edge exposure model for testing diisocyanate toxicity to human skin. Validation of this model test system can provide a more appropriate tool than is currently available using 2D in vitro and animal models to investigate the role of skin exposure to a chemical respiratory sensitization. As such, the scope of the research proposed is consistent with and amplifies NIOSH goals and mission to protect worker health. We have an opportunity to contribute to the NIOSH Research to Practice initiative by providing new knowledge and tools to identify potential susceptible subpopulations and, thus, strategies for intervention and control as well as to provide critical data to set exposure limits by taking into account individual variation that alter exposure classification.