Abstract Atopic dermatitis (AD) affects up to 20% of children worldwide and is an increasing public health problem. Although AD in infants and young children can resolve, there is a substantial risk of progression from AD to other atopic diseases, including food allergy (FA), allergic rhinitis, and asthma, i.e. the atopic march. The mechanisms underlying the atopic march are incompletely understood and this is a critical gap in knowledge as this information is necessary in order to design preventive strategies that can hijack or abort the atopic march. The crucial role of Filament aggregating protein (or filaggrin, FLG) in the development of AD and the atopic march has been demonstrated and extensively reviewed. The Mechanisms of Progression of Atopic Dermatitis to Asthma in CHildren (MPAACH) cohort is an early life cohort of children with AD with extensive longitudinal clinical health outcomes data and longitudinal biospecimens. Using MPAACH, we recently found that low non-lesional, but not lesional, skin FLG expression is associated with the development of co-sensitization and moderate- severe AD, both key risk factors for the future development of allergic co-morbidities of AD. Project 2 will test the central hypothesis that genetic factors including CARD14 act in concert with environmental factors and inflammation to determine FLG expression in AD skin, driving transient or persistent skin barrier dysfunction, which may differentially impact long-term AD outcomes. Project 2 builds upon prior cycles of AADCRC funding and leverages the extensive longitudinal clinical and molecular data and biospecimens available from MPAACH and Project 1. In Aim 1, we will identify genetic variants that modulate skin FLG expression and integrate this with clinical, exposure, and molecular data from MPAACH to identify the combination of factors that determine skin FLG expression. We found that rs11652075 in CARD14 results in increased expression of CARD14 and leads to attenuated FLG expression. In Aim 2, we will build on this data and determine the mechanisms that link rs11652075 and enhanced CARD14 signaling to FLG homeostasis. While LoF FLG mutations are important in the atopic march, the relationship between the trajectory of FLG expression in early life and over time with longitudinal AD outcomes is unclear. In Aim 3, we will determine the impact of low skin FLG expression (persistent or transient) over time and the genetic loci identified in Aim 1 on AD outcomes. Based on reports that early application of emollients to the skin may prevent the development of AD, allergic sensitization, and food allergy, this study is clinically important because it may help identify individuals with the magnitude and duration of dysregulated barrier function that may benefit from prevention and intervention strategies designed to enhance skin barrier function. Moreover, our study will help determine the optimal timing to deliver these interventions. Finally, this project ad...