To cite this article: Kezic S, O’Regan GM, Yau N, Sandilands A, Chen H, Campbell LE, Kroboth K, Watson R, Rowland M, Irwin McLean WH, Irvine AD. Levels of filaggrin degradation products are influenced by both filaggrin genotype and atopic dermatitis severity. Allergy 2011; 66: 934–940. Abstract Background: Filaggrin, coded by FLG, is the main source of several major components of natural moisturizing factor (NMF) in the stratum corneum (SC), including pyrrolidone carboxylic acid (PCA) and urocanic acid (UCA). Loss-offunction mutations in FLG lead to reduced levels of filaggrin degradation products in the SC. It has recently been suggested that expression of filaggrin may additionally be influenced by the atopic inflammatory response. In this study, we investigated the levels of several breakdown products of filaggrin in the SC in healthy controls (CTRL) and patients with atopic dermatitis (AD) in relation to FLG null allele status. We examined the relationship between NMF (defined here as the sum of PCA and UCA) and AD severity. Methods: The SC levels of filaggrin degradation products including PCA, UCA, histidine (HIS) and tyrosine were determined in 24 CTRL and 96 patients with moderate-to-severe AD. All subjects were screened for 11 FLG mutations relevant for the study population. Results: The levels of PCA, UCA and HIS correlated with FLG genotype. Furthermore, these levels were higher in the CTRL when compared to AD patients with no FLG mutations. Multiple regression analysis showed that NMF levels were independently associated with FLG genotype and severity of disease. Conclusion: Decreased NMF is a global feature of moderate-to-severe AD; within AD, FLG genotype is the major determinant of NMF, with disease severity as a secondary modifier. NMF components are reliably determined by a noninvasive and relatively inexpensive tape stripping technique.

Loss-of-function (LOF) mutations in the filaggrin gene (FLG) are a well-replicated risk factor for atopic dermatitis (AD) and are known to cause an epidermal barrier defect. The nature of this barrier defect is not fully understood. Patients with AD with FLG LOF mutations are known to have more persistent disease, more severe disease, and greater risk of food allergies and eczema herpeticum. Abnormalities in corneocyte morphology have been observed in patients with AD, including prominent villus-like projections (VP); however, these ultrastructural features have not been systematically studied in patients with AD in relation to FLG genotype and acute and convalescent status. We sought to quantitatively explore the relationship between FLG genotype, filaggrin breakdown products (natural moisturizing factor [NMF]), and corneocyte morphology in patients with AD. We studied 15 children at first presentation of AD and after 6 weeks of standard therapy. We applied atomic force microscopy to study corneocyte conformation in patients with AD stratified by FLG status and NMF level. By using a new quantitative methodology, the number of VPs per investigated corneocyte area was assessed and expressed as the Dermal Texture Index score. Corneocytes were also labeled with an anti-corneodesmosin antibody and visualized with scanning electron microscopy. We found a strong correlation between NMF levels and Dermal Texture Index scores in both acute and convalescent states (respective r = -0.80 and -0.75, P < .001 and P = .002). Most, but not all, VPs showed the presence of corneodesmosin abundantly all over the cell surface in homozygous/compound heterozygous FLG patients and, to a lesser extent, in heterozygous and wild-type patients. NMF levels are highly correlated with corneocyte morphology in patients with AD. These corneocyte conformational changes shed further insight into the filaggrin-deficient phenotype and help explain the barrier defect in patients with AD with FLG LOF mutations.