Transient epidermal barrier deficiency and lowered allergic threshold in filaggrin‐hornerin (FlgHrnr−/−) double‐deficient mice.

Background: Filaggrin (Flg) and hornerin (Hrnr) share similar structural and functional features. Both proteins have been implicated as essential proteins for skin barrier maintenance. Loss‐of‐function mutations of these genes constitute a risk factor for atopic dermatitis and eczema‐related asthma. Furthermore, both FLG and HRNR protein levels are downregulated in patients with atopic dermatitis. Thus, mice deficient for Flg and Hrnr provide a novel model to study skin barrier impairment and the susceptibility for cutaneous inflammation. Methods: By using appropriate targeting vectors and breeding strategies, we established a homozygous FlgHrnr double‐deficient (FlgHrnr−/−) mouse model lacking both genes including the intergenomic sequence. Results: Neonates appeared normal, but developed a transient scaly phenotype with overall flaky appearance, but no overt skin phenotype in adulthood, thereby reflecting a subclinical barrier defect seen in humans. Structurally, FlgHrnr−/− mice displayed a markedly reduced granular layer and a condensed cornified layer. Functionally, FlgHrnr−/− mice showed permeability abnormalities and metabolic aberrations regarding the production of natural moisturizing factors (NMFs) in the stratum corneum. Surprisingly, although the immune system revealed no aberrations under steady‐state conditions, FlgHrnr−/− mice are predisposed to mount an allergic contact dermatitis, especially at hapten threshold levels eliciting allergic reactions. Conclusions: Together, our FlgHrnr−/− mouse model nicely reflects the epicutaneous sensitization susceptibilities and inflammatory reactions to environmental insults in humans with impaired skin barrier functions. [ABSTRACT FROM AUTHOR]