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Netherton syndrome (NS) is a severe genodermatosis characterized by abnormal scaling and constant atopic manifestations. NS is caused by mutations in SPINK5 (Serine Protease INhibitor Kazal-type 5), which encodes LEKTI (LymphoEpithelial Kazal Type-related Inhibitor). Lack of LEKTI causes stratum corneum detachment secondary to epidermal proteases hyperactivity. Whereas a skin barrier defect is generally regarded as a major cause for atopy, we previously identified a cell-autonomous signaling cascade that triggers pro-Th2 cytokine thymic stromal lymphopoietin (TSLP) production in LEKTI-deficient epidermis. This signaling is initiated by unrestricted kallikrein 5 (KLK5) activity, which directly activates proteinase-activated receptor 2 (PAR2)-mediated expression of TSLP and favors a cutaneous proallergic microenvironment independently of the environment and of the adaptive immune system. To further confirm these results in vivo, we generated Spink5/Par2 double knockout (DKO) mice. At embryonic day 19.5, these mice display a dramatic decrease in TSLP expression, although stratum corneum detachment persists, confirming the role of the KLK5-PAR2 cascade in TSLP-mediated early proallergic signaling. However, deletion of Par2 in adult DKO-grafted skin does not rescue the inflammatory phenotype probably resulting from stratum corneum detachment. We conclude that several mechanisms trigger and maintain the inflammatory phenotype in NS. These include skin barrier impairment, mechanical stress secondary to stratum corneum detachment, as well as protease-induced proinflammatory and proallergic pathways, including PAR2-mediated overexpression of TSLP.

The human epidermis serves 2 crucial barrier functions: it protects against water loss and prevents penetration of infectious agents and allergens. The physiology of the epidermis is maintained by a balance of protease and antiprotease activities, as illustrated by the rare genetic skin disease Netherton syndrome (NS), in which impaired inhibition of serine proteases causes severe skin erythema and scaling. Here, utilizing mass spectrometry, we have identified elastase 2 (ELA2), which we believe to be a new epidermal protease that is specifically expressed in the most differentiated layer of living human and mouse epidermis. ELA2 localized to keratohyalin granules, where it was found to directly participate in (pro-)filaggrin processing. Consistent with the observation that ELA2 was hyperactive in skin from NS patients, transgenic mice overexpressing ELA2 in the granular layer of the epidermis displayed abnormal (pro-)filaggrin processing and impaired lipid lamellae structure, which are both observed in NS patients. These anomalies led to dehydration, implicating ELA2 in the skin barrier defect seen in NS patients. Thus, our work identifies ELA2 as a major new epidermal protease involved in essential pathways for skin barrier function. These results highlight the importance of the control of epidermal protease activity in skin homeostasis and designate ELA2 as a major protease driving the pathogenesis of NS.

Background: Erythrodermas are often life-threatening conditions in infants. Determination of the underlying cause is crucial. Microscopic changes in adult erythroderma lack specificity., Objective: To determine if an early skin biopsy is helpful for the diagnosis of neonatal and infantile erythroderma., Methods: Seventy-two patients admitted for erythroderma in the first year of life were retrospectively included. One hundred and eleven skin biopsies (12-year period) were examined by 3 pathologists blinded to the clinical diagnosis, and classified into atopic dermatitis, immunodeficiency (ID), psoriasis, Netherton syndrome (NS), ichthyosis, other. From year 2000, LEKTI antibody was performed when NS was suspected. Pathological diagnosis was then compared with clinical diagnosis., Results: The final diagnosis was made in 69.3% of the cases. In 57.6%, pathological diagnosis was in accordance, and in 11.7%, it was in accordance, but other diagnosis had also been proposed. For ID, sensitivity and specificity were 58.5 and 98.5%, respectively. Before year 2000, NS was frequently misdiagnosed with psoriasis, but with the use of LEKTI antibody, sensitivity and specificity were 100%., Conclusion: Skin biopsy is helpful for etiologic diagnosis of early erythroderma of infancy, particularly in ID and NS, the most severe diseases. Consequently, these results justify an early systematic skin biopsy for a better and earlier management.

Netherton syndrome is an autosomal recessive disorder, characterized by ichthyosis, atopic manifestations, and hair shaft abnormalities (trichorrhexis invaginata). The diagnosis of Netherton syndrome may be established on the basis of just one abnormal hair, but it is often difficult to find a hair with pathognomonic features on light microscopic examination. Every attempt requires pulling new hairs. We present the case of an 11-year-old female patient with Netherton syndrome in whom hair and scalp videodermoscopy (trichoscopy) was used to visualize typical bamboo hairs and hairs with golf tee type endings in the scalp hairs and eyebrows. Trichoscopy is a method, which allows noninvasive viewing of hair shafts in many-fold magnification without the need of pulling hair for diagnostic purposes. This case shows that trichoscopy may be employed to significantly improve the chance of establishing the diagnosis of Netherton syndrome in patients.