Loss-of-function variants in EPHX3 cause nonsyndromic epidermal differentiation disorders

Epidermal differentiation disorders (EDDs) are inherited keratinization disorders with remarkable clinical and genetic heterogeneity. Defects in epidermal lipid metabolism and corneocyte lipid envelope assembly account for a subset of EDDs. Epoxide hydrolase 3 (EH3), encoded by EPHX3, mediates the hydrolysis of linoleate-derived lipid epoxides during barrier formation. While Ephx3-deficient mice exhibit impaired skin barrier function, no human disorders caused by EPHX3 variants have been reported to date.

To determine the genetic basis of a new subtype of nonsyndromic EDD (nEDD) presenting from early infancy with generalized erythematous patches and plaques, xerosis and skin peeling; and to assess the impact of EH3 deficiency on epidermal lipid processing.

Whole-exome sequencing was performed in three unrelated families with nEDDs to investigate the underlying genetic causes. The skin biopsy from one representative patient was analysed using immunofluorescence staining and lipid staining. Liquid chromatography-mass spectrometry was used to quantify EH3 activity. Patient-derived primary keratinocytes were used to generate three-dimensional (3D) skin equivalents to assess epidermal differentiation.

Three affected individuals from Chinese, Pakistani Irish and Russian families with nEDDs were enrolled. Biallelic EPHX3 variants (c.74G>A, p.Trp25*; c.289G>T, p.Gly97*; c.524G>A, p.Gly175Asp; c.908G>A, p.Trp303*; c.1033delC, p.Gln345Argfs*51) were identified in the patients. Total trihydroxyoctadecenoic acid was reduced in tape-stripping stratum corneum extracts from affected individuals, suggesting reduced EH3-dependent epoxide hydrolysis in vivo. Consistently, carriers of EPHX3 variants showed markedly reduced epoxide hydrolase activity in vitro. Nile red staining of the patient's skin lesion revealed reduced and redistributed lipids, suggesting a disturbed stratum corneum lipid barrier. The patient's keratinocyte-constructed 3D skin equivalent showed disrupted stratum corneum, more extensive keratin 1, increased loricrin and Ki-67, and reduced claudin-1 staining.

We report biallelic loss-of-function EPHX3 variants causing a new subtype of nEDD. Our study also supports an important role of EH3-dependent epoxide hydrolysis in epidermal barrier lipid processing.