ICHTHYOSIS: Clinical and molecular update. Part 1: Introduction and non-syndromic ichthyoses

Ichthyoses are a heterogeneous group of diseases sharing symptoms and a common etiopathogenic mechanism. Clinically, these diseases are characterized by the presence of erythema and variable degrees of skin thickening and desquamation. Although the affected area, severity, and molecular substrate are very variable, they are all signs of a disruption of the barrier formed during epidermal differentiation. Ichthyoses follow patterns of Mendelian inheritance and present symptoms since birth or shortly thereafter. Clinically, they can be categorized into non-syndromic (when symptoms are caused exclusively by the epidermal barrier dysfunction) and syndromic ichthyoses (when the causal gene has extracutaneous functions impacting other organs). Knowledge of molecular mechanisms has improved dramatically over the past few years, and we currently know not only most causal genes, but also the functions of the encoded proteins and their impact on skin barrier formation. In the first part of this review, we'll be introducing ichthyosis physiopathology, along with a clinical and genetic update of non-syndromic entities (those included in the consensus classification and those clinically and/or molecularly characterized since then).