Updated molecular genetics and pathogenesis of ichthiyoses

Abstract

Research into the molecular genetics and pathomechanisms of ichthyoses have advanced considerably, resulting in the identification of several causative genes and molecules underlying the disease. In 2009, the First Ichthyosis Consensus Conference was held to establish a consensus for the nomenclature and classification of inherited ichthyoses, by which an international consensus for the classification of inherited ichthyosis was achieved. In this review, the pathogeneses of various ichthyoses are summarized based on their revised classification and terminology. Skin barrier defects are involved in the pathogenesis of various types of ichthyosis. The known causative molecules underlying ichthyosis include ABCA12, lipoxygenase-3, 12R-lipoxygenase, CYP4F22, ichthyin and steroid sulfatase, all of which are thought to be related to the intercellular lipid layers. ABCA12 is a known keratinocyte lipid transporter associated with lipid transport in lamellar granules and a loss of ABCA12 function leads to defective lipid transport in the keratinocytes, resulting in the most severe, harlequin ichthyosis phenotype. Other causative molecules for ichthyoses are transglutaminase 1, keratins and filaggrin. Transglutaminase 1 plays a role in cornified cell envelope formation. Keratins 1, 10 and 2 are involved in the keratin network of suprabasal keratinocytes and filaggrin is essential for the formation of keratohyalin granules. It is important to obtain information concerning genetic defects and to elucidate ichthyotic disease pathomechanisms for the establishment of an effective therapy and beneficial genetic counseling, including a prenatal diagnosis for families affected by ichthyotic disease.

Fig. 1

Major components of skin barrier in stratum corneum consist of intercellular lipid layers, cornified cell envelope and keratin/filaggrin degradation products. Figure modified from Ref. No. 1.

Fig. 2

Clinical features of ichthyosis. (A) An HI patient harboring a homozygous ABCA12 splice site mutation. Thick, plate-like scales are seen on the whole body. Figure modified from Ref. No. 11. (B) A CIE patient carrying compound heterozygous ABCA12 nonsense and missense mutations. Fine, whitish scales are observed on erythrodermic skin. Figure modified from Ref. No. 42.