Molecular Pathogenesis of Genetic and Inherited Diseases Premature Terminal Differentiation and a Reduction in Specific Proteases Associated with Loss of ABCA12 in Harlequin Ichthyosis

One of the primary functions of skin is to form a defensive barrier against external infections and water loss. Disrupted barrier function underlies the most severe and often lethal form of recessive congenital ichthyosis, harlequin ichthyosis (HI). HI is associated with mutations in the gene that encodes the ABC transporter protein, ABCA12. We have investigated the morphological and biochemical alterations associated with abnormal epidermal differentiation and barrier formation in HI epidermis. An in vitro model of HI skin using human keratinocytes retrovirally transduced with shRNA targeting ABCA12 in a three-dimensional , organotypic co-culture (OTCC) system has also been developed. A robust reduction in ABCA12 expression had a dramatic effect on keratinocyte differentiation and morphology comparable with that observed in HI skin, including a thicker epidermis and abnormal lipid content with a reduction in nonpolar lipids. As seen in HI epidermis, proteins that are normally expressed in late differentiation were highly dysregulated in the ABCA12-ablated OTCC system. These proteins were expressed in the stratum basale and also in the stratum spinosum, indicative of a premature terminal differentiation phenotype. Expression of the proteases kallikrein 5 and cathepsin D was dramatically reduced in both HI epidermis and the OTCC model. These data suggest that ABCA12 is a key molecule in regulating keratinocyte differentiation and transporting specific proteases associated with desquamation. (Am J Pathol 2009, 174:970–978; DOI: 10.2353/ajpath.2009.080860) Harlequin ichthyosis (HI; MIM 242500) is the most severe and often lethal form of recessive congenital ichthyosis. Infants born with HI have hard, thick skin covering most of their body. The skin forms large diamond-shaped plates resembling armor plating separated by deep red fissures, which restrict movement. These skin abnormalities also affect the shape of the eyelids and lips, causing ectropion and eclabion, respectively. HI sufferers can also have malformations or autoamputation of the fingers and toes because of constricting bands of skin in utero. Because of the impaired barrier function of the skin, neonates struggle to control water loss, regulate temperature, and are more susceptible to infection and have feeding difficulties. In addition, the tightened skin produced can cause breathing difficulties leading to respiratory failure. From our cohort data of more than 70 unrelated families, 45% of HI-affected neonates die soon after birth (if not still-born). Disease from 6 months onwards resembles a nonbullous congenital ichthyosiform erythroderma. However, the skin barrier still remains severely compromised in HI patients, and they will always have problems with thermal regulation, have increased trans-epidermal water loss, and be at a greater risk of microbial infection. The gene defective in HI is ABCA12, a member of the adenosine triphosphate (ATP)-binding cassette (ABC) superfamily of active transporters. ABCA12 mutations are responsible for all HI cases analyzed to date with the majority of mutations being either nonsense substitution or frameshift. ABCA12 localizes to lamellar granules (LG) in normal epidermal keratinocytes. The exact role of the ABCA12 protein in LG lipid transport is unknown, but it is hypothesized that ABCA12 transports glucosyl-