Harlequin ichthyosis is a congenital scaling syndrome of the skin in which affected infants have epidermal hyperkeratosis and a defective permeability barrier. Mutations in the gene encoding a member of the ABCA transporter family, ABCA12, have been linked to harlequin ichthyosis, but the molecular function of the protein is unknown. To investigate the activity of ABCA12, we generated Abca12 null mice and analyzed the impact on skin function and lipid content. Abca12 ؊/؊ mice are born with a thickened epidermis and die shortly after birth, as water rapidly evaporates from their skin. In vivo skin proliferation measurements suggest a lack of desquamation of the skin cells, rather than enhanced proliferation of basal layer keratinocytes, accounts for the 5-fold thickening of the Abca12 ؊/؊ stratum corneum. Electron microscopy revealed a loss of the lamellar permeability barrier in Abca12 ؊/؊ skin. This was associated with a profound reduction in skin linoleic esters of long-chain -hydroxyceramides and a corresponding increase in their glucosyl ceramide precursors. Because -hydroxyceramides are required for the barrier function of the skin, these results establish that ABCA12 activity is required for the generation of longchain ceramide esters that are essential for the development of normal skin structure and function.
Harlequin ichthyosis (HI)2 is the most severe of the congenital, autosomal ichthyoses with affected infants developing large, hard, plate-like scales over all of their epidermal surfaces. Abnormal temperature regulation, enhanced water loss, and bacterial super-infections develop as a consequence of defects in the barrier functions of the skin, making survival through the neonatal period difficult. In recent years, several groups have linked HI, as well as less severe forms of ichthyosis, to mutations in the gene encoding a member of the ABCA family of transporters, ABCA12 (1-7). The A class of ABC transporters consists of at least eleven transporters in humans and mice, several of which are known to play critical roles in human disease. ABCA1, the best studied of the proteins, is causally linked to Tangier disease and is associated with defective phospholipid and cholesterol transport from intracellular lipid stores to the major amphipathic helical apoprotein of high density lipoprotein, apolipoprotein A-1 (8 -11). ABCA3 mutations cause a form of neonatal respiratory failure that arises from a failure to transport pulmonary phospholipids comprising surfactant from their storage site in the lamellar bodies of alveolar type II cells to the alveolar space (12)(13)(14). This transport process, like the one involving ABCA1, appears to depend on the lipidation of an acceptor amphipathic helical protein, surfactant protein B (15, 16). ABCA4 causes Stargadt macular degeneration and visual loss that is associated with a defect in the transport of phosphatidylethanolamine-retinylidene adducts out of retinal pigment epithelial cells (17)(18)(19). The role of any acceptor proteins in this process is unknown. T...