The fatty aldehyde dehydrogenase (FALDH) ALDH3A2 is the causative gene of Sjögren Larsson syndrome (SLS
Sjögren-Larsson syndrome (SLS)2 is a hereditary neurocutaneous disorder caused by mutations in the fatty aldehyde dehydrogenase (FALDH) gene, ALDH3A2. The major symptoms of SLS are mental retardation, spastic di-or tetraplegia, and ichthyosis, with crystalline macular dystrophy sometimes comorbid (1). ALDH3A2 catalyzes the conversion of fatty aldehydes with medium-chain (MC) to very-long-chain fatty acids (FA) (MC, C5-C10; long-chain (LC), C11-C20; and very-longchain, ՆC21), with the most preferred substrates being C16 and C18 aldehydes (2, 3). To date, more than 70 mutations have been found in the ALDH3A2 gene of SLS patients, and most of them cause Ͼ90% reduction in enzyme activity (4). Because aldehyde molecules are reactive and toxic in general, it is considered that accumulated fatty aldehydes cause the SLS pathology by reacting with certain important proteins in the nervous system and epidermis and compromising their functions. Furthermore, the literature lacks a detailed description of the characteristics of Aldh3a2 knock-out mice.The skin symptom ichthyosis is characterized by dry, thickened, and scaly skin, often likened to fish scales, and is caused by a skin permeability barrier defect related to multilayered lipids (lipid lamellae) in the epidermis. The epidermis is composed of four cell layers, the stratum basale, stratum spinosum, stratum granulosum, and the stratum corneum, the last of which is the site of these lipid lamellae, and accordingly has the most important role in skin barrier formation (5, 6). Keratinocytes proliferate in the stratum basale and migrate outward, differentiating into cell layer-specific cell types. The major lipid components of lipid lamellae are ceramides, cholesterol, and FAs, with ceramides being the most abundant (5, 6). A variety of ceramide species exist in the epidermis. Among them, acylceramides are a class of epidermis-specific ceramides that play an essential role in skin barrier formation (7,8). Ceramides are normally composed of a long-chain base (LCB) and a FA (9). However, acylceramides contain an additional hydrophobic chain (linoleic acid), which is esterified with the hydroxylated -carbon of the FA. In the epidermis of SLS patients, some ceramide species, including the acylceramide EOS (a combination of an esterified -hydroxy FA and the LCB sphingosine), are greatly reduced (10).Several metabolic pathways generate the substrates of ALDH3A2, fatty aldehydes. These include metabolic pathways of leukotriene B 4 , diet-derived phytol, plasmalogens, and fatty alcohols (11)(12)(13)(14)(15). Furthermore, we recently revealed that metabolism of LCBs also generates fatty aldehydes, i.e. hexadecanal (C16:0 aldehyde) from dihydrosphingosine (DHS) and