Plasmalogens and fatty alcohols in rhizomelic chondrodysplasia punctata and Sjögren-Larsson syndrome.
Plasmalogens are a special class of ether-phospholipids, best recognized by their vinyl-ether bond at the sn-1 position of the glycerobackbone and by the observation that their deficiency causes rhizomelic chondrodysplasia punctata (RCDP). The complex plasmalogen biosynthetic pathway involves multiple enzymatic steps carried-out in peroxisomes and in the endoplasmic reticulum. The rate limiting step in the biosynthesis of plasmalogens resides in the formation of the fatty alcohol responsible for the formation of an intermediate with an alkyl-linked moiety. The regulation in the biosynthesis of plasmalogens also takes place at this step using a feedback mechanism to stimulate or inhibit the biosynthesis. As such, fatty alcohols play a relevant role in the formation of ether-phospholipids. These advances in our understanding of complex lipid biosynthesis brought two seemingly distinct disorders into the spotlight. Sjögren-Larsson syndrome (SLS) is caused by defects in the microsomal fatty aldehyde dehydrogenase (FALDH) leading to the accumulation of fatty alcohols and fatty aldehydes. In RCDP cells, the defect in plasmalogens is thought to generate a feedback signal to increase their biosynthesis, through the activity of fatty acid reductases to produce fatty alcohols. However, the enzymatic defects in either glyceronephosphate O-acyltransferase (GNPAT) or alkylglycerone phosphate synthase (AGPS) disrupt the biosynthesis and result in the accumulation of the fatty alcohols. A detailed characterization on the processes and enzymes that govern these intricate biosynthetic pathways, as well as, the metabolic characterization of defects along the pathway should increase our understanding of the causes and mechanisms behind these disorders.