N-linked glycosylation pathway with known defects

The figure depicts the biosynthesis of the 14-mer lipid-linked oligosaccharide (LLO) precursor of N-glycosylation in the endoplasmic reticulum. After completion, the oligosaccharide is transferred to an asparagine acceptor in the target protein. After transfer, the oligosaccharide is trimmed by specific glycosidases and subsequently rebuilt. The result can be structures in any of the 3 groups: high mannose, hybrids, or complex types. Every biosynthetic step where pathogenic mutations in the corresponding gene have been detected in at least 1 patient is indicated with an “X,” and the gene name is indicated next to it. Some genes (eg, COGs and ATP6V0A2) are not directly involved in the biosynthesis of the N-inked chain but, rather, take part in Golgi function, causing secondary effects on the glycosylation. (Contributed by Dr. Erik A Eklund.)