Reynders et al. (2009) reported a Portuguese boy with congenital disorder of glycosylation type IIj (CDG2J; 613489) and identified compound heterozygosity for a 2185C-T transition in exon 18 of the COG4 gene, resulting in an arg729-to-trp (R729W) substitution at a conserved residue, and a large deletion (606976.0002) of approximately 400 kb with the distal breakpoint between intron 2 and exon 5 of the COG4 gene and the proximal breakpoint upstream of the FUK gene (608675). The FUK gene encodes L-fucose kinase, which is necessary for the reutilization of fucose after the degradation of oligosaccharides. Because no decreased fucosylation was observed in the N-glycans of the patient, the authors concluded that the 'partial monosomy' of this gene was not pathogenic. The R729W mutation was not identified in over 100 European control alleles. Western blot analysis of patient fibroblasts showed reduced COG4 protein levels compared to control, and downregulation of COG4 expression additionally affected expression or stability of other lobe A subunits. Despite this, full complex formation was maintained albeit to a lower extent. Subunits were present in a cytosolic pool and full complex formation assisted tethering preceding membrane fusion. The unrelated father and mother were heterozygous for the R729W mutation or the deletion, respectively, and Western blot analysis of parental fibroblasts showed normal COG4 protein levels.
By determining the crystal structure of a COG4 C-terminal fragment, Richardson et al. (2009) determined that the R729 residue occupies a key position at the center of a salt bridge network, thereby stabilizing the small C-terminal domain. Knockdown of COG4 in HeLa cells by use of a COG4-specific shRNA plasmid resulted in disruption of glycosylation of cell surface proteins. A full-length COG4 containing the R729W mutation failed to rescue the glycosylation defect in these knockdown cells.
