Structural and functional consequences of aspartate/asparagine-β-hydroxylase variants causing Traboulsi syndrome

Traboulsi syndrome is an autosomal recessive hereditary disease associated with developmental defects, in particular of the ocular system. SNPs affecting the ASPH gene, which encodes for the 2-oxoglutarate (2OG)-dependent oxygenase aspartate/asparagine-β-hydroxylase (AspH), are associated with Traboulsi syndrome. AspH catalyzes hydroxylations of conserved aspartate/asparagine residues in epidermal growth factor-like domain (EGFD) proteins. We report studies on the clinically observed Traboulsi syndrome-associated R688Q, R735Q, and R735W AspH variants. The results reveal that pathogenic active site substitutions substantially reduce, though do not ablate, EGFD hydroxylase activity compared with wt AspH. They imply that efficient AspH-catalyzed EGFD hydroxylation is important during human development. Crystallographic studies reveal conservation of the overall AspH fold, but that the preferred conformations of 2OG in complex with the R735Q and R735W AspH variants differ from those with wt AspH. Screening of potential 2OG cosubstrate substitutes reveals that certain 2-oxoacids, including naturally present metabolites, manifest enhanced catalytic efficiency of Traboulsi syndrome-associated AspH variants compared with 2OG. The results thus provide proof of principle for a therapeutic strategy involving rescue of impaired activities of pathogenic active site AspH variants by use of 2-oxoacids, or 2-oxoacid precursors, other than 2OG.