Antithrombin Murcia (K241E) causing antithrombin deficiency: a possible role for altered glycosylation

Identification of mutations in the SERPINC1 gene has revealed different mechanisms responsible for antithrombin deficiency. Deletions and nonsense mutations associate with type I deficiency. Certain missense mutations cause type II deficiency by affecting the heparin binding site or the reactive center loop, while others result in type I deficiency by intracellular retention or RNA instability.

We studied the molecular, biochemical, proteomic and glycomic characterization of a new natural mutant (K241E) that may be classified as pleiotropic.

The mutation caused a significant decrease in the anticoagulant activity mainly due to a reduced heparin affinity and a modification of the electrostatic potential that might explain the impaired ability of the mutant protein to form complexes with the target protease in the absence of heparin. Mass spectrometry and glycomic analyses confirmed an increased molecular weight of 800 Da in the mutant protein possibly due to core-fucosylation, provoking the loss of heparin affinity. Additionally, carriers of this mutation also have a minor mutant isoform that still followed normal glycosylation, retaining similar heparin affinity to wild-type alpha-antithrombin, and certain anticoagulant activity, which may explain the milder thrombotic risk of patients carrying this mutation. Similar results were observed using recombinant K241E antithrombin molecules.

Our data suggest a new mechanism involved in antithrombin type II deficiency by indirectly affecting the glycosylation of a natural variant. Additional studies are required to confirm this hypothesis.