The most severe congenital thrombophilia, antithrombin deficiency, may be underestimated because certain pathogenic variations were not discovered by usual functional approaches. In 4 unrelated thrombophilic patients with early and recurrent thrombosis having normal antithrombin activity, researchers found two novel SERPINC1 variations, p.Glu227Lys and p.Asn224His.

In one case, the mutation was discovered using whole-genome sequencing, but in the other 3 cases, it was discovered through SERPINC1 sequencing based on a single functional positive finding suggesting deficiency. According to a eukaryotic expression model, the two variations shared a common functional defect: impaired or null N-glycosylation of Asn224. When the plasma of the carriers was incubated for 1 hour at 41°C, they displayed normal anti-FXa or anti-FIIa activities but diminished anti-FVIIa activity and a discernible loss of inhibitory function.

Furthermore, the variations’ β glycoform, which lacked two N-glycans, showed lower secretion, higher heparin affinity, no inhibitory function, and a probable dominant-negative impact. The findings explained why carriers produced more thrombin. Mutation tests disclosed that Lysine residues near the N-glycosylation sequon impair N-glycosylation efficiency. The findings revealed some additional factors involved in the control of N-glycosylation, a critical posttranslational modification that, according to the findings, impacted folding, secretion, and function, giving new evidence of the pathogenic consequences of improper antithrombin N-glycosylation.