Molecular basis of type I antithrombin deficiency in two women with recurrent venous thromboembolism in the first trimester of pregnancy

Inherited antithrombin (AT) deficiency carries a 50% risk of venous thromboembolism (VTE) during pregnancy. Here, we investigated the molecular basis of type I AT deficiency in two women with recurrent VTE in the first trimester of pregnancy. Phenotype analysis showed both probands had almost 50% of normal AT levels. Two novel heterozygous AT mutations were identified: g.7920C > T resulting in a Trp225Cys mutation in case 1 and g.13863C > A causing an Ala404Asp mutation in case 2. Transient expression of either wild-type (WT) or mutant AT expression vectors in HEK293T and CHO cells showed impaired secretion of both AT mutant proteins. Immunofluorescence analysis revealed that the staining of AT-Trp225Cys in both endoplasmic reticulum (ER) and Golgi apparatus was similar to that of AT-WT, and the staining of AT-Ala404Asp was mainly present in ER but was weaker than that of AT-WT. These results revealed that the type I AT deficiency in two patients was caused by impaired secretion of the AT-Trp225Cys and AT-Ala404Asp mutant proteins, respectively. The two mutations are associated with a high risk of thrombotic onset and women with these AT mutations are prone to VTE in early pregnancy.