Gene analysis of inherited antithrombin deficiency and functional analysis of abnormal antithrombin protein (N87D)

Phenotypic and Genotypic Analysis of a Hereditary Antithrombin Deficiency Pedigree Due to a Novel SERPINC1 Mutation (p.Met281Thr)

Antithrombin (AT) is one of the physiological anticoagulants that are mainly synthesized in the liver. As a protease inhibitor belonging to the serpin superfamily, AT is able to inactivate thrombin and inhibit activated coagulation factors IX, X, XI, and XII (FIXa, FXa, FXIa, and FXIIa).1 Moreover, it has been found that AT can inhibit activated FVII (FVIIa) by accelerating dissociation of FVIIa-tissue factor complex and preventing it from recombining.2 The AT gene (SERPINC1), located on chromosome 1 at q23.1-23.9 and spreads 13.5 kb, is composed of seven extrons and six introns.3 Hereditary AT deficiency is a rare thrombotic disorder caused by defects in SERPINC1 gene.4 It is inherited in an autosomal-dominant manner with an incidence of roughly 0.02 to 0.25% in the general population and 2 to 5% in patients with a history of venous thromboembolism (VTE).1 5 The most common thrombotic manifestations of patients with AT deficiency are VTEs, and their risks of VTE are approximately 20 times higher than those of nondeficient individuals.6 And the consequences of thrombophilia caused by AT deficiency are more severe than those of protein C and S protein deficiency,2 so it should be given sufficient attention.

Identification of a novel alternatively spliced isoform of antithrombin containing an additional RCL-like loop

Antithrombin (AT3) is one of the most important inhibitors of blood coagulation proteases that belong to the serpin family of protease inhibitors. In this study, a novel alternatively spliced isoform of AT3 was identified, both at transcript and protein level. This novel transcript contains an additional region in the continuation of exon 3b that was included in the transcript due to use of an alternate 5' splice site. The existence of the novel transcript was confirmed in human brain and liver through RT-PCR. An analysis of the complete transcript indicated that the native reactive centre loop (RCL) of AT3 is maintained; however the novel amino acid sequence projects out as an additional loop as evident from MD simulation studies. A unique amino acid sequence present in the novel isoform was used for the development of polyclonal antibody. The expression of novel isoform was confirmed in human brain and liver tissue using Western blot analysis. Interestingly an alignment of RCL like domain with other inhibitory serpins showed significant similarity with the neuroserpin RCL. To the best of our knowledge, this is the first evidence of alternatively spliced AT3 sequence containing an additional loop and could have physiological relevance.

Two Novel Mutations Cause Hereditary Antithrombin Deficiency in a Chinese Family

OBJECTIVE

To study the molecular basis of hereditary antithrombin (AT) deficiency in a Chinese family. It will help us understand the pathogenesis of this type of disease.

METHOD

AT activity (AT:A) and the AT antigen (AT:Ag) level were tested by chromogenic substrate and immunoturbidimetry, respectively. To identify the novel mutations, SERPINC1 gene sequencing was carried out. The possible impact of the mutations was analyzed by model and bioinformatic analyses.

RESULTS

AT:A and the AT:Ag level of the proband were 43% and 113 mg/L (normal range: 98-119% and 250-360 mg/L), respectively. Sequencing analysis revealed compound heterozygous mutations, including a frameshift mutation (c.318_319insT) resulting in Asn75stop and a missense mutation (c.922G>T) resulting in Gly276Cys. The bioinformatic and model analyses indicated that these mutations may disrupt the function and structure of the AT protein.

CONCLUSION

We detected 2 novel heterozygous mutations (c.318_319insT and c.922G>T) in the proband, and these were associated with decreased AT:A.