Two Novel Mutations Cause Hereditary Antithrombin Deficiency in a Chinese Family

Clinical and functional characterization of rare compound heterozygous mutations in the SERPINC1 gene causing severe thrombophilia

INTRODUCTION

Hereditary antithrombin (AT) deficiency is a rare autosomal dominant disorder with significant clinical heterogeneity. In the study, we identified a patient with AT deficiency caused by compound heterozygous mutations in the SERPINC1 gene.

METHODS

A total of 9 individuals from three generations were investigated. The mutations were identified by direct sequencing of SERPINC1. Multiple in silico tools were programmed to predict the conservation of mutations and the effect on the AT structure. The coagulation state was evaluated by the thrombin generation assay. Recombinant AT was overexpressed in HEK293T cells; the mRNA level was determined using RT-qPCR. Western blotting, ELISA, and immunocytofluorescence were applied to characterize the recombinant AT protein.

RESULTS

The proband was a 26-year-old male who experienced recurrent venous thrombosis. He presented the type I deficiency with 33 % AT activity and a synchronized decrease in AT antigen. Genetic screening revealed that he carried a heterozygous c.318_319insT (p.Asn107*) in exon 2 and a heterozygous c.922G > T (p.Gly308Cys) in exon 5, both of which were completely conserved in homologous species and resulted in enhanced thrombin generation capability. Hydrophobicity analysis suggested that the p.Gly308Cys mutation may interfere with the hydrophobic state of residues 307-313. In vitro expression studies indicated that the levels of the recombinant protein AT-G308C decreased to 46.98 % ± 2.94 % and 41.35 % ± 1.48 % in transfected cell lysates and media, respectively. After treatment with a proteasome inhibitor (MG132), the quantity of AT-G308C protein in the cytoplasm was replenished to a level comparable to that of the wild type. The mRNA level of AT-N107* was significantly reduced and the recombinant protein AT-N107* was not detected in either the lysate or the culture media.

CONCLUSION

These two mutations were responsible for the AT defects and clinical phenotypes of the proband. The p.Gly308Cys mutation could lead to proteasome-dependent degradation of the AT protein in the cytoplasm by altering local residue hydrophobicity. The c.318_319insT could eliminate aberrant transcripts by triggering nonsense-mediated mRNA degradation. Both mutations resulted in type I AT deficiency.

Analysis of phenotype and gene mutation in three pedigrees with inherited antithrombin deficiency

BACKGROUND

Inherited AT deficiency is an autosomal-dominant thrombophilic disorder usually caused by various SERPINC1 defects associated with a high risk of recurrent venous thromboembolism. In this article, the phenotype, gene mutation, and molecular pathogenic mechanisms were determined in three pedigrees with inherited AT deficiency.

METHODS

Coagulation indices were examined on STAGO STA-R-MAX analyzer. The AT:Ag was analyzed by ELISA. All exons and flanking sequences of SERPINC1 were amplified by PCR. AT wild type and three mutant expression plasmids were constructed and then transfected into HEK293FT cells. The expression level of AT protein was analyzed by ELISA and Western blot.

RESULTS

The AT:A and AT:Ag of probands 1 and 3 were decreased to 49% and 52 mg/dL, 38% and 44 mg/dL, respectively. The AT:A of proband 2 was decreased to 32%. The SERPINC1 gene analysis indicated that there was a p.Ile421Thr in proband 1, a p.Leu417Gln in proband 2, and a p.Met252Thr in proband 3, respectively. The AT mRNA expression level of the three mutants was not significantly different from AT-WT by qRT-PCR. The results of ELISA and Western blot tests showed that the AT-M252T and AT-I421T mutants had a higher AT expression than the AT wild type (AT-WT), and the AT protein expression of AT-L417Q mutants had no significant difference compared with AT-WT in the cell lysate. The AT expression levels of AT-M252T and AT-I421T mutants were lower than that of AT-WT, and there was no significant difference between AT-L417Q mutant and AT-WT in the supernatant.

CONCLUSION

The p.I421T and p.M252T mutations affected the secretion of AT protein leading to type I AT deficiency of probands 1 and 3. The p.Leu417Gln mutation was responsible for the impaired or ineffective activity AT protein in proband 2 and caused type II AT deficiency.

Type II antithrombin deficiency caused by a novel missense mutation (p.Leu417Gln) in a Chinese family

THE AIM OF THE REPORT WAS T

o explore the phenotype and genotype of a hereditary antithrombin deficient Chinese family. Functional and molecular analysis of the proband and his family members was performed. Online bioinformatics software was used to predict the pathogenicity of the novel mutation. ClustalX-2.1-win and PyMol software were applied to conservative analysis and generate molecular graphic images, respectively. Functional analysis had shown that the antithrombin (AT):A of the proband was reduced to 32% whereas AT:Ag was normal. Molecular analysis revealed a heterozygous missense mutation p. Leu417Gln in exon 7 of SERPINC1 gene. Bioinformatics and model analysis indicated that this mutation could affect the integrity of local intermolecular structures, resulting in a mild type of antithrombin deficiency but when combined with other genetic or acquired thrombophilic factors, patients may develop venous thrombosis. The p.Leu417Gln mutation was responsible for the decrease of AT:A in this family and caused type II antithrombin deficiency.