Thrombophilia is a group of disorders in which blood has an increased tendency to clot. It is a multicausal disease triggered by interactions between inherited and acquired conditions. The acquired risk factors include antiphospholipid antibody syndrome, malignancy, oral contraceptive use, hormonal replacement therapy, surgery, obesity, smoking, prolonged travel, immobility and pregnancy3. The reported patient had no acquired risk factors while the plasma levels of AT antigen and activity were significantly decreased. As the acute thrombosis event may cause a transient reduction in AT level, which could be misread to suggest AT deficiency, measurements were repeated after the patient had recovered. The results revealed that the levels of AT antigen and AT activity were still significantly reduced, which demonstrated the patient had AT deficiency. Moreover, A mutation (c.233_236dup p.Val80Alafs*26) in the SERPINC1 gene, encoding AT, was identified by WES. The deletion mutation can lead to a frameshift at the 80th codon (Val) and premature termination at the 26th downstream amino acid of the SERPINC1 protein (p.Val80Alafs*26), resulting in a failure in expression of AT. Thus, we believe that multisystem VTE in this case should be associated with AT deficiency caused by SERPINC1 mutation.
The genetic burden of thrombophilia is estimated from 35 to 60%, indicating a strong heritability affecting the function of coagulation or antithrombin system 7. Generally, inherited thrombophilia is mainly attributed by either the loss of anticoagulant function (i.e., mutations in SERPINC1, PROC genes)8, or the gain of procoagulant function (i.e., mutations in F5, F2 genes)9. Among all of those mutations, the factor V Leiden mutation in F5 is the most common inheritance-related cause of thrombophilia. F2-related thrombophilia is the second most common genetic form of thrombophilia, occurring in about 1.7-3% of the European and US general populations2. Hereditary AT deficiency has a prevalence of 1:500–5000 in the general population10. AT is the endogenous anticoagulant of normal hemostasis, which regulates the coagulation cascade by inhibiting serine proteases of the intrinsic pathway (i.e., thrombin, factors IXa, Xa and XIa) 11. Up to 80% of cases with suspicion of inherited AT deficiency are caused by defects of SERPINC1 that encodes AT11. In this case, only the SERPINC1 gene mutation was detected by WES.
Inherited AT deficiency is divided into type I deficiency, in which both the functional activity and antigenic levels AT are proportionately reduced (quantitative deficiency), and type II deficiency, in which normal antigen levels are found in association with low AT activity due to a dysfunctional protein (qualitative deficiency) 5. Further specialized tests help to subclassify type II deficiencies. As the plasma AT antigen and activity were both decreased significantly, it was suggested that this patient had type I AT deficiency. Clinically, the thrombotic events often occur at an earlier age if someone has a genetical AT deficiency. VTE occurred in 85% of AT deficient relatives before 55 years of age in family studies 12. Moreover, homozygous individuals with type I AT deficiency have a higher risk of severe venous thromboembolism (VTE) in childhood13. However, this patient as well as part patients in other cases are older than 6014, which indicates the pathogenesis mechanisms of genetical AT deficiency may also involve with acquired risk factors.
SERPINC located on chromosome at q23.1–23.9 d spreads 13.5 kb, is composed of seven extrons and six introns 15. According to the Human Gene Mutation Database (HGMD), more than 250 mutations in the SERPINC1 have been already identified, and missense or nonsense mutations constitute more than 50% to the genetic defects, followed by small deletions, gross deletions, and small insertions 16. Most mutation change single protein-building blocks in AT, resulting in disrupting in its ability to control coagulation. In addition, genetic mutations could alter the domains of antithrombin-associated conformational instability, leading to protein polymerization 17. Missense and null mutations are widely reported in VTE families. For example, there is a G to T substitution at nucleotide position 13,268, resulting in the replacement of the normal alanine residue at position 384 by serine and the synthesis of a dysfunctional AT with a reduction in anti-IIa activity18, 19. A heterozygous missense mutation (c.848_849insGATGT) is partly responsible for the AT deficiency in a server VTE patients20. The combination of different SERPINC1 mutations or variants are also involved in multiple thrombophilic disorder 21. There are some correlations between AT deficiency phenotype and genotype. Generally, Type I deficiency is caused by nonsense mutations or short insertions and deletions within SERPINC1 which lead to frameshifts and result in a failure in expression of AT. Type II deficiency is usually caused by missense mutations affecting residues that are involved in AT function22. In this study, we reported the case of a severe multisystem VTE patient with type I AT deficiency caused by a heterozygous frameshift mutation of SERPINGC1. The c.233_236dup of SERPINGC1 is a frameshift that contributes to the protein synthesis termination, leading to protein destruction and type I AT deficiency, which has not been reported yet. Hotpot in SERPINC1 gene is rare but also been detected in other Chinese cohorts, including genetic variants c.881G > T (p.Arg294Leu) and c.883G > A (p.Val295Met)23–26.
The initial management of VTE in patients with AT deficiency should generally be no different from its management in those without AT 27. Occasionally, it is worthwhile to consider AT concentrate in the individual with severe thrombosis 28. In determining the length of oral anticoagulation therapy, the circumstances of the thrombotic event, all the patients risk factors for recurrence and all risk factors for bleeding should be weighed together. It is suggested that individuals carried with hereditary AT deficiency and already developed to VTE should receive long-term anticoagulation 27.
Based on this recommendation, a long-term oral anticoagulant dabigatran was applied in this case. No thromboembolic and bleeding event occur during the 5-year follow-up period. Dabigatran is a reversible, potent, competitive direct thrombin inhibitor. In comparison to similar anticoagulant strategy like warfarin, the benefits of dabigatran include decreased risk in major ischemic and bleeding event, as well as good compliance since there is no need of regularly laboratory test in monitoring clotting indices 29. More importantly, we have shown the successful treatment with dabigatran of multisystem VTE associated with hereditary type I AT deficiency. However, long-term effectiveness and safety of dabigatran for hereditary AT deficiency need to be confirmed in a big number of cases.
There are also some limitations in this case. A disadvantage of direct sequence analysis is that it is inadequate for revealing large gene rearrangements in all coding regions of SERPINC1 gene. Multiplex Ligation-dependent Probe Amplification (MLPA) should be performed simultaneously. Genetic counseling for his whole family is necessary. We are not able to determine AT deficiency in the asymptomatic family members based on these genetic data. Bioinformatic analysis is highly useful in clinical laboratories with limited experimental facilities. However, functional studies are extremely desirable to evaluate the possible effects of a genetic variation. Further studies are required to verify the frameshift for mutations described in SERPINC1, which may include recombinant models, analysis of SERPINC1 transcripts and minigene models.
In conclusion, we have identified a novel frameshift variation within the SERPINC1 gene in a multisystem VTE patient, suggesting frameshift mutations may play a significant role in human hereditary AT deficiencies. Our study enriches the insights of genetic factors for VTE and will facilitate the genetic diagnosis of this disease. In type I AT deficiency, oral anticoagulant dabigatran may be promising in prevention of VTE. However, further studies with larger sample size should be performed.