All nonsynonymous variants detected in coagulation pathway genes are presented in Table 2. One variant in SERPINC1 (Table 2, in bold) has not been reported previously. This single-nucleotide polymorphism causes a missense mutation (A to G base change [T to C on the complementary strand] in chromosome 1: 173873157) in exon 7 of the SERPINC1 gene. This results in an amino acid change of isoleucine to threonine in position 422 of antithrombin III (ATIII; NP_000479.1). The presence of this genome mutation was subsequently verified by both direct Sanger sequencing of the patient’s serum and post-mortem liver samples, and mRNA transcription of the SERC1A gene obtained from the patient’s liver sample (Figure 2). The potential for this mutation having functional consequences was demonstrated using DUET, a web-based computational program designed to study missense mutations in proteins (http://bleoberis.bioc.cam.ac.uk/duet/), predicting the destabilizing effects of new mutations on ATIII (Supplemental Digital Content 3, Supplemental Figure, http://links.lww.com/AACR/A86). No other potentially damaging variants were observed in any coagulation pathway gene (Table 2).
Verification of expression of the SERPINC1 mutation in our patient indicates that she was a heterozygous carrier of a new type of deleterious mutation in this gene, which is responsible for ATIII protein coding. ATIII is a α2-globulin protease inhibitor synthesized in the liver and inhibits factors IIa and Xa. Hereditary ATIII deficiency has a prevalence of 1:500 to 1:50003 and results from an autosomal-dominant heterozygous mutation (homozygous variant is likely incompatible with life). Hereditary ATIII deficiency (caused by >220 known genetic variants) can be classified as type I or type II based on the specific SERPINC1 mutation. Individuals with type I mutations have only 1 working copy SERPINC1 in each cell, resulting in ATIII activity approximately half normal. Type I mutations account for only 12% of ATIII deficiency but are responsible for 60% of pathologic clotting events.3
Type II deficiency (88% ATIII mutations, but responsible for only 40% of clotting events) results in production of ATIII with reduced activity.3 Type II deficiency can be subdivided on the basis of whether there is altered binding to the target protease (type IIa), altered heparin binding (type IIb), or an altered interaction between ATIII and thrombin (type IIc). This distinction has important clinical significance and indicates that the type of ATIII defect modulates not only the risk of thromboembolism but also the location of the clot (arterial versus venous system).4 Lack of sufficient biological material from our patient prevented us from further characterizing the ATIII deficiency subtype.
Although no significant thrombotic events had been reported in our patient before transplantation, a catastrophic thrombosis occurred intraoperatively. For affected individuals, the incidence of thromboembolism usually increases with age and can be as high as 50% by 50 years of age.3 Considering the heterozygous character of this mutation, the clinical presentation of thrombotic events is likely to be related to additional factors, such as the administration of antifibrinolytics, the use of continuous CRRT, and/or hypercoagulability related to ESLD itself.
Severe thrombotic complications associated with antifibrinolytic administration in patients with ATIII pathway mutations have been described for other genetic errors in the coagulation pathway, and, as in our case, those patients had no history of thrombosis.5 Because 30% to 80% of carriers of ATIII mutations have thrombotic complications during their lifespan, any modulation of the coagulation system in such patients can be associated with significant thromboembolic risks.
Liver failure is associated with profound disruption in a patient’s coagulation profile that includes changes not only in the production of coagulation factors and platelets but also in endothelial function, which results in a new unstable balance.6 There is up to a 75% decrease in levels of liver-dependent coagulation (factors II, V, VII, IX, and X) and anticoagulation factors such as proteins C, S, ATIII, plasminogen, antiplasmin, and ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13) (responsible for inactivation of von Willebrand factor) and up to a 200% increase in levels of liver-independent factors, including factor VIII, von Willebrand factor, and plasminogen activator inhibitor 1 in vascular endothelium.6,7 A significant reduction in endothelial production of tissue factor pathway inhibitor necessary for inactivation of factor Xa also has been demonstrated.6 These changes in factor levels could be responsible for shifting the coagulation equilibrium toward hypercoagulability.6,8,9 The use of antifibrinolytics in this unbalanced situation might precipitate thrombotic complications. Antifibrinolytics (lysine analogs), such as ε-aminocaproic acid and TXA, block lysine binding sites on plasminogen. This prevents binding between plasminogen activator–plasmin complexes and fibrin with subsequent inhibition of fibrinolysis.
Despite significant reductions in transfusion requirements, antifibrinolytic use, in fact, has been associated with severe thrombotic complications.2 Gologorsky et al10 reported severe thrombotic events after graft reperfusion in patients receiving TXA. Warnaar et al11 demonstrated that >50% of cases of pulmonary emboli and intracardiac clotting during OLT were associated with the administration of antifibrinolytics. The risks of antifibrinolytic-induced thrombosis during OLT can be exacerbated by pathologic conditions such as acute renal failure or hemodialysis and genetic factors including ATIII deficiency.
It also has been demonstrated that dialysis catheters used for CRRT can be associated with an increased risk of thrombosis, which is directly related to the duration the catheter was in place.12 CRRT in our patient was performed using a 9-Fr catheter placed in right femoral vein. This catheter was placed, and CRRT started, 3 days preoperatively and continued throughout surgery. Although this may have been an additional factor contributing to the thrombotic event in our patient, thromboses identified by Stavroulopoulos et al12 were associated with catheters that were in place for weeks, not days.
Although we were unable to measure the concentration and activity of ATIII preoperatively in our patient, and therefore do not have direct proof that this ATIII pathway mutation was the cause of the thrombotic event, considering the timing, extent, and fulminant character of thrombosis, it is very likely that this mutation, in combination with the other factors mentioned, contributed to this event.
It is important to recognize that hypercoagulability is an essential characteristic of ESLD. This understanding has already led to significant changes in patients’ preoperative evaluation in many transplant centers including routine screening for hypercoagulable syndromes. As genomic screening tests become more affordable and precise, screening for functional variants in the coagulation pathway should be considered.
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