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SPLTRAK Abstract Submission
Identification and Characterization of a Novel Thrombomodulin Lectin-Like Domain Variant in a Patient with Venous Thrombosis
Sean X. Gu1, Lauren M. Shevell2, Christopher A. Tormey1, Henry M. Rinder1, Noffar Bar2,3, Alfred I. Lee2,3
1Department of Laboratory Medicine, Yale School of Medicine, New Haven, CT, United States/2Department of Internal Medicine, Yale School of Medicine, New Haven, CT, United States/3Section of Hematology, Department of Internal Medicine, Yale School of Medicine, New Haven, CT, United States

BACKGROUND: Thrombomodulin (THBD) is an anticoagulant protein with anti-inflammatory properties mediated by an N-terminal lectin-like domain. Prior studies identified THBD variants conferring increased venous thromboembolism (VTE) risk in African-Americans, but no study has implicated the lectin-like domain in thrombosis. Using whole exome sequencing (WES), we identified a novel THBD variant, A184T, in an African-American female with deep venous thrombosis (DVT). The location of the variant was in a linker segment of THBD adjacent to its lectin-like domain. We hypothesized that THBD A184T might exert effects on both inflammation and coagulation. OBJECTIVES: To characterize the effects of a novel THBD variant adjacent to the lectin-like domain on laboratory markers of inflammation and coagulation in an African-American female with VTE. METHODS: The patient is a 33 year-old African-American female with a history of recurrent miscarriages and iron deficiency, who early in her 7th pregnancy developed a proximal left lower extremity DVT. DNA was extracted from her blood and WES performed utilizing an Illumina HiSeq 2500 sequencer; mean coverage of the exome was 100X with 96% of the exome covered ≥8 times. Variants were filtered based on population frequency (≤2.5% allele frequency in gnomAD), linkage to a disease in OMIM, and variant type and classified according to ACMG criteria. A focused analysis of the exome utilizing our previously-published 55-gene extended thrombophilia panel identified a novel heterozygous variant of uncertain significance in THBD, A184T. Further analysis of THBD A184T was performed via sequence homology comparisons and blood measurements of coagulation studies, cytokines, and thromboelastography. RESULTS: Analysis of THBD A184T showed it to be located in a short hydrophobic segment linking the lectin-like domain to the rest of the protein. This variant is extremely rare, with an allele frequency of 0% in gnomAD. THBD A184T is adjacent to conserved hydrophobic residues, and substitution of a nonpolar alanine to a more polar threonine at this position is predicted to disrupt the hydrophobic linker region. Laboratory testing on the patient’s blood showed elevated D-dimer (1.23 mg/L; reference range, ≤0.5), thrombin-antithrombin (TAT) complex (5.3 mcg/L; reference range, <4), and plasminogen activator inhibitor-1 (PAI-1) (59 ng/mL; reference range, 4-43) with normal fibrinogen, von Willebrand factor antigen, tissue plasminogen activator, heparin cofactor II, and activities of coagulation factors II, V, VII, VIII, IX, and XI. Thromboelastography showed a shortened R time (4.6 min; reference range, 5-10) with other normal parameters. A cytokine panel showed normal TNF-alpha, soluble interleukin-2 receptor, and interleukins 1 beta, 2, 4, 5, 6, 8, 10, 13, and 17. CONCLUSIONS: In this African-American female with VTE, we identified a novel THBD variant, A184T, adjacent to the lectin-like domain. This variant is extremely rare and hypothesized to disrupt protein function. Laboratory testing revealed elevated D-dimer, TAT, and PAI-1 levels. Other studies have shown PAI-1 to be upregulated in inflammation, suggesting that THBD A184T may indirectly promote thrombosis through dysregulation of thrombomodulin anti-inflammatory function, providing evidence of a link between inflammation and thrombosis. Further studies examining in vitro activity of THBD A184T function are in development.