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Test Catalog

Test ID: THBNG    
THBD Gene, Next-Generation Sequencing, Varies

Useful For Suggests clinical disorders or settings where the test may be helpful

Ascertaining a pathogenic alteration in the THBD gene in patients with atypical hemolytic uremic syndrome

 

Ascertaining a pathogenic alteration in the THBD gene in patients with reduced thrombin generation and a strong family or personal history of excessive bleeding that is not explained by results of conventional and specialized coagulation testing

 

This test is not intended for prenatal diagnosis

Genetics Test Information Provides information that may help with selection of the correct genetic test or proper submission of the test request

This test detects alterations in the THBD gene, which have been associated with one of several clinical phenotypes and hereditary disorders, namely an increased risk for thrombosis, atypical hemolytic uremic syndrome (aHUS), and hereditary excessive bleeding following physical trauma or surgery. A genetic consultation is strongly recommended prior to ordering THBD sequencing.

 

The gene target for this test is:

Gene name (transcript): THBD (GRCh37 (hg19) NM_000361)

Chromosomal location: 20p11.21

Testing Algorithm Delineates situations when tests are added to the initial order. This includes reflex and additional tests.

No screening tests exist for defects in THBD.

 

If thrombotic tendency is a concern, a set of clinical guidelines from the British Society for Haematology on testing for heritable thrombophilia is freely available.(1)

 

If atypical hemolytic uremic syndrome (aHUS) is a concern, it is strongly recommended an etiological diagnosis of aHUS be attempted prior to genetic testing in order to focus on timely and informed treatment of the patient. Refer to the consensus statement on the identification and diagnosis of thrombotic microangiopathies by the Mayo Clinic Complement Alternative Pathway-Thrombotic Microangiopathy Disease-Oriented Group.(2) Additionally, a recommended list of laboratory investigations for patients identified as having aHUS has been developed by The European Paediatric Study Group.(3)

 

If a THBD-related bleeding disorder is a concern, this disorder has only been recently characterized and no algorithmic testing methodology has been developed that is specific to this disorder. A systematic diagnosis through conventional coagulation testing is recommended prior to considering genetic testing for any suspected bleeding disorder.

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Thrombomodulin (TM) is an endothelial cell membrane protein found mainly in capillary beds that functions as a co-factor to thrombin in the activation of protein C. TM binds to thrombin and switches its substrate specificity from procoagulant substrates fibrinogen, factor V, factor VIII, and platelets to anticoagulant protein C, enhancing activation by 1000-fold. Activated protein C downregulates further thrombin generation, suppressing clot formation. TM also has an anti-inflammatory role as a negative regulator of the complement arm of the innate immune system. TM enhances activation of thrombin activatable fibrinolysis inhibitor (TAFI, also known as procarboxypeptidase B), which inhibits fibrinolysis and inactivates complement-derived anaphlatoxins C3a and C5a. TM, in a thrombin-independent manner, also interferes with inflammation by suppressing leukocyte trafficking and dampening complement activation through lectin-like domain (4).

 

Despite its role in coagulation, it is unclear whether thrombomodulin gene alterations play a significant role in venous thromboembolism (5). The THBD gene encodes thrombomodulin and  pathogenic alterations in the gene appear rare among thrombophilic patients, even those with severe thrombophilia (4). No severe TM deficiencies have been identified in patients with thrombosis, indicating that thrombomodulin alterations, in the absence of protein C deficiency, might not be associated with large vessel thrombosis (5). Since strong genotype-phenotype correlation has yet to be demonstrated between these alterations and venous thrombosis, indiscriminate testing for alterations in THBD or other genes related to coagulation in unselected patients presenting with a first episode of venous thrombosis is not recommended. A set of clinical guidelines from the British Society for Haematology on testing for heritable thrombophilia (1) is freely available. There is somewhat stronger evidence of an association between thrombomodulin gene polymorphisms and a moderately increased risk for arterial thrombosis, although more studies including a larger number of patients are needed to more firmly establish this risk (5).

 

Some alterations in THBD are associated with atypical hemolytic uremic syndrome (aHUS), which is characterized by microangiopathic hemolytic anemia, thrombocytopenia, and renal failure. An estimated 5% of patients with aHUS have alterations in THBD (4). However, it appears that an alteration of a single THBD allele is not sufficient by itself to cause aHUS and additional factors are probably required, such as virus-like illness (4). A method to screen patients of aHUS based on presentation and a recommended list of investigations has been developed by The European Paediatric Study Group for HUS (3).

 

Finally, a specific alteration in THBD, c.1611C>A (p.Cys537*), is associated with an autosomal dominant hereditary bleeding disorder characterized by excessive bleeding following physical trauma or surgery and extremely elevated levels of soluble TM (6,7,8). As of January 2019, this is the only alteration in THBD associated with a bleeding disorder reported in the Human Gene Mutation Database (HGMD Professional 2018.4). A systematic diagnosis through conventional coagulation testing is recommended prior to considering genetic testing for any suspected bleeding disorder. THBD-related bleeding is associated with normal results for these tests (i.e., normal prothrombin time, activated partial thromboplastin time, thrombin time, and fibrinogen assays) (7). Individuals with THBD-related bleeding have been observed to have normal results for the following assays as well: Coagulation factors II, V, VII, VIII, IX, X, XI, XII, and XIII, von Willebrand factor antigen, von Willebrand cofactor activity, plasma antithrombin, protein C and S levels, and activated protein C ratio (7). Additionally, there should be no evidence of platelet dysfunction. In individuals with THBD-related bleeding, prothrombin consumption index has been observed to be elevated (7) and levels of soluble TM are extremely elevated (6). If an assessment of thrombin generation and measurement of plasma TM levels finds reduced thrombin generation or there are extremely high levels of soluble TM and no other explanation for bleeding can be found, a diagnosis of THBD-related bleeding should be considered and molecular testing is clinically indicated (7,9).

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

An interpretive report will be provided.

Interpretation Provides information to assist in interpretation of the test results

An interpretive report will be provided.

 

Evaluation and categorization of variants is performed using the most recent published American College of Medical Genetics and Genomics recommendations as a guideline. Variants are classified based on known, predicted, or possible pathogenicity and reported with interpretive comments detailing their potential or known significance.

 

Consultations with the Mayo Clinic Special Coagulation Clinic, Molecular Hematopathology Laboratory, or Thrombophilia Center are available for DNA diagnosis cases. This may be especially helpful in complex cases or in situations where the diagnosis is atypical or uncertain.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

Clinical:

Some individuals may have a mutation that is not identified by the methods performed. The absence of a mutation, therefore, does not eliminate the possibility of thrombophilia or atypical hemolytic uremic syndrome (aHUS). This assay does not distinguish between germline and somatic alterations, particularly with variant allele frequencies (VAF) significantly lower than 50%. Test results should be interpreted in context of clinical findings, family history, and other laboratory data. Misinterpretation of results may occur if the information provided is inaccurate or incomplete.

 

If there is a family history of thrombophilia or atypical hemolytic uremic syndrome, it is often useful to test first-degree family members to help establish the clinical significance of variants of unknown significance.

 

Technical Limitations:

Next generation sequencing (NGS) may not detect all types of genetic variants. Additionally, rare polymorphisms may be present that could lead to false negative or positive results. Therefore test results should be interpreted in the context of activity and antigen measurements, clinical findings, family history, and other laboratory data. If results do not match clinical findings, consider alternative methods for analyzing these genes, such as Sanger sequencing or large deletion/duplication analysis. Misinterpretation of results may occur if the information provided is inaccurate or incomplete.

 

If multiple alterations are identified, NGS is not able to distinguish between alterations that are found in the same allele ("in cis") and alterations found on different alleles ("in trans"). This limitation may complicate diagnosis or classification and has implications for inheritance and genetic counseling. To resolve these cases, molecular results must be correlated with clinical history, activity and antigen measurements, and/or family studies.

 

Unless reported or predicted to cause disease, alterations found deep in the intron or alterations that do not result in an amino acid substitution are not reported. These and common polymorphisms identified for this patient are available upon request.

 

Reclassification of Variants Policy: At this time, it is not standard practice for the laboratory to systematically review likely pathogenic variants or variants of uncertain significance that are detected and reported. The laboratory encourages health care providers to contact the laboratory at any time to learn how the status of a particular variant may have changed over time.

 

Contact the laboratory if additional information is required regarding the transcript or human genome assembly used for the analysis of this patient's results.

Clinical Reference Recommendations for in-depth reading of a clinical nature

1. Baglin T, Gray E, Greaves M, et al: Clinical guidelines for testing for heritable thrombophilia. Br J Haematol 2010 Apr;149(2):209-220

2. Go RS, Winters JL, Leung N, et al: Thrombotic Microangiopathy Care Pathway: A Consensus Statement for the Mayo Clinic Complement Alternative Pathway-Thrombotic Microangiopathy (CAP-TMA) Disease-Oriented Group. Mayo Clin Proc. 2016; Sep;91(9):1189-1211. doi: 10.1016/j.mayocp.2016.05.015

3. Ariceta G, Besbas N, Johnson S, et al: Guideline for the investigation and initial therapy of diarrhea-negative hemolytic uremic syndrome. Pediatr Nephrol 2009 Apr;24(4):687-696. doi: 10.1007/s00467-008-0964-1

4. Delvaeye M, Noris M, De Vriese A, et al: Thrombomodulin mutations in atypical hemolytic-uremic syndrome. N Engl J Med. 2009 Jul 23;361(4):345-357

5. Anastasiou G, Gialeraki A, Merkouri E, et al: Thrombomodulin as a regulator of the anticoagulant pathway: implication in the development of thrombosis. Blood Coagul Fibrinolysis. 2012 Jan;23(1):1-10. doi: 10.1097/MBC.0b013e32834cb271

6. Dargaud Y, Scoazec JY, Wielders SJ, et al: Characterization of an autosomal dominant bleeding disorder caused by a thrombomodulin mutation. Blood. 2015;125(9):1497-1501. doi: 10.1182/blood-2014-10-604553

7. Langdown J, Luddington RJ, Huntington JA, Baglin TP: A hereditary bleeding disorder resulting from a premature stop codon in thrombomodulin (p.Cys537Stop). Blood 2014 Sep 18;124(12):1951-1956

8. Maclachlan A, Dolan G, Grimley C, el al: On Behalf of the UK GAPP Study Group: Whole exome sequencing identifies a mutation in thrombomodulin as the genetic cause of a suspected platelet disorder in a family with normal platelet function. Platelets 2017 Sep;28(6):611-613. doi: 10.1080/09537104.2017.1283011

9. Philippou H: Unexplained bleeding: another player to look out for! Blood 2014 Sep 18;124(12);1850-1851

10. Faioni EM, Franchi F, Castaman G, et al: Mutations in the thrombomodulin gene are rare in patients with severe thrombophilia. Br J Haematol. 2002 Aug;118(2):595-599

11. Kunz G, Ireland HA, Stubbs PJ, et al: Identification and characterization of a thrombomodulin gene mutation coding for an elongated protein with reduced expression in a kindred with myocardial infarction. Blood 2000 Jan 15;95(2):569-576

Special Instructions Library of PDFs including pertinent information and forms related to the test