The Coagulation and Molecular and Immunodiagnostics Laboratories of ITxM Diagnostics are pleased to offer testing for the HR2 haplotype, which is associated with mild resistance to activated protein C (APC) and with an increased risk of thrombosis.

BACKGROUND

Venous thromboembolism has been associated with molecular defects in several hemostatic components.  The most frequent genetic risk factor for deep venous thrombosis (DVT) is a poor anticoagulant response to APC.  Resistance to APC (APCr) is found in ~23% of our local unselected patient population with venous thrombosis.  The majority of these patients have APCr due to the presence of the Factor V Leiden mutation.  Researchers have actively studied the factor V gene to determine the cause of APCr in the 5-10% of patients that do not carry the Leiden mutation.  Recently, evidence has emerged that shows that Factor V genetic components other than the Leiden mutation contribute to the APCr phenotype and are associated with increased thrombotic risk.

METHODS

A polymerase chain reaction (PCR) based assay amplifies an 828 bp fragment of the factor V genomic DNA containing the R2 allele.  The amplified fragment is then restriction endonuclease digested using Rsa I.  Digest fragments are separated by agarose gel electrophoresis to allow detection and discrimination of homozygosity or heterozygosity for the polymorphism in the factor V gene.

CLINICAL SIGNIFICANCE

A haplotype (HR2) including nine polymorphisms in exon 13 is always found in subjects carrying the Factor V A4070G allele (R2 allele).  The R2 allele was found to affect the APC anticoagulant response both in noncarriers of the Leiden mutation and in carriers, pointing to an association between R2 allele and APCr. Homozygosity for HR2 has been shown to increase thrombotic risk.    In addition, combined heterozygosity of HR2 and Factor V Leiden confers a 3 to 4- fold increase in relative risk of thrombosis as compared with Leiden alone.  The clinical significance of the HR2 haplotype in combination with other genetic factors such as the Prothrombin Gene Variant has not yet been determined.  Testing for the presence of this haplotype should not be used for screening.  It should be done on patients with a family or patient history of thrombosis under the following circumstances:

1. Markedly abnormal APC resistance test, heterozygosity for the Leiden mutation, and multiple thrombotic episodes
2.  Abnormal APC resistance test and negative for the Leiden mutation
3. Exclusion of acquired APC resistance; negative testing for lupus anticoagulant, patient is not pregnant and normal levels of factor VIII.

SPECIMEN REQUIREMENTS

 In order to assure the accuracy of our test results, the Factor V HR2 Haplotype genetic assay must be performed on whole blood.

1. Blood must be collected in one (1) yellow top tube (ACD).  DO NOT centrifuge the specimen and DO NOT separate plasma.
2. Ship sample at room temperature.
3. Specimen must be received within 72 hours of collection.
4. We request that a genetic consent form be submitted with specimen

TEST INFORMATION 

The Factor V HR2 Haplotype genetic testing fee is $220.00.

TEST CODE:  5591               CPT CODES:  83890, 83898, 83892, 83894, 83912

This test was developed and its performance characteristics determined by ITxM Diagnostics.  It has not been cleared or approved by the U.S. Food and Drug Administration.  This test is used for clinical purposes and should not be regarded as investigational or for research use.  This laboratory is certified under the Clinical Laboratory Improvement Amendments of 1988 (CLIA) as qualified to perform high complexity clinical testing.

FOR FURTHER INFORMATION CALL:  412-209-7270  OR  1-800-967-9672

REFERENCES 

1. Bernardi F., Faioni, EM, Custoldi, E, et al.  A factor V component differing from factor V R506Q contributes to the activated protein C resistance phenotype.  Blood 1997; 90; 1552-7.
2. Faioni, EM, Franchi, F, Bucciarelli, P., et al.  Co-inheritance of the HR2 haplotype in factor V confers an increased risk of venous thromboembolism to carriers of the factor V R506Q (factor V Leiden).  Blood 1999; 94; 3062-6.
3. Alhenc-Gelas, M, Nicaud, V, Gandrille, S, et al.  The factor V gene A4090G mutation and risk of venous thrombosis.  Thromb. Hemost.  1999; 81; 193-7.
4. DeVisser, MC, Guasch, JF, Kamphuisen, PN, et al.  The HR2 haplotype of factor V effects on factor V levels, normalized activated protein C sensitivity ratios and risk of venous thrombosis.  Thromb. Hemost.  2000; 83; 577-83.