During routine screening for the prothrombin 20210G>A mutation as a risk factor for venous thrombosis, we and others have reported cases of prothrombin 20209C>T heterozygotes that are enriched in individuals of non-Caucasian descent who have had thrombotic events and/or obstetrical complications. The prothrombin 20209C>T variant, as well as six other variants close to position 20210, have been found by melting curve analysis using the LightCycler Instrument (Roche Molecular Biochemicals) and hybridization probes. Unlike assays which use restriction enzymes or allele-specific PCR, melting curve analysis can distinguish multiple base changes that lie within the region of a hybridization probe as long as the base change causes a significant change in melting temperature from expected results. We describe thirteen new prothrombin 20209C>T heterozygous cases from unrelated individuals and one family study which were found during routine screening of clinical samples for the prothrombin 20210G>A mutation. Ethnic and clinical information were available in 11/13 cases: Ten individuals were of African descent while one was Hispanic. Eight individuals had thrombotic events (venous thrombosis, pulmonary embolism, and/or stroke), one had a pregnancy loss, one had bilateral below the knee amputations due to severe peripheral vascular disease, and one had a questionable pulmonary embolism. We next examined the prevalence of the prothrombin 19911A>G polymorphism in 20209C>T heterozygotes since alleles 19911A and 20210A are in full linkage and since the 19911G allele has been associated with increases in both plasma prothrombin activity and venous thrombosis risk. Of twenty 20209C>T heterozygotes, eighteen were 19911GG homozygous and two were 19911AG heterozygous. These results are consistent with linkage of alleles 20209T and 19911G and contrasts with the linkage of alleles 20210A and 19911A. Since the 19911G allele was found in all 20209C>T patients, it is possible that the 19911G-20209T haplotype increases the risk for thrombosis and/or obstetric complications, especially among individuals of African descent.

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