title: Homozygous factor V-deficient patients show resistance to activated protein C whereas heterozygotes do not.
abstract: Resistance to activated protein C (APC resistance) was assessed in plasma from patients with heterozygous and homozygous factor V (FV) deficiency. Because of the identity of the new APC cofactor with non-activated FV, it was expected that the lower the FV level the higher the APC resistance in plasma. Heterozygotes for the FV defect (both antigen and activity levels around 40%) did not show APC resistance in plasma. In contrast, homozygous patients for the same defect (less than 1% antigen and activity levels), had obvious APC resistance. Whether this finding was consistent with a spurious APC resistance or whether it truly reflected the lack of the APC cofactor activity in congenital FV deficiency remains to be clarified. Mixing (1:1) plasma from patients with a homozygous FV defect with pooled normal plasma (PNP) corrected both procoagulant and anticoagulant FV activities. Whenever severely APC-resistant plasma was used in place of PNP, only procoagulant activity was corrected and APC resistance was not affected. This suggests that homozygous FV-deficient plasma completely lacks the cofactor, i.e. the second APC cofactor, which can correct APC resistance in plasma. This indirectly confirms that the second APC cofactor is related to FV.
authors.affiliation: IV Chair of Internal Medicine, Padua University Medical School, Italy.
authors.forename: P
authors.initials: P
authors.lastname: Simioni
authors.forename: A
authors.initials: A
authors.lastname: Girolami
mesh_terms: D004351:Drug Resistance; D005166:Factor V Deficiency / Q000097:blood / Q000235:genetics*; D006579:Heterozygote; D006720:Homozygote; D006801:Humans; D010314:Partial Thromboplastin Time; D011486:Protein C / Q000494:pharmacology*
chemical_list: D011486:Protein C