Incidence of activated protein C resistance caused by the ARG 506 GLN mutation in factor V in 113 unrelated symptomatic protein C-deficient patients. The French Network on the behalf of INSERM Free

Because multiple risk factors in one patient may increase the clinical expression of thrombophilia, we assessed the presence in protein C- deficient patients of the factor V Arg 506 Gln mutation responsible for activated protein C resistance. Using a strategy allowing rapid screening of factor V exon 10, we studied 113 patients with protein C deficiency and 104 healthy volunteers. We detected the Arg 506 Gln mutation in 15 patients (14%) and in one healthy subject (1%). We identified a previously unpublished sequence variation leading to an Arg 485 Lys substitution in three normal subjects and seven protein C- deficient patients. A significant difference in the allelic frequency of the Arg 506 Gln factor V mutation was found between protein C- deficient patients heterozygous for an identified protein C mutation (n = 84; allelic frequency, 4.8%) and protein C-deficient patients with no identified mutation in the protein C gene coding regions (n = 25; allelic frequency, 14%). The results demonstrate that a significant subset of thrombophilic patients has multiple genetic risk factors although additional secondary genetic risk factors remain to be identified for the majority of symptomatic protein C-deficient patients.