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We have previously reported that the 3363 inserted (Ins) C mutation in exon 6 of the protein C gene was present in four unrelated French patients and in four French Canadian families with type I protein C deficiency as well as in a large Vermont protein C deficient kindred of French Canadian origin. The present study was designed to investigate the likelihood of the existence of a founder effect for this mutation in protein C deficient individuals of French origin living in France, Quebec and Vermont. In order to demonstrate a possible founder effect for the 3363 InsC mutation, we have previously constructed a high-resolution genetic map to locate several highly polymorphic markers close to the protein C locus. Thereafter, the markers D2S347, D2S2339, D2S383, D2S2271 and D2S2215 were genotyped in 117 heterozygotes from France (n = 7), Quebec (n = 36) or Vermont (n = 74). The allelic frequency distribution of these five markers was also determined in fifty control French Canadian subjects and thirty-two unaffected members of the Vermont kindred with normal protein C levels and compared with their frequency in our cohort of heterozygotes. Our data suggest that patients from Quebec and Vermont carry a common haplotype at the protein C locus. Moreover, in order to study the evolutionary history of the 3363 InsC mutation, we traced back the ascending genealogy of one proband in each of the families with this mutation. These results showed that the 3363 InsC mutation was most probably introduced in North America by a couple of French settlers who established themselves in 1669 on Isle d'Orleans located near Quebec City. All heterozygotes for the 3363 InsC mutation living in North America are related to these founders within 10 generations. Thus, these families afford a unique opportunity to evaluate the role of the protein C system in thrombophilia due to the high degree of linkage disequilibrium at the protein C gene, which in essence holds that variable more constant than in a more heterogeneous population.

A family with a high incidence of venous thromboembolism was investigated. We performed medical evaluations on 184 of the 411 surviving members of the pedigree, which allowed assignment of individuals into positive, equivocal, or negative categories with respect to their clinical histories of thrombosis. Subjects with antigenic levels of protein C less than 66% of a normal plasma pool were classified as having protein C deficiency. Positive thrombotic histories were found in 13 of the 46 family members determined to be protein-C deficient and in only five of their 138 biochemically unaffected relatives. Statistical analysis of the association between thromboembolic disease and protein-C deficiency was strongly positive chi 2 = 24.95, P less than .0001 with n = 184), indicating that heterozygous protein-C deficiency is an important independent risk factor for the development of thrombotic manifestations in this pedigree. However, the absence of thromboembolic manifestations in many of the protein-C deficient family members to date indicates that other, as yet undefined, factors must play an important role in the clinical expression of this disorder.