Recurrence of a Phe31Ser mutation in the Gla domain of blood coagulation factor X, in unrelated Algerian families: a founder effect?

The presence of gene lesions in blood coagulation factor X (FX) was investigated in eight FX-deficient patients with severe bleeding symptoms, originating from five unrelated Algerian families (FX coagulant activity <1%, FX antigen ranging from 2% to 16%). A missense mutation (p.Phe31Ser) in the Gla domain was found in homozygous form for all patients but one, who is a compound heterozygote for the Phe31Ser mutation and for a non-sense mutation, Tyr130Term in EGF-2 domain. The haplotypes of FX alleles were determined by the following allelic variants located in the promoter: g.1323_1330delTTGTGA (A1/A2), g.1449T>C, g.1451C>A, upstream to exon 3: g.17257C>T and downstream to exon 3: g.17396A>C. The A1-C-A-T-C haplotype was found on each allele bearing the Phe31Ser mutation in the eight FX deficient patients contrasting with its low frequency (8%) in a control Algerian population (in which the Phe31Ser substitution was absent). The patients came from the same geographical area of Algeria (5/8 are certainly from Kabyle origin) and the haplotype analysis suggests a founder effect. Transient expression study reveals that, for the mutant FX-Phe31Ser, FX antigen level was 60% in conditioned media and 140% in cell lysates compared with the wild type FX. The partial retention and intracellular accumulation of the mutant FX might be due to impaired folding and/or conformational changes, and the discrepancies observed between the FX antigen level in COS-7 cell supernatant (60%) and in the patients plasma (2-16%) to an in vivo increased clearance of the secreted unstable FX mutant.