Identification and Characterization of a New Mutation in Coagulation Factor VII (Carmel mutation), with Different Reactivity Towards Tissue Factors of Different Origin

Inherited factor VII (FVII) deficiency is a rare autosomal recessive disorder manifested by bleeding tendency in homozygotes or compound heterozygotes. The bleeding tendency is variable and does not always correlate with the measured amount of FVII. More than 30 mutations causing FVII deficiency have been identified. The aim of this study was to identify the molecular defect in a family with FVII abnormality and to characterize the functional defect and its clinical consequences.

An 80 year old woman showed various prolonged prothrombin time (PT) dependent on the origin of tissue factors (TF) in the reagents, in the range of 5-18%. FVII activity measured using placenta human TF was ~4% and less than 1% using recombinant human TF or rabbit brain TF. Direct sequencing of PCR-amplified FVII genomic DNA exons demonstrated a single homozygous nucleotide exchange G>A in exon 6 predicting missense mutation Tyr164Cys, in close vicinity to the active site of the protease and to the binding site of FX.

Sequencing of exon 6 in six members of the family comprising three generations was performed. All of them exhibited low activity of FVII. Direct sequencing of PCR-amplified exon 6 demonstrated that her son, daughter and grand-daughter are heterozygote to the novel mutation, Tyr164Cys, henceforth, Carmel mutation. The grand -daughter (third generation) was identified as a compound heterozygote carrying in addition an Ala244Val mutation along with a Arg353Gln polymorphism, a well-known linkage previously characterized in the Israeli population, inherited from the paternal side. Her FVII activity was 3-7% and 8% FVII antigen. In the individuals heterozygote for Tyr164Cys we detected 41-54% activity and 46-66% antigenic FVII.

The measurement of VIIa in the proband and her grand-daughter could not demonstrate any activity in plasma, possibly because of perturbation of the interaction between the mutated FVII and TF or FX which are part of the assay. The heterozygote for Tyr164Cys showed approximately 30% FVIIa of normal controls .

Despite the dramatic laboratory presentation, the clinical manifestation of the Tyr164Cys genotype is very mild, suggesting that a low level of FVII is sufficient to prevent bleeding. The homozygous patient reached an old age, gave birth to two children and did not suffer any major bleedings during her life. The compound heterozygote presents a similar phenotype with slightly higher FVII activity and antigen, no measurable FVIIa activity and mild clinical presentation. Further investigation into this novel mutation is needed in order to characterize the interaction of the FVII with TF, phospholipids and FX and to understand the reason for such low amount of FVII protein in plasma.