Molecular Characterization of a Type I Quantitative Factor V Deficiency in a Thrombosis Patient that Is “Pseudo Homozygous” for Activated Protein C Resistance

Abstract

SummaryResistance to activated protein C (APC), which is associated with the FV Leiden mutation in the large majority of the cases, is the most common genetic risk factor for thrombosis. Several laboratory tests have been developed to detect the APC-resistance phenotype. The result of the APC-resistance test (APC-sensitivity ratio, APC-SR) usually correlates well with the FV Leiden genotype, but recently some discrepancies have been reported. Some thrombosis patients that are heterozygous for FV Leiden show an APC-SR usually found only in homozygotes for the defect. Some of those patients proved to be compound heterozygotes for the FV Leiden mutation and for a type I quantitative factor V deficiency. We have investigated a thrombosis patient characterized by an APC-SR that would predict homozygosity for FV Leiden. DNA analysis showed that he was heterozygous for the mutation. Sequencing analysis of genomic DNA revealed that the patient also is heterozygous for a G5509→A substitution in exon 16 of the factor V gene. This mutation interferes with the correct splicing of intron 16 and leads to the presence of a null allele, which corresponds to the “non-FV Leiden” allele. The conjunction of these two defects in the patient apparently leads to the same phenotype as observed in homozygotes for the FV Leiden mutation.

Factor V: a combination of Dr Jekyll and Mr Hyde

“…The main laboratory findings are the presence of a low APC sensitivity ratio (APC-SR) and low plasma values of factor V antigen and activity. [168][169][170][171] In this case the normal factor V allele is silent, and only the factor V LEIDEN allele is expressed. As a consequence, the patient who is genetically heterozygous for factor V LEIDEN is phenotypically homozygous for the mutation.…”

Aetiology of Venous Thrombosis

“…Plasma of such patients contain mutant FVL molecules only, similar to what is present in the plasma of people homozygous for the FVL mutation, resulting in what is called "Pseudohomozygous APC-R". Such cases have similar risk value to develop VTE as homozygous APC-R/FVL cases Simioni et al, 1996;Guasch et al, 1997). Acquired risk factors, including trauma, surgery, immobilization, pregnancy, use of oral contraceptives and LA have been reported to greatly increase the risk of developing VTE in individuals who have FVL.…”

Arg2074Cys missense mutation in the C2 domain of factor V causing moderately severe factor V deficiency: molecular characterization by expression of the recombinant protein

“…An additional 8 mutations causing FV deficiency have been reported in the heterozygous state, 5 of which were found in patients who carried the FV Leiden mutation on the second allele, leading to the "pseudohomozygous APC-resistance" condition. [14][15][16][17][18] Only one genetic defect associated with type II deficiency has been so far reported. 19 The molecular characterization by transient expression in COS-1 cells of the Arg2074Cys missense mutation in F5 causing moderately severe FV deficiency in an Italian patient is reported here for the first time.…”