Severe factor V deficiency: exon skipping in the factor V gene causing a partial deletion of the C1 domain

Background: Severe factor V (FV) deficiency is a rare coagulation disorder, characterized by very low or unmeasurable plasma levels of functional and immunoreactive FV. Among rare inherited coagulopathies, FV deficiency is the least characterized from a molecular point of view (only 12 mutations have been reported). Objectives: The aim of this work was to investigate, at the molecular level, the pathogenetic mechanisms responsible for a case of severe FV deficiency. Patients and methods: A 19-year-old Iranian man showing unmeasurable FV activity and severely reduced FV antigen level in plasma was studied. Mutation screening was performed by sequencing. The effect of the identified mutation was investigated both at the mRNA and at the protein level. Results: Molecular analysis of the factor V (FV) gene identified a novel homozygous A!T transversion at position + 3 of the donor splice site of intron 19 (IVS19+3A→T). Production of mutant mRNA in HeLa cells demonstrated that this mutation causes the entire exon 19 to be skipped from the FV mRNA. The mutant processed transcript codes for a deleted FV, lacking the first 24 amino acids of the C1 domain. Expression of the mutant FV protein in COS-1 cells showed that the deleted protein was synthesized but not secreted; moreover, the intracellular amount of deleted FV was reduced compared to wild type, suggesting intracellular degradation of mutant FV. Conclusions: This work reports the molecular characterization of the first mutation causing a partial deletion in the FV molecule, resulting in a severe impairment of protein secretion.