Molecular basis of quantitative factor V deficiency associated with factor V R2 haplotype

To investigate the molecular mechanisms of the quantitative factor V (FV) deficiency associated with the FV R2 haplotype, 4 missense mutations, Met385Thr, His1299Arg, Met1736Val, and Asp2194Gly, identified in the R2 haplotype allele, were analyzed by in vitro expression studies. The FV variant carrying all 4 mutations showed a markedly lower steady-state expression level than wild-type FV because of low synthesis rate and impaired secretion of the mutant protein. The Asp2194Gly mutation was found to play a key role in the impaired secretion of the mutant FV by interfering with its transport from the endoplasmic reticulum to the Golgi complex. The deleterious effect of the Asp2194Gly mutation was shown to be dominant among the 4 mutations. The Met385Thr mutation and His1299Arg mutation had no effect on steady-state expression levels, but the secretion rates of the mutant proteins were moderately decreased by these mutations. The His1299Arg mutation partially impaired glycosylation in the C-terminal part of the B-domain of the mutant FV, which was supposed to affect the secretion rate, but not the steady-state expression level. It was also suggested that the Met385Thr mutation partially impairs posttranslational modification of the mutant FV without affecting the steady-state expression level. No deleterious effect of the Met1736Val mutation was observed in terms of expression and intracellular processing. Our in vitro data strongly suggest that the naturally existing R2 haplotype mutant FV, which carries all 4 mutations, has the potential to result in quantitative FV deficiency in vivo owing to impaired expression of the mutant protein when the Asp2194Gly mutation is present.