Molecular defects in the factor X gene caused by novel heterozygous mutations IVS5+1G>A and Asp409del

Factor X (FX) deficiency is a rare autosomal-recessive bleeding disorder caused by diverse mutations in the F10 gene. To investigate the molecular basis of severe FX deficiency in a mildly hemorrhagic patient, variants of the F10 gene were detected by sequencing. A missense mutation was analysed by in vitro expression and modelling analysis, and a splice mutation using ectopic transcript analysis. The levels of activity of FX (FX:C) were <1% in both intrinsic and extrinsic pathway assays and 1.71% in chromogenic assay, the level of FX antigen (FX:Ag) was 53.36% in the proband. Two novel heterozygous mutations (IVS5+1G>A and Asp409del) were identified in the F10 gene. Ectopic transcript expression combined with informative marker (heterozygous Asp409del) analysis of the splice mutation (IVS5+1G>A) revealed and confirmed that the transcript from the mutated allele was absent, likely caused by the nonsense-mediated mRNA decay pathway. In vitro expression analysis showed that the Asp409del mutant led to a loss of enzymatic activity rather than impaired expression. Molecular modelling analysis confirmed that the Asp409del mutant dramatically altered the conformation of the 185-189 loop and impaired binding of the loop to sodium ions (Na(+) ), diminishing the enzymatic activity of FXa. This is the first report to clarify the molecular mechanisms of two naturally occurring F10 gene variants that cause severe FX deficiency.