OBJECTIVETo investigate the molecular mechanism of haemophilia B caused by the novel mutation of Arg327Ile (R327I) in FIX gene.

METHODSThe R327I, R327Ala(A), R327Lys(K), R327Asn(N) and a replacement mutant (FIXβFVII), in which FIX β strand 324-329 was replaced by that of FVII 298-303, expression plasmids were constructed with site-directed mutagenesis method based on the wild-type (WT) FIX expression plasmid. The HEK293 cell was transiently transfected, then the activity of FIX (FIX:C) was assayed by one stage method in the conditioned medium, while the FIX:Ag in both the conditioned media and the cell lysates was measured by ELISA. The molecular weight and the semi-quantity of expressed FIX were analyzed by Western blot. Fluorescent protein expression plasmid was constructed to investigate the synthesis and secretion of the FIX R327I mutation in the viable cells.

RESULTSFIX:C of the R327I mutant protein was 4.49% of the level of the WT in the conditioned medium, and the FIX:Ag of the R327I mutant protein in the conditioned medium and the cell lysates was 31.02% and 129.29% compared to that of WT, respectively. The mutation was characterized as cross-reaction material reduced (CRMR). The viable cell fluorescent assays showed that the R327I protein was more in both the viable cells and in lysosome than that of WT. The FIX:C of the R327A, R327K, R327N and FIXβFVII mutants was reduced compared to that of WT, the reduction of FIX:C of FIXβFVII was the most significantly amount among all the mutants in medium. FIX:Ag of all the mutants in the medium, except that the R327K increased, was reduced. The result of Western blot showed that the molecular weight of R327I protein was the same as that of WT, but the amount of the protein was much less compared with WT in the conditioned medium.

CONCLUSIONThe abnormal synthesis and secretion as well as the abnormal function of the R327I mutant protein causes haemophilia B. The residue of R327 as well as the β strand domain of R327 located play important roles of the specific function of FIX.