THE CARBOXYL-TERMINAL REGION OF FACTOR IX IS ESSENTIAL FOR ITS SECRETION

The carboxyl-terminal region of factor IX (residues 403−415) contains several natural mutations which result in mild to severe forms of hemophilia B. A battery of factor IX minigene expression vectors carrying various mutations in the C-terminal region were constructed and studied by transient expression assay using HepG2 cells. Mutations included in this study are Y404P, I408N, T412N, T412S, T415G, T415S, T415L, and T415R as well as five selected naturally occurring mutations in the region, R403Q, R403W, Y404H, W407R, and T412K. In comparison to the normal factor IX, these mutations neither significantly affected the factor IX mRNA level nor affected the stability of the secreted factor IX in the culture medium but did decrease to various extents the intracellular and secreted factor IX protein levels as quantified by enzyme-linked immunosorbent assay. T415L, T415S, and T415R showed only minor reductions in the intracellular and minor to moderate reductions in the secreted factor IX levels. T415G showed ...