Active γ-carboxylated human factor IX expressed using recombinant DNA techniques

Abstract

Factor IX (Christmas factor), a vitamin K-dependent plasma protein made in the liver, functions in the middle phase of the intrinsic pathway of blood coagulation¹. A functional deficiency of factor IX underlies haemophilia B, a chromosome X-linked recessive disease for which the major therapeutic approach is replacement treatment using factor IX concentrates. The cloning and characterization of the gene for human factor IX^2–4 would mean that human factor IX could be produced in greater yield and purity through using recombinant DNA techniques⁵. We have now used a human factor IX cDNA clone⁴, inserted into a vaccinia virus-derived vector, to infect human hepatoma cells which normally produce no factor IX⁶, and mouse fibroblasts. Fully active factor IX was produced by the hepatoma cells, whereas the fibroblasts produced a protein less active than natural factor IX, even in the presence of high levels of vitamin K. Human factor IX is extensively post-translationally modified, and thus represents probably the most complex protein produced in active form by recombinant DNA techniques to date. Our study also illustrates the potential of vaccinia virus-based vectors for expressing significant amounts of complex, clinically useful proteins in eukaryotic cells, in addition to its already demonstrated usefulness for producing live recombinant vaccines^7–9.