Presence of a T383G Mutation in the Vitamin K Epoxide Reductase Gene (VKORC1) in a Patient Resistant to Four Different Vitamin K Antagonists.

Resistance to vitamin K antagonists is a rare disorder which until recently has not been associated to a genetic factor. The existence of inherited warfarin resistant rat strains has been related to mutations of the genes involved in the vitamin K cycle. Vitamin K dependent carboxylase was cloned in 1991¹, while human vitamin K epoxide reductase complex 1 (VKORC1), another enzyme implicated in vitamin K cycle, has been cloned more recently^2,3. Mutations in this gene are responsible for both human and rat warfarin resistance ³. In 1997, we reported ⁴ a 63-year old patient with recurrent pulmonary embolism and deep vein thrombosis without known hereditary or acquired thrombophilia who was found resistant to warfarin (up to 45mg/day), fluindione (up to 80mg/day), acenocoumarol (up to 12 mg/day) and phenprocoumon (up to 30 mg/day). With phenprocoumon, long-acting vitamin K antagonist, drug concentration reached 85 mg/L (usual range 1–5 mg/L) but the INR remained around 1. Daily low molecular weight heparin (LMWH) was continued 3 months. Treatment was discontinued on 2 occasions and a recurrent thrombotic episode was observed. The patient is now receiving a single daily dose of low molecular weight heparin (80 mg Enoxaparin, body weight 120 kg, 67 IU/kg once a day) for the past 10 years without thrombotic or bleeding episodes. Osteodensitometry remains normal after 9 years of treatment. Careful biochemichal investigation had demonstrated a deficiency in vitamin K dependent carboxylation and absence of blockade of vitamin K epoxide reductase by phenprocoumon. This year, we sequenced the three exons of VKORC1 in this patient and detected a heterozygous T383G transversion resulting in a leucine to arginine substitution (L128R). This mutation has been recently identified by Rost³ et al. in an individual with warfarin resistance. It was not found in 259 control subjects that we had tested. Interestingly, another gene mutation of the VKORC1 can be responsible for a combined deficiency of vitamin K dependent clotting factors ³. In conclusion, a resistance to all vitamin K antagonists, including warfarin up to 45 mg/day is extremely rare and the mutation T383G of the VKORC gene has been reported in only one patient before the case reported here. Testing for mutation in VKORC1 will help in explaining some cases of anti vitamin K resistance.