Since the discovery of vitamin K in 1943, named after its crucial role in koagulation (Scandinavian spelling), little progress has been made in identifying the protein components of the VKOR complex or their corresponding genes. VKOR promotes blood clotting and is inhibited by warfarin, which, in return, is underused due to difficulties in dosage and fear of bleeding.

"The story starts with two families suffering from a deficiency of all vitamin-K-dependent factors (VKCFD type 2)," senior investigator Dr Johannes Oldenburg (University of Würzburg) explained to heartwire. "In these families, we mapped the candidate gene for this rare genetic disorder to a 20-centimorgan-large interval of chromosome 16. At the same time, we hypothesized that VKCFD type 2 and warfarin resistance are allelic variants of the same gene." The German group performed a systematic mutation screen of this gene sequence in two VKCFD2 subjects and four subjects with warfarin resistance. They found missense mutations in one gene in all participants and in a rat strain with warfarin resistance and named the gene VKOR complex subunit 1 (VKORC1). The gene encodes a small transmembrane protein, which the researchers found to be inactive in VKCFD2.

"Surprisingly, though, the mutant proteins from warfarin-resistant people are fairly sensitive to warfarin when tested in cultured cells," writes Dr J Evan Sadler (Washington University School of Medicine, St Louis, MO) in a News and Views article in the journal.3

"It is not yet known how the mutation causes warfarin resistance in vivo," said Oldenburg, "probably by interfering with the binding to warfarin. However, it is astonishing that the mutation in warfarin resistance spread over almost the whole protein, thus not allowing identification of a specific warfarin-binding epitope."

In future studies, other subunits of the multiprotein complex need to be isolated to entirely understand the pharmacological action of warfarin and the development of new coagulants targeting VKOR, Rost's team explains.

Although it seems a heavy burden for a small protein...this molecule alone may be responsible for recycling vitamin K.

Li's group, on the other hand, on the basis of their findings, suggests that only one protein, VKORC1, might regulate the activity of the VKOR complex. "Although it seems a heavy burden for a small protein without obvious relationship to other reductases, this molecule alone may be responsible for recycling vitamin K," says Sadler. Li and colleagues selected 13 genes on chromosome 16 that encode transmembrane proteins. Using short interference RNA pools against individual genes to study their ability to inhibit VKOR activity, they also identified the VKORC1 gene, because it was the only one that caused a reduction in the activity of the VKOR complex. Their findings suggest that "the activity is the product of a single gene," the authors write. However, it is also possible that cofactors act in conjunction with this gene. "It is important to characterize further VKOR as it may lead to more accurate dosing or to the design of safer, more effective anticoagulants than are currently available," Li et al conclude.

Oldenburg's group is very enthusiastic about their discovery. "The VKOR complex was the subject of intensive research for more than 30 years without anyone being able to resolve its structure," he told heartwire. "With the identification of the VKOR gene and the corresponding protein, the main component of this complex is now known. It represents the molecular target of coumarins, which are the most frequent drugs used for oral anticoagulation therapy worldwide." Further, he pointed out that the discovery has established the basis of two different genetic disorders. "Both VKCFD type 2 and warfarin resistance are caused by either homozygous or heterozygous mutations in the VKORC1 gene." Further studies are now needed to analyze the VKORC1 gene to assess the genetic variation of different warfarin dosages used in patients for oral anticoagulant therapy and to identify the functional domains and kinetics of the VKORC1 protein and the structure of the whole VKOR complex.