VKORC1 mutations detected in patients resistant to vitamin K antagonists are not all associated with a resistant VKOR activity

BACKGROUND

The VKORC1 gene codes for the VKORC1 enzyme, which is responsible for the reduction of vitamin K epoxide into vitamin K. VKORC1 enzyme is the target of vitamin K antagonists (VKA). Twenty-eight rare single mutations in the VKORC1 coding sequence have been reported from resistant patients receiving unusually high doses of VKA to achieve therapeutic anticoagulation.

OBJECTIVES

It has been suggested that these mutations are responsible for the resistant phenotype, while biochemical consequences of these mutations on the VKORC1 enzyme have not yet been evaluated. Therefore, the aim of this study was to investigate the causality of the VKORC1 mutations in the resistance phenotype.

METHODS

Wild-type VKORC1 and its spontaneous mutants were expressed in Pichia pastoris and susceptibility to VKA was assessed by the in vitro determination of kinetic and inhibition constants.

RESULTS AND CONCLUSIONS

The in vitro analysis revealed that six mutations only (A26P, A41S, V54L, H68Y, I123N and Y139H) were associated with increase in K(i) , suggesting their involvement in the resistance phenotype observed in patients. A41S and H68Y led to selective resistance, respectively, to indane-1,3-dione and 4-hydroxycoumarine derivatives. The other mutations did not increase the K(i). Furthermore, 10 mutations (S52L, S52W, W59L, W59R, V66M, V66G, G71A, N77S, N77T and L128R) led to an almost complete loss of activity. These results suggest the existence of other resistance mechanisms.

Celiac disease risk in the USA: high prevalence of antiendomysium antibodies in healthy blood donors

BACKGROUND

Recent epidemiologic studies in Europe using antigliadin (AGA) and anti-endomysium antibodies (AEA) for initial screening have shown that the overall prevalence of celiac disease (CD) is about 1:300. There are no comparable scientific data for the USA, where CD is considered rare. The main aim of this study was to determine the prevalence of increased AEA in healthy blood donors in the USA.

METHODS

Sera from 2000 healthy blood donors were screened for IgG AGA and IgA AGA with an enzyme-linked immunosorbent assay test. All those with increased AGA levels, those with intermediate levels, and random samples with low levels were tested for AEA, using both monkey esophagus (ME) and human umbilical cord (HUC) cryosections as substrates.

RESULTS

The mean age of the blood donors was 39 years, with 52% being men, 85.2% being Caucasian, 11.8% African-American, 1.5% Asian, and 1.5% Hispanic. Eight healthy blood donors had positive AEA tests on both monkey esophagus and human umbilical cord. Among the eight subjects with increased AEA levels seven were Caucasian and one was African-American. All the four examined AEA-positive donors carried the known susceptibility alleles for CD.

CONCLUSIONS

The prevalence of increased AEA levels in healthy blood donors in the USA is 1:250 (8:2000). This is similar to that reported in countries in Europe, where subsequent small-intestinal biopsies have confirmed CD in all those with AEA positivity. On the basis of a high positive predictive value of the AEA antibody test, it is likely that the eight blood donors identified in this study have CD. These data suggest that CD is not rare in the USA and that there is need for a large-scale epidemiologic study to determine the precise prevalence of the disease in the USA.