ACC: Rapid Genotyping Assay Helps Determine Warfarin Dose Response

March 24, 2007

NEW ORLEANS -- A rapid genotyping assay may help predict warfarin (Coumadin) dose response, researchers found.

NEW ORLEANS, March 24 -- A rapid genotyping assay designed to ease the management of warfarin (Coumadin) therapy is likely to be the first genetic test used in cardiology, researchers asserted here.

The one-hour assays showed 100% concordance with slower, direct sequencing for the warfarin sensitivity variant VKORC1 and 99% agreement for the warfarin metabolism variant CYP2C9.

So reported Jeffery L. Anderson, M.D., of Intermountain Healthcare and the University of Utah, both in Salt Lake City, at the American College of Cardiology meeting.

"Currently we treat everyone the same way with this drug," he said, despite variation in response with some patients needing as little as 1 mg and others needing up to 15 mg.

The assay represents the first broad application of genetics to medical cardiovascular practice, he said.

The FDA has recommended genotyping of both variants to optimize warfarin dosing, "but application has been limited, in part because of cumbersome assays," Dr. Anderson said.

"It looks like warfarin is here to stay," he added. "This will make it much safer, I think."

In the study, 139 patients contributed DNA samples swabbed from their mouth. The DNA was then run through polymerase chain reaction using Simple Probes on either Rapid Cycler 2 or RAPID LT equipment, which are commercially available.

Then, the samples were tested with high-resolution melting profile analysis on the HR-1 or RAPID LT to identify genotypes. Accuracy was compared with results of direct sequencing using Big Dye terminator chemistry.

Among the results, the researchers reported:

  • For VKORC1, both sequencing and melting curve analysis found 57 wild type, 60 heterogeneous variants, and 22 homogenous variants,
  • For CYP2C9, both methods found 126 wild type, 20 heterogeneous variants, and no homogeneous variants whereas the melting curve method found one heterogeneous variant labeled as a false negative, and
  • Overall there were 183 true negatives, 102 true positives, and one false positive with the melting curve method versus gene sequencing.

The one false positive was found to be due to a second, unexpected and rare variant, which Dr. Anderson said could be eliminated by probe redesign.

He concluded that there is excellent concordance between the methods, which "allows for genotype-guided warfarin dosing in clinical 'real-time.'"