Surgical Innovations

Blood Test Replaces Heart Biopsy

Since the 1970's, heart transplant patients have had to regularly undergo an invasive, uncomfortable, and potentially risky test for signs of rejection, a leading cause of death among heart transplant recipients. The test, endomyocardial biopsy, or EMB, involves inserting a catheter into a vein in the neck and threading it into the heart, so that a tiny amount of the heart muscle can be sampled for analysis. Now, a quick, easy-to-administer blood test is rapidly replacing EMB as the gold standard for diagnosing rejection of the donor heart.

So-called "heat maps" like the one above show which genes are upregulated (red) and downregulated (green) during rejection of a transplanted heart.

The test, a product of a five-year, multicenter study conducted with biomedical company XDx, was based on the hypothesis that a genetic test could detect the absence of rejection in heart transplant recipients. "We believed that using the knowledge gained by the mapping of the human genome, it might be possible to detect gene expression variations that correspond to immune activity during rejection. If so, these changes could be identified by testing a regular blood sample," explains Mario C. Deng, MD, Director of Cardiac Transplantation Research at Columbia University, Department of Medicine, Division of Cardiology, and Co-Principal Investigator of the Cardiac Allograft Rejection Gene Expression Observational study (CARGO).

CARGO proceeded in three phases, and involved testing of over 600 patients in eight U.S. centers.

During the first phase of the study, the researchers determined the genetic changes associated with the immune process involved in rejection after heart transplantation. They constructed DNA microarrays and analyzed over 7000 genes identified in medical literature as involved in immune activation by cells known as leukocytes. In contrast to older methods of studying the activity of single genes, powerful DNA microarrays arrange the entire genome on a single chip, and can provide a picture of the activation status of thousands of genes at once. Screening of more than 7000 genes during Phase 1 refined the researchers' gene selection to 252 candidate genes.

Further analysis of the 252 candidate genes then narrowed the pool to a set of 62. During this phase, Dr. Deng and the study team worked with XDx to analyze each of these genes in patients who did and did not experience rejection to determine their levels of activity. From there, further analysis produced an 11-gene set of the most significant genes associated with the immune changes in organ rejection. Using these 11 genes and another 9 for control, the company developed the AlloMap™ molecular expression test – a tiny chip with 20 genes used to evaluate a cardiac patient's blood sample.

During this phase, a prospective, blinded study of post-transplant patients was conducted to verify whether the 20-gene test could accurately detect the absence of organ rejection.

Results: The test was able to consistently detect the absence of rejection. Patients with low scores had a very low risk of rejection, while those with higher scores were more likely to experience moderate to severe rejection.

As a result of this clinical trial, AlloMap™ testing for rejection after heart transplantation has been certified in all 50 states and is now covered by insurance. "This represents a paradigm change in how transplant rejection will be monitored," Dr. Deng states. "For many patients, this simple blood test can now be used instead of invasive biopsy." Results of the CARGO study were published in 2006 in the American Journal of Transplantation.