Gene Expression is the process by which the genetic information contained in DNA is transcribed into messenger RNA (mRNA) for subsequent translation into the functional protein gene product. RNA levels can be measured and analyzed to provide current diagnostic information about a patient’s disease state.
Complex diseases, such as cardiovascular disease, are the result of a combination of physiological phenomena influenced by the interaction of genes, gene products, and the environment. In recent years, a number of molecular diagnostic companies have successfully developed and commercialized multi-gene expression based tests, which have enabled physicians to make more informed clinical decisions. Gene expression provides valuable, tissue and cell-specific information about the molecular mechanisms involved in disease processes, allowing a clinician to, for example, evaluate cardiovascular disease state, activity, and/or progression at a point in time.
It is well established that the immune system plays a key role in the development of atherosclerotic plaque in coronary artery disease (CAD).1 This knowledge makes it plausible to study circulating immune cells that are easily accessed in peripheral blood as a means of assessing the state of atherosclerotic occlusion in the coronary vasculature. With the introduction of Corus™ CAD, CardioDx has expanded the application of genomic medicine into the cardiovascular disease field. Corus CAD is the first and only clinically validated gene expression test that objectively quantifies the likelihood of obstructive* CAD in stable chest pain patients. Corus CAD has been validated in a rigorous multi-center trial and integrates the expression levels of 23 genes and other patient characteristics that have been demonstrated to indicate obstructive CAD. With Corus CAD, CardioDx has applied robust, proven industry practices as well as implemented novel, state-of-the-art gene expression techniques.
State-of-the-Art Gene Expression Techniques
Stabilization of RNA in Whole Blood Samples (Collection and Preservation of Gene Expression in Blood)
The collection and extraction of ribonucleic acid (RNA) from blood is a sensitive and sophisticated process. In order to accurately quantify the levels of cellular RNA from blood samples, the natural process of RNA degradation must be prevented. Until recently, stabilization of RNA in a whole blood sample required a complex collection process, making tests of this nature cumbersome for clinical application. However, CardioDx has conducted a large scale clinical trial, PREDICT (Personalized Risk Evaluation and Diagnosis In the Coronary Tree) to assess gene expression utilizing the PreAnalytix PAXGene® blood collection system.
This system effectively ensures the reliable preservation and stabilization of RNA molecules, dramatically simplifying the genomic sample collection process for clinicians, as no exacting specimen storage or pre-analytic processing steps are required. This advancement has enabled CardioDx to introduce Corus CAD, a gene expression test in the cardiovascular disease field that is suitable for broad clinical utilization. After the blood sample arrives at the CLIA2-licensed CardioDx Commercial Laboratory, highly reproducible automated methods are used to isolate the RNA, and convert it to a DNA copy of a messenger RNA (cDNA), and quantify expression levels by quantitative real-time polymerase chain reaction (qRT-PCR). These expression levels enable the assessment of obstructive CAD likelihood using the clinically validated product.
Microarrays
State of the art microarrays, providing the simultaneous measurement of gene expression across the entire human genome, allowed CardioDx scientists to determine which gene expression levels are altered in the presence of CAD. CardioDx has used the results of such arrays in the discovery phase of product development to compare the gene expression profiles between individuals with and without disease. This led to the identification of a smaller set of only a few hundred genes that became candidates for inclusion in the final Corus CAD test.
Quantitative Real Time Polymerase Chain Reaction
qRT-PCR is a highly sensitive and reproducible technique recognized as the gold standard for precise quantification of gene expression. Because qRT-PCR is practical for use on a smaller number of genes, it is perfectly suited for the final stages of test development - the final selection of genes - and is incorporated into the CardioDx Commercial Laboratory workflow to produce the final Corus CAD test result.
CLIA Lab
CardioDx operates a Clinical Laboratory Improvement Act (CLIA)-licensed laboratory on site in Palo Alto, California. All CardioDx tests are performed in this state-of-the-art, licensed facility ensuring high quality output with objective and reproducible results.
Biostatistical Analysis
The final Corus CAD algorithm transforms the gene expression levels of associated genes into the statistical likelihood of obstructive CAD. CardioDx utilized a range of statistical approaches, including logisitic regression, to create this final algorithm.
Gene Expression at CardioDx
CardioDx designed the PREDICT study with the goals of developing and validating a multi-gene expression based test that uses peripheral blood to assess the likelihood that a stable chest pain patient has obstructive CAD.3 The result of this work has led to the Corus CAD product.
Corus CAD is a gene expression test based on a clinically validated algorithm that integrates the expression levels of 23 genes and other patient characteristics that have been demonstrated to indicate obstructive CAD. The test was designed to provide clinicians with robust, objective biological information, which is distinct from the existing functional and anatomical information used to assess CAD in chest pain patients today.
* Obstructive CAD is defined as at least one atherosclerotic plaque causing ≥50% luminal diameter stenosis in a major coronary artery (≥1.5 mm lumen diameter) as determined by invasive quantitative coronary angiography (QCA).
- Seo D, Ginsburg G, Goldschmidt-Clermont P. Gene Expression Analysis of Cardiovascular Diseases. Novel Insights Into Biology and Clinical Applications. JACC. 2006;48(2):227-235.
- CLIA# 05D1083624.
- PREDICT trial. Clinical trial summary found at: www.clinicaltrials.gov, NCT00500617.
