**2. Clinical overview of congenital heart diseases**

Congenital heart defects, affecting most heart's parts (Figure 1), can be classified into three categories: cyanotic heart disease, left-sided obstruction defects and septation defects (Bruneau, 2008). In cyanotic heart disease, the mixing of oxygenated and deoxygenated

Gene Expression Profiling – A New Approach in the Study of Congenital Heart Disease 189

In the pre-human genome sequencing era, the possibility to identify a relevant set of causative genes for multigenic diseases such as cardiomyopathies was limited. Classical genetic approaches were developed to find single loci or genes with the power to cause Mendelian disorders. In this case, disorders can result from a single base change in the deoxyribonucleic acid (DNA) that leads to significant alteration in protein abundance or function. However in disorders resulting from multiple gene variants that collectively contribute to an individual's multigenic defect, new genomics approaches are needed. With the arrival of such genomics technologies, genes microarrays approach has emerged as a real opportunity allowing the performance of gene expression analysis of disease-relevant tissues. If DNA defines the inherent genetic make up of a person, it is the transcription of the DNA into RNA (that could be translated into protein) that integrates the dynamic interaction of an individual with the environment. Consequently, microarrays provide us with an opportunity to measure mRNA abundance that correlate with a particular disease state, clinical outcome, or therapeutic response, giving us an unprecedented opportunity to investigate the genomic contribution to cardiovascular diseases (Cook and Rosenzweig,

Expression profiling is the study of the expression level changes of large numbers of genes simultaneously. The concept of microarray technology is simple: specific DNA sequences, called "probes," are selected to "target" genes of interest. Microarrays refer to solid substrates of glass, plastic, or silicon containing hundreds or thousands of microscopic spots of DNA (Figure 2). Each of the DNA spots, apposed to the solid material, contains hundreds to thousands of identical probes. Each probe has a sequence of nucleotide bases that is complementary and unique to a single gene. Because of the technology's advances, it is common to use microarrays carrying the entire complement of the human genome in

Fig. 2. Schematic of a microarray experiment. cDNA, complementary deoxyribonucleic acid;

In a typical microarray experiment, RNA is extracted from a tissue or sample of interest. The small part of the RNA population that represents the transcribed genes, mRNA, is

**3. Gene expression profiling** 

**3.1 Microarray technology** 

today's microarray experiments.

cRNA, complementary ribonucleic acid.

2002; Goldsmith and Dhanasekaran, 2004; Napoli et al., 2003).

blood results in the blue appearance of affected infants. Defects contributing to this condition include transposition of the great arteries (TGA), tetralogy of Fallot (TOF), tricuspid atresia, pulmonary atresia, Ebstein's anomaly of the tricuspid valve, double outlet right ventricle (DORV), persistent truncus arteriosus (PTA) and total anomalous pulmonary venous connection. The second main type of congenital heart disease, left-sided obstructive lesions, includes hypoplastic left heart syndrome (HLHS), mitral stenosis, aortic stenosis, aortic coarctation and interrupted aortic arch (IAA). The third type of congenital heart disease, septation defects, can affect septation of the atria (atrial septation defects, ASDs), septation of the ventricles (ventricular septal defects, VSDs) or formation of structures in the central part of the heart (atrioventricular septal defects, AVSDs). Other types of congenital defect that do not fit into the above three main categories are bicuspid aortic valve (BAV) and patent ductus arteriosus (PDA).

Fig. 1. Congenital heart defects. Diagram of heart illustrating the structures that are affected by congenital heart diseases. AC, aortic coarctation; AS, aortic stenosis; ASD, atrial septal defect; AVSD, atrioventricular septal defect; BAV, bicuspid aortic valve; DORV, double outlet right ventricle; Ebstein's, Ebstein's anomaly of the tricuspid valve; HLHS, hypoplastic left heart syndrome; HRHS, hypoplastic right heart; IAA, interrupted aortic arch; MA, mitral atresia; MS, mitral stenosis; PDA, patent ductus arteriosus; PS, pulmonary artery stenosis; PTA, persistent truncus arteriosus; TA, tricuspid atresia; TAPVR, total anomalous pulmonary venous return; TGA, transposition of the great arteries; TOF, tetralogy of Fallot; VSD, ventricular septal defect.

Depending on the severity of the congenital heart disease, mortality and morbidity varies but can be serious. The number of surgeries needed to correct many of the anatomical defects can weaken the affected children and considerably compromise their quality of life. Congenital heart surgery has made tremendous gains over the past 10 years; however, recovery and outcome statistics continue to point out the need for improvements in this increasingly younger patient population. Paediatric patients undergoing cardiac surgery continue to need mechanical assist devices, as well as prolonged inotropic support or an open chest despite a technically perfect repair. Moreover, perioperative myocardial damage with low cardiac output remains the most common cause of morbidity and death after repair of congenital lesions (Hammon, 1995). It is therefore essential to improve our understanding of the genetic mechanisms and pathways associated with the different congenital heart conditions and their response to the surgical stress of ischaemia and reperfusion injury and cardiopulmonary bypass.
