The heart is a muscular organ divided into four chambers that allows deoxygenated blood from the body to be pumped to the lungs to be oxygenated. From the lungs, the oxygenated blood enters back into the heart where it is pumped back to the rest of the body to supply the cells and tissues with oxygen.
The heart pumps continuously from 4 weeks after fertilization until death to maintain the continuous and controlled movement of blood through thousands of miles of capillaries that infuse every tissue and every cell in the body. In this way, nutrients and other essential materials are passed from the capillaries into the surrounding cells, where waste products are removed at the same time.
The energetic requirements of the heart are therefore highly demanding, and for these requirements to be met, fuel usage by the myocardial cells has to be at optimum levels. Cardiovascular diseases (CVD) describe a class of diseases that affect the cardiac muscle or blood vessels. Generally, these diseases are associated with a build-up of fatty deposits in blood vessels, causing an inflammatory response within the vessels, as well as reducing blood flow to the heart. Research has found that single nucleotide polymorphisms (SNPs) in the genes encoding various proteins responsible for lipid and glucose metabolism have significant effects on heart health.
Low-density lipoprotein (LDL) are the main carriers of cholesterol in the bloodstream and are often described as the ‘bad’ cholesterol since it accumulates on the walls of blood vessels if present in high amounts. LDL is confirmed to be a main independent risk factor for the process of atherosclerosis (blood vessel damage) 1.
Reduced LDL clearance
Studies have identified that SNPs in a particular protein that is secreted from the liver cause its enhanced expression and increased binding to LDL. In this way, the transport of LDL is greatly reduced, thereby preventing its recycling and degradation within the cells, which then leads to reduced LDL-cholesterol clearance 2. LDL then accumulates within the vessels and significantly increases the risk of cardiovascular diseases.
The cell surface receptor for LDL is one of the regulating mechanisms the liver uses for cholesterol metabolism. Normally, LDL will bind to this receptor where it is internalized and degraded. This process frees the attached cholesterol for use in other processes. Interestingly, research has described the role of SNPs in the LDL receptor as having pathological consequences for the heart 3,4 due to changes in its function.
Familial hypercholesterolemia, an inherited disorder characterized by consistently elevated levels of cholesterol is also due to SNPs in these important transport proteins or their receptors. The inherited SNP in the gene encoding for the LDL apolipoprotein receptors causes an impairment in the break-down of LDL by preventing the binding and internalization of LDL into cells 5. Levels of LDL and cholesterol in the blood, therefore, become elevated, leading to hypercholesterolemia.
Enhanced cholesterol uptake
Cholesterol transporters in the gut and liver and SNPs found in the genes of these transporters have been implicated in the development of atherosclerotic cardiovascular disease6. Normally, these transporters promote cholesterol and plant sterol elimination from the body by limiting their uptake into the gut and liver. SNPs in these genes cause sitosterolemia, a condition characterized by cholesterol and plant sterols accumulation in the blood, leading to premature cardiovascular disease7.
Dysfunctional lipoprotein metabolism
For cholesterol and triglycerides to be transported in the blood, they bind to specific proteins. This lipid-protein complex is referred to as lipoprotein and enzymes play a role in separating the two components when the lipoprotein has arrived at its destination. SNPs in these enzymes result in changes in the functionality of the enzyme. For example, increased activity of this enzyme causes the rate of transfer of cholesteryl esters from high-density lipoprotein to be elevated, resulting in hypertriglyceridemia (high triglyceride levels) 8.
Therefore, optimal lipid transport, degradation, and metabolism prevents the build-up of unhealthy cholesterols in blood vessels and reduces the risk of CVDs.
The heart muscle omnivorous and can use any readily available substrate. Normally, energy is derived from fatty acid oxidation, however, under conditions of stress, the heart switches to glucose as the main source of fuel 9. In the pathological remodeling of the heart, the predominant fuel is switched to glucose.
Research has found that SNPs in genes responsible for glucose regulation predispose individuals to Type II diabetes and CVD 10. This same gene was found to be associated with increased trafficking of immune cells to blood vessel walls, which contribute to inflammation 9. There is therefore a close link between altered glucose metabolism and its association with blood vessel remodeling and inflammation.
Nitric Oxide synthesis
Nitric oxide is a critical signaling molecule in the cells of blood vessels. When produced, its presence causes the relaxation of the smooth muscle in the vessels, and also allows the vasodilatation of these vessels to increase the volume of blood traveling through them. The synthesis of nitric oxide is dependent on an enzyme, which has varying levels of activity depending on the physiological need of the body (i.e. exercise, rest, stress, etc.). Deficiencies in this enzyme due to the presence of SNPs lead to reduced levels of nitric oxide which is associated with the increased occurrence of cardiovascular disease, including coronary artery disease 11.