Asthma Health Article

Genetic profile

Current medical research continues to refine our understanding of how genes influence the development and severity of asthma symptoms in individual patients. It has been clearly established that asthma tends to run in families. Recent research, including studies that trace the appearance of asthma in families with twins, suggests that one's genetic makeup rather than environment is the major factor in determining an individual's predisposition—or potential—for developing asthma. Studies show that identical twins are more likely to share a genetic predisposition for asthma than are fraternal (non-identical) twins. Still, it is the presence of allergens and other substances in the environment that actually stimulate or "turn on" the genes that are related to asthma.

Determining the role of inheritance in asthma is made more difficult because many different genes seem to be involved in controlling the development and expression of asthma. Thus, there is no clear Mendelian pattern of inheritance of asthma such as in sickle cell anemia disease, which is clearly controlled by the presence or absence of a single gene for that disease.

Some scientists suspect that as many as 20 or more different genes may control an individual's potential for developing asthma. Scientists refer to this multi-gene component as polygenic heritability. Children of asthmatic parents have about a 30% chance of developing chronic asthma.

The task of identifying the specific genes responsible for various asthma symptoms will be made easier by the Human Genome Project. This mammoth research project has identified all of the genes that make up the 23 pairs of chromosomes in human cells. Much work remains in learning the role of each of these genes in the human body.

Asthma and the immune system

Research studies show that specific symptoms experienced by asthma patients, such as the inflammation of the airways and lungs, are initiated by the action of genes that regulate the activity of the human immune system. In other words, these genes control how the immune system responds to the presence of substances that can potentially trigger asthma symptoms.

Like a modern army, the human immune system consists of a wide array of specialized devices that work together to "neutralize enemy forces." In human terms, the "enemy forces" are antigens, the term given to any foreign agent invading the body. Antigens include disease producing organisms and toxic chemicals in the environment. The human equivalent of "specialized devices" is a complex network of cells in the immune system. Some of these cells produce antibodies, large molecules made up of proteins, that attack specific types of antigens.

The immune system "remembers" its contact with specific antigens, such as viruses, bacteria, and other pathogenic organisms, house dust mites, and plant pollen. Any subsequent—or future—encounter with a "known" antigen stimulates the immune system to produce antibodies that specifically target that antigen.

IgE antibodies

In more detail, scientists have identified a specific set of genes (on the long arm of Chromosome 5, to be exact) that force the immune system to make above normal amounts of the allergic antibody called Immunoglobulin E (IgE) in asthmatic patients. IgE is an antibody composed of a large Y-shaped protein molecule. The immune system produces this antibody in response to the presence of foreign substances—allergens—such as dust mites or pet hair. IgE is made by the plasma cells of the immune system. It is the key culprit in the process that creates the symptoms of asthma. IgE plays a critical role in initiating the inflammation of the respiratory tract, which is a primary cause of asthma attacks. A research study suggests that asthmatic patients produce higher levels of IgE antibodies in response to allergens such as house dust mites than do people without asthma.

A possible explanation for this overproduction of IgE antibodies could be related to a lack of exposure to common childhood illnesses. For example, cold viruses and other respiratory illnesses stimulate the human immune system to produce a certain type of helper T cell that specifically targets these disease agents. However, in the absence of such stimuli, the immune system instead produces another type of helper T cell that initiates the production of the IgE antibody.

IgE antibodies coat the surfaces of mast cells and white blood cells, called basophils, which are part of the immune system. The base of the Y of the IgE molecules attach to basophils in the blood and to mast cells, which are found in the connective tissue of the lungs, skin, tongue, and lining of the nose. Mast cells are sentries that rapidly react to the presence of antigens that trigger acute asthmatic incidents.

Some of the foreign antigens entering the respiratory airways will become attached to the extended arms of IgE molecules on the surface of the mast cells. This combination of antigen and antibody triggers these cells to release histamines and other substances into nearby tissues. Histamines are a type of chemical signal that initiates the inflammatory response, one of the primary symptoms of asthma. Inflammation involves increased blood flow to affected tissues. Histamines stimulate the dilation—widening—of the walls of blood vessels and make them more porous so that more blood fluid and proteins leak out of the blood vessels and into surrounding tissue, causing the swelling and reddening typical of inflammation. This inflammation, along with the constriction of the muscles in walls of the bronchial airways, narrows the air passages and makes breathing more difficult. These changes are what is referred to as an asthma attack.

Other studies have also suggested that the genes that are responsible for making the bronchial passageways "over reactive" (increasing the tendency of constricting or narrowing) in asthmatic patients are quite distinct from the genes that regulate the action of the immune system.

Recent genetic research may result in some major changes in our understanding of the role of specific genes in asthma. British scientists have tentatively identified a single gene that could be responsible for as many as 40–50% of all asthma cases. The U.K. scientists also suggest that four other genes may also play a significant role in the development of asthma. It is generally believed that some genes may simply enhance—magnify or reinforce—the action of other genes that are primarily responsible for triggering asthma. This task of unraveling the genetics of asthma is made more complicated by the variety of ways in which these genes can interact in different people.