Asthma is believed to result from a complex interplay between a person’s genes and various environmental fac-tors at a specific time in his or her life. It can thus be viewed as the result of interactions that occur between internal (genetic) elements and external (environmental) exposures. Environmental factors that have been studied include viruses such as RSV (respiratory syncytial virus), cockroaches, cigarette smoke, exhaust, farm animals, med-ications (including acetaminophen), pesticides, pets, and wood smoke. The data are intriguing and raise further questions that continue to stimulate research both into genetic and environmental features. Recent attention has, for example, focused on factors that may affect the unborn child’s immune system.
Given the elevated rate of childhood asthma and the observation that more than half of children with asthma are diagnosed by the age of 3, researchers have also sought to examine the possible influence of prenatal environments on the developing fetus’s risk of developing asthma early in life. Some studies in particular have suggested the possibility of (but not proved) a link between lower-than-normal vitamin D levels in mothers during pregnancy and the development of asthma in their children. More research into why some persons, but not others, go on to develop asthma in the course of their lifetimes is desperately needed.
In the case of vitamin D’s importance, for example, a clinical trial was scheduled to begin in late 2009 to investigate if adequate maternal vitamin D supplementation in pregnancy leads to a decrease in asthma during the child’s first three years of life. The trial is titled, “Maternal Vitamin D Supplementation to Prevent Childhood Asthma (VDAART)”; Vitamin D is also thought to perhaps play a role in asthma symptom severity. John Brehm and colleagues published “Serum Vitamin D Levels and Markers of Severity of Childhood Asthma in Costa Rica” in the May 2009 American Journal of Respiratory and Critical Care Medicine and provided evidence of an inverse relationship between vitamin D levels and measures of allergy and asthma severity in Costa Rican children with asthma. The authors also point out that the “Results of some, but not all, epidemiologic studies suggest that vitamin D deficiency is associated with an increased incidence of asthma symptoms . . . higher maternal intakes of vitamin D during pregnancy are associated with decreased risks for recurrent wheeze in young children suggesting that vita-min D may play a role in the development of asthma.”
Clinicians have long noted that certain viral infections seem to be related to the development of asthma in pre-disposed individuals, both children and adults. Physicians refer to particular “asthmagenic” viruses (such as RSV) that cause typical respiratory infection and symptoms at first, only to leave the patient with an asthma-like condition. Not all infectious agents are viruses, however, and attention has also been directed to other infectious organ-isms and their possible role in asthma development. Lung infection with a common bacterium (not a virus) called Chlamydia pneumoniae (recently renamed Chlamy-dophila pneumoniae) has in particular been suggested (but not proven) as a possible cause of reversible asthma in adults. As of this writing, a clinical trial, AZMATICS: AzithroMycin/Asthma Trial in Community Settings, is under way at several locations in the United States to attempt to further address the possible link between persistent infection with the bacteria and the development of adult asthma.
One of the challenges in studying asthma is that asthma is a heterogeneous condition, which means that it is far from a single disease in terms of symptoms, response to treatment, and associations. Despite their both experiencing episodic cough, wheeze, and shortness of breath, a 6-year-old boy with food allergies, eczema, and year-round asthma does not “have the same asthma” as a 56-year-old woman without any allergy who cannot make it through the New England winters without her inhaler medications, for example. Similarly, a 30-year-old male baker with wheezing and cough due to a type of asthma called occupational asthma that is related to specific industrial settings and exposures has a different asthma than the aforementioned 6-year-old boy and 56-year-old woman. The development of asthma also reflects the genetic endowment we inherit from our parents.
The tendency to develop asthma, particularly in young people, has an inherited basis. If one or both parents have asthma, for example, their child has a greater likelihood of developing asthma as com-pared to another child whose parents have no personal history of asthma. A study of 344 families residing in Arizona revealed that among children diagnosed with asthma, 6% were from families in which neither parent had asthma, 20% had one parent with asthma, and 60% of children with asthma shared the condition with both parents. Similarly, studies of large populations of twins, comparing asthma and allergy in pairs of identical and fraternal twins, point to an inherited factor required for asthma development. Although it has long been observed that asthma runs in families, there is no one particular gene known to be responsible for the development of asthma.
It appears instead that several genes contribute to an individual’s tendency to develop asthma. Still other genes likely influence the disease. Genes located on several chromosomes, such as chromosomes 2, 5, 6, 7, 12, 16, 17, 19, and 20, are important in asthma. Further scientific investigations will hopefully delineate the identification of primary asthma genes, which are those that contribute to the development of asthma in the first place, as well as genes responsible for disease severity (asthma severity-modifying genes), and those determining a response to standard treatments (asthma treatment-modifying genes).
These observations, along with a large body of research findings, have led to the current view that the development of asthma is the result of a complex interaction between a susceptible individual and specific environ-mental conditions at a certain point in time. Gemma’s comment: It’s especially important to remember that asthma is tricky: changing weather conditions, humidity holding particles in the air, smoke, and volatile organic compounds all can produce asthma symptoms. In the city where renovations, construction, street repairs, cleaning and heating systems, and blowing stacks can occur at any time, it’s important to keep a sharp eye (ear and nose, as well) on the ever-changing environment.