DENVER—The early environment may be important in determining whether a child develops atopy or asthma, but children’s responses to their environment are not all alike: The disease phenotype may be determined by genetic background as well as by exposure. At the annual meeting of the Academy of Allergy, Asthma, and Immunology, researchers provided examples of how understanding the interplay of genetic background and environment may aid in choosing and maximizing appropriate therapy.[1]

EARLY EXPOSURE PROTECTIVE

Erika von Mutius, MD, MSc, Head of the Allergy and Asthma Outpatient Clinic at the University Children’s Hospital in Munich, discussed the protective effects of raw milk consumption and maternal or infant exposure to traditional European dairy farms. If these factors are combined early in life, said Dr. von Mutius, “then we have a tremendous protective effect on atopic outcomes.” Timing of the exposure is important, she added: Exposure during the first year of life, but not after, appears to help prevent asthma, whereas cumulative exposure during the preschool years may protect against atopic sensitization and hay fever.

Disease phenotypes also differ in their responsiveness to environmental factors, such as endotoxin. For example, endotoxin levels in a child’s mattress correlate inversely with the prevalence of hay fever and physician-diagnosed asthma, but the relationship between those levels and wheeze prevalence is a bimodal curve. Low endotoxin loads have been linked to an increased prevalence of atopic wheeze, whereas prevalence of nonatopic wheeze was increased only with high loads. “Endotoxin exposure decreases the risk for atopic asthma but increases the risk for nonatopic asthma and wheeze,” emphasized Dr. von Mutius, “whereas for the early life farm exposure, both phenotypes are affected.” She added, “Certainly, the susceptibility to environmental exposures differs according to the susceptibility to different phenotypes.”

Genetic factors may shape this susceptibility. Dr. von Mutius cited findings relating exposure effects and disease phenotype with expression of CD-14, toll-like receptor (TLR) 2 and TLR-4. “Among the farmers’ children, the expression of CD-14 and TLR-2 is enhanced,” she pointed out. Thus, innate differences in expression of these genes could affect responsiveness to environmental factors.

GENOTYPE CAN GUIDE THERAPY

Identifying single nucleotide polymorphisms (SNPs) that affect disease phenotype may be of more than academic interest. “Implicit in the notion of pharmacogenetics is the notion that variability from one person to another in the response to a drug can, in part, be based on the genetic makeup of an individual,” stressed Stephen B. Liggett, MD, Professor of Medicine and Pharmacology at the University of Cincinnati. In the general population, response to a therapeutic drug such as albuterol is variable, following a broad normal distribution. He pointed out, “One of the goals of future therapies is that we could shift this curve to the right” by steering individuals who respond poorly to a drug toward other therapies.

Genotyping might help identify such patients. “We were very surprised to find that there’s a lot of genetic variability in the human ß2-adrenergic receptor,” said Dr. Liggett. Also, just as ethnic groups differ in prevalence and degree of responsiveness to ß-agonist therapy, they may also differ in frequencies of adrenoreceptor alleles.

Dr. Liggett and colleagues compared receptor gene down-regulation in cultured fibroblasts after in vitro exposure to ß-agonists. “In the so-called ‘wild-type’ receptor, about 25% of the receptors are lost after a chronic agonist exposure.” A variant with glycine at position 16, however, showed 41% down-regulation, and a third variant showed no change in expression. “When we look at human airway smooth muscle cells, … the amount of down-regulation is much more extensive: 77% in the ‘wild-type,’ almost complete disappearance with [the Gly16] polymorphism, … and a meager amount of down-regulation—29%” with the third.

Individual genotype was also related to decline in peak expiratory flow in a trial of daily albuterol use. Morning peak flow progressively decreased in patients homozygous for Arg16, but not in Gly16 homozygotes. Dr. Liggett said, “This is pretty clear evidence … that we have at least one gene identified that can predict long-term responsiveness to ß-agonists.” Other work has shown an increased number of asthma exacerbations in individuals homozygous for the Arg16 allele who were given long-term albuterol treatment, but not in those homozygous for the Gly16 variant. Dr. Liggett observed, “While it’s true that short-acting ß-agonists given on a regular basis appear to have a deleterious effect, it is very much dependent on your ß2 receptor genotype.”

Discovery of additional polymorphisms in the promoter region of the receptor genes prompted his group to consider the effects of particular sets of polymorphisms, known as haplotypes. After examining the genotypes of patients receiving ß-agonists, “there seemed to be a correlation between the haplotype and FEV1 [forced expiratory volume in one second] 30 minutes after inhalation of two puffs of albuterol.” Compared with patients homozygous for haplotype 4, those who were homozygous for haplotype 2 had robust short-term improvements in FEV1. “It turns out that haplotype 2 has increased expression of the receptor itself … and increased messenger RNA as compared to haplotype 4 receptor,” he noted. “We’re now convinced that we have a way to predict acute responsiveness to ß-agonists.”

Haplotype frequencies differ among ethnic groups. Haplotype 9 occurs in 5% of the African-American population, but not in whites. This haplotype includes “an SNP that is exactly in a glucocorticoid response element in the 5´ promoter region,” said Dr. Liggett. “So, we may have found one potential cause of steroid unresponsiveness (or poor responsiveness) in African-Americans.”

IL-13 POLYMORPHISM PROTECTIVE

Donata Vercelli, MD, also explored asthma’s diversity. She remarked, “We are probably not dealing with a single disease but a spectrum of phenotypes,” each with distinct pathogenetic mechanisms and associated genetic loci. “Subtle genetic defects may impact on the pathogenesis of asthma,” emphasized Dr. Vercelli, Professor of Cell Biology at the University of Arizona in Tucson. Both interleukin 13 (IL-13) and its receptor are expressed during allergic inflammation. And, she noted, “There is very strong evidence that IL-13 may be necessary and sufficient to induce an asthma-like phenotype in the mouse”; thus, variation in the IL-13 gene could affect adaptive immunity. Indeed, IL-13 gene polymorphisms have been linked with elevated immunoglobulin (Ig) E levels, atopic dermatitis, and asthma. Furthermore, she said, “Having polymorphisms in both the IL-13 promoter and the receptor … gives a fivefold increase in risk,” relative to having each SNP individually.

How might these variations affect disease? Dr. Vercelli described experiments comparing the effect of a single amino-acid substitution on IL-13’s ability to stimulate peripheral blood cells in vitro. If the variant rather than the “wild-type” IL-13 is added, “you see a massive response—the majority of the cells … now become positive” for CD23 expression. (“Wild-type” IL-13 induces expression in only 10%.) This increased activity may be linked to increased IgE levels and allergy. By contrast, a polymorphism in the CD14 gene promoter is associated with increases in serum CD14 levels. She noted, “In the carriers of this same polymorphism, IgE levels in the serum are decreased, so this is a protective effect.”

—Mimi Zucker, PhD