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Asthma is a classic example of gene-environment interaction. A host of environmental triggers, from cigarette smoke to cockroaches, can set it off. A dozen or so genes for various molecules, including cytokines, have been implicated in asthma and related disorders such as eczema. The question is: How do these environmental triggers and the disease-implicated genes communicate?
It's a question quite a few want to answer. Researchers are using genetic screens and positional cloning, and some are looking at the evolution of allergens. One group of collaborators found multiple polymorphisms in the gene ADAM33--a discovery that raises even more questions. In Finland, researchers conducted a genome scan of a homogenous population and found linkage in a region of chromosome 7 for three phenotypes. In New York, researchers have started a gene bank for phenotypic data on patients with asthma.
Many could benefit from successful research. According to the National Institutes of Health, about 18 million Americans have asthma, five million of them children. The 18 million represents an increase of more than 150% since 1980. Asthma accounted for $12.7 billion (US) in medical expenses and lost earnings in 2000 alone. Worldwide, according to the World Health Organization, the economic costs associated with asthma are estimated to exceed those of tuberculosis and HIV/AIDS combined.
Primarily, asthma researchers approach gene-environment investigations by relating genes important in asthma and allergy to the environmental exposures considered significant to immune system ontogeny and asthma symptom phenotypes, says Scott Weiss. For example, Weiss's team at Harvard Medical School studies sequence variation in genes for several immune system-related cytokines and relates that to environmental exposures. Many of these molecules are related to innate immunity, which is inborn, as opposed to acquired immunity, which is obtained through prolonged contact with pathogens and allergens. In collaboration with Fernando D. Martinez, director of the Arizona Respiratory Center at the University of Arizona in Tucson, Weiss and colleagues are sequencing every gene in the Tlr (toll-like receptor) pathway; these genes are important in mediating responses to bacterial endotoxins and other environmental exposures.
ALLERGENS AND PARASITES Asthma has a strong genetic component, based on family and twin studies, a fact known for years, notes Tim P. Keith, senior director of human genetics at Genome Therapeutics in Waltham, Mass. Based on heritability studies, 60% to 70% of the variation in asthma susceptibility is due to genetic factors.
Since the 1980s, Oxford University's William Cookson, professor of human genetics, and colleague Miriam Moffatt, university research lecturer, have been conducting genome screens and positional cloning studies to find the genes that underlie asthma and other allergic diseases such as eczema.1 "Now the field has matured," says Cookson. About a dozen of complete genome screens exist worldwide that have been published; others have been done in the private sector. He says there is now "very good agreement" among these studies that about 10 chromosomes are involved.2-4
Cookson and Moffat speculate that evolutionarily, these immune system genes probably were used originally to fend off parasitic infections. Moffat has recently looked at a set of genes in a population of aboriginal Australians that are heavily infected with hookworm.5 She found a stronger genetic effect in the presence of parasites. "In Western society, obviously, we don't have the same element of parasitism," she says. "Allergens have replaced the parasites, that's what we're recognizing."
Keith and colleagues embarked on a genetic study to find asthma susceptibility genes.6 Together with UK colleague Stephen Holgate, they associated genetic information with clinical data such as bronchial hyper-responsiveness in a large family study. They were interested in whether a particular allele of a given polymorphism shows up more frequently in affected individuals versus controls. They found multiple polymorphisms in the ADAM33 gene, which were associated significantly with an increased risk of asthma.
ADAM33 is a membrane-bound metalloprotease with eight domains. Genome Therapeutics and its pharmaceutical partner, Schering-Plough, are investigating the exact function of ADAM33. It is known that family members display shedding of cell-surface proteins such as cytokines and cytokine receptors, but no one yet has established a link between this shedding and ADAM33.
In identifying the candidate chromosomal region, Keith and colleagues looked at different asthma phenotypes. In their population they saw increased statistical evidence for linkage to the ADAM33 chromosomal region in patients with both asthma and bronchial hyper-responsiveness. Studies like these are "key" to finding better asthma therapies, says Keith.
Physician Tarja Laitinen, University of Helsinki, and colleagues conducted a genome scan of a homogenous population from the Kainuu province of Finland.7 They found linkage in a region of chromosome 7 for three phenotypes: asthma, a high level of immunoglobulin E (IgE), and a combination of the two, with the strongest being the IgE level. IgE is the most well-known molecular marker for allergies. Now the Helsinki group is trying to narrow the region on chromosome 7 to find a candidate gene. Along with colleagues at the Karolinska Institute in Stockholm, the Helsinki group is conducting an epidemiological study of young children in Finland and Sweden; they are looking for environmental triggers in conjunction with the chromosome 7 genotype.
SUSCEPTIBILITY AND SEVERITY Environmental triggers shape asthma susceptibility and severity. Diet, allergens, and infections can trigger an asthmatic episode in a person at risk. Peyton Eggleston, a professor of pediatrics at John Hopkins University School of Medicine, studies allergens and their relation to asthma. The disease will not develop in genetically susceptible people unless they are exposed. "If [such a] person is raised in Antarctica, they don't ever get sensitized," states Eggleston.
Cockroach allergens, an environmental trigger, has received much attention in recent years, especially for populations living in urban environments. Eggleston and Hopkins colleague John Ford are embarking on a study of asthma genetics and sensitization to rodent and cockroach allergens in a Baltimore, Md., population.
Recently, Linda Rogers, an assistant professor of medicine at New York University (NYU) School of Medicine, found that exposure to cockroaches and subsequent sensitization is linked to asthma symptoms in elderly patients with asthma.8 Cockroach allergens comprise two proteins, Bla g 1 and Bla g 2, whose functions are unknown. The Bellevue Hospital Asthma Clinic, where Rogers works, keeps a database on 3,000 patients. She and colleagues tested half of those age 60 and older, finding that 47% were sensitized to cockroach allergens, as measured by a radioallergosorbent blood test; this rate is similar to that found in studies of children. Airflow and pulmonary function also were significantly affected in patients with cockroach sensitivity.
Rogers' colleague and clinic director Joan Reibman, an associate professor of medicine at NYU, notes, "We think there are genes that might make one susceptible to asthma, but also genes that might modify the severity of the disease, which is what we're really interested in."
Reibman, Rogers, and colleagues look at asthma as a disease of many traits, and they aim to define those traits and find genes associated with them. To do this, Reibman and others have started an asthma gene bank. They are phenotyping patients with asthma to catalogue their symptoms and severity, to go along with a repository of gene and protein samples obtained from the patients. They eventually hope to use proteomics techniques to find polymorphisms in specific genes, and to compare polymorphism distribution in the clinic's asthmatic population to a carefully matched control population. "Our hope is to look at gene-environment interactions," says Reibman.
NATURE VS. NURTURE In the past five or so years, many asthma studies have blended environment with genetics, and many more are in the works. Marsha Wills-Karp, professor and director of immunobiology at Children's Hospital, Cincinnati, came across C5 as a susceptibility gene in a genome-wide screen in a murine model.9 She and colleagues identified two loci on chromosome 2, and using microarray technology, they identified C5 as a gene of interest. As it turns out, susceptible A/J mice have a deletion of two base pairs that leads to impaired messenger RNA expression and a nonfunctional protein that can't be cleaved into its active fragments, C5a and C5b.
"As C5a had previously been identified as a mediator of anaphylaxis, which could induce several features of asthma, it was difficult to see how a defective C5 gene could confer susceptibility," recalls Wills-Karp. "Taken together these data suggested that C5 may play a previously unrecognized role in immune responses."
To sort this out, her husband, Chris Karp, showed that the measles virus invades cells through a complement receptor called CD46, whose activation turns off production of the Th1-regulating cytokine, IL-12, leading to Th2-mediated immune responses. These studies suggest that the complement protein provides a link between the innate and adaptive immune system. Thus, sufficient C5 protein, as seen in the resistant mice, conferred protection through a Th1 immune response, but the deletion in C5 in A/J mice leads to a default Th2 immune response and asthma.
They also hypothesized that where C5a may induce IL-12 and Th1, another type of complement protein, C3a, may inhibit IL-12 and result in Th2 cytokine production. This situation is true in both respiratory syncytial virus (RSV) and ambient particulate matter-induced asthmatic symptoms, which researchers showed were inhibited in C3a-deficient mice.10 "Taken together, these studies suggest that allergen-induced asthma or environmentally induced symptoms such as RSV and pollution likely are due to activation of complement at the airway surface," concludes Wills-Karp. Susceptibility, she explains, probably results from genetic alterations in genes controlling C5 or C3 expression or activity, which could confer an imbalance in these two pathways.11
Steven Kleeberger, chief of the laboratory of respiratory biology at the National Institute of Environmental Health Sciences, did a genome scan and found regions on chromosome 17 and 11 that are important contributors to ozone allergen susceptibility, as manifested by inflammation in the murine model's lung. The candidate gene is TNF-* on chromosome 17, a potent pro-inflammatory cytokine.12
In later work, Kleeberger and colleagues showed a link between the phenotype of ozone-induced increased permeability of lung tissue in a murine model and the Tlr-4 gene, which determines susceptibility to decreased lung epithelial cell function.13 If there is a mutation in Tlr-4, the mouse is protected against ozone-induced lung hyperperme- ability. Interestingly, Tlr-4 is also turned on during an endotoxin response. Says Kleeberger: "All this points to a critical role for the innate immune system" in asthma, and a fruitful direction for research.
Karen Young Kreeger (firstname.lastname@example.org) is a freelance writer in Philadelphia.
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