© 2005 American Thoracic Society DOI: 10.1165/rcmb.F303
The Role of Prostaglandin D Receptor Gene in Asthma PathogenesisBrigham and Women's Hospital, Boston, Massachusetts Correspondence and requests for reprints should be addressed to Craig M. Lilly, Brigham and Women's Hospital, 75 Francis St., Boston, MA 02115. E-mail: clilly{at}partners.org
Efforts to define the genetic contribution to asthma susceptibility are starting to change our understanding of asthma pathogenesis and provide new opportunities for intervention. Asthma has been characterized as a disease to which genetic variation and environmental exposure contribute in approximately equal measure. Disease explanatory models have advanced from a concept of asthma as eosinophilic bronchitis, to models that include the primary allergic immune response, polarized subsets of lymphocytes, airway recruitment of circulating blood cells, the role of cytokines and chemokines and structural changes in the airway, and the episodic release of substances that cause the transient airway narrowing that results in the clinical manifestations of asthma. Genetic studies have linked variations in several genes to asthma susceptibility, including genes such as ADAM-33 that may affect bronchial hyperresponsiveness by altering the availability of growth factors and their receptors; complement protein genes; genes such as IL4R- To date, asthma-associated genetic variants appear to function through more subtle mechanisms than by simply altering protein coding sequences. The exact mechanisms by which susceptibility variants affect the timing, splicing, and cellular or tissue expression of the gene in the context of the disease can be difficult to establish. We reported the association of variants of the prostaglandin D receptor gene (PTGDR), located on human chromosome 14q22.1, with asthma susceptibility (2). We investigated this gene because chromosome 14q22.1 is associated with asthma susceptibility in an increasing number of family-based linkage studies, and because abrogation of the receptor encoded by PTGDR is protective against the asthma-like phenotype in a mouse model (3). In studies comparing the impact of abrogation of the alternative eicosanoid receptors, the mouse homolog of PTGDR (designated DP1) had the greatest impact on expression of an asthma-like phenotype in allergen sensitization and challenge models (4). The effects of abrogation of DP1 on expression of the asthma phenotype were dependent on the intensity of allergen exposure; genotype-associated limitation of lymphocyte recruitment occurred at moderate- but not high-intensity allergen exposure (3). In our human studies, some individuals with genetic variants that impaired PTGDR expression developed asthma. These findings are consistent with the speculation that intense environmental exposures can overcome the protective effects of this genotype. Careful phenotyping of DP1–/– mice provided insights into the mechanisms by which DP-1 abrogation is protective against the asthma-like phenotype. DP1–/– mice failed to efficiently recruit lymphocytes to their airways, which is required for expression of the asthma-like phenotype in this model. This failure is thought to be an indirect consequence of DP1 abrogation, as direct PGD2-induced chemotaxis of murine lymphocytes depends on the DP2 or CRTH2 receptor rather than the DP1 receptor (5). Allergen exposure in the mouse is associated with increased DP1 expression in ciliated, nonciliated, and type II alveolar epithelial cells. Mouse airway epithelial cell CCL22 (MDC) expression is increased by PGD2 exposure, and allergen-sensitized and -challenged mice recruit CCR4-expressing lymphocytes to the airways in a CCL22-dependent process that is facilitated by pretreatment with aerosolized PGD2 (5). These changes appear to be relevant to the asthmatic airway, as our microarray study showed that bronchoscopically obtained human airway epithelial cell PTGDR and CCL22 expression tended to be elevated after segmental allergen exposure (6). The indirect effects of DP1 abrogation on lymphocyte recruitment could be explained by the relative inability of airway epithelial cells to secrete CCL22 (5) after allergen challenge. Although the exact mechanisms that limit airway lymphocyte recruitment or retention remain undefined, the weight of evidence from mouse models suggests that effects of the murine homolog of PTGDR on lymphocyte recruitment contribute to the creation of the asthma-like phenotype induced by repeated allergen exposure. PTGDR also affects other cells in the allergic immune response, for example, enhanced survival of human eosinophils by inhibition of apoptotic pathways and altered efficiency of anti-IgE–induced histamine release by rat peritoneal mast cells. Pharmacologic inhibition of the guinea pig homolog has abrogated upper airway congestion and acute allergen-induced contractile responses and has limited the recruitment of eosinophils to the lower airways after allergen sensitization and challenge (5). PTGDR is thus pleiotropic in its effects on allergic airway inflammation, and apparently uses multiple mechanisms to affect expression of the asthma phenotype (Figure 1).
Prior knowledge of the functional role of PTGDR in expression of the asthma-like phenotype in the mouse allowed us to generate a priori biological hypotheses and determine whether any observed genetic associations were consistent with these hypotheses. We began our study by determining the functional effects of variants that were common enough to have significant explanatory power in our datasets. We identified three PTGDR promoter region single-nucleotide polymorphisms (SNPs) that occurred in transcription factor–binding sites and hence altered the avidity of nuclear protein binding. We identified some haplotypic combinations of these PTGDR variants that increased transcription factor binding and synergistically increased reporter gene expression and others that decreased transcription factor binding. We then tested the hypothesis that haplotypes that decrease gene expression are associated with a reduced risk of asthma in two genetically distinct U.S. populations using a case–control design. We excluded major population substructure within both our European-American and African-American case–control populations using genomic controls (7). Combinations of variants that were functionally shown to limit the expression of PTGDR were associated with reduced asthma risk of similar magnitude in both study populations (2). We limited type I error by using functional priors to inform our hypotheses regarding the direction of genetic association, by using the same functional priors that describe the effects of the variants on phenotype to limit the number of empirical analyses, and by replicating the findings in a genetically distinct population. Defining the functional effects of genetic variants made it easier to formulate hypotheses about the potential manipulation of the PTGDR receptor–ligand system for therapeutic benefit in asthma. Because the effects of manipulating this system in animal models of asthma are congruent with our findings, we can use these results to corroborate the value of our approach in a field where the initial association of genetic variants with the disease has been difficult to replicate (8). Many important questions remain unanswered. We do not know whether the PTGDR variants are associated with asthma in other populations and ethnic groups, including Latinos and Asians. Uncertainty about the true frequency of the haplotypes in the populations from which our case–control subjects were drawn prevents us from making a legitimate estimate of the population-attributable risk of asthma. Defining the fraction of the genetic risk of asthma that is accounted for by these variants is critical for judging their overall importance to clinical medicine and public health; further population-based studies will be necessary to define the etiologic fraction attributable to these variants. The effects of PTGDR variants on disease severity/progression and on response to therapeutic interventions also remain to be defined. The evidence that PTGDR is required for expression of the asthma phenotype in more than one animal model, its association with asthma susceptibility in humans, and the availability of safe and effective oral agents that inhibit the receptor provides a clear and compelling justification for human clinical trials. An asthma clinical trial that studied a DP1-selective agent would have the advantage of defining the contribution of this receptor in isolation and have the potential safety benefit of not directly perturbing lymphocyte surface receptors. On the other hand, using an agent that also blocked CRTH2 would also abrogate the direct recruitment of CRTH2-bearing lymphocytes by PGD2 to the airway and might be more effective than a DP1-selective agent. How best to design clinical trials of PGD2 receptor antagonists is not completely clear from the available data. The mouse models suggest that the airway epithelial DP1 receptor response to PGD2 is necessary to mobilize the chemokines required for lymphocyte recruitment only at modest levels of allergen exposure (5). Thus, including effective allergen avoidance strategies in a trial of DP1 inhibitor therapy could influence the efficacy of DP1 antagonists. Trials of new anti-asthma drugs sponsored by pharmaceutical companies have traditionally focused on short-term improvement in symptom scores and lung function to facilitate registration by regulatory agencies. The fact that PGD2 is the most abundant prostanoid released into the airway after allergen challenge and can induce bronchoconstriction, mucus production, and vascular permeability (5) suggests that abrogation of its receptor(s) would have salutary effects on airway tone and asthma symptoms. Our experience with other receptor antagonists and anti-IgE monoclonal antibodies suggests that baseline airway tone and asthma symptoms depend on many mediators and that therapeutic responses vary considerably among individuals. Periodic asthma symptoms appear to be caused by the episodic release of combinations of bronchoconstrictors that vary among individuals, and individuals have varying capacities to respond to procontractile mediators. Controlling asthma symptoms at the level of these mediators or their receptors depends on our ability to abrogate the effects of all of the mediators to which an individual responds with significant airway constriction. Residual asthma symptoms in some treated individuals with asthma are probably caused by the airway action of PGD2 and its action at the receptor encoded by PTGDR.
The ability to identify individuals bearing a PTGDR haplotype that increases gene expression allows trial designs that target interventions to individuals with greater expression and therefore greater expected benefits. The increased asthma risk of the high-transcriptional-efficiency promoter haplotypes, and the reduced asthma risk of the low-efficiency haplotypes, were accounted for using a dominant model. Thus, we can perform a "proof of concept" study of the effects of a PGD2 receptor inhibitor targeted to individuals who bear one or more copies of the high-efficiency haplotype and no copies of the low-efficiency haplotype. More important, this ability to genetically stratify therapeutic trials allows subgroups of individuals to be identified who are more likely to benefit from treatment because of genetic differences. If therapeutic response were solely a function of PTGDR genotype, A central issue in the design of clinical trials is what the primary outcome parameters should be. When the primary aim is to demonstrate efficacy in treating asthma, a combination of effects on symptom scores and lung function allow comparison of the magnitude of therapeutic effects with those reported for established asthma therapies. PGD2 is the most abundant eicosanoid produced by the lung after segmental allergen challenge; thus, an antagonist should have efficacy alone or in combination with other agents for controlling asthma exacerbations. Alternatively, the effect of a PGD2 receptor antagonist on rates of exacerbation is an outcome parameter highly relevant to the costs of providing asthma care. Finally, the association of PTGDR genetic variants that reduce receptor expression with reduced risk of having an asthma diagnosis suggests that an agent that inhibits this receptor could be effective for the primary prevention of asthma. Designing this kind of intervention would only be possible if the active agent has a firmly established record of safety and would be aided by an ability to identify a population at risk and to know the timing of the window of opportunity for successful intervention. The association of PTGDR promoter variants with asthma raises new questions about how this receptor influences expression of the asthma phenotype. Differences in promoter structure among species make it difficult to use animal models to predict with reliability how human PTGDR promoter region variants will affect the temporal and spatial expression of the receptor within the airway. The paucity of human airway tissue obtained as a function of donor genotype is a significant obstacle to understanding how genetic variants affect the timing and abundance of PTGDR receptor expression in the asthmatic airway. In light of these obstacles and the availability of safe and effective receptor antagonists, we argue that investigation of the effects of these agents in human clinical trials is a desirable next step.
Conflict of Interest Statement: Neither author has a financial relationship in a commercial entity that has an interest in the subject of this manuscript. Received in final form June 22, 2005
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and IL-13 that affect the allergic immune response; genes involved in pattern recognition such as CD14, TIM-1, and TLR-2; and an increasing number of genes of the airway epithelium that may be important for barrier defense, e.g., DPP10, GPRA, and SPINK5 (
70% of U.S. individuals with asthma would benefit from treatment with a receptor antagonist. Being able to identify the 
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