Editorial
Copyright ©The Author(s) 2015. Published by Baishideng Publishing Group Inc. All rights reserved.
World J Clin Pediatr. May 8, 2015; 4(2): 13-18
Published online May 8, 2015. doi: 10.5409/wjcp.v4.i2.13
Clinical asthma phenotyping: A trial for bridging gaps in asthma management
Magdy Mohamed Zedan, Wafaa Nabil Laimon, Amal Mohamed Osman, Mohamed Magdy Zedan
Magdy Mohamed Zedan, Wafaa Nabil Laimon, Amal Mohamed Osman, Mohamed Magdy Zedan, Department of Asthma, Allergy and Immunology, Mansoura Faculty of Medicine, Mansoura University, Mansoura 35516, Egypt
Author contributions: Zedan MM designed the research; Zedan MM and Laimon NW performed the research; Zedan MM, Laimon NW, Osman MA and Zedan MM wrote the paper.
Conflict-of-interest: None.
Open-Access: This article is an open-access article which was selected by an in-house editor and fully peer-reviewed by external reviewers. It is distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited and the use is non-commercial. See: http://creativecommons.org/licenses/by-nc/4.0/
Correspondence to: Magdy Mohamed Zedan, MD, Department of Asthma, Allergy and Immunology, Mansoura Faculty of Medicine, Mansoura University, Mansoura 35516, Egypt. magdyzedan@mans.edu.eg
Telephone: +20-50-2242526 Fax: +20-50-2234092
Received: January 28, 2015
Peer-review started: January 29, 2015
First decision: March 6, 2015
Revised: March 19, 2015
Accepted: April 16, 2015
Article in press: April 20, 2015
Published online: May 8, 2015

Abstract

Asthma is a common disease affecting millions of people worldwide and exerting an enormous strain on health resources in many countries. Evidence is increasing that asthma is unlikely to be a single disease but rather a series of complex, overlapping individual diseases or phenotypes, each defined by its unique interaction between genetic and environmental factors. Asthma phenotypes were initially focused on combinations of clinical characteristics, but they are now evolving to link pathophysiological mechanism to subtypes of asthma. Better characterization of those phenotypes is expected to be most useful for allocating asthma therapies. This article reviews different published researches in terms of unbiased approaches to phenotype asthma and emphasizes how the phenotyping exercise is an important step towards proper asthma treatment. It is structured into three sections; the heterogeneity of asthma, the impact of asthma heterogeneity on asthma management and different trials for phenotyping asthma.

Key Words: Asthma phenotypes, Asthma heterogenity, Endotypes, Asthma subtypes, Asthma syndrome

Core tip: Although asthma diagnosis is based mainly on clinical basis using history taking and physical examination, treatment options are not tailored according to the clinical phenotype. We still do not have a way to work up a given patient with asthma and to easily delineate the specific pathobiology that leads to her or his airway dysfunction. We can recognize the clinical syndromes, but we cannot spell out the steps that lead from genetic or biochemical defects to disease presentation. Thus we are left with a paradox in the study of asthma; we have effective treatments that are not biologically informative, and we have informative treatments that are less effective.



INTRODUCTION

Asthma is a chronic inflammatory airway disease characterized by episodic reversible airway obstruction that variably presents with cough, wheezing, shortness of breath, or chest tightness[1,2].

Asthma affects nearly 300 million people worldwide. Moreover, western countries have shown rising prevalence over the past three decades[2]. The prevalence of bronchial asthma in the Nile Delta region of Egypt was found to be 7.7%[3].

Asthma is a heterogeneous and genetically determined disease with different presentation, disease progression, and response to therapy. However, despite this recognition, the treatment approaches for asthma have been uniformly applied irrespective of its “subtype”[4]. Guidelines advocate a universal stepwise approach in which medications are initially prescribed based on patient age and asthma severity, and then the number, frequency, and dose of medications are titrated upward when asthma control is inadequate.

The fact that there are some asthmatic patients with variable presentations who do not respond to commonly used medications and classified as severe asthma[5,6], has driven the urgent need to phenotype the disease using unbiased approaches, and depending mainly on essential clinical and physiologic features for better treatment approaches.

Different researches have tried to shape asthma phenotypes through their clinical, physiologic and cellular parameters. A phenotype was defined as “observable characteristics” of subtypes of asthma ranging from clinical presentation, triggering factors to therapeutic responses. While, the fact of asthma pathogenesis is far beyond that as these different asthma phenotypes are based on different underlying biologic processes in each patient[7].

For better understanding of this heterogeneity and the complexity of clinical and research results, the term “endotype” was evolved reflecting a specific pathogenic mechanism. Endotype was defined according to molecular pathogenesis or therapeutic responses. Thus, accurate identification of those asthma endotypes could help in tailoring of asthma therapy[2].

ASTHMA AS A HETEROGENEOUS DISEASE

Heterogeneity of asthma reflects different clinical presentations with different responses to asthma medicines. This makes a challenge for the appropriate choice of asthma treatment. Thus, a better understanding of this heterogeneity could be of great help. Asthma heterogeneity could be explained at four levels: clinical, biological, therapeutic and molecular levels (Table 1).

Table 1 Examples of proposed clinical phenotypes.
Clinical levelClinical characteristicsBiological levelMolecular levelTherapeutic level
Xie et al[2]Exercise induced asthmaMild asthma and reactive bronchospasm in response to sustained exerciseMast-cell activation; Th2 cytokines; cysteinyl leukotrienesResponsive to cysteinyl leukotriene modifiers, beta agonists and antibody to IL-9
Obesity asthma phenotypeAdult onset Mostly females Severe symptomsLack of Th2 biomarkers; oxidative stress ADMAResponsive to weight loss, antioxidants and possibly to hormonal therapy
Early-onset Allergic asthma> 3 episodes per year Mild, moderate, or severe Family history of asthmaSpecific IgE; Th2 cytokines; thick SBM17q12; Th2-related genesCorticosteroid-responsive; Th2-targeted
Late-onset Eosinophic asthmaAdult onset Often severe Sinus diseaseIncreased both peripheral eosinophils and IL-5Responsive to antibody to IL-5 and cysteinyl leukotriene modifiers; corticosteroid-refractory
Zedan et al[8]Shortness of breath phenotype> 10 yr No sex predominance Longer disease duration Negative family history of asthmaIncreased total serum IgEHigher prevalence of TT genotype of SNP IL-4C 590T (Zedan et al[18])Responsive to fluticasone alone
Cough phenotype< 10 yr Female predominance Shorter disease duration Positive family history of asthmaIncreased both peripheral eosinophis and sECPResponsive to montelukast alone
Wheezy phenotypeIncrease in peripheral eosinophilis and sECPResponsive to both fluticasone and montelukast
Clinical level

Different studies suggest phenotypic classification of asthma depending on clinical basis. These phenotypes include allergic and non allergic asthma. Other phenotypes defined by clinical or physiological categories (i.e., severity, age at onset, and chronic airway obstruction), by asthma triggers (i.e., viral, exercise, occupational allergens, or irritants), or their course (i.e., early transient/persistent/late onset wheeze) have also been proposed[8]. Other asthma phenotypes include cough variant asthma and obese asthma phenotype. Cough variant asthma was recognized as an asthma phenotype that solely presents with coughing[9]. This asthma phenotype was defined by the presence of atopic features, milder eosinophilic airway inflammation and airway remodeling[10].

Obese asthma phenotype is assumed to be non T helper type 2 (Th2) mediated in which low fractional exhaled nitric oxide, eosinophils and IgE levels were observed[11,12]. Preliminary studies have suggested that in late onset obese asthma, there is increased asymmetric dimethylarginine which inhibits inducible nitric oxide synthetase leading to superoxide generation increasing the oxidative stress in such patients[13].

Biological level

The heterogeneity of asthma as an inflammatory disorder is well established. Simpson et al[14] have detected specific inflammatory endotypes in sputum samples consistent with cytokine profiles in asthmatic patients forming different endotypes that ranged from: eosinophilic, neutrophilic, mixed, and paucigranuloctic subtypes[15]. Severe asthma was found to be a different disease entity dating since birth with a specific underlying biology[16,17]. Thus, it should be viewed like this rather than the end result of airway remodeling. Recently, Zedan et al[18] correlated clinical asthma phenotypes with the underlying cytokine and genetic pattern. Serum cytokines levels were found to be a reflection of cytokine gene polymorphisms via affecting the transcriptional regulation. The clinical asthma phenotypes showed statistically significant differences in cytokine profile and genotyping distribution of IL4RA-175V and IL4C-590T. Further a different genotyping was noticed when dealing with asthma as a group and after its clinical phenotyping.

Therapeutic level

In spite of asthma heterogeneity, the prescribed medications for asthma management are similar involving B2 agonists, leukotriene receptors antagonists and inhaled corticosteroids. These asthma medicines were found to control the symptoms in the majority of patients and failed in others. Several researches tried to explain this variable response to asthma therapy in different patients.

The polymorphism of β2 receptors could adversely affect the response to regular short-acting drugs[19]. On the other aspect, 15% of asthmatics show a reasonable response to leukotriene modifiers. However, polymorphism in 5 lipoxygenase (ALOX) and the receptor (CYSLTR2) were found to affect the drug response[20,21].

Further, differences in corticosteroid responsiveness may be caused by complex genetic variations as more than 100 genes were found to be involved in allergic asthma, but these genetics variations are influenced by environmental exposures. Moreover, in many cases, it has been difficult to establish clear linkage because of disease heterogeneity and poorly stratified populations[22].

By stratifying patients according to phenotype, future studies may be better able to identify the genes or other biomarkers associated with various aspects of asthma. Currently, the use of specific biomarkers to diagnose and monitor asthma is in its infancy but is being evaluated in clinical trials[23].

Molecular level

Despite the importance of Th2 cytokines in atopic asthma, recent data in both adults and children has challenged the concept of a Th1/Th2 imbalance and has showed an evidence of Th1 profile.

Th2 imprint was present in only 50% of the mild asthmatics and those patients were characterized by lung eosinophils, mast cells, higher IgE levels, hyperreactive airway, higher tissue expression of Th2 cytokines and thicker subepithelial basement membrane[24,25]. In addition, they showed a good response to inhaled corticosteroids in contrast to those without the type-2 cytokine profile.

IMPACT OF ASTHMA HETEROGENEITY ON ASTHMA MANAGEMENT

Asthma therapy can markedly improve quality of life, morbidity, and mortality of asthma patients and decrease health care costs if applied to the right patient at the right time[26]. The choice of controller medications should be based on clinical efficacy, patient phenotype, previous responses to treatment, patient’s compliance, side effects, and drug safety[27].

Although asthma diagnosis is based mainly on clinical basis using history taking and physical examination[28], treatment options are not tailored according to the clinical phenotype. We still do not have a way to work up a given patient with asthma and to easily delineate the specific pathobiology that leads to her or his airway dysfunction. We can recognize the clinical syndromes, but we cannot spell out the steps that lead from genetic or biochemical defects to disease presentation. Thus we are challenged by a paradox in the management of asthma; we have effective treatments that are not biologically informative, and we have informative treatments that are less effective[29].

Recently, the new emerging cytokine based therapies flare the importance of asthma phenotyping. Effective application of these new therapies to certain patients depends on the development of biomarkers of molecular heterogeneity in asthma[30].

DIFFERENT TRIALS FOR ASTHMA PHENOTYPING

A phenotype is defined as the “observable properties of an organism that are produced by the interactions of the genotype and the environment”[31]. Multiple approaches have been proposed for asthma phenotyping, none has been widely agreed upon. Many of those approaches have been biased; depends mainly on clinical characteristics without obvious inflammatory biomarkers. Also, the selection of a single dominant characteristic to categorize a given patient is mostly inaccurate as it ignores the overlap between groups[32]. These include categorization of patients based upon atopic state, symptomatic trigger, disease severity, and pattern of airflow obstruction. In addition, pathological classifications, despite being important for clarifying disease pathogenesis, they are extremely invasive clinically.

Several studies have approached asthma phenotyping in a less biased and more statistically based manner[31] as a result of lacking of specific biomarkers for each asthma phenotype. Three large clinically oriented analyses performed in Europe[33,34] and the United States[35,36] developed clusters based upon age of onset of asthma, gender, allergic features, asthma symptoms, and lung function as well as other factors that varied between the three studies. The resulting phenotypes include early-onset mild allergic asthma, later-onset asthma associated with obesity, and severe non-atopic asthma with frequent exacerbations.

Four clusters were identified based upon asthma duration, the number of asthma controller medications, baseline Forced Expiratory Volume in 1 s, skin test, and total serum IgE. All of them were strongly associated with histories of atopic dermatitis and atopy, although the degree of atopy as measured by number of positive allergy skin tests and total serum IgE did differ significantly between groups[34]. However, these studies lacked the accurate pathological and immunological definition of asthma phenotypes[31].

Asthma has a strong genetic component[37]. Most of genetic association studies have dealed with asthma as a one disease[38] and only few studies[39] delineated specific asthma phenotypes. Asthma subtypes cannot be termed phenotypes without linking the observable characteristics to the genetic signature of the patients[40]. Woodruff et al[41] began to identify molecular phenotypes of asthma. They analyzed airway epithelial brushings and they identified “TH2 high” and “TH2 low” individuals.

Zedan et al[27] proposed three clinical asthma phenotypes based on symptomatology after validation of these symptoms[42,43]. The study tried to characterize three clinical phenotypes [shortness of breath (SOB), cough, and wheeze] by specific inflammatory biomarkers and their response to asthma medications. They found that children who had SOB had older age, and longer disease duration, higher levels of total serum IgE, and responded to fluticasone alone. While cough group was found to have younger age, shorter disease duration, higher levels of eosinophilic% and serum Eosinophilic Cationic Protein, and responded to montelukast alone. Moreover, wheezy group showed mixed pattern and responded to both medications[27].

Two single nucleotide polymorphisms (SNPs) in IL-4 and IL4RA were studied in Egyptian asthmatics with different clinical phenotypes[18]. They found that asthma as a group had AG heterozygosity genotype, whereas cough with SOB group showed AA and GG homozygosity genotype. In addition, cough group and SOB group revealed significant increase in serum levels of IL-17 among patients with CC homozygote variant of IL-4C 590T compared to patients with CT heterozygote variant. In turn, phenotyping based on symptomatology makes a sense in endotyping of asthma, which may have a reflection on heterogeneity in response to asthma medications[18].

The previously mentioned clinical asthma phenotypes were found to express a significant value in detection of specific cytokine and genotype profiles for asthma; hence this may help in prediction and diagnosis of clinical asthma phenotypes which might have a potential value in tailoring asthma therapies.

The concept of personalized medicine, however, gained most traction from the application of genetics to patients with disease. Polymorphisms in genes (both single nucleotide polymorphisms, SNPs, and other more substantive coding variations) are associated with therapeutic responses to short-acting beta2 agonists, leukotriene inhibitors, corticosteroids, and several others; other polymorphisms confer risk of developing diseases: asthma, atopy, bronchial hyperresponsiveness, etc[44].

CONCLUSION

Asthma is a complex disease with evidenced heterogeneity at different levels. Despite this heterogeneous nature, the same treatment approaches are still applied for different patients with variable responses. Phenotyping/endotyping dilemma is the only roadmap to bridge the gap between this variable disease and the single approach of management. A key obstacle in this field is a lack of “gold standard” phenotypes with underlying identifiable biomarkers, genetic, or molecular profiles. A novel conceptual framework to link the clinical signature of every patient to her/his inflammatory phenotype is very essential to meet the challenges of tailoring the right medications for to the right patient at the right time.

Footnotes

P- Reviewer: Al-Biltagi M, Classen CF S- Editor: Tian YL L- Editor: A E- Editor: Wu HL

References
1.  Zedan M, Settin A, Farag MK, El-Bayoumi M, El Regal ME, El Baz R, Osman E. Gene polymorphisms of tumor necrosis factor alpha-308 and interleukin-10-1082 among asthmatic Egyptian children. Allergy Asthma Proc. 2008;29:268-273.  [PubMed]  [DOI]
2.  Xie M, Wenzel SE. A global perspective in asthma: from phenotype to endotype. Chin Med J (Engl). 2013;126:166-174.  [PubMed]  [DOI]
3.  Zedan M, Settin A, Farag M, Ezz-Elregal M, Osman E. Prevalence of Bronchial asthma among Egyptian school children. Egypt J Bronchol. 2009;3:124-130.  [PubMed]  [DOI]
4.  Anderson GP. Endotyping asthma: new insights into key pathogenic mechanisms in a complex, heterogeneous disease. Lancet. 2008;372:1107-1119.  [PubMed]  [DOI]
5.  Chung KF, Godard P, Adelroth E, Ayres J, Barnes N, Barnes P, Bel E, Burney P, Chanez P, Connett G. Difficult/therapy-resistant asthma: the need for an integrated approach to define clinical phenotypes, evaluate risk factors, understand pathophysiology and find novel therapies. ERS Task Force on Difficult/Therapy-Resistant Asthma. European Respiratory Society. Eur Respir J. 1999;13:1198-1208.  [PubMed]  [DOI]
6.  Proceedings of the ATS workshop on refractory asthma: current understanding, recommendations, and unanswered questions. American Thoracic Society. Am J Respir Crit Care Med. 2000;162:2341-2351.  [PubMed]  [DOI]
7.  Wenzel SE. Asthma: defining of the persistent adult phenotypes. Lancet. 2006;368:804-813.  [PubMed]  [DOI]
8.  Zedan M, Attia G, Zedan MM, Osman A, Abo-Elkheir N, Maysara N, Barakat T, Gamil N. Clinical asthma phenotypes and therapeutic responses. ISRN Pediatr. 2013;2013:824781.  [PubMed]  [DOI]
9.  Niimi A. Cough, asthma, and cysteinyl-leukotrienes. Pulm Pharmacol Ther. 2013;26:514-519.  [PubMed]  [DOI]
10.  Ioan I, Poussel M, Coutier L, Plevkova J, Poliacek I, Bolser DC, Davenport PW, Derelle J, Hanacek J, Tatar M. What is chronic cough in children. Front Physiol. 2014;5:322.  [PubMed]  [DOI]
11.  Amelink M, de Nijs SB, de Groot JC, van Tilburg PM, van Spiegel PI, Krouwels FH, Lutter R, Zwinderman AH, Weersink EJ, ten Brinke A. Three phenotypes of adult-onset asthma. Allergy. 2013;68:674-680.  [PubMed]  [DOI]
12.  Holguin F, Bleecker ER, Busse WW, Calhoun WJ, Castro M, Erzurum SC, Fitzpatrick AM, Gaston B, Israel E, Jarjour NN. Obesity and asthma: an association modified by age of asthma onset. J Allergy Clin Immunol. 2011;127:1486-93.e2.  [PubMed]  [DOI]
13.  Holguin F, Comhair SA, Hazen SL, Powers RW, Khatri SS, Bleecker ER, Busse WW, Calhoun WJ, Castro M, Fitzpatrick AM. An association between L-arginine/asymmetric dimethyl arginine balance, obesity, and the age of asthma onset phenotype. Am J Respir Crit Care Med. 2013;187:153-159.  [PubMed]  [DOI]
14.  Simpson JL, Scott R, Boyle MJ, Gibson PG. Inflammatory subtypes in asthma: assessment and identification using induced sputum. Respirology. 2006;11:54-61.  [PubMed]  [DOI]
15.  Brasier AR, Victor S, Boetticher G, Ju H, Lee C, Bleecker ER, Castro M, Busse WW, Calhoun WJ. Molecular phenotyping of severe asthma using pattern recognition of bronchoalveolar lavage-derived cytokines. J Allergy Clin Immunol. 2008;121:30-37.e6.  [PubMed]  [DOI]
16.  Chanez P, Wenzel SE, Anderson GP, Anto JM, Bel EH, Boulet LP, Brightling CE, Busse WW, Castro M, Dahlen B. Severe asthma in adults: what are the important questions. J Allergy Clin Immunol. 2007;119:1337-1348.  [PubMed]  [DOI]
17.  Phelan PD, Robertson CF, Olinsky A. The Melbourne Asthma Study: 1964-1999. J Allergy Clin Immunol. 2002;109:189-194.  [PubMed]  [DOI]
18.  Zedan M, Bakr A, Shouman B, Zaghloul H, Al-Haggar M, Zedan M, Osman A. Single Nucleotide Polymorphism of IL4C-590T and IL4RA 175V and Immunological Parameters in Egyptian Asthmatics with Different Clinical Phenotypes. J Allergy Ther. 2014;5:1-7.  [PubMed]  [DOI]
19.  Hawkins GA, Weiss ST, Bleecker ER. Clinical consequences of ADRbeta2 polymorphisms. Pharmacogenomics. 2008;9:349-358.  [PubMed]  [DOI]
20.  Klotsman M, York TP, Pillai SG, Vargas-Irwin C, Sharma SS, van den Oord EJ, Anderson WH. Pharmacogenetics of the 5-lipoxygenase biosynthetic pathway and variable clinical response to montelukast. Pharmacogenet Genomics. 2007;17:189-196.  [PubMed]  [DOI]
21.  Pillai SG, Cousens DJ, Barnes AA, Buckley PT, Chiano MN, Hosking LK, Cameron LA, Fling ME, Foley JJ, Green A. A coding polymorphism in the CYSLT2 receptor with reduced affinity to LTD4 is associated with asthma. Pharmacogenetics. 2004;14:627-633.  [PubMed]  [DOI]
22.  Szefler SJ, Martin RJ, King TS, Boushey HA, Cherniack RM, Chinchilli VM, Craig TJ, Dolovich M, Drazen JM, Fagan JK. Significant variability in response to inhaled corticosteroids for persistent asthma. J Allergy Clin Immunol. 2002;109:410-418.  [PubMed]  [DOI]
23.  National Asthma Education and Prevention Program. Expert Panel Report 3 (EPR-3): Guidelines for the Diagnosis and Management of Asthma-Summary Report 2007. J Allergy Clin Immunol. 2007;120:S94-S138.  [PubMed]  [DOI]
24.  Dougherty RH, Sidhu SS, Raman K, Solon M, Solberg OD, Caughey GH, Woodruff PG, Fahy JV. Accumulation of intraepithelial mast cells with a unique protease phenotype in T(H)2-high asthma. J Allergy Clin Immunol. 2010;125:1046-1053.e8.  [PubMed]  [DOI]
25.  Woodruff PG, Modrek B, Choy DF, Jia G, Abbas AR, Ellwanger A, Koth LL, Arron JR, Fahy JV. T-helper type 2-driven inflammation defines major subphenotypes of asthma. Am J Respir Crit Care Med. 2009;180:388-395.  [PubMed]  [DOI]
26.  Weiss ST. New approaches to personalized medicine for asthma: where are we. J Allergy Clin Immunol. 2012;129:327-334.  [PubMed]  [DOI]
27.  Zedan M, Gamil N, El-Chennawi F, Maysara N, Hafeez HA, Nasef N, Fouda A. Evaluation of Different Asthma Phenotype Responses to Montelukast Versus Fluticasone Treatment. Pediatric Asthma, Allergy & Immunology. 2009;22:63-68.  [PubMed]  [DOI]
28.  Global Initiative for Asthma. Global strategy for asthma management and prevention [Accessed May 5 2012].  Available from: http://www.ginasthma.org/.  [PubMed]  [DOI]
29.  Drazen JM. Asthma: the paradox of heterogeneity. J Allergy Clin Immunol. 2012;129:1200-1201.  [PubMed]  [DOI]
30.  Bhakta NR, Woodruff PG. Human asthma phenotypes: from the clinic, to cytokines, and back again. Immunol Rev. 2011;242:220-232.  [PubMed]  [DOI]
31.  Wenzel SE. Asthma phenotypes: the evolution from clinical to molecular approaches. Nat Med. 2012;18:716-725.  [PubMed]  [DOI]
32.  Corren J, Lemanske RF, Hanania NA, Korenblat PE, Parsey MV, Arron JR, Harris JM, Scheerens H, Wu LC, Su Z. Lebrikizumab treatment in adults with asthma. N Engl J Med. 2011;365:1088-1098.  [PubMed]  [DOI]
33.  Haldar P, Pavord ID, Shaw DE, Berry MA, Thomas M, Brightling CE, Wardlaw AJ, Green RH. Cluster analysis and clinical asthma phenotypes. Am J Respir Crit Care Med. 2008;178:218-224.  [PubMed]  [DOI]
34.  Siroux V, Basagaña X, Boudier A, Pin I, Garcia-Aymerich J, Vesin A, Slama R, Jarvis D, Anto JM, Kauffmann F. Identifying adult asthma phenotypes using a clustering approach. Eur Respir J. 2011;38:310-317.  [PubMed]  [DOI]
35.  Moore WC, Meyers DA, Wenzel SE, Teague WG, Li H, Li X, D’Agostino R, Castro M, Curran-Everett D, Fitzpatrick AM. Identification of asthma phenotypes using cluster analysis in the Severe Asthma Research Program. Am J Respir Crit Care Med. 2010;181:315-323.  [PubMed]  [DOI]
36.  Fitzpatrick AM, Teague WG, Meyers DA, Peters SP, Li X, Li H, Wenzel SE, Aujla S, Castro M, Bacharier LB. Heterogeneity of severe asthma in childhood: confirmation by cluster analysis of children in the National Institutes of Health/National Heart, Lung, and Blood Institute Severe Asthma Research Program. J Allergy Clin Immunol. 2011;127:382-389.e1-13.  [PubMed]  [DOI]
37.  Martinez FD, Vercelli D. Asthma. Lancet. 2013;382:1360-1372.  [PubMed]  [DOI]
38.  Ober C, Hoffjan S. Asthma genetics 2006: the long and winding road to gene discovery. Genes Immun. 2006;7:95-100.  [PubMed]  [DOI]
39.  Bel EH. Clinical phenotypes of asthma. Curr Opin Pulm Med. 2004;10:44-50.  [PubMed]  [DOI]
40.  Wenzel SE. Phenotypes in asthma: useful guides for therapy, distinct biological processes, or both. Am J Respir Crit Care Med. 2004;170:579-580.  [PubMed]  [DOI]
41.  Woodruff PG, Khashayar R, Lazarus SC, Janson S, Avila P, Boushey HA, Segal M, Fahy JV. Relationship between airway inflammation, hyperresponsiveness, and obstruction in asthma. J Allergy Clin Immunol. 2001;108:753-758.  [PubMed]  [DOI]
42.  Manual for the International Study of Asthma and Allergies in Childhood (ISAAC) Bochum and Auckland: ISAAC Coordinating Committee, 1992 .  [PubMed]  [DOI]
43.  Zedan M, Settin A, Farag M, EL-Bayoumi M, Ezz-Elregal M, Abd-Elkader A, Osman E, Fouda A. How do Egyptian children describe asthma symptoms. Egyptian J of Bronchology. 2009;3:74-80.  [PubMed]  [DOI]
44.  Calhoun WJ, Brassier AR. Conclusions and Future Directions. Heterogeneity in Asthma. Heidelberg, Dordrecht London: Springer New York 2014; 340-342.  [PubMed]  [DOI]