Rationale: The Severe Asthma Research Program cohort includes subjects with persistent asthma who have undergone detailed phenotypic characterization. Previous univariate methods compared features of mild, moderate, and severe asthma. Objectives: To identify novel asthma phenotypes using an unsupervised hierarchical cluster analysis. Methods: Reduction of the initial 628 variables to 34 core variables was achieved by elimination of redundant data and transformation of categorical variables into ranked ordinal composite variables. Cluster analysis was performed on 726 subjects. Measurements and Main Results: Five groups were identified. Subjects in Cluster 1 (n 5 110) have early onset atopic asthma with normal lung function treated with two or fewer controller medications (82%) and minimal health care utilization. Cluster 2 (n 5 321) consists of subjects with early-onset atopic asthma and preserved lung function but increased medication requirements (29% on three or more medications) and health care utilization. Cluster 3 (n 5 59) is a unique group of mostly older obese women with late-onset nonatopic asthma, moderate reductions in FEV 1 , and frequent oral corticosteroid use to manage exacerbations. Subjects in Clusters 4 (n 5 120) and 5 (n 5 116) have severe airflow obstruction with bronchodilator responsiveness but differ in to their ability to attain normal lung function, age of asthma onset, atopic status, and use of oral corticosteroids. Conclusions: Five distinct clinical phenotypes of asthma have been identified using unsupervised hierarchical cluster analysis. All clusters contain subjects who meet the American Thoracic Society definition of severe asthma, which supports clinical heterogeneity in asthma and the need for new approaches for the classification of disease severity in asthma.

Background-Severe asthma causes the majority of asthma morbidity. Understanding mechanisms that contribute to the development of severe disease is important.

During the past decade a plethora of studies have unravelled the multiple roles of nitric oxide (NO) in airway physiology and pathophysiology. In the respiratory tract, NO is produced by a wide variety of cell types and is generated via oxidation of l-arginine that is catalyzed by the enzyme NO synthase (NOS). NOS exists in three distinct isoforms: neuronal NOS (nNOS), inducible NOS (iNOS), and endothelial NOS (eNOS). NO derived from the constitutive isoforms of NOS (nNOS and eNOS) and other NO-adduct molecules (nitrosothiols) have been shown to be modulators of bronchomotor tone. On the other hand, NO derived from iNOS seems to be a proinflammatory mediator with immunomodulatory effects. The concentration of this molecule in exhaled air is abnormal in activated states of different inflammatory airway diseases, and its monitoring is potentially a major advance in the management of, e.g., asthma. Finally, the production of NO under oxidative stress conditions secondarily generates strong oxidizing agents (reactive nitrogen species) that may modulate the development of chronic inflammatory airway diseases and/or amplify the inflammatory response. The fundamental mechanisms driving the altered NO bioactivity under pathological conditions still need to be fully clarified, because their regulation provides a novel target in the prevention and treatment of chronic inflammatory diseases of the airways.

Nitrogen oxides (NOx), regarded in the past primarily as toxic air pollutants, have recently been shown to be bioactive species formed endogenously in the human lung. The relationship between the toxicities and the bioactivities of NOx must be understood in the context of their chemical interactions in the pulmonary microenvironment. Nitric oxide synthase (NOS) is a newly identified enzyme system active in airway epithelial cells, macrophages, neutrophils, mast cells, autonomic neurons, smooth muscle cells, fibroblasts, and endothelial cells. The chemical products of NOS in the lung vary with disease states, and are involved in pulmonary neurotransmission, host defense, and airway and vascular smooth muscle relaxation. Further, certain patients with pulmonary hypertension, adult respiratory distress syndrome and asthma may experience physiologic improvement with NOx therapy, including inhalation of nitric oxide (NO.) gas. Both endogenous and exogenous NOx react readily with oxygen, superoxide, water, nucleotides, metalloproteins, thiols, amines, and lipids to form products with biochemical actions ranging from bronchodilation and bacteriostasis (S-nitrosothiols) to cytotoxicity and pulmonary capillary leak (peroxynitrite), as well as those with frank mutagenic potential (nitrosamines). Recent discoveries demonstrating the relevance of these species to the lung have provided new insights into the pathophysiology of pulmonary disease, and they have opened a new horizon of therapeutic possibilities for pulmonary medicine.

In this issue of TheJournal Clerch et al. (1) show that hyperoxia regulates lung manganese superoxide dismutase (MnSOD) through a nonreceptor-mediated pathway involving G proteins. These observations suggest a number of possibilities regarding the cell involved and signaling mechanism used. Regu lation of MnSOD expression is a critical element in the lung's response to multiple forms of oxidant stress. It is likely that the location of MnSOD in mitochondria imparts protection to electron transport chain components enabling maintenance of cell energy sources under conditions of metabolic stress. Exposure to hyperoxia leads to a specific upregulation ofMnSOD in the mitochondria of alveolar epithelial type II cells (2). Selective over-expression of MnSOD in the mitochondria oftype II cells protects mice against hyperoxic stress (3). Indeed, the enhanced whole lung expression of MnSOD identified by Clerch et al. (1) is likely to have occurred in type II cells. These cells have a number of specialized functions designed to protect the host against inflammatory stimuli, microbes, and pollutants. In particular, they express extracellular (EC)-SOD and produce the bioactive radicals superoxide (02-) and nitric oxide (NO) (4). Selected biological functions of these radicals may derive from their rapid interaction to form peroxynitrite (OONO-). Which species predominates at the cell surface will depend on the rates and sites ofproduction ofthe primary radicals and the local concentrations of antioxidant enzymes such as EC-SOD. In this context one wonders if EC-SOD is not also regulated in the model of Clerch et al. (1). Two important questions are raised by this study. First, what is the molecular mechanism by which small diffusible ligands are recognized (i.e., what is the molecular sensor)? Second , how are these redox signals transduced into changes in gene expression? It has become increasingly clear that redox-active species such as O2 and NO-play important servoregu-latory roles through activation of cytosolic enzymes and transcription factors. Signaling by these redox species may be initiated in or at the plasma membrane (1, 5). Indeed, NO and H202 have been shown to activate G proteins (6). A distinctive feature shared by redox-active biomolecules is that they exert biological activity by virtue of their chemical reactivity, as opposed to the traditional noncovalent interactions of ligands with receptors. Metal-or sulfur-containing proteins are molecular targets for these diffusible signals. Critical thiols on the G protein are, therefore, candidate regulatory sites. Such activation of G proteins by S-nitrosylation (7) would be consistent with reports that NO. opposes pertussis toxin-mediated ADP-ribosylation ofcysteinyl residues (6). Analogous covalent interactions of O2T with protein thiols should be entertained. Con-formational changes induced in the protein likely serve as a switching mechanism to transduce the chemical signal into a physiological response. It is possible that redox-active biomolecules (O2 and ...