© 2005 American Thoracic Society doi: 10.1164/rccm.2412002
Searching for Acute Respiratory Distress Syndrome GenesAren't We There Yet?University of Toronto Toronto, Ontario, Canada Acute respiratory distress syndrome (ARDS) is one of biggest challenges in critical care medicine. Over the past three decades, much progress has been made in its diagnosis, prognosis, and management. However, its morbidity and mortality remain high. The etiology of ARDS is complicated, but its clinical and pathologic manifestations are similar. It has been speculated that a common injury pathway may contribute to the damage of lung tissue (1). Acute inflammatory responses have been considered as the major mechanisms that lead to acute lung injury (ALI) and ARDS. Several proinflammatory cytokines have been targeted with specific antagonists. However, these trials did not demonstrate clinical benefit (2). Mechanisms specific to different underlying causes of ARDS need to be explored. Several groups have published gene profiling data related to ALI (3–6), ventilator-induced lung injury (3, 7), and sepsis (3, 4). In the current issue of the Journal (pp. 361–370), Ye and coworkers report a new candidate gene for ALI, pro–B cell colony-enhancing factor (PBEF) (8). This gene is upregulated in ALI, both in animal models and clinical samples. The authors further elucidated its regulatory mechanisms in cell culture, identified different genotypes of PBEF among human subjects, and proposed potential connection between its polymorphism and the incidence of ARDS. This work raises interesting questions about the applications of gene profiling in ALI and strategies for uncovering genes involved in pathogenesis of this process. HOW TO IDENTIFY ARDS-RELATED GENES Most genes encoding cytokines and chemokines are inducible, that is, their basal levels in normal tissue are very low, and expression is rapidly elevated upon exposure to inflammatory stimuli. For instance, PBEF was first reported as a factor for the maturation of B cell precursors (9). Its expression was increased in neutrophils of patients with sepsis, and it can prolong the survival of neutrophils, which may consequently contribute to ALI (10). In another study, Jeyaseelan and colleagues reported that LIX (LPS-induced CXC chemokine) was significantly upregulated by LPS challenge in a rat model and that blocking LIX reduced ALI (4). The limitation of this strategy, however, is that our work could be restricted by the extent of current knowledge. Although we may complete the map of cytokine networks related to ARDS, the genes that are important for ARDS through mechanisms other than inflammatory responses could be overlooked. WHAT MODEL SYSTEM SHOULD BE USED? In recognition of the multiple faces of ARDS in clinical setting, several groups have used animal models (4, 7, 11). The genetic background of animal strains, the health conditions before experimentation, and the etiology of lung injury are well defined. However, with the infiltration of inflammatory cells into the lung, the dramatic changes of gene profiles could be simply due to changes in pulmonary cell populations. Therefore, cell culture models have been considered to be preferable in certain ways as they allow determination of the inflammatory responses in a particular cell type to a particular stimulus (3). On the other hand, cell–cell interactions, neural-humoral regulation, and other confounding factors cannot be simulated in vitro. Ye and coworkers used several animal models from different species and samples from patients with ARDS to determine changes of PBEF. They also used a cell culture model and performed genetic analysis in human patients (8). This multidisciplinary approach makes the conclusion more convincing. HOW TO VALIDATE THE MICROARRAY DATA Currently, quantitative real-time RT-PCR (qRT-PCR) is commonly used for confirmation of microarray data. The amplicons of qRT-PCR are very small, as they represent only a portion of a gene transcript. To confirm a particular gene of interest, primers should be designed against several regions of the gene. Copland and colleagues performed in situ hybridization on genes identified by microarray (7), which not only confirms alterations in expression, but also locates the cell types responsible for these changes. Ye and coworkers connected PBEF gene expression with its polymorphism to explain its importance in ALI (8). Inducible genes related to inflammatory responses usually require de novo transcription and translation. Thus, their protein levels should change in the same direction as their mRNAs. However, this may not be true for all genes. SHALL WE FOCUS ON GENES OR GENE PATTERNS?
Many investigators believe that microarrays are hypothesis-generating tools. Each gene identified will have to be tested individually. This concept is true; however, it discounts the power of gene profiling. dos Santos and colleagues reported that multiple CXC chemokines that were upregulated by TNF- AREN'T WE THERE YET? The technology of microarray and bioinformatics has been successfully applied in the journey of searching for ARDS genes. Focusing on the specific patterns of gene expression, with clinical confirmation of genes and gene patterns may lead to new discovery. We are at the end of a good beginning. FOOTNOTES Conflict of Interest Statement: M.L. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. REFERENCES
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in human lung epithelial cells are located closely on the same human chromosome (
