Airway Mucin Concentration as a Marker of Chronic Bronchitis

Abstract

Background: Chronic obstructive pulmonary disease (COPD) is characterized by chronic bronchitic and emphysematous components. In one biophysical model, the concentration of mucin on the airway surfaces is hypothesized to be a key variable that controls mucus transport in healthy persons versus cessation of transport in persons with muco-obstructive lung diseases. Under this model, it is postulated that a high mucin concentration produces the sputum and disease progression that are characteristic of chronic bronchitis.

Methods: We characterized the COPD status of 917 participants from the Subpopulations and Intermediate Outcome Measures in COPD Study (SPIROMICS) using questionnaires administered to participants, chest tomography, spirometry, and examination of induced sputum. Total mucin concentrations in sputum were measured with the use of size-exclusion chromatography and refractometry. In 148 of these participants, the respiratory secreted mucins MUC5AC and MUC5B were quantitated by means of mass spectrometry. Data from chronic-bronchitis questionnaires and data on total mucin concentrations in sputum were also analyzed in an independent 94-participant cohort.

Results: Mean (±SE) total mucin concentrations were higher in current or former smokers with severe COPD than in controls who had never smoked (3166±402 vs. 1515±152 μg per milliliter) and were higher in participants with two or more respiratory exacerbations per year than in those with zero exacerbations (4194±878 vs. 2458±113 μg per milliliter). The absolute concentrations of MUC5B and MUC5AC in current or former smokers with severe COPD were approximately 3 times as high and 10 times as high, respectively, as in controls who had never smoked. Receiver-operating-characteristic curve analysis of the association between total mucin concentration and a diagnosis of chronic bronchitis yielded areas under the curve of 0.72 (95% confidence interval [CI], 0.65 to 0.79) for the SPIROMICS cohort and 0.82 (95% CI, 0.73 to 0.92) for the independent cohort.

Conclusions: Airway mucin concentrations may quantitate a key component of the chronic bronchitis pathophysiologic cascade that produces sputum and mediates disease severity. Studies designed to explore total mucin concentrations in sputum as a diagnostic biomarker and therapeutic target for chronic bronchitis appear to be warranted. (Funded by the National Heart, Lung, and Blood Institute and others.).

Figure 1. Associations between Total Mucin Concentrations and Phlegm Production and Disease Severity in Chronic Obstructive Pulmonary Disease (COPD)

Panel A shows a model representing the progression from normal lung to cigarette smoke–induced chronic bronchitis. In healthy persons, the balance of active ion absorption (Na⁺) versus secretion (Cl⁻), passive osmotically entrained water transport, and mucin secretion generates a mucus layer with secreted mucin concentrations that are lower than the tethered mucin and other glycoconjugate concentrations in the periciliary layer (PCL). The result is a well-hydrated PCL and efficient mucociliary clearance (MCC). In persons with cigarette smoke–induced chronic bronchitis, an imbalance of ion transport coupled with mucin hypersecretion increases the mucin concentration in the mucus layer, producing osmotic compression of the PCL, adhesion of hyperconcentrated mucus to airway surfaces, and cessation of MCC. The adherent mucus may be expelled as phlegm or sputum by cough. Mucus that cannot be expelled by cough continues to accumulate, concentrates, and ultimately becomes the basis for airflow obstruction and the nidus for intermittent infection or exacerbation. CFTR denotes cystic fibrosis transmembrane regulator, and ENaC epithelial sodium channel. Panel B shows total mucin concentration in controls who had never smoked and who reported no phlegm (59 participants; 10 of 69 participants did not answer the questionnaire), current or former smokers who reported no phlegm (397 participants), and current or former smokers who reported bringing up phlegm (434 participants). Panel C shows total mucin concentrations and spirometrically defined disease severity in controls who had never smoked (69 participants) and in current or former smokers with a Global Initiative for Chronic Obstructive Lung Disease (GOLD) stage of 0 (indicating an increased risk of disease; 303 participants), 1 (indicating mild COPD; 165 participants), 2 (indicating moderate COPD; 293 participants), or 3 (indicating severe COPD; 85 participants). Panel D shows total mucin concentration and the prospective annualized exacerbation rate from enrollment until the end of the study (defined as the number of days until follow-up or death). Rates were classified as zero exacerbations per year (596 participants), more than zero but fewer than two exacerbations per year (262 participants), and two or more exacerbations per year (36 participants). In Panels B through D, the P values shown but not connected by a bracket are for the comparison between the designated group and the first group shown. Other significant differences between groups are shown with a bracket. The horizontal line in the boxes represents the median, the cross represents the mean, and the bottom and top of the boxes represent the 25th and 75th percentiles, respectively. I bars represent the upper adjacent value (75th percentile plus 1.5 times the interquartile range) and the lower adjacent value (25th percentile minus 1.5 times the interquartile range), and the dots outliers. Bar plots of the data in Panels B through D are provided in the Supplementary Appendix, available with the full text of this article at NEJM.org. All P values were adjusted for multiple comparisons with the use of the Tukey–Kramer method.

Figure 2. Association between Potential Etiologic Pathways and Total Mucin Concentrations in Sputum

Panel A shows total mucin concentration and smoking history. Data are for 69 participants who had never smoked, 460 former smokers, and 374 current smokers. Panel B shows total mucin concentration and asthma status in participants with COPD (GOLD stages 1 through 3). Data are for 84 participants with current asthma and 389 participants who had never received a diagnosis of asthma. P values are for the comparison with participants who had never smoked or who had never received a diagnosis of asthma. The horizontal line in the boxes represents the median, the cross represents the mean, and the bottom and top of the boxes represent the 25th and 75th percentiles, respectively. I bars represent the upper adjacent value (75th percentile plus 1.5 times the interquartile range) and the lower adjacent value (25th percentile minus 1.5 times the interquartile range), and the dots outliers. All P values were adjusted for multiple comparisons with the use of the Tukey–Kramer method.

Figure 3. Absolute Concentrations of MUC5B and MUC5AC and Disease Severity

Panels A and B show absolute concentrations of MUC5B and MUC5AC, respectively, in controls who had never smoked (19 participants), current or former smokers without spirometric evidence of COPD (42 participants), current or former smokers with mild-to-moderate COPD (59 participants), and current or former smokers with severe COPD (28 participants). P values are for the comparison with controls who had never smoked. The horizontal line in the boxes represents the median, the cross represents the mean, and the bottom and top of the boxes represent the 25th and 75th percentiles, respectively. I bars represent the upper adjacent value (75th percentile plus 1.5 times the interquartile range) and the lower adjacent value (25th percentile minus 1.5 times the interquartile range), and the dots outliers. All P values were adjusted for multiple comparisons with the use of the Tukey–Kramer method.

Figure 4. Total Mucin Concentration and Diagnosis of Chronic Bronchitis

Panel A shows total mucin concentrations in current or former smokers who were identified as having chronic bronchitis by a questionnaire that reflected the classic definition of the disorder (199 participants), current or former smokers who were identified as having chronic bronchitis by the St. George’s Respiratory Questionnaire (SGRQ) (382 participants), and controls who had never smoked and were not identified as having chronic bronchitis by either questionnaire (58 participants). P values are for the comparison with healthy controls who had never smoked. The horizontal line in the boxes represents the median, the cross represents the mean, and the bottom and top of the boxes represent the 25th and 75th percentiles, respectively. I bars represent the upper adjacent value (75th percentile plus 1.5 times the interquartile range) and the lower adjacent value (25th percentile minus 1.5 times the interquartile range), and the dots outliers. All P values were adjusted for multiple comparisons with the use of the Tukey–Kramer method. Panel B shows receiver-operating-characteristic (ROC) curves for total mucin concentration in participants with classically defined chronic bronchitis, as compared with controls who had never smoked, in the SPIROMICS cohort (orange curve; area under the curve [AUC], 0.72 [0.65 to 0.79]) and in the independent cohort (green curve; AUC, 0.82 [0.73 to 0.92]). The blue curve represents total mucin concentration in participants with classically defined chronic bronchitis, as compared with all participants without chronic bronchitis, in the entire SPIROMICS cohort (AUC, 0.62 [0.58 to 0.67]).