Airway mucin MUC5AC and MUC5B concentrations and the initiation and progression of chronic obstructive pulmonary disease: an analysis of the SPIROMICS cohort

Abstract

Background: We previously described the contributions of increased total airway mucin concentrations to the pathogenesis and diagnosis of the chronic bronchitic component of chronic obstructive pulmonary disease (COPD). Here, we investigated the relative contribution of each of the major airway gel-forming mucins, MUC5AC and MUC5B, to the initiation, progression, and early diagnosis of airways disease in COPD.

Methods: SPIROMICS was a multicentre, observational study in patients aged 40-80 years recruited from six clinical sites and additional subsites in the USA. In this analysis, MUC5AC and MUC5B were quantitated by stable isotope-labelled mass spectrometry in induced sputum samples from healthy never-smokers, ever-smokers at risk for COPD, and ever-smokers with COPD. Participants were extensively characterised using results from questionnaires, such as the COPD assessment test (CAT) and St George's Respiratory Questionnaire; quantitative CT, such as residual volume/total lung capacity ratio (RV/TLC) and parametric response mapping-functional small airway disease (PRM-fSAD); and pulmonary function tests, such as FEV₁, forced vital capacity (FVC), and forced expiratory flow, midexpiratory phase (FEF_25-75%). Absolute concentrations of both MUC5AC and MUC5B were related to cross-sectional (baseline, initial visit) and 3-year follow-up longitudinal data, including lung function, small airways obstruction, prospective acute exacerbations, and smoking status as primary outcomes. This study is registered with ClinicalTrials.gov (NCT01969344).

Findings: This analysis included 331 participants (mean age 63 years [SEM 9·40]), of whom 40 were healthy never-smokers, 90 were at-risk ever-smokers, and 201 were ever-smokers with COPD. Increased MUC5AC concentrations were more reliably associated with manifestations of COPD than were MUC5B concentrations, including decreased FEV₁ and FEF_25-75%, and increased prospective exacerbation frequency, RV/TLC, PRM-fSAD, and COPD assessment scores. MUC5AC concentrations were more reactive to cigarette smoke exposure than were MUC5B concentrations. Longitudinal data from 3-year follow-up visits generated a multivariate-adjusted odds ratio for two or more exacerbations of 1·24 (95% CI 1·04-1·47, p=0·015) for individuals with high baseline MUC5AC concentration. Increased MUC5AC, but not MUC5B, concentration at baseline was a significant predictor of FEV₁, FEV₁/FVC, FEF_25-75%, and CAT score decline during the 3-year follow-up. Moreover, current smokers in the at-risk group showed raised MUC5AC concentrations at initial visits and decreased lung function over 3 years. By contrast, former smokers in the at-risk group showed normal MUC5AC concentrations at the initial visit and preserved lung function over 3 years.

Interpretation: These data indicate that increased MUC5AC concentration in the airways might contribute to COPD initiation, progression, exacerbation risk, and overall pathogenesis. Compared with MUC5B, greater relative changes in MUC5AC concentrations were observed as a function of COPD severity, and MUC5AC concentration seems to be an objective biomarker to detect disease in at-risk and pre-COPD individuals. These data suggest that MUC5AC-producing pathways could be potential targets for future therapeutic strategies. Thus, MUC5AC could be a novel biomarker for COPD prognosis and for testing the efficacy of therapeutic agents.

Funding: National Institutes of Health; National Heart, Lung, and Blood Institute.

Figure 1:

Participant flow diagram

Figure 2:. Individual mucin concentrations and COPD severity, and the association between lung function and mucin subtypes and their ratio

(A–C) Absolute mean concentrations of MUC5B and MUC5AC, and their ratio, in never-smoker controls, ever-smokers without evidence of COPD by spirometry (at-risk; formerly known as GOLD stage 0), and ever-smokers with mild (GOLD1), moderate (GOLD2), and severe (GOLD3) COPD. (D–F) Mean FEV₁ percentage predicted in tercile groups of MUC5AC concentration, MUC5B concentration, and MUC5AC/MUC5B ratio. (G–I) Mean FEV₁/FVC in tercile groups of MUC5AC concentration, MUC5B concentration, and MUC5AC/MUC5B ratio. Error bars represent SEM. Sample size for each group is shown in the middle of the bar. COPD=chronic obstructive pulmonary disease. FVC=forced vital capacity. GOLD=Global Initiative for Chronic Obstructive Lung Disease. *p≤0.01. †p≤0.005. ‡p≤0.001. §p≤0.05.

Figure 3:. Association between individual mucin concentrations and small airway patency and total future exacerbations

Mean FEF_25–75% predicted (A–C) and mean RV/TLC (D–F) in tercile groups of MUC5AC concentration, MUC5B concentration, and MUC5AC/MUC5B ratio. (G–I) Association between individual mucin concentrations and total future exacerbations (in all participants who completed 3-year follow-up visits). Panels G and H show mean MUC5AC and MUC5B concentrations and the prospective yearly total exacerbation rates (calculted by dividing the number of total exacerbations by follow-up time expressed in days multiplying by 365) from enrolment until the end of the study (until 3-year follow-up or death). Total exacerbations shown as zero exacerbations in next 3 years (92 participants), more than zero but fewer than two exacerbations (30 participants), and two or more exacerbations (44 participants). The multivariate adjusted odd ratios to predict future exacerbations is shown in the appendix (p 12). Error bars represent SEM. Sample size for each group is shown in the middle of the bar. FEF_25–75%=forced expiratory flow, midexpiratory phase. RV/TLC=residual volume/total lung volume ratio. *p≤0.01. †p≤0.001. ‡p≤0.05. §p≤0.005.

Figure 4:. Associations between mucin concentrations and assessment scores

Mean CAT scores (A–C) and mean SGRQ scores (D–F) in tercile groups of MUC5AC concentration, MUC5B concentration, and MUC5AC/MUC5B ratio. MUC5AC concentration (D; p=0.013) and MUC5AC/MUC5B ratio (F; p=0.0032), but not MUC5B concentration (E; p=0.094), were associated with overall SGRQ scores. CAT score ranges from 0 to 40, with higher scores denoting a more severe impact of COPD on a patient’s life. SGRQ score ranges from 0 to 100, with higher scores indicating more limitations. Error bars represent SEM. Sample size for each group is shown in the middle of the bar. CAT=COPD assessment test. COPD=chronic obstructive pulmonary disease. SGRQ=St George’s Respiratory Questionnaire. *p≤0.05. †p≤0.005. ‡p≤0.01.

Figure 5:. Effect of smoking cessation on individual mucin concentrations and prospective lung function over follow-up visits

(A–B) Effect of smoking status (former smoker vs current smoker) on MUC5AC and MUC5B concentration in at-risk ever-smokers and patients with COPD (GOLD stage 1–3). Data are means and error bars represent SEM. Sample size for each group is shown in the middle of the bar. (C) Comparison of lung function longitudinally over four visits. Comparison of mean FEV₁ percentage predicted in healthy never-smokers, at-risk former smokers, and at-risk current smokers from enrolment until the end of the study (until 3-year follow-up). Error bars represent SEM. COPD=chronic obstructive pulmonary disease. GOLD=Global Initiative for Chronic Obstructive Lung Disease. *p≤0.05. †p≤0.01. ‡p≤0.005. §p≤0.001.

Figure 6:. ROC curves of mucin concentrations

MUC5AC and MUC5B ROC curves in participants with COPD (FEV₁/FVC <0.7) compared with never-smokers. The AUC for MUC5AC in all smokers, with or without COPD, was 0.80 (95% CI 0.73–0.87) compared with never-smokers (appendix p 20). AUC=area under the ROC curve. COPD=chronic obstructive pulmonary disease. FVC=forced vital capacity. ROC=receiver-operating characteristic.