Severity of cough in idiopathic pulmonary fibrosis is associated with MUC5 B genotype

In our cohort of 68 IPF patients, we replicate the reported association between the minor T allele of SNP rs35705950 in the MUC5B promoter region and ILD. Our data demonstrate a minor allele frequency (MAF) of 37% in the group with LCQ and 33% in the group without the LCQ, similar to previously published IPF data [1, 12]. Seibold et al. [1], reported a MAF of 37.5% in cases and 9.1% in controls. Data from the 1000 Genomes Project gives a frequency of the T allele of 5.1% for controls. Our study is therefore consistent with the current observations that the minor allele is associated with IPF.

Within the subset of patients for whom we have data on cough severity, we found statistically significant association of the minor T allele with cough severity. This association is independent of effects of age, gender and other clinical variables (GERD, UACS, smoking status, sleep apnea, FVC, FEV1/FVC and DLCO). This lack of association with the FEV1/FVC ratio and DLCO suggests that concomitant emphysema or other obstructive disease does not account for any of the observed cough differences. There were an additional 68 patients who did not return the cough questionnaire. This is in part because 45 of these patients had died prior to the initiation of this study. It is notable that this group matches the questionnaire group in age, sex and genotype frequency. However, the pulmonary function was significantly lower, likely related to the late phase of their disease at the time of recruitment. The IPF subjects demographics are consistent with the current literature in that they were predominantly male and older in age. Of note, our population does have a smaller percentage of smokers than is typically reported. This reflects the population of Utah where smoking is less common.

There are several possible mechanisms for our observation that MUC5B genetic variation is related to cough severity. MUC5B expression is upregulated in patients with IPF who carry the minor allele [1]. The upregulation may impact airway clearance of mucus or increase mucus secretion resulting in a symptomatic cough. Murine models suggest that quantities of intracellular mucin in airway epithelial cells results from the balance between mucin production, clearance and secretion. This balance is tightly controlled by MUC5AC and MUC5B [13]. In normal airways, MUC5B appears to be the primary gel forming mucin in the small airways. Expression of both of these mucins can be altered by several inflammatory factors [14] although it appears that MUC5B expression is less influenced in inflammatory states than is MUC5AC [15].

The MUC5B polymorphism relationship to cough in IPF observed in our study may be consistent with recent findings by Seibold et al. [16] demonstrating that MUC5B is the dominant mucin in the normal distal airway epithelium and in the honeycomb lesion seen in IPF [16]. Mucin may be developing distally in those patients with overexpression driven by the MUC5B mutations. Distal mucin accumulation may trigger a cough that may or may not be productive. This phenomenon is postulated in other diseases such as asthma, bronchiolitis and emphysema where the presence of excess mucus from the surface epithelium primarily impacts the distal airways [17]. The previous study by Siebold suggested that mucus production is enhanced in all patients with IPF regardless of the MUC5B genotype [1, 18]. However, MUC5B is the overexpressed mucin in these patients and may possess specific qualities that trigger an exaggerated cough. Interestingly, in our study, the MUC5B polymorphism did not appear to impact sinus disease suggesting there is no role of this genetic variant in upper airway tract disease.

The presence of the minor (T) allele has been associated with improved survival in IPF [18]. This finding supports the genetic basis of disease heterogeneity associated with the MUC5B genotype. The presence of cough would be expected to increase shear stress postulated to be a factor in IPF [19] and thus, from a biomechanical perspective, it is difficult to reconcile increased cough and improved survival. While it is not immediately apparent how an enhanced cough and improved survival are interrelated, it could be postulated that the cough encourages clearance of infectious agents or decreases time of exposure to excess MUC5B and other deleterious molecules in the respiratory bronchioles that potentially interfere with alveolar repair.

This study has a number of limitations. The number of patients who responded to the questionnaire is relatively small, mostly reflective of the death of subjects who were accrued in earlier years prior to the current study with the cough questionnaire. For these subjects, descriptive information including sinus disease, acid reflux disease or estimates of cough severity on subjects are not uniformly available. Thus, although the genotype frequencies were similar in subjects with and without cough questionnaires, we do not know if this group’s cough severity has any relation to the T allele. A second limitation is that our measure of cough was a self-report rather than an objective cough count. This methodology may introduce some reporter bias. However, it should be noted that a well-accepted validated questionnaire was utilized. Our analysis of the comorbidities known to impact cough is suboptimal. Subjects were assigned a categorization of affectation by self-report and medical record review when available. In the case of GERD, current estimates that up to 90% of patients with idiopathic pulmonary fibrosis have GERD [20]. Our reported percentage of GERD likely underestimates true prevalence of the GERD in our subjects. Moreover, we do not have validated data quantifying the severity of reflux or the effectiveness of treatment. Future studies to follow up on our observation would benefit from a larger study population, more precise measures of the comorbidities known to produce cough and an objective cough measure. In addition, evaluating the relationship between the MUC5B polymorphism and the Cough Quality of Life questionnaire, which has recently been validated for use in IPF [21], should be considered. Evaluation of the association between mucus characteristics, MUC5B allele and cough severity might also be interesting.

The variable expression of cough in IPF patients is not predictable. Cough is a problematic symptom in many, but not all, patients with this disease. While well-known cough risk factors such as GERD or ACE inhibitors can sometimes be controlled, clinicians are often unsuccessful in mitigating this troubling symptom. Recently, use of thalidomide has been shown to decrease cough severity in IPF [22], but there are very few effective therapeutic options for treating cough. Understanding genetic underpinnings of this and other specific clinical features of IPF may allow for personalized therapeutic approaches with potential to improve cough or even slow disease progression.

Our findings represent a potentially important application of the role MUC5B plays in the phenotypic expression of IPF. Phenotypic heterogeneity is observed clinically in patients with IPF but the genetic components of this heterogeneity are not understood. As further genetic contributions to IPF are elucidated, it is important to determine the associations between genotypes and phenotypic expression. Our observation of the possible relationship between MUC5B and cough suggests a possible influence of this polymorphism and suggests that the presence of the minor allele in MUC5B may account for a specific phenotypic component of IPF.