In this study, we confirm that the MUC5B rs35705950 minor allele (T) is associated with IPF. The TOLLIP gene variant rs5743890 appears to correlate with survival and disease progression, therefore being of potential utility to stratify IPF patients at diagnosis.
The MUC5B promoter variant rs35705950 has been recognized as the dominant risk factor for developing (IPF) (7, 17, 20, 21). A recent metanalysis encompassing 2733 IPF patients and 5044 controls from 8 different cohorts confirmed that the minor T allele is significantly and strongly associated with an increased risk of IPF compared with the G allele (odds ratio [OR] 4.85, 95% confidence interval [CI] 3.79–6.21) in an allele dose-dependent manner. The strength of this association was more remarkable in Caucasians (22). In line with these previous studies, we found that the minor allele T (G/T and T/T genotype) is more frequent in IPF patients than in healthy controls (p < 0.0001). There was no difference with regard to age, gender or functional impairment at diagnosis between carriers or non-carriers of the minor allele T in our cohort. Borie et al. already reported similar results for French patients (23). We did not observe any association of MUC5B variants with survival, disease progression or acute exacerbation. In previous studies the associations with survival were contradictory. A significant association of the minor allele T with improved survival has been described in sporadic IPF in Caucasians (significant reduction in 2-year cumulative incidence of death)(3) whereas an association with increased mortality was reported in Han patients (24). A recent study from the Netherlands on a total of 170 IPF patients did not find an association of the minor allele T with survival in patients with sporadic IPF (N = 115), in contrast to MUC5B minor allele carriers with familial IPF (N = 55), who had a better survival (carriers 53 months vs non-carriers 37 months, p = 0.01) (25). Consistently with previous studies (3, 26), Van der Vis et al. did not find any association of MUC5B minor allele with severity of lung function impairment at diagnosis both in sporadic and familiar IPF(27). In 187 Han patients with IPF, carriers of MUC5B minor allele had significantly lower FVC and DLco values compared to those with homozygote major allele genotype (p < 0.0001 for both) (24). Further, Stock et al. did not observe any association of MUC5B minor T allele with lung function impairment at diagnosis, but with a longer time to decline in FVC (HR 0.59, p = 0.052), while no association was seen with time to decline in DLco (p = 0.9)(26). We did not find a significant association of the minor allele T with lung function impairment at diagnosis or decline over time. These findings are in line with a previous study from our group comparing the MUC5B polymorphism in German and Japanese IPF patients(18).
We did not find an association between TOLLIP SNPs alleles and IPF. This is in contrast to two previous genome-wide studies where the TOLLIP rs5743890 minor allele (C) was found to be protective and associated with reduced susceptibility to IPF(2, 20). Intriguingly, in one of these studies, IPF patients carrying this protective minor allele had increased mortality in three different cohorts (on average HR 1.72 with 95% CI 1.24–2.38, p = 0.0012). In the lung tissue from these patients the expression of TOLLIP protein was reduced by 20% (2). We also observed that patients carrying this minor allele had a reduced survival compared to those with the T/T genotype, with a remarkable difference of 20 months in median survival time. This association with survival was confirmed by multivariate analysis, after adjustment for a number of covariates. Consistently, patients with the C/T genotype had a higher rate of disease progression and tended to have a greater decline in FVC over time than patients with the T/T genotype. We could also demonstrate that the TOLLIP rs5743890 C/T genotype was the strongest predictor of disease progression in a multivariate analysis (HR 3.212, p = 0.003). So far, this is the first study showing an association of the minor allele (C) in TOLLIP rs5743890 with disease progression in IPF. Taken together, these data suggest a potential role of this SNP as a genetic biomarker for risk stratification of IPF patients in a real-world setting.
With respect to the other investigated TOLLIP SNP rs3750920, we did not find any significant association of the genotypes with survival or disease progression. However, a tendency for a protective role for AE of the minor allele (T) was observed (Fig. 3). Oldham et al. found that the TOLLIP rs3750920 polymorphisms may influence the response to NAC therapy in patients with IPF. Patients with the TOLLIP rs3750920 homozygote genotype for the minor allele may benefit from NAC therapy, whereas NAC may cause harm to patients carrying the homozygote genotype for the major allele. (4) .Due to the low number of patients treated with NAC in our cohort (24%), we were not able to replicate these observations. We did not find any association of MUC5B or TOLLIP SNPs with clinical response to treatment with antifibrotic drugs.
The importance of TOLLIP in IPF is emphasized by the relationship between biological impact and potential disease outcome. Besides the well-known role in inflammation and autoimmunity by enhancing signaling pathways associated with IL-1β, TNF-a, IFN-b, IL-13, toll-like receptor (TLR) (28) and autophagy(29), a recent study revealed that TOLLIP protects bronchial epithelial cells from bleomycin-induced apoptosis(30). A study investigating TOLLIP expression in lung tissue of IPF patients revealed that the TOLLIP expressing cells were macrophages, alveolar type II, and basal cells. A mechanistic assay using basal cells showed that TOLLIP was able to reduce mitochondrial ROS accumulation following bleomycin-induced mitochondrial damage of basal cells (30). All these data together corroborate the hypothesis of a protective effect of TOLLIP in several cell populations against oxidative damage and fibrosis. This could be a possible explanation of the link between TOLLIP SNPs and worse outcome in IPF. Further validation in bigger cohorts is needed.
Despite the novel findings of this study, it has several limitations. First, the number of patients is rather low for a genetic study and a validation cohort was not included. We found for example, that alleles in MUC5B and TOLLIP rs5743890, despite the highest linkage disequilibrium, were differently distributed, suggesting a possible genetic instability of the locus or, simpler, that a larger number of subjects is needed to confirm allele distribution. Second, there was an imbalance in age and smoking habits between patients and healthy controls. Further, only two subjects had familial IPF, so that a comparison with sporadic IPF patients, as other studies did(27), was not possible in our cohort. In conclusion, we confirm that the minor allele T at MUC5B rs35705950 is associated with IPF and that the minor allele C at TOLLIP rs5743890 is associated with an increased risk of death and disease progression. Although the results are promising, further validation in multi-center and multi-ethnic studies is needed.