In the present study, the annual decline in lung function was estimated for a population-based cohort in Korea. Current smokers showed significantly higher rate of decline in FEV1 than healthy never smokers. We also performed GWIS for genetic variants associated with COPD susceptibility over time. The interaction was evaluated using a joint test near the previously described FAM13, MFAP3L, and AADAT loci on chromosome 4 and at the DNAH11 locus on chromosome 7. Importantly, we found evidence for temporal interaction between DNAH11, MFAP3L, and AADAT and FEV1/FVC; minor alleles of the selected SNPs near DNAH11, MFAP3L, and AADAT tended to have low FEV1/FVC values. However, the statistical model for the analysis of gene-time interactions should be selected carefully. This was because the effects of time on FEV1/FVC were very strong, and the means and variances may differ according to genetic variants.
FAM13A was reported to be associated with the FEV1/FVC ratio in the CHARGE consortium [9] and was strongly associated with COPD susceptibility in the previous GWAS [8, 18, 19]. In this study, FAM13A was strongly associated with the FEV1/FVC ratio. Although this locus neither reached genome-wide significance nor was replicated in the GENIE cohort, this previously reported region could be associated with lung function in the Korean population.
The rs75679995 SNP was the second-most significant SNP in our cohort, which was found in the intron of DNAH11, a dynein-coding gene. The biological importance of dyneins (microtubule motor protein complexes) in DNAH11 is well understood. They are required for the initiation of MAPK3/6 and p38 signal transduction, which in turn regulate many biological processes, such as cell survival, differentiation, migration [20, 21], and immune and inflammatory responses [22, 23].
Primary ciliary dyskinesia (PCD) is a rare autosomal recessive genetic disorder characterised by dysfunction of motile cilia. DNAH11 mutations are reported in 6% of all PCDs, and 22% of those with normal ultrastructure [24–26]. Generally, ciliary dysmotility causes poor mucociliary clearance and leads to the impairment of pulmonary function and severe respiratory infections. The mechanism of lung function decline that was observed in the cohort could be attributed to ciliary dysfunction.
The results of eQTL analysis of rs9991425 revealed high expression of both MFAP3L and AADAT. We further analysed MFAP3L and AADAT expression using the GTEx portal, which revealed that MFAP3L was upregulated in muscle while AADAT was expressed in cultured fibroblasts. The numbers of transcripts per million kilobases for MFAP3L and AADAT in lung tissue were 1 and 6.286, respectively. Thus, it was not clear as to which gene contributed to the significant effect of rs9991425 on FEV1/FVC ratio; however, AADAT might be a more promising candidate, as its expression in lung tissue is higher than that of MFAP3L. This gene plays an important role in thyroid hormone regulation [27] and lung cancer [28] and is a major negative regulator of amino acid metabolism involving the mitochondria in cultured fibroblasts, suggesting that AADAT may be involved in the pathogenesis of COPD or in lung function decline.
Increasing evidence indicates that the mitochondria are involved in cellular functions beyond oxygen sensing and energy production. Mitochondria can sense upstream processes such as inflammation, infection, presence of tobacco smoke, and environmental insults [29–31], which play important roles in these diseases, and can subsequently respond to these stimuli by altering mitochondrial protein expression and structure (and resultant dysfunction). In contrast, mitochondrial dysfunction affects cytosolic and mitochondrial calcium regulation, airway contractility, expression of housekeeping genes, responses to oxidative stress, proliferation, apoptosis, fibrosis, and metabolism, which are all key aspects of airway disease pathophysiology [32]. To the best of our knowledge, this is the first study to demonstrate a potentially novel COPD susceptibility locus in AADAT on chromosome 4.
A report has shown that > 70% patients with COPD have GOLD stage 1 (mild) or 2 (moderate) disease with no apparent respiratory symptoms such as dyspnoea on exertion [33]. Previous studies have shown that tiotropium can ameliorate the annual decline in FEV1 and reduce the frequency of acute exacerbation compared to that observed in placebo-treated patients with GOLD stage 1 or 2 COPD [34, 35]. Therefore, earlier detection of COPD or lung function decline may lead to early effective intervention, reducing disease progression and the associated socioeconomic burden. Our observations are clinically important as they provide meaningful insights regarding the genetic risk of lung function decline in COPD. We proposed a method that revealed the complexity of gene-time interaction analyses, identified consistent gene-time interactions, and proposed a statistical model for association. Interest in early detection of the disease has been increasing in the USA in recent years, where COPD monitoring is performed by the National Lung Health Education Program. However, considering that individuals may suddenly undergo rapid decline in lung function, performing selective PFT only in patients with genetic susceptibility to COPD, rather than in all patients, may be cost-effective. The results of our study are especially noteworthy as studies investigating the genetic risk factors for early COPD or lung function decline are lacking despite substantial progress in evaluating the genetic susceptibility to COPD.
Although we were able to identify susceptibility loci related to decrease in lung function, unfortunately, these data were not identically replicated in the GENIE study. For example, although an interaction between time and SNPs was found for the KARE data, the interaction between pack years and SNPs was not significant for the GENIE data. However, this may be attributed to the fact that in our replication of the GENIE study, the characteristics of the patients differed from that obtained from the KARE data. In particular, the KARE data were based on rural and urban community populations, while the GENIE data comprised participants who underwent regular health screening and received routine medical care. Medical care and routine health check-ups are often positively related to socioeconomic status, which could have resulted in selection bias and influenced the results of our study [36].
Our study provides novel findings but has several notable limitations. First, the definition of COPD is based on pre-bronchodilator data. Second, we did not perform functional in vitro analysis regarding whether these genetic variants affect early COPD. Thus, further studies are required to validate our findings.
In conclusion, our study suggests that several SNPs located on chromosome 4 or in FAM13 are possibly associated with early COPD. We identified that DNAH11 and AADAT might be involved in susceptibility to lung function decline or COPD. Further studies are required to validate our findings.