Several groups have compared eQTLs in different tissues from the same individual, finding both overlapping and tissue-specific eQTLs [59]–[61]. Multiple tissues are known to be important in COPD biology, including large and small airways, lung parenchyma and immune cells. By only surveying sputum, we may have missed significant eQTLs for COPD genes that are expressed in other tissues. Multiple cell types may be present in sputum, yet neutrophils have been shown to be the predominant cell type in the sputum samples from COPD subjects in ECLIPSE [16]. Despite these limitations, sputum is a clinically important tissue in COPD and is more accessible for genomic and biomarkers studies than lung tissue. Studying diseased individuals may be advantageous to identify eQTL SNPs for potential disease genes, which may only be expressed, or may be expressed at higher levels, in patients compared to healthy controls.
