Gain-of-function MUC5B Promoter Variant, Strongly Linked to Idiopathic Pulmonary Fibrosis, Defines an Epigenetically Poised Enhancer (ATAC-seq)

The G/T transversion, rs35705950, located approximately 3 kb upstream of the MUC5B start site, is the cardinal risk factor for IPF. Here, we investigate the function and chromatin structure of this -3 kb region and provide evidence that it functions as a classically defined enhancer subject to epigenetic programming. We use nascent transcript analysis to show that RNA polymerase II loads within 10 bp of the G/T transversion site, definitively establishing enhancer function for the region. By integrating ATAC-seq analysis of fresh and cultured human airway epithelial cells with nuclease sensitivity data, we demonstrate that this region is in accessible chromatin that affects the expression of MUC5B. Through applying paired single nucleus RNA- and ATAC-seq to frozen tissue from IPF lungs, we extend these findings directly to disease, with results indicating that epigenetic programming of the -3 kb enhancer in IPF occurs in both MUC5B-expressing and non-expressing lineages. In aggregate, our results indicate that the MUC5B-associated variant, rs35705950, resides within an enhancer that is subject to epigenetic remodeling and contributes to pathologic misexpression in IPF. The Omni method for the Assay for transposase-accessible chromatin by sequencing (Omni-ATAC-seq) was performed in primary human airway epithelial cells cultured at air-liquid interface (ALI), cells from fresh bronchial brushes or BEAS-2B airway epithelial cells. All samples were sequenced in duplicate.