Global Methylation Patterns in Idiopathic Pulmonary Fibrosis

BACKGROUND: Idiopathic Pulmonary Fibrosis (IPF) is characterized by profound changes in the lung phenotype including excessive extracellular matrix deposition, myofibroblast foci, alveolar epithelial cell hyperplasia and extensive remodeling. The role of epigenetic changes in determining the lung phenotype in IPF is unknown. In this study we determine whether IPF lungs exhibit an altered global methylation profile.\ud \ud METHODOLOGY/PRINCIPAL FINDINGS: Immunoprecipitated methylated DNA from 12 IPF lungs, 10 lung adenocarcinomas and 10 normal histology lungs was hybridized to Agilent human CpG Islands Microarrays and data analysis was performed using BRB-Array Tools and DAVID Bioinformatics Resources software packages. Array results were validated using the EpiTYPER MassARRAY platform for 3 CpG islands. 625 CpG islands were differentially methylated between IPF and control lungs with an estimated False Discovery Rate less than 5%. The genes associated with the differentially methylated CpG islands are involved in regulation of apoptosis, morphogenesis and cellular biosynthetic processes. The expression of three genes (STK17B, STK3 and HIST1H2AH) with hypomethylated promoters was increased in IPF lungs. Comparison of IPF methylation patterns to lung cancer or control samples, revealed that IPF lungs display an intermediate methylation profile, partly similar to lung cancer and partly similar to control with 402 differentially methylated CpG islands overlapping between IPF and cancer. Despite their similarity to cancer, IPF lungs did not exhibit hypomethylation of long interspersed nuclear element 1 (LINE-1) retrotransposon while lung cancer samples did, suggesting that the global hypomethylation observed in cancer was not typical of IPF.\ud \ud CONCLUSIONS/SIGNIFICANCE: Our results provide evidence that epigenetic changes in IPF are widespread and potentially important. The partial similarity to cancer may signify similar pathogenetic mechanisms while the differences constitute IPF or cancer specific changes. Elucidating the role of these specific changes will potentially allow better understanding of the pathogenesis of IPF.\ud \u

Methylation profile of LINE-1 retrotransposon.

<p>Global methylation levels of the LINE-1 retrotransposons were defined using the EpiTYPER mass array assay. Each bar represents the methylation in one of the 15 CpG dinucleotides or CpG units that span the LINE-1 sequence. Methylation levels are calculated as the average of all samples in each group (Control, IPF, Cancer) and standard error bars are included. The X axis shows the CpG dinucleotide or the CpG unit and the Y axis shows the percentage of methylation. The LINE-1 promoter is indicated in purple. The red arrowheads indicate units of 2-3 CpG dinucleotides.</p

Comparison of IPF and Adenocarcinoma to control samples.

<p>(A) 3-D plot representation of the results after Principal Component Analysis of all 3 sample groups. Each color-coded dot represents a sample (red-control, green-IPF and blue-cancer) and each sample is positioned in the 3-D space according to its similarity or difference to the others. (B) Comparison of differentially methylated CpG islands that overlap between IPF and Lung Cancer. The heatmap consists of 402 differentially methylated CpG islands that are found to overlap between IPF and Lung Cancer. High methylation levels of a CpG islands are shown in yellow while low methylation levels of methylation are shown in purple. Grey stands for no difference between the two groups being compared.</p

Human subjects.

<p>Human subjects.</p

Expression of genes with differentially methylated promoters.

<p>qRT-PCR assay on 3 genes with hypomethylated promoter-associated CpG islands showed increase in the expression of the downstream gene. Y-axis shows fold change of detected transcripts in IPF samples when the expression in controls is set to baseline equal to 1. * denotes p-values <0.05. Error bars are based on Standard Deviation.</p

Differentially methylated promoters between IPF and control samples.

<p>Differentially methylated promoters between IPF and control samples.</p

Functional annotation clustering of differentially methylated CpG islands.

<p>Functional annotation clustering of differentially methylated CpG islands.</p

CpG islands are differentially methylated in IPF and control samples

<p>(A) Human CpG Island Microarray data: the heatmap on the left is the visual comparison of global methylation profiles between the 10 Control and the 12 IPF samples. The heatmap on the right consists only of the differentially methylated CpG probes highlighted by the red rectangle (<i>p</i>-value < 0.05, FDR<5%). Methylated CpG islands are shown in progressively brighter shades of yellow, depending on the fold difference, and hypomethylated CpG islands are shown in progressively brighter shades of purple. Grey stands for no difference between the two sample groups being compared. (B) EpiTYPER confirmation of differentially methylated CpG islands. Bars represent the average methylation of all the samples per study group. The X-axis shows the genomic location of each CpG island and Y-axis shows the percentage of methylation.</p