Abstract 5177: Spatial co-fragmentation pattern of cell-free DNA recapitulates in vivo chromatin organization and identifies tissue-of-origin
Authors: Liu Yaping , Liu Tzu-Yu , Weinberg David , Torre Chris J. De La , Tan Catherine L. , Schmitt Anthony D. , Selvaraj Siddarth , Tran Vy , Laurent Louise C. , Bidard François-Clément , Haque Imran S. 
Published: August 2019

Abstract
Three-dimensional chromatin organization varies across different cell types and is essential for gene regulation. Functional genomic elements that reside kilobases to megabases away can be brought into spatial proximity by chromatin folding. In fixed cells, DNA fluorescence in situ hybridization and super-resolution microscopy can measure the distances between loci at ~10-100nm resolution, while chromosome conformation capture followed by next-generation sequencing (Hi-C) is able to profile genome-wide chromatin organization at kilobase-pair level resolution by measuring contact probabilities between pairs of loci. These methods provide a static snapshot of genome compaction and organization in different cellular states. However, assessing in vivo genome-wide chromatin organization changes non-invasively and longitudinally in patients is challenging due to the limitations of current technologies. Recently, circulating cell-free DNA (cfDNA) in blood has been shown as a promising biomarker to capture the genetic and local epigenetic changes within patients.
Here, we inferred in vivo chromatin organization in blood cells from co-fragmentation patterns of cfDNA by using fragment lengths estimated from paired-end whole genome sequencing (WGS). We performed cfDNA WGS on 100 healthy, 34 colorectal cancer, 48 lung cancer, and 19 melanoma patients. The inferred chromatin organization is highly concordant with Hi-C performed on white blood cells and not explained by technical biases, sequence composition, or other epigenetic factors. Further, we developed methods to identify the tissue-of-origin of cfDNA based on its co-fragmentation pattern and Hi-C signal in reference cell types, which confirmed that most cfDNA in healthy individuals is derived from hematopoietic cells. In cancer patients, we observed an increased contribution to cfDNA from cancer cells that was quantitatively correlated with estimated tumor fraction in cfDNA and qualitatively matched tumor type. We also verified the results using publicly available cfDNA WGS data from different healthy and cancer patients. These results are consistent with previous studies that directly measured DNA methylation or that inferred nucleosome positions from WGS on cfDNA. However, our method has distinct advantages including using only low-coverage WGS, not requiring bisulfite treatment, and providing a more robust and quantitative estimation of cell type contributions. Collectively, our results demonstrate the potential of using cfDNA WGS to non-invasively assess the in vivo three-dimensional chromatin organization and determine tissue-of-origin in different physiological and pathological conditions, which may be useful for detecting, monitoring and treating different diseases.
Citation Format: Yaping Liu, Tzu-Yu Liu, David Weinberg, Chris J. De La Torre, Catherine L. Tan, Anthony D. Schmitt, Siddarth Selvaraj, Vy Tran, Louise C. Laurent, François-Clément Bidard, Imran S. Haque. Spatial co-fragmentation pattern of cell-free DNA recapitulates in vivo chromatin organization and identifies tissue-of-origin [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 5177.