Authored by Chen Hsiung Yeh*
Short Communication
High-throughput sequencing of circulating cell-free DNA (cfDNA) as liquid biopsy has revolutionized tumor genome profiling by providing a more accurate, longitudinal, real-time and non-invasive mean for precision and personalized medicine. Current knowledge on cfDNA characteristics revealed that it exists mainly as double-stranded molecules, resulting from biological fragmentation into both short (<1 kb) and long segments (>10 kb) [1,2]. Short fractions are mostly derived from apoptosis via the activation of cellular endonucleases leading to the cleavage of chromatin DNA into inter-nucleosomal fragments [3], whereas necrosis generates relatively long fragments of DNA. It is now believed that circulating cfDNA pool of cancer patients is originated from a combination of apoptosis, necrosis, and active release [4]. Within the nucleosomal core, cfDNA is protected from blood nucleases by histones, whereas the linker is vulnerable to digestion. As a result, regions showing high or low frequency of fragmentation correspond to the sequences between or occupied by nucleosomes, respectively, with the majority of short-fragment length corresponds to single nucleosome size of 160-170 bp. These observations strongly support the notion that the patterns of cfDNA fragmentation are guided by chromatin structure particularly the interplay between nucleosome positioning, epigenetic regulation, and gene expression machinery [5,6].
Furthermore, cfDNA is highly heterogeneous since it represents numerous different tissues each of which has its own gene expression profiles. Indeed, biological fragmentation of nucleosome-bound DNA is never random leading to biased representation of cfDNA sequences and unbalanced read coverage and uniformity, especially near genomic regions of transcription start sites and exonic boundaries, where nucleosome positioning is highly phased [6,7]. Hence, it is believed that chromatin changes associated with loci overall expression level contribute to the cfDNA fragmentation pattern, i.e., cfDNA patterning reflects a general picture of gene expression [8]. Accordingly, gene expression directed patterns of cfDNA fragmentation could have important impacts for next-generation sequencing (NGS) analysis. The required level of resolution for a NGS assay is achieved by providing sufficient coverage, which generally refers to the average number of reads that align to each base within the targeted gene regions. Theoretically, the coverage uniformity among different loci/alleles should be high and even to make calls with confidence. However, not all genes are born equally in terms of functionality and chromosome location, i.e., biological representation bias exists.
Information about the nature and mechanism of cfDNA fragmentation in their chromatin context is essential to understand not only the genetic representation but also the complex interactions that are responsible for tumor chromatin architecture. Changes in gene expression can alter dynamic chromatin states: gene activity is usually low with condensed and higher order packaged chromatin, whereas active gene expression always leads to relaxed and open chromatin where DNA fragmentation readily occurs (Figure 1).
Therefore, cfDNA fragmentation is not a random process evenly spread across the entire genome. As a result, it is expected that some genes in fragmented cfDNA are over-represented and others are under-represented. Most importantly, the real picture of uneven genetic representation of cfDNA caused by non-random fragmentation is greatly hindered by current DNA extraction methodology that depleted nucleosomes and other DNA-protein complexes in order to reach high sequencing uniformity.
The disparity in genetic representation in cfDNA fragments has been well documented [9-12]. Studies have shown that repetitive sequences Alu and certain satellite markers were found to be overrepresented, while L1 and L2 repeats were under-represented in the cell-free apoptotic DNA. L1 elements are mainly located in the transcriptionally inactive heterochromatin and Alu repeats are associated with gene-rich euchromatin regions that have high frequency of gene expression.
Examination of the fragment lengths present in cfDNA following silica extraction, we corroborated the findings in literature that fragmentation is primarily between nucleosomes with subsequent intra-nucleosomal cleavage along the DNA helical turn [13]. The fragment sizes corresponding to mono-, di- and tri-nucleosomal subunits appear to be prevalent (Figure 1). Only around 10-20 % of the cfDNA population are 170 bp increments with a laddering pattern, which is related to DNA wrapped around multiple nucleosomes and protected from nuclease cleavage. The higher abundance of longer fragment cfDNA population (>1 kb) could be due to the regions being generally inaccessible by enzymes because of the dense packaging. In addition, utilizing NGS analysis with each purified fraction, we are able to show that longer fragments exhibited much higher coverage uniformity, on-target rate and mean depth than shorter fragments (Table 1).
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