Circulating cell-free DNAs (cfDNAs) are DNA fragments which can be isolated from mammalian blood serum or plasma. In order to gain deeper insight into their origin(s), we have characterized the composition of human and cattle cfDNA via large-scale analyses of high-throughput sequencing data. We observed significant differences between the composition of cfDNA in serum/plasma and the corresponding DNA sequence composition of the human genome. Retrotransposable elements and non-telomeric satellite DNA were particularly overrepresented in the cfDNA population, while telomeric satellite DNA was underrepresented. This was consistently observed for human plasma, bovine serum and for the supernatant of human cancer cell cultures. Our results suggest that reverse transcription of retrotransposable elements and secondary-structure formation during the replication of satellite DNA are contributing to the composition of the cfDNA molecules in the mammalian blood stream. We believe that our work is an important step towards the understanding of the biogenesis of cfDNAs and thus may also facilitate the future exploitation of their diagnostic potential.

中文翻译：

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循环的无细胞DNA主要由逆转座因子和非端粒卫星DNA组成。

循环无细胞DNA（cfDNA）是可从哺乳动物血清或血浆中分离的DNA片段。为了更深入地了解它们的起源，我们通过对高通量测序数据的大规模分析来表征人和牛cfDNA的组成。我们观察到血清/血浆中的cfDNA组成与人类基因组的相应DNA序列组成之间存在显着差异。在cfDNA群体中，逆转座因子和非端粒卫星DNA的含量特别高，而端粒卫星DNA的含量却不足。对于人血浆，牛血清和人癌细胞培养物的上清液，一直观察到这一点。我们的结果表明，在卫星DNA复制过程中，可逆转座因子的逆转录和二级结构的形成有助于cfDNA分子在哺乳动物血流中的组成。我们相信，我们的工作是迈向了解cfDNA的生物发生的重要一步，因此也可能促进其诊断潜力的进一步开发。