Abstract
Plasma transports blood cells with a mixture of compounds, including nutrients, waste, antibodies, and chemical messengers...throughout the body. Non-soluble factors such as circulating DNA and extracellular vesicles have recently been added to the list of these components and have been the subject of extensive research due to their role in intercellular communication. Circulating DNA (cirDNA) is composed of cell-free and particle-associated DNA fragments, which can be released by all cell types. cirDNA is derived not only from genomic DNA but also from extrachromosomal mitochondrial DNA. Numerous studies carried out lately indicate that the quantitative and qualitative analysis of cirDNA represents a breakthrough in clinical applications as a non-invasive biomarker for diagnosis, prognosis and therapeutic follow-up. However, despite the promising future of cirDNA in clinical applications, particularly in oncology, knowledge regarding its origins, composition and functions, that could considerably optimize its diagnostic value, is still lacking.The main goal of my thesis was to identify and characterize the structural properties of extracellular DNA of mitochondrial origin. By examining the integrity of this DNA, as well as the size and density of associated structures, this work revealed the presence of dense particles larger than 0.2 µm containing whole mitochondrial genomes. We characterized these structures by electron microscopy and flow cytometry and identified intact mitochondria in the extracellular medium in vitro and ex vivo (in plasma samples from healthy individuals). Oxygen consumption by these mitochondria was detected by the Seahorse technology, suggesting that at least some of these intact extracellular mitochondria may be functional.In addition, I contributed to other studies carried out in the team, such as studies aiming at evaluating (1) the influence of pre-analytical and demographic parameters on the quantification of nuclear and mitochondrial cirDNA on a cohort of 104 healthy individuals and 118 patients with metastatic colorectal cancer, (2) the influence of hypoxia on the release of cirDNA in vitro and in vivo, and (3) the potential of cirDNA analysis in the early detection and screening of cancer.This manuscript present a recent review on cirDNA and its different mechanisms of release, which go hand in hand with the structural characterization of this DNA, its functional aspects and its clinical applications. In addition, this thesis provides new knowledge on the structure of extracellular mitochondrial DNA and opens up new avenues for reflection, particularly on the potential impact that could have those circulating mitochondria on cell-cell communication, inflammation and clinical applications.