Circulating free DNA (cfDNA) (also known as cell-free DNA) are degraded DNA fragments released to body fluids such as blood plasma, urine, cerebrospinal fluid, etc. Typical sizes of cfDNA fragments reflect chromatosome particles (~165bp), as well as multiples of nucleosomes, which protect DNA from digestion by apoptotic nucleases. [1] The term cfDNA can be used to describe various forms of DNA freely circulating in body fluids, including circulating tumor DNA (ctDNA), cell-free mitochondrial DNA (ccf mtDNA), cell-free fetal DNA (cffDNA) and donor-derived cell-free DNA (dd-cfDNA). [2] Elevated levels of cfDNA are observed in cancer, especially in advanced disease. [3] There is evidence that cfDNA becomes increasingly frequent in circulation with the onset of age. [4] cfDNA has been shown to be a useful biomarker for a multitude of ailments other than cancer and fetal medicine. This includes but is not limited to trauma, sepsis, aseptic inflammation, myocardial infarction, stroke, transplantation, diabetes, and sickle cell disease. [5] cfDNA is mostly a double-stranded extracellular molecule of DNA, consisting of small fragments (50 to 200 bp) [6] [7] and larger fragments (21 kb) [8] and has been recognized as an accurate marker for the diagnosis of prostate cancer and breast cancer. [9]
Recent studies have laid the foundation for inferring gene expression from cell-free DNA, with EPIC-seq emerging as a notable advancement. [10] This method has substantially raised the bar for the noninvasive inference of expression levels of individual genes, thereby augmenting the assay's applicability in disease characterization, histological classification, and monitoring treatment efficacy. [10] [11] [12]
Other publications confirm the origin of cfDNA from carcinomas and cfDNA occurs in patients with advanced cancer. Cell‐free DNA (cfDNA) is present in the circulating plasma and in other body fluids. [13]
The release of cfDNA into the bloodstream appears by different reasons, including apoptosis, necrosis and NETosis. Its rapidly increased accumulation in blood during tumor development is caused by an excessive DNA release by apoptotic cells and necrotic cells. Active secretion within exosomes has been discussed, but it is still unknown whether this is a relevant or relatively minor source of cfDNA. [14]
cfDNA circulates predominantly as nucleosomes, which are nuclear complexes of histones and DNA. [15] cfDNA can also be observed in shorter size ranges (e.g. 50bp) and associated with regulatory elements. [16] They are frequently nonspecifically elevated in cancer but may be more specific for monitoring cytotoxic cancer therapy, mainly for the early estimation of therapy efficacy. [17]
Circulating nucleic acids were first discovered by Mandel and Metais in 1948. [18] It was later discovered that the level of cfDNA is significantly increased in the plasma of diseased patients. This discovery was first made in lupus patients [19] and later it was determined that the levels of cfDNA are elevated in over half of cancer patients. [20] Molecular analysis of cfDNA resulted in an important discovery that blood plasma DNA from cancer patients contains tumor-associated mutations and it can be used for cancer diagnostics and follow up. [21] [22] The ability to extract circulating tumor DNA (ctDNA) from the human plasma has led to huge advancements in noninvasive cancer detection. [23] Most notably, it has led to what is now known as liquid biopsy. In short, liquid biopsy is using biomarkers and cancer cells in the blood as a means of diagnosing cancer type and stage. [24] This type of biopsy is noninvasive and allows for the routine clinical screening that is important in determining cancer relapse after initial treatment. [25]
The intracellular origin of cfDNA, e.g., either from nucleus or mitochondria, can also influence the inflammatory potential of cfDNA. mtDNA is a potent inflammatory trigger. [26] mtDNA, due to its prokaryotic origin, holds many features that are similar to bacterial DNA, including the presence of a relatively high content of unmethylated CpG motifs, which are rarely observed in nuclear DNA. [27] The unmethylated CpG motifs are of particular importance as TLR9, the only endolysosomal DNA-sensing receptor, has a unique specificity for unmethylated CpG DNA. mtDNA was shown to activate neutrophils through TLR9 engagement [28] unless coupled to carrier proteins, mtDNA, but not nuclear DNA, can be recognized as a danger-associated molecular pattern inducing pro-inflammation through TLR9. [29] Collins et al. reported that intra-articular injection of mtDNA induces arthritis in vivo, proposing a direct role of mtDNA extrusion in the disease pathogenesis of RA . [30] [29]
MtDNA, in contrast to nuclear DNA, is characterized by elevated basal levels of 8-OHdG, a marker of oxidative damage. The high content of oxidative damage in mtDNA is attributed to the close proximity of mtDNA to ROS and relatively inefficient DNA repair mechanisms that can lead to the accumulation of DNA lesions. [30] [31]
They have shown that oxidative burst during NETosis can oxidize mtDNA and the released oxidized mtDNA by itself, or in complex with TFAM, can generate prominent induction of type I IFNs. [26] Oxidized mtDNA generated during programmed cell death is not limited to activate TLR9, but was shown to also engage the NRLP3 inflammasome, leading to the production of pro-inflammatory cytokines, IL-1β, and IL-18. [30] [32] MtDNA can also be recognized by cyclic GMP-AMP synthase (cGAS), a cytosolic dsDNA sensor to initiate a STING-IRF3-dependent pathway that in turn orchestrates the production of type I IFNs. [30] [33]
cfDNA purification is prone to contamination through genomic DNA due to ruptured blood cells during the purification process. [34] Because of this, different purification methods can lead to significantly different cfDNA extraction yields. [35] [36] At the moment, typical purification methods involve collection of blood via venipuncture, centrifugation to pellet the cells, and extraction of cfDNA from the plasma. The specific method for extraction of cfDNA from the plasma depends on the protocol desired. [37]
In general, the detection of specific DNA sequences in cfDNA can be done by two means; sequence specific detection (PCR based) and general genomic analysis of all cfDNA present in the blood (DNA sequencing). [38] The presence of cfDNA containing DNA from tumor cells was originally characterized using PCR amplification of mutated genes from extracted cfDNA. [21] PCR based analysis of cfDNA typically rely on the analytical nature of qPCR and digital PCR. Both of these techniques can be sensitive and cost-effective for detecting limited number of hotspots mutations. For this reason the PCR based method of detection is still very prominent tool in cfDNA detection. This method has the limitation of not being able to detect larger structural variant present in ctDNA and for this reason massively parallel next generation sequencing is also used to determine ctDNA content in cfDNA
Massively parallel sequencing (MPS) has allowed the deep sequencing of cfDNA. This deep sequencing is required to detect mutant ctDNA present in low concentrations in the plasma. Two main sequencing techniques are typically used for targeted analysis of mutant cfDNA; PCR amplicon sequencing [39] and hybrid capture sequencing. [40] Other forms of genetic alterations can be analysed using ctDNA (e.g. somatic copy number alterations or genetic rearrangements). Here, methods based on untargeted sequencing, like WGS or low coverage WGS, are mainly used.
The majority of cfDNA research is focused on DNA originating from cancer (ctDNA). In short, the DNA from cancer cells gets released by cell-death, secretion or other mechanisms still not known. [41] The fraction of cfDNA released by tumor cells in circulation is influenced by the size of the tumor as well as the tumor stage and type. Early stage cancers and brain tumor are among the most difficult to detect with liquid biopsy. [42] [43] [44]
Elevated cfDNA has been detected with acute blunt trauma [45] and burn victims. [46] In both of these cases cfDNA concentration in the plasma were correlated to the severity of the injury, as well as outcome of the patient.
It has been shown that an increase cfDNA in the plasma of ICU patients is an indicator of the onset of sepsis. [47] [48] Due to the severity of sepsis in ICU patients, further testing in order to determine the scope of cfDNA efficacy as a biomarker for septic risk is likely. [5]
Patients showing signs of myocardial infarction have been shown to have elevated cfDNA levels. [49] This elevation correlates to patient outcome in terms of additional cardiac issues and even mortality within two years. [50]
Foreign cfDNA has been shown to be present in the plasma of solid organ transplant patients. This cfDNA is derived from the grafted organ and is termed dd-cfDNA (donor-derived cell-free DNA). Dd-cfDNA values spike initially after a transplant procedure (>5%) with values heavily depending on the transplanted organ and typically drop (<0.5%) within one week for most organs. [51] If the host body rejects the grafted organ the ddcfDNA concentration in the blood (plasma) will rise to a level greater than 5-fold higher than those without complications. This increase in ddcfDNA can be detected prior to any other clinical or biochemical signs of complication. [51] Besides dd-cfDNA in plasma, some research also focused on the excretion of ddcfDNA through urine. This is of special interest in kidney allografts transplantation. When dd-cfDNA is measured using targeted next-generation sequencing, assays were used with a population specific genome wide SNP panel. [52] Attaching barcodes to the ligated adapters prior to NGS during library preparation make absolute ddcfDNA quantification possible without the need for prior donor genotyping. [53] This has been shown to provide additional clinical benefits if the absolute number of cfDNA copies is considered combined together with the fraction of ddcfDNA over cfDNA from the recipient to determine whether the allograft is being rejected or not. [52]
cfDNA allows a rapid, easy, non-invasive and repetitive method of sampling. A combination of these biological features and technical feasibility of sampling, position cfDNA as a potential biomarker of enormous utility for example for autoimmune rheumatic diseases and tumors. It offers also a potential biomarker with its own advantages over invasive tissue biopsy as a quantitative measure for detection of transplant rejection as well as immunosuppression optimisation. However, this method lacks uniformity on the type of sample (plasma/serum/synovial fluid/urine), methods of sample collection/processing, free or cell-surface bound DNA, cfDNA extraction and cfDNA quantification, and also in the presentation and interpretation of quantitative cfDNA findings. [30]
cfDNA is quantified by fluorescence methods, such as PicoGreen staining and ultraviolet spectrometry, the more sensitive is quantitative polymerase chain reaction (PCR; SYBR Green or TaqMan) of repetitive elements or housekeeping genes, or deep sequencing methods. Circulating nucleosomes, the primary repeating unit of DNA organization in chromatin, are quantified by enzyme-linked immunosorbent assays (ELISA). [54]
NucPosDB: a database of nucleosome positioning in vivo and nucleosomics of cell-free DNA
A biopsy is a medical test commonly performed by a surgeon, an interventional radiologist, or an interventional cardiologist. The process involves the extraction of sample cells or tissues for examination to determine the presence or extent of a disease. The tissue is then fixed, dehydrated, embedded, sectioned, stained and mounted before it is generally examined under a microscope by a pathologist; it may also be analyzed chemically. When an entire lump or suspicious area is removed, the procedure is called an excisional biopsy. An incisional biopsy or core biopsy samples a portion of the abnormal tissue without attempting to remove the entire lesion or tumor. When a sample of tissue or fluid is removed with a needle in such a way that cells are removed without preserving the histological architecture of the tissue cells, the procedure is called a needle aspiration biopsy. Biopsies are most commonly performed for insight into possible cancerous or inflammatory conditions.
Noninvasive genotyping is a modern technique for obtaining DNA for genotyping that is characterized by the indirect sampling of specimen, not requiring harm to, handling of, or even the presence of the organism of interest. Beginning in the early 1990s, with the advent of PCR, researchers have been able to obtain high-quality DNA samples from small quantities of hair, feathers, scales, or excrement. These noninvasive samples are an improvement over older allozyme and DNA sampling techniques that often required larger samples of tissue or the destruction of the studied organism. Noninvasive genotyping is widely utilized in conservation efforts, where capture and sampling may be difficult or disruptive to behavior. Additionally, in medicine, this technique is being applied in humans for the diagnosis of genetic disease and early detection of tumors. In this context, invasivity takes on a separate definition where noninvasive sampling also includes simple blood samples.
Digital polymerase chain reaction is a biotechnological refinement of conventional polymerase chain reaction methods that can be used to directly quantify and clonally amplify nucleic acids strands including DNA, cDNA, or RNA. The key difference between dPCR and traditional PCR lies in the method of measuring nucleic acids amounts, with the former being a more precise method than PCR, though also more prone to error in the hands of inexperienced users. A "digital" measurement quantitatively and discretely measures a certain variable, whereas an “analog” measurement extrapolates certain measurements based on measured patterns. PCR carries out one reaction per single sample. dPCR also carries out a single reaction within a sample, however the sample is separated into a large number of partitions and the reaction is carried out in each partition individually. This separation allows a more reliable collection and sensitive measurement of nucleic acid amounts. The method has been demonstrated as useful for studying variations in gene sequences — such as copy number variants and point mutations — and it is routinely used for clonal amplification of samples for next-generation sequencing.
Minimal residual disease (MRD), also known as Molecular residual disease, is the name given to small numbers of cancer cells that remain in a person either during or after treatment when the patient is in remission. Sensitive molecular tests are either in development or available to test for MRD. These can measure minute levels of cancer cells in tissue samples, sometimes as low as one cancer cell in a million normal cells, either using DNA, RNA or proteins.
A circulating tumor cell (CTC) is a cell that has shed into the vasculature or lymphatics from a primary tumor and is carried around the body in the blood circulation. CTCs can extravasate and become seeds for the subsequent growth of additional tumors (metastases) in distant organs, a mechanism that is responsible for the vast majority of cancer-related deaths. The detection and analysis of CTCs can assist early patient prognoses and determine appropriate tailored treatments. Currently, there is one FDA-approved method for CTC detection, CellSearch, which is used to diagnose breast, colorectal and prostate cancer.
A cancer biomarker refers to a substance or process that is indicative of the presence of cancer in the body. A biomarker may be a molecule secreted by a tumor or a specific response of the body to the presence of cancer. Genetic, epigenetic, proteomic, glycomic, and imaging biomarkers can be used for cancer diagnosis, prognosis, and epidemiology. Ideally, such biomarkers can be assayed in non-invasively collected biofluids like blood or serum.
Natera, Inc. is a clinical genetic testing company based in Austin, Texas that specializes in non-invasive, cell-free DNA (cfDNA) testing technology, with a focus on women’s health, cancer, and organ health. Natera’s proprietary technology combines novel molecular biology techniques with a suite of bioinformatics software that allows detection down to a single molecule in a tube of blood. Natera operates CAP-accredited laboratories certified under the Clinical Laboratory Improvement Amendments (CLIA) in San Carlos, California and Austin, Texas.
Extracellular RNA (exRNA) describes RNA species present outside of the cells in which they were transcribed. Carried within extracellular vesicles, lipoproteins, and protein complexes, exRNAs are protected from ubiquitous RNA-degrading enzymes. exRNAs may be found in the environment or, in multicellular organisms, within the tissues or biological fluids such as venous blood, saliva, breast milk, urine, semen, menstrual blood, and vaginal fluid. Although their biological function is not fully understood, exRNAs have been proposed to play a role in a variety of biological processes including syntrophy, intercellular communication, and cell regulation. The United States National Institutes of Health (NIH) published in 2012 a set of Requests for Applications (RFAs) for investigating extracellular RNA biology. Funded by the NIH Common Fund, the resulting program was collectively known as the Extracellular RNA Communication Consortium (ERCC). The ERCC was renewed for a second phase in 2019.
Circulating tumor DNA (ctDNA) is tumor-derived fragmented DNA in the bloodstream that is not associated with cells. ctDNA should not be confused with cell-free DNA (cfDNA), a broader term which describes DNA that is freely circulating in the bloodstream, but is not necessarily of tumor origin. Because ctDNA may reflect the entire tumor genome, it has gained traction for its potential clinical utility; "liquid biopsies" in the form of blood draws may be taken at various time points to monitor tumor progression throughout the treatment regimen.
A liquid biopsy, also known as fluid biopsy or fluid phase biopsy, is the sampling and analysis of non-solid biological tissue, primarily blood. Like traditional biopsy, this type of technique is mainly used as a diagnostic and monitoring tool for diseases such as cancer, with the added benefit of being largely non-invasive. Liquid biopsies may also be used to validate the efficiency of a cancer treatment drug by taking multiple samples in the span of a few weeks. The technology may also prove beneficial for patients after treatment to monitor relapse.
CAPP-Seq is a next-generation sequencing based method used to quantify circulating DNA in cancer (ctDNA). The method was introduced in 2014 by Ash Alizadeh and Maximilian Diehn’s laboratories at Stanford, as a tool for measuring Cell-free tumor DNA which is released from dead tumor cells into the blood and thus may reflect the entire tumor genome. This method can be generalized for any cancer type that is known to have recurrent mutations. CAPP-Seq can detect one molecule of mutant DNA in 10,000 molecules of healthy DNA. The original method was further refined in 2016 for ultra sensitive detection through integration of multiple error suppression strategies, termed integrated Digital Error Suppression (iDES). The use of ctDNA in this technique should not be confused with circulating tumor cells (CTCs); these are two different entities.
Maurice Stroun was a Swiss researcher and professor at the University of Geneva in the Department of Plant Biochemistry and Physiology. He is known for first hypothesizing and demonstrating the existence of disease-specific circulating nucleic acids as well as first developing techniques for the detection of tumor-related characteristics of circulating DNA and RNA in plasma and serum, or liquid biopsies as this field is now known.
Prognostic markers are biomarkers used to measure the progress of a disease in the patient sample. Prognostic markers are useful to stratify the patients into groups, guiding towards precise medicine discovery. The widely used prognostic markers in cancers include stage, size, grade, node and metastasis. In addition to these common markers, there are prognostic markers specific to different cancer types. For example estrogen level, progesterone and HER2 are markers specific to breast cancer patients. There is evidence showing that genes behaving as tumor suppressors or carcinogens could act as prognostic markers due to altered gene expression or mutation. Besides genetic biomarkers, there are also biomarkers that are detected in plasma or body fluid which can be metabolic or protein biomarkers.
Circulating mitochondrial DNA, also called cell-free circulating mitochondrial DNA and circulating cell-free mitochondrial DNA(ccf mtDNA), are short sections of mitochondrial DNA (mtDNA) that are released by cells undergoing stress or other damaging or pathological events. Circulating mitochondrial DNA is recognized by the immune system and activates inflammatory reactions. It is also a biomarker that can be used to detect the degree of damage from myocardial infarctions, cancers and ordinary stress. In certain situations it acts as a hormone.
Urinary cell-free DNA (ucfDNA) refers to DNA fragments in urine released by urogenital and non-urogenital cells. Shed cells on urogenital tract release high- or low-molecular-weight DNA fragments via apoptosis and necrosis, while circulating cell-free DNA (cfDNA) that passes through glomerular pores contributes to low-molecular-weight DNA. Most of the ucfDNA is low-molecular-weight DNA in the size of 150-250 base pairs. The detection of ucfDNA composition allows the quantification of cfDNA, circulating tumour DNA, and cell-free fetal DNA components. Many commercial kits and devices have been developed for ucfDNA isolation, quantification, and quality assessment.
Cancer Likelihood in Plasma (CLiP) refers to a set of ensemble learning methods for integrating various genomic features useful for the noninvasive detection of early cancers from blood plasma. An application of this technique for early detection of lung cancer (Lung-CLiP) was originally described by Chabon et al. (2020) from the labs of Ash Alizadeh and Max Diehn at Stanford.
EPIC-seq,, is a high-throughput method that specifically targets gene promoters using cell-free DNA (cfDNA) sequencing. By employing non-invasive techniques such as blood sampling, it infers the expression levels of targeted genes. It consists of both wet and dry lab stages.
Nitzan Rosenfeld is a professor of Cancer Diagnostics at the University of Cambridge. He is a Senior Group Leader at the Cancer Research UK Cambridge Institute and co-founder of Inivata, a clinical cancer genomics company.
Focused-ultrasound-mediated diagnostics or FUS-mediated diagnostics are an area of clinical diagnostic tools that use ultrasound to detect diseases and cancers. Although ultrasound has been used for imaging in various settings, focused-ultrasound refers to the detection of specific cells and biomarkers under flow combining ultrasound with lasers, microbubbles, and imaging techniques. Current diagnostic techniques for detecting tumors and diseases using biopsies often include invasive procedures and require improved accuracy, especially in cases such as glioblastoma and melanoma. The field of FUS-mediated diagnostics targeting cells and biomarkers is being investigated for overcoming these limitations.
mRNA-based disease diagnosis technologies are those using messenger RNAs.. as molecular diagnostic tools to discover the relationships between patient's DNAs and their specific biological features. The mRNA-based disease diagnosis technologies have been applied to medical field widely in recent years, especially on early diagnosis of tumors. The technology can be applied to various types of samples depending on how easily the samples are accessible and whether the samples reliably contain the mRNA that related to specific diseases. For example, in hepatocellular carcinoma, the tumor tissues excised during the operation are a good resource for mRNA-based test to analysis. Among those most commonly used samples, blood sample is one of the most easily accessible via minimally invasive method. degenerative diseases. Blood has been used in early diagnosis of some cancers, such as non-small lung cancerand neuroendocrine tumors.
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