Presence of DNA Fragments and DNA products such as mRNA, miRNA, lncRNA, circRNA and other nucleic acidsHas revolutionised the diagnostic technologies specially for the diseases, namely colorectal cancer, inflammatory liver diseases, acute myocardial infarction, acute myocardial infarction,  diabetes, preeclampsia. These components can be used as biomarkers for diagnosis of cancer. Most of the biofluids can be obtained by noninvasive methods and therefore certain prenatal diagnoses are made easy. 

The presence of DNA in cell free body fluids was first noticed by Lo et al [1] as an aggregate of fragmented DNA materials released from different cells and tissues into the circulation, including cancer cells if present. Analysis of tumor-derived DNA in circulation, or circulating tumor DNA (ctDNA), forms the basis of plasma DNA-based cancer diagnostics.

Later, on analysis the presence of DNA fragments were also seen in the human body fluids such as   urine [2], saliva [3] and CSF [4, 5]. In patients with lung cancer, ctDNA has also been detected in bronchial washings [6] and pleural fluids [7].Amniotic fluid [8].

Until 1970 nobody suspected free-floating DNA in the body fluids or plasma; Williamson [9], was the first one who indicated the presence of oligomeric fragments of DNA in primary neonatal liver cultures. Spontaneous or accidental DNA fragmentation when occured that gradually accumulates in a cell. Later, it is released in the blood plasma.  Circulating free DNA (cfDNA) are degraded DNA fragments of varying sizes (50 - 200 bp).  There may be three sources of cfDNA, namely, tumor DNA (ctDNA), cell-free mitochondrial DNA (ccf mtDNA), and/or cell-free fetal DNA (cffDNA) circulating not only in the bloodstream but also in other body fluids [10]. Apoptosis or necrosis is the major source of circulating free DNA [11], however still the mechanism of cfDNA being released by cells into circulation is not fully understood. Nevertheless as early as 1968, we localized DNA in the ooplasm of Bufo by using the Feulgen reagent. The unfertilized degenerating eggs show a DNA fragmentation scattered all over the ooplasm [12]. This finding was consistent with the hypothesis that these DNA fragments were a specific degradation product of nuclear DNA

The mechanism that cells use to release free circulating DNA into the blood may enhance the diagnostic and prognostic value of these nucleic acid molecules.

Using electrophoresis, we can see how many different DNA fragments are present in a sample and how large they are relative to one another.The separation and identification of DNA fragments based on their size is possible by using a gel electrophoresis [13]. In search of foolproof diagnosis researchers started using DNA sequence single-cell DNA amplification and sequencing strategy. A team of biochemists from Baylor College of Medicine, USA  has identified “damagenome,” parts of the genome that appear prone to spontaneous DNA damage in human brain cells that may contribute to disease [14-17]. These observations opened the door for new clinical and diagnostic applications for cancer, cardiovascular diseases, neurological diseases, infectious diseases, genetic disorders etc. The application of cfDNA is already a success story in prenatal diagnosis of genetic diseases.

A great proportion of cfDNA comes from normal cells of the body, a small part of which is related to tumors, coming from primary tumors, metastatic sites or CTCs, and is called ctDNA. Lately, it was found that not only DNA fragments but DNA products such as mRNA, miRNA, lncRNA, circRNA and other nucleic acids are also found in the body fluids. They are present in different body fluids and offer the possibility to use them in diagnosis of different diseases (10). 

The presence of cell-free DNA (cfDNA) was observed by Mandel and Metais in the sera of cancer patients in 1948[18]. Tan et al. [19] reported a higher concentration of cfDNA in samples of cancer patients in 1966. The real clinical application started when Dennis Lo detected the fetal gender and RhD group in the maternal plasma in 1997 [1].   

Analysis of cell-free DNA and circulating tumor cells has the potential to change clinical practice by exploiting blood rather than tissue as a source of information [20]. The current standard treatment for patients with early-stage colon cancer consists of surgical resection, followed by adjuvant therapy in a select group of patients deemed at risk of cancer recurrence. The decision to administer adjuvant therapy, intended to eradicate the clinically inapparent minimal residual disease (MRD) to achieve a cure is guided by clinicopathologic characteristics of the tumor. However, the risk stratification based on clinicopathologic characteristics is imprecise and results in under or overtreatment in a substantial number of patients. Emerging research indicates that the circulating tumor DNA (ctDNA), a fraction of cell-free DNA (cfDNA) in the bloodstream that originates from the neoplastic cells and carry tumor-specific genomic alterations, is a promising surrogate marker of MRD. Several recent studies suggest that ctDNA-guided risk stratification for adjuvant therapy outperforms existing clinicopathologic prognostic indicators. Preliminary data also indicate that, aside from being a prognostic indicator, ctDNA can inform on the efficacy of adjuvant therapy, which is the underlying scientific rationale for several ongoing clinical trials evaluating ctDNA-guided therapy escalation or de-escalation. Furthermore, serial monitoring of ctDNA after completion of definitive therapy can potentially detect cancer recurrence much earlier than conventional surveillance methods that may provide a critical window of opportunity for additional curative-intent therapeutic interventions. This article presents a critical overview of published studies that evaluated the clinical utility of ctDNA in the management of patients with early-stage colon cancer, and discusses the potential of ctDNA to transform the adjuvant therapy strategies. 

ctDNA is a promising biomarker that can obtain quantitative and qualitative comprehensive tumor DNA in a minimally invasive manner [21]. ctDNA assays will guide the choice of the most appropriate chemotherapy treatments, and predict early recurrence, chemotherapeutic response, and resistance. ctDNA tests work by looking for MRD in a patient's blood. “All cancers have a special genetic makeup—they have mutations that can be picked up in the bloodstream,” said Namrata Vijayvergia, [22], a hematologist/oncologist at Fox Chase Cancer Center.