Circulating cell-free DNA

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 My last post gave you a glimpse into the basics of circulating nucleic acids. Today, we will look at circulating cell-free DNA (cfDNA). 


What is cell-free DNA?


Live or dying cells can release DNA into the blood stream. In case of diseases, it can be released by normal as well as diseased cells. Persons with infectious diseases may have viral DNA in their blood. Likewise, during pregnancy, fetal DNA can be detected in the mother's blood. But a large part of cell-free DNA arises from normal cells in case of diseases and from maternal cells, in case of pregnancy. But, the genetic and the epigenetic markers the DNA carries is different from the normal cells. So, researchers try to diagnose disease or fetal abnormalities based on these changes. For example, cancer cells contain mutations in specific genes.  These can be detected in cell-free DNA. In this case, the DNA containing the mutation is specific to the tumor and is thus called  "circulating tumor DNA (ctDNA)". 

Now, there are three groups of cell-free DNA, depending on its origin. 

  • The first is total circulating DNA, that is, all the DNA that is present in the blood, regardless of which cell has released it. 
  • The next is the DNA released by tumor, virus or fetus. 
  • The third is mitochondrial DNA, DNA usually present in the mitochondria or the "powerhouse" of the cell, the part of the cell producing energy. 




So when we do liquid biopsy with cfDNA, we are trying to detect changes which are specific to the condition being diagnosed. I will discuss the different changes in cfDNA  attributes such as concentration, DNA integrity index, methylation and mutation, and how they can be used for disease testing, in a future post. 

But how does the cell release DNA into the blood? Watch out for my next post, where we will discuss how cell-free DNA is released into the bloodstream. If you have any comments or questions, please post them in the Comments section below!

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