What is the Difference Between ctDNA and cfDNA

The main difference between ctDNA and cfDNA is that ctDNA is circulating tumor DNA or tumor-derived fragmented DNA in the bloodstream that is not associated with cells, whereas cfDNA is cell-free DNA, which circulates freely in the bloodstream, and has no origin of the tumor.

ctDNA and cfDNA are two types of DNA freely circulating in the bloodstream with no cells. Normally, a single cell can release a great proportion of free DNA into the bloodstream.

Key Areas Covered

1. What is ctDNA
   • Definition, Characteristics, Importance
2. What is cfDNA
   • Definition, Characteristics, Importance
3. Similarities Between ctDNA and cfDNA
   • Outline of Common Features
4. Difference Between ctDNA and cfDNA
   • Comparison with Key Differences

Key Terms

cfDNA, ctDNA

What is ctDNA

ctDNA is circulating tumor DNA or DNA fragments with a tumor cell origin. The main feature of ctDNA is that ctDNA reflects the entire tumor genome. Also, this DNA is important in monitoring tumor progression throughout the treatment process. The technique involved in the monitoring of tumor DNA from ctDNA is liquid biopsies. If ctDNA is present in someone’s bloodstream, it reflects the presence of a tumor. Scientists can detect the mutations of tumor cells by capturing ctDNA from the bloodstream and monitoring them through next-generation sequencing.

Figure 1: ctDNA

Furthermore, ctDNA originates directly from the tumor or circulating tumor cells. Generally, circulating tumor cells refer to the viable, intact tumor cells that are shed from primary tumors and enter the bloodstream or lymphatic system. These cells undergo apoptosis or necrosis, releasing their DNA into the bloodstream, and another way of originating ctDNA in the bloodstream is the active release from the tumor cells.  

What is cfDNA

cfDNA is circulating free DNA, which are DNA fragments of 50-200 bp in size in the blood plasma. There are several forms of cfDNA in the bloodstream. These generally include the circulating tumor DNA (ctDNA), circulating cell-free mitochondrial DNA (ccf mtDNA), and cell-free fetal DNA (cffDNA). Here, the circulating DNA is double-stranded and extracellular. Meanwhile, cell-free mitochondrial DNA refers to the short sections of mitochondrial DNA released by cells during stress, damaging, or pathological events. On the other hand, cell-free fetal DNA refers to the fetal DNA circulating in the maternal blood. But cffDNA is not detectable in maternal blood two hours after delivery.

Figure 2: cfDNA Origin

Moreover, the elevated levels of cfDNA are detectable in cancer. This increases the levels of ctDNA. Other than cancer, cfDNA is important in trauma, sepsis, aseptic inflammation, myocardial infarction, stroke, transplantation, diabetes, and sickle cell disease.

Similarities Between ctDNA and cfDNA

• ctDNA and cfDNA are two types of DNA that freely circulate in the bloodstream with no cells.
• The origin of free DNA is apoptosis and necrosis of the corresponding cells in the bloodstream.
• The fragment size of the free DNA is 166 bp, which corresponds to the length of DNA wrapped around a nucleosome plus a linker. This reflects that the formation of free DNA has the origin of apoptotic DNA fragmentation.
• A single cell can release a great proportion of free DNA into the bloodstream.

Difference Between ctDNA and cfDNA

Definition

ctDNA refers to fragments of DNA derived from tumor cells and circulating in the blood together with cell-free DNA from other sources, while cfDNA refers to all non-encapsulated DNA in the bloodstream.

Origin

Usually, ctDNA originates directly from a tumor or from circulating tumor cells, while cfDNA enters the bloodstream after apoptosis or necrosis.

Carries

ctDNA carries information from the tumor cell genome, while cfDNA carries genome-wide DNA information. 

Type

ctDNA is a type of cfDNA, while the other types of cfDNA include cell-free mitochondrial DNA (cf mtDNA) and cell-free fetal DNA (cffDNA).

Conclusion

In brief, ctDNA and cfDNA are two freely-circulating types of DNA in the bloodstream. However, ctDNA is circulating tumor DNA that elevates in the presence of cancer. Apart from ctDNA, there are other types of cfDNA; these are ccfDNA and cffDNA. Therefore, ctDNA is a type of cfDNA that circulates free DNA. Usually, cfDNA occurs in the bloodstream as a result of apoptosis and necrosis of cells. However, ctDNA originates with the apoptosis of tumor cells. Therefore, the main difference between ctDNA and cfDNA is their origin.

References:

1. Liquid biopsies, sequencing, ctdna & cfdna: Explained: Fight CRC. Fight Colorectal Cancer. (n.d.). Retrieved September 19, 2022.

Image Courtesy:

1. “CtDNA in circulation” By Racheljunewong – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Source-and-biology-of-ctDNA” By Wei-Lun Huang- Own Work (CC BY 4.0) via Commons Wikimedia