The circulating tumor DNA: a promising cancer biomarker

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Aishat Motolani

What if I say a floating DNA in the blood could broadcast a glimpse of a diseased system’s future, particularly that of cancer?  

Diagnosing cancer commonly involves the use of invasive procedures, such as tumor tissue biopsy. This involves the examination of tissues extracted from a primary or metastatic tumor site. Although tumor tissue biopsy can give insights into the molecular profile of a tumor, the process can cause complications and pain in patients. For example, the risk of displacing a tumor from its primary site to the surrounding tissue or blood is high during a needle biopsy. So, blood sampling (liquid biopsy) offers a better alternative. This process involves the screening of the blood for disease biomarkers. Hence, these blood biomarkers— measurable substances that indicate the presence and extent of diseases— provides a minimally invasive alternative to avert clinical complications. As such, blood biomarkers that accurately represent a tumor profile are currently being studied.  

Normally, DNA, which makes up our genes, resides in the nucleus of cells. It passes on its information to RNA. Then, RNA travels out of the nucleus to make proteins. DNA is composed of a sequence of four distinct building blocks known as nucleotides: Adenosine, Thymine, Guanine, and Cytosine. A combination of any three nucleotides holds the information to make a single building block of proteins called amino acids.There 20 different amino acids, and sequences of any of the amino acids makes a protein. So when there is a change in a DNA sequence, the amino acid changes, causing a change in the protein structure, and sometimes, its function as well. This process is known as mutation. It is just like altering the words in an already composed email; the whole message will take on a different meaning. In cancer, cells often change the words in their already composed message to ensure their growth and survival. 

Sometimes DNA can exist outside of a cell. This type of DNA is known as circulating cell-free DNA (ccfDNA). It can be released into the blood either by itself or by being enveloped in a small sac-like material (exosomes). Healthy cells use DNA secretion in exosomes to remove excess cell DNA, thus, maintaining balanced cellular functions. But, tumor cells hijack this function to spread cancer to healthy cells. On the other hand, DNA by itself in the blood is mainly released by dying cells. In healthy individuals, this form of DNA mainly originates from blood cells. But in cancer patients, the circulating tumor DNA (ctDNA) accounts for most of the ccfDNA in the blood. 

So what are ctDNAs? They are fragments of DNA released into the blood from dying tumor cells. ccfDNA was first described in 1948 by researchers. A few decades later, researchers discovered that cancer patients had elevated ccfDNA, most of which originated from tumors. CtDNAs differ from normal ccfDNA by the presence of genes with specific cancer-causing mutations. Also, the levels of ctDNAs can vary depending on the cancer stage and cancer type. These DNAs can be detected using technologies such as deep-sequencing or digital Polymerase Chain reaction (dPCR), among others. dPCR has higher sensitivity in detecting ctDNAs in the blood that reflects the profile of advanced tumors. Also, dPCR can be paired with deep sequencing to identify rare types of ctDNAs in tumors. 

Several studies have reported the promising potential of ctDNA in cancer diagnosis and drug development. For example, ctDNA has played a key role in understanding the evolution of tumors within a patient. The molecular profile of tumors is highly varied. Some tumors may contain different mutations suited for survival in either a low oxygen or an oxygen-rich environment. Also, mutations present at the primary tumor site are different from those at the metastatic site. These mutations continue to evolve as patients receive cancer treatments or as the disease progresses. Because tissue biopsy of primary tumors may not capture these differences, genotyping the ctDNAs in the blood offers a better alternative for understanding the tumor dynamics in real-time. Such real-time insight into tumor progression is possible due to the relatively short half-life of ctDNA, about 2 hours.

CtDNAs are beneficial for detecting mutation that occurs during cancer treatment. Certain cancer drugs are designed to target the activity of a single protein. Decreasing the activity of that single protein slows down cancer growth. But, cancer cells are smart. They create a shield for themselves by undergoing mutation, hence, changing the targeted protein’s structure. This often renders the drug ineffective against cancer, leading to drug resistance. 

Recently, researchers observed that patients with non-small cell lung cancer (NSCLC) tend to develop resistance to a class of drug called EGFR inhibitors. This drug targets a protein called Epidermal Growth Factor Receptor (EGFR). Later, researchers found out that the resistance is caused by a specific mutation on EGFR. This mutation involves the change of an amino acid threonine(T) to a different amino acid methionine (M) at the 790th position of the EGFR protein sequence.  Blood testing of ctDNAs in NSCLC patients was shown to detect this mutation early before it manifests clinically. Measuring the ctDNAs in such patients can guide treatment options by enabling an early switch to another class of drug that hinders the cancer-promoting effect of T790M mutation. This strategy ensures that a patient is receiving the optimal treatment at any point in time as the disease progresses. 

Also, ctDNAs may predict the recurrence of cancer following treatment. Most cancer patients typically undergo surgery or chemotherapy to eliminate tumors or slow down their growth. However, relapse is quite common because the absolute absence of residual tumors cannot be confirmed. This is particularly the case for patients with advanced-stage cancer. 

In 2008, a study assessed the levels of ctDNAs in the plasma of 20 patients after the surgical removal of tumors. The detection of these ctDNAs was based on probes that can identify unique gene mutations specific to a tumor type. In about two years, the researchers observed that patients with no detectable ctDNA showed no tumor recurrence while those with detectable ctDNAs had tumor recurrence. So, examining post-operative ctDNA levels in cancer patients may offer a promising approach to predicting cancer recurrence in patients. Ultimately, this approach will enable the swift administration of necessary treatments and prevent the sprawling growth of residual tumors in patients.  

A wealth of information about cancer progression lies in the blood. CtDNAs are rapidly emerging as a promising biomarker to monitor disease progression and to guide prompt treatment options. Currently, various clinical trials are exploring the link between ctDNA’s evolution and clinical outcomes in cancer patients. It is noteworthy to mention that ctDNA detection faces certain drawbacks such as the difficulty in differentiating between ctDNA and normal ccfDNA, the low levels of ctDNA in the blood circulation, and the accurate measurement of ctDNAs in the plasma. Nonetheless, as advanced technologies and new studies continue to emerge, these floating soothsayers (ctDNAs) may soon offer new hope to cancer patients globally.

Edited by Manisit Das

Header image by Gerd Altmann from Pixabay

Works Discussed

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