Cancer constitutes a heterogeneous class of diseases, characterized by uncontrolled cell growth that results from a combination of both environmental and hereditary risk factors. Many different tissue types can become malignant, such as breast, lung, liver, and skin, and even within a particular tumor there is heterogeneity, with certain cancer cells in a patient bearing mutations which are lacking in other cells nearby. Only in recent years have DNA sequencing and other genetic analysis techniques progressed enough to enable researchers to understand many cancers at a cellular and molecular level, attribute specific cancers to associated genetic changes and molecular pathways, and determine the extent to which these changes are seen in a patient’s tumor.
Types of Tumors
Tumors consist of two major types of cells: the malignant cells and the stroma cells. While stroma cells are normal cells that support and nourish the tumor, the malignant cells are a heterogeneous group of cancerous cells that also have the potential to give rise to disease metastasis via dissemination of CTCs. CTCs are recognized as cancer cells that detached from primary or secondary tumors and enter the bloodstream, traveling nearby or to distant organs and forming new tumors.
Primary tumors are rarely the determining survival factor among different types of cancers, since primary tumors can typically be removed effectively by localized surgical resection and/or radiation. Even if cancer is diagnosed at an early stage and treated by one of these methods, cancer is a major cause of mortality worldwide because of the lethal effect of tumors which metastasize. Metastasis is more difficult to treat than a primary tumor due to its systemic nature.
What Are CTCs?
Just after the Civil War (1869), an autopsy discovered the existence of cancer cells in the blood of a patient who had died from a metastasized tumor. From the 1940s through the 1970s, researchers learned that:
A gram of a metastasizing tumor produces roughly one million CTCs each day
CTCs are generated by virtually all of the over 25 types of solid tumors
CTCs are responsible for seeding metastatic secondary tumors at distant sites throughout a patient’s body, most often through the circulatory system.
In other words, CTCs are both the messengers for propagating the disease and carry within their DNA the blueprint for a given cancer variant.
CTCs detach from primary or secondary tumors — via mechanisms such as necroptosis, apoptosis or secretion — and enter the bloodstream (intravasation), traveling to distant organs (blood circulation) and forming new tumors (extravasation). CTCs can leave the primary tumor earlier in its formation process or after the tumor reaches a certain critical mass of cells.
CTCs in the Bloodstream
The mechanism of intravasation — the process by which CTCs enter the bloodstream — is believed to be facilitated by the epithelial-mesenchymal transition (EMT). This transition appears to occur in primary tumor cells that have stem cell-like properties and are therefore the most invasive ones. During EMT, tumor cells lose their epithelial characteristics and acquire migratory properties that enhance their invasiveness and permit them to enter the blood circulation.
While in the bloodstream, CTCs are believed to possess a semi-mesenchymal phenotype. However, the vast majority of CTCs will die within 24 hours in that harsh environment due to attacks by WBCs and the high shear rates in the heart; or persist in a dormant, inactive state for years. Very few CTCs are capable of metastasizing, and even fewer can form distant tumors. Nevertheless, this small percentage of CTCs — estimated at 1-2% — have been called the decathlon athletes of CTCs and are sufficient to initiate and continue the blood-borne metastatic process, which leads to the progression of the disease and is ultimately responsible for the overwhelming majority of cancer-related deaths.
In order to reach distant organs and form new tumors, CTCs need to extravasate — exit the blood vessels — and survive in the new environment; such cells may potentially acquire more aggressive phenotypes that initiate the tumor formation process, or may re-enter the blood circulation to travel to other organs or back to the initial tumor.
After CTCs extravasate in new locations, they are believed to undergo a reverse EMT process — the mesenchymal-to-epithelial transition (MET) –- and assimilate in the new environment. While some CTCs have genes that might make them more benign or less likely to survive in other locations in the body, others have different patterns of gene expression that enable them to lodge themselves in new locations, establishing a new tumor. Such cells initiate micrometastases (fewer than 100 cells) that may eventually develop into overt metastases that are finally visible using current MRI, CT scan or ultrasound imaging techniques.
In theory, CTCs can reach the majority of organs in the body and initiate metastasis. However, research indicates that different types of cancer give rise to metastasis at different sites; for example, prostate cancer metastasizes preferentially in the bone, while breast cancer cells give rise to new tumors in the liver, lungs, or bone. This preferential behavior is due to the heterogeneity of tumors and their CTCs, and the consequently different ability of cells to extravasate and survive in new locations.