Detecting the rare, renegade cancer cells
Very early in tumour development, the tumour may send out some extremely sneaky cells into the bloodstream. The cells may circulate in the bloodstream as dormant cancer cells for months or years, and may even survive cancer treatment aimed at treating a primary cancer. The cells are called Circulating Tumour Cells, and are the cause of cancer spread, metastases. It is the metastases, rather than the primary tumour, which is most often responsible for patient fatality. Circulating Tumour Cells have become a hot topic in cancer research laboratories and Oncology conferences worldwide: if these cells can be targeted, then it is possible to stop cancer in its tracks.
Circulating Tumour Cells, abbreviated as 'CTCs,' were first discovered over 150 years ago. The reason that it has taken so long for CTCs to become the focus of much research attention is because they it has been near impossible to develop a reliable way of detecting them. You see, CTCs are incredibly rare. There may be one CTC in amongst millions of blood cells. It has been only recently that scientist have been able to develop methods of finding, counting and analysing these cells.
One way of finding the cells is based on their physical properties: CTCs are larger than most blood cells, so passing a blood sample through a sieve-like membrane, CTCs can be captured. The problem with this method is that smaller CTCs slip through and remain undetected. Another property is the CTC's density: centrifuging a blood sample will separate its constituents via a density gradient. Again, the issue is some CTCs with irregular densities will remain undetected. Other physical properties that have been used are electric charge and deformability.
A popular method is to separate CTCs based on their biological properties. Epithelial-dervived cancers (breast, prostate, lung, liver, skin, oral, colorectal) will have molecule called EpCAM on the surface of their cells. Immunomagnetic cell enrichment finds CTCs via magnetic-bound anti-EpCAM antibodies. This sounds complex, but it's really not. Basically an antibody is a molecule that attaches specifically to things. This one is made to specifically attach to the molecule EpCAM that is found on the outside of CTCs. The antibody also has a magnetic bead, so that the entire complex, together with the detected cell, can be separated from the blood with the use of a magnet. There's a couple of issues with this method: firstly, the cell has to have enough EpCAM on its cell surface for there to be enough bound magnetic beads to attract the cell to a magnet. Secondly, there's something called EMT, or 'Epithelial to mesenchyme transition.'
EMT is a process that escapee cancer cells go through before they enter the blood. They de-differentiate and become more like stem-cells, and thus lose some of their biological features. One feature that they are very likely to lose, is EpCAM. This means that cells with very low EpCAM on their cell surface will not be detectable by Immuno Magnetic separation as there's not enough of the molecule to attract the cells to a magnet.
Genetic based methods of detection involve detecting patterns of genetic activity associated with cancer cells. These methods show a lot of promise, and a lot of attention is currently being devoted to the question of which array of genes is the best choice. The issue is that every single person's cancer is unique, and will have a unique pattern of genetic activity. Finding a blanket range of genes to test that is the ideal set of genes is the tricky thing here.
One technology that has been critically acclaimed is Maintrac, from Germany. Maintrac detects cells via their cell surface receptor EpCAM. However, instead of using magnetic-bead bound antibodies, they use anitbodies labelled with a fluorochrome bead. When this antibody binds to an EpCAM on the surface of a CTC, it fluoresces. This fluorescence is detected by an automated microscope. There are two main advantages to this technology:
- Cells that have undergone EMT and only have a miniscule amount of EpCAM on them are still able to be found
- The CTC remains alive and available for further testing
Maintrac technology has been clinically validated and published in peer-review journals for over twelve years. Maintrac tests are acknowledged and utilised by many universities, Oncologists and cancer clinics worldwide.
Practitioners: Pachmann et. al (2005) Quantification of the response of circulating epithelial cells to neoadjuvant treatment for breast cancer: a new tool for therapy monitoring, Breast Cancer Research (7) 975-979 ...Download article