The fraction of dying cells would be expected to be of less scientifically and clinically interesting. It would be good to know the fraction and to know which are actually living and may do damage, so they can be targeted for further analyses.
A practical use of measuring apoptosis would be to follow effectiveness of therapies. Previously we used M30 in the marker channel; however, that antibody is no longer available. We have not developed any caspase anbodies for usage on the CellSearch as yet, but they may be a way of assessing apoptosis.
The question of whether the circulating tumor cells that are captured is not so much a question of whether they are alive as it is "whether they still have the capability of dividing". That question has not been answered and proven to date. Yes, cells have been grown out but as to whether they are truely tumor cells is still up to debate. What is actually captured in the peripheral blood are those cells remaining following the first pass through the periphery. During that first pass any cell with the capability of adhering has been loss to capture and those cells are the cells which could form metastatic seedings. That is an unknown number of events and could only be answered if blood was collected prior to or at the time of entry into the peripheral circulation.
I think it really depends of the context: the proportion of dying cells may tell you a lot if compared pre- and early during therapy for example.
In regards to possibly metastatic cells and for that matter testing viable vs non-viable cells it may not be enough to look strictly at apoptotic markers (DAPI, M30 have been used in CTCs) or anti-apoptotic markers (Bcl-2, Mcl-1 have been used in CTCs) as cells may die by other means (the passage through the bloodstream is though!).
Proliferation markers as Ki67 (been used in CTCs) may be helpful but they do not detect dormant cells (assuming dormant cells would be in G0-phase of the cell cycle, where Ki67 is not expressed).
I think in the end of the day to really test how many CTCs can form metastasis, they have to be introduced into nude mouse models, but even then it will be hard to determine how many dormant cells may hide away and not form tumors (but could) in the time frame of the experiment.
So, quite exciting there is still some research to do and strategies to optimize to answer this question!
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