Most cells in the human body are not dividing. Some are in a permanently non-dividing state whereas others, such as adult stem cells, can re-enter the proliferative state in response to certain triggers like the need to repair damaged tissue. These reversibly non-dividing cells ‒ so-called dormant cells ‒ are important for tissue homeostasis, repair and regeneration, for example in the gut. In the context of cancer, dormant tumour cells can evade therapy that frequently targets fast-dividing cells and present a risk of cancer recurrence.
Existing methods to identify dormant stem cells use a variety of markers on the cell surface that are specific for certain types of stem cell, rather than a uniform measure of all stem cells that is independent of tissue type or species. These methods also require prior knowledge of the stem cell population of interest that precludes discovery of new stem cell types.
In this paper published in the journal Nature Communications, Dr Rasmus Freter and colleagues from the groups of Professor Colin Goding (Ludwig Oxford) and Dr Francesco Neri (Leibniz-Institute on Ageing, Fritz-Lipmann-Institute and Friedrich-Schiller-University, Jena, Germany) set out to develop a more general method for identifying living dormant stem cells that would be applicable to all cell types, including cancer, and which would not rely on pre-existing knowledge of the location of the stem cells. The researchers made use of the fact that dormant stem cells have low activity of an enzyme called CDK9, a kinase which adds phosphate chemical groups to protein targets and which is required for genes to be active.
The team designed a genetically encoded assay for CDK9 activity called Optical Stem Cell Activity Reporter (OSCAR) that fluoresces in dormant stem cells but not in cells with active CDK9. They were able to use OSCAR to detect dormant stem cells in time-lapse microscopy of intestinal organoid cultures, in an OSCAR mouse model, and by fluorescence-activated cell sorting (FACS).
This method provides a useful tool for characterising dormant cells both in cultured cells and in living tissues including cancers. The ability to isolate live dormant cells, understand why they are dormant and how they are reactivated will be crucial for both regenerative medicine and for effective anti-cancer therapies.