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The dark side of stem cell quiescence

Adult stem cells a are non-dividing or slow cycling cell population, which reside in the organs largely in G-0 phase of cell cycle (quiescent). Currently, there is a general assumption that quiescence serves as a protective mechanism that saves stem cells from environmental stress and DNA damaging agents. Also, there is an assumption that quiescence protects cancer stem cells from radiation and chemotherapy, making them resistant. New findings [1], which came out from the Emmanuelle Passegue laboratory [2], demonstrate that stem cell quiescence also has a dark side due to intrinsic vulnerability to mutations.

Nonhomologous recombination end joining DNA repair coupled with stem cell quiescence
The authors showed that quiescent hematopoietic stem cells (HSCs), in response to damage (irradiation), use different mechanism of DNA repair compared to cycling HSCs and myeloid progenitors. Nonhomologous recombination end joining (NHEJ) is a mechanism of DNA repair in quiescent HSCs, coupled with acquisition of errors and mutations. Unlike quiescent HSCs, cycling HSCs and progenitors use a different and effective mechanism of DNA repair – homologous recombination (HR). But no wonder here, it is known that NHEJ is a common mechanism of DNA repair for any type of quiescent cells in mammals [3]:

Interestingly, repair of DSBs by homologous recombination is downregulated during G0, G1 and early S phases in the somatic cells of multicellular eukaryotes 5. This means that NHEJ is the predominant – if not exclusive – mechanism for the repair of DSBs during G0, G1 and early S phases, and NHEJ continues to repair a minority of breaks during late S and G2 phases.

Now it was discovered in adult stem cells. Most importantly, authors demonstrated that DNA mutations can persist or expanded in aberrant clone during HSC serial transplantation from primary to secondary recipients.

Taken together, these results demonstrate that quiescence dramatically restricts HSPCs’ ability to use the high-fidelity HR-mediated repair and instead forces them to rely on the more error-prone NHEJ mechanism to repair DSBs.

Our transplantation experiments directly demonstrate that damaged HSCs, which have undergone DNA repair and acquired mutation(s) during this process, can persist in vivo at relatively high frequencies and contribute either to the clonal expansion of aberrant cells or to the maintenance of cells with genomic alterations.

What is strange to me is that they didn’t get leukemia in mice after serial transplantation of mutated expanded HSC clones. Seem like protection mechanisms are very strong and disease onset requires additional hit mutations.

Radioresistance is uncoupled of quiescence
We know that stem cells in our organs are extreme survivers. They survive high doses of irradiation, exposure to DNA damaging agents and many chemotherapeutic drugs. The authors showed that active (cycling) HSCs magically retain radioresistance (by unknown mechanisms), even using HR mechanism for DNA repair. So, active cycling HSCs – that’s what we need! They are good guys! The consequence of this finding can put under question the assumption that cancer stem cell radioresistance could be explained by quiescence. Seem like it’s not true. Something else, different from quiescence make stem cells radioresistant.

The first puzzle is the evidence that NHEJ are actually coupled with radio- [4] and chemoresistance [5]. How can active cycling HSCs using the HR mechanism be radioresistant but myeloid progenitors are not? Yet another puzzle is the whole way of comparing quiescent HSCs and cycling HSCs. I don’t think authors did phenotype control of quiescent HSC and active (cycling) HSCs. Maybe a significant part of HSCs turned into multipotent progenitors, even after one day of stimulation protocol? More than that, what about nearly 50% of quiescent fraction of multipotent progenitors (LSK/Flk2+)? How are they resistant to irradiation? Well, we need to do more work to find some answers.

Aging, chemotherapy and some translational points
I like the point that authors make in discussion about how their findings can explain loss of HSCs function with age:

Our findings suggest that accumulation of NHEJ-mediated mutation(s) over a lifetime could dramatically hinder HSC performance and be a major contributor to the loss of function observed in aged HSCs and the development of age-related hematological disorders.

Another good point is the possible explanation of increased rate of chemotherapy-related malignancies (solid cancer or leukemia) after treatment with DNA damaging agents:

…cytotoxic therapies might inadvertently mutate the patient’s own quiescent HSCs by forcing them to undergo DNA repair using a mutagenic mechanism. Specifically, we show that proliferating HSCs have significantly decreased mutation rates, with no observed changes in their radioresistance, suggesting that it might be beneficial to induce HSCs to cycle prior to therapy with DNA damaging agents to enhance DNA repair fidelity and reduce the risk of leukemia development. Although this possibility remains to be tested…

Even though the possibility of this mechanism should be tested further, I wonder if it is universal for all adult stem cells in our organs? If all adult stem cells reside mostly in quiescent state, why not? It gives us some ideas about how we can play with chemotherapeutic drug combinations and therapeutic irradiation in clinical oncology. If your drug targets cycling cells, the approach mentioned above is not going to work. But what could work is the following sequence: 1 – drug killing cycling cells, but not causing severe DNA damage; 2 – drug which targets quiescent cells causing them to enter into cell cycle; 3 – DNA damaging drug. What do you think?

pre-published abstract [1]