An Immortalised Leukaemia Cell Line Models the Quiescence of Leukaemic Stem Cells and Shows Impaired Double Strand Break Repair Following Daunorubicin Treatment

Approximately 50% of patients with acute myeloid leukaemia respond to remission-induction chemotherapy, but later relapse. Relapse is thought to be due to the continued presence of a quiescent, chemoresistant leukaemic cell subpopulation. Understanding the damage response in these cells might help to guide targeted therapies. We therefore developed an in vitro model of the quiescent subpopulation and used it to study drug-induced damage and repair in quiescent multidrug resistant cells.

We cultured CD34+ CD38- multidrug resistant KG1a AML cells under several conditions reported to induce cell cycle arrest. We used Pyronin Y to measure RNA content and 7-aminoactinomycin D to measure cell viability. Chemosensitivity, reactive oxygen species (ROS), mitochondrial pore transition and oxidative damage were measured flow cytometrically. gammaH2A.X foci were quantified to measure the double strand break response and DNA damage response and repair gene expression was studied using PCR microarrays and confirmed by real time PCR.

mTOR inhibitors induced an increase in G₀ without induction of apoptosis. 48 hours' exposure to rapamycin increased the proportion of G₀ cells from 13.3% (SD 2.3%) to 46.1% (SD 6%) and decreased mean cell volume. Delayed re-entry into cell cycle following rapamycin withdrawal confirmed the G₀ status of these cells. Differentiation markers remained negative. Although several of the other conditions studied resulted in reduced cell growth, they also induced apoptosis, as did combinations of rapamycin with other growth inhibitors.

The toxicity of the chemotherapy drug daunorubicin, which acts in part by inducing ROS, was reduced in the quiescence-enriched cells. Sensitivity to mitochondrial pore transition was similar in proliferating and quiescence-enriched cells, indicating that apoptotic pathways are not impaired. However, both basal and drug-induced ROS were significantly lower in quiescence-enriched than in the proliferating cells (p=0.006 for basal ROS and 0.013 for daunorubicin-induced ROS). Furthermore, several DNA repair genes were differentially regulated following daunorubicin treatment of the quiescence-enriched compared to the proliferating cells – these included genes responsible for the repair of double strand breaks. On treatment with daunorubicin, double strand breaks, but not oxidative damage to DNA were observed in both cell populations. However, strikingly, although quiescence-enriched cells sustained fewer DNA damage foci than proliferating cells, they were unable to resolve the damage after daunorubicin was removed.

By using rapamycin to enrich KG1a cells for quiescence, we have shown low basal and drug-induced ROS to be associated with chemoresistance in these cells. However, we also found that quiescence gave rise to an impaired double strand break response, which might force these cells to rely on alternative repair pathways and thus be sensitive to synthetic lethal targeting.

No relevant conflicts of interest to declare.