Introduction: Medulloblastoma (MB), the most common malignant brain tumor in children, is often incurable upon recurrence. Recurrence is largely driven by treatment-resistant quiescent tumour cells. While biomarkers have identified quiescent SHH MB cell populations, the mechanisms underlying their quiescence and chemoresistance remain poorly understood. Here, we investigate the role of the cell cycle inhibitor p57 in inducing quiescence in SHH MB and demonstrate that dexamethasone, a drug widely used in MB management, induces p57-mediated quiescence, potentially reducing treatment efficacy.
Methods: To evaluate p57’s role in chemoresistance, we introduced a TMP-inducible p57 construct into Ptch1+/- SHH MB cells. p57-overexpressing and wildtype cells were then treated with vincristine or vehicle control. Next, to evaluate the role of dexamethasone in p57 expression, Ptch1+/- SHH MB cells were treated with dexamethasone, or vehicle control, and p57 levels and cell cycle states were quantified using high-throughput single-cell immunofluorescent imaging.
Results: In culture, nuclear p57 expression was elevated in Sox2+ and Nestin+ stem-like SHH MB cells compared to rapidly cycling Atoh1+ cells. Stabilizing p57 with TMP in Ptch1+/-;p57DHFR cells increased the number of G0 cells six-fold. TMP-treated Ptch1+/-;p57DHFR cells demonstrated significant resistance to the chemotherapy agent vincristine, showing robust survival at doses as high as 10µM, where untreated cells exhibited 100% cell death. Furthermore, treatment with 100nM dexamethasone increased nuclear p57 expression by 40% (p=0.0026) in Ptch1+/- cells, correlating with a 15% increase in cells in G0 phase (p=0.0017). In Ptch1+/-;Tp53-/- cells, dexamethasone treatment led to a doubling of G0-phase cells (p < 0.0001), suggesting dexamethasone may induce p57-mediated quiescence and contribute to chemotherapy resistance in SHH MB cells.
Conclusion : Our findings suggest that dexamethasone induces p57 expression, promoting quiescence and potential chemoresistance in SHH MB cells. These results raise important considerations around the use of dexamethasone in MB treatment, as it may inadvertently contribute to tumor recurrence by enhancing cell quiescence.
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