Introduction: Glioblastoma (GB) is a devastating form of brain tumor with a very poor prognosis. Quiescent Glioblastoma Stem Cells (qGSCs) are believed to initiate GB and be partly responsible for the fatal recurrence following treatment. Importantly, no single molecular marker exists to precisely identify qGSCs. Recent in vivo evidence has shown the nucleocytoplasmic ratio of poly-adenylated (poly (A)) RNA to be higher in quiescent neural stem cells (qNSCs) when compared to active neural stem cells (aNSCs), and in qGSCs compared to aGSCs in vitro. Moreover, preliminary evidence revealed a bimodal distribution of the poly (A) ratio among Ki67- cells. Whether or not staining combinations with Ki67 (expressed mainly in G2) and other proliferative markers such as MCM2 (expressed mainly in G1) would improve the identification of qGSCs was investigated.
Methods: Freshly-resected GB samples were stained for SOX2, Ki67, MCM2 and oligo(dT). The nucleocytoplasmic poly (A) ratio was calculated for supposedly qGSCs (both Ki67- and MCM2-) and aGSCs (either Ki67+, MCM2+, or both). Only SOX2+ cells (GSCs) were included.
Results: MCM2 identified a fraction of GSCs that were not identified by Ki67. GSCs positive for MCM2 displayed three patterns of MCM2 expression: MCM2nuc (nuclear), MCM2cyto (cytoplasmic) and MCM2nuc+cyto (equal distribution). All Ki67+ cells were MCM2nuc. Therefore, the nucleocytoplasmic poly (A) ratio of supposedly active MCM2nuc GSCs was compared with the ratio of supposedly quiescent MCM2cyto cells, which was significantly higher. No difference in the ratio was found between Ki67+ cells and MCM2nuc/Ki67- cells. Eventually, a negative correlation was established between the nucleocytoplasmic MCM2 and poly (A) ratios.
Conclusion : MCM2 identifies more aGSCs than Ki67, while identifying all the aGSCs already identified by Ki67. Thus, absence of nuclear MCM2 appears to be a better marker than absence of Ki67 to identify qGSCs. Further validation of this concept with higher numbers of samples is nevertheless needed.
