Fig. 1

ALT+ GSCs were detected by quantifying telomere (a) and DNA C-Circle content (b) in a panel of 24 cell lines. Using a threshold cut-off value of 0.5 (dashed line) for telomere content and CCs, 2 ALT+ GSCs were identified, GS 8–18 and GS 5–22. Both GS 5–22 and GS 8–18 lack detectable ATRX protein (c). Additionally, these cell lines have negligible mRNA expression for TERT (d), indicating lack of telomerase activity. U-2 OS, a commercially available ALT+ osteosarcoma cell line which is ATRX mutant was used as a positive control for ALT and negative control for ATRX immunoblotting. Conversely, TS 603 and TS 543 which are known ATRX wild-type GSCs, were used as negative controls for ALT and positive controls for ATRX immunoblotting. GS 5–22 cells, stably expressing the luciferase reporter, were injected intracranially into nude mice and formed tumors within 4 weeks (e)