Figure 3.

β1L-mediated activation of the mutant TERT promoter is required for telomere maintenance in GBM. (A) Telomere length at days 44, 61, and 78 in TERT promoter mutant lines or days 44, 61 and 83 in TERT promoter wild-type lines post-editing relative to day 33 post-editing for CTRL or β1L-reduced clones. *p value<0.05, two-sided Student’s t-test comparing values between CTRL and β1L-reduced clones at day 78/83 for each respective line. Values are mean ± S.D. of at least three independent assays. (B) Relative telomere length after transfection of an empty (VECTOR), β1L, or TERT expression vector in TERT promoter-mutant lines 78 or 83 days post-editing. Red dotted line indicates time of transfection (at day 58 [LN229] or 61 [GBM1 and T98G] post-editing). *p value<0.05, two-sided Student’s t-test of values of β1L or TERT versus VECTOR at day 78/83. Values are mean ± S.D. of at least three independent experiments. (C) Representative DAPI images (left images) and quantification (right graphs) of chromatin bridges (arrow) in CTRL or β1L-reduced clones at days 70–75 post-editing. Scale bar = 20 μm. *p value<0.05, **p value<0.01, two-sided Student’s t-test compared to CTRL. Quantification values are weighted mean ± S.D. of at least ten independent fields of view. See also Figure S4.