Fig. 6

HGPRT-mediated guanine salvage in H3K27M-expressing tumors protects against the effects of RT. A Schematic showing mechanism of in vivo resistance to MMF via increased intratumoral guanine abundance. Schematic was created using BioRender.com. B Hypoxanthine and guanine abundances in orthotopic DIPGXIII-GFP/LUC xenograft tumors and contralateral normal brain tissue. Tissues were harvested by GFP fluorescence-guided mechanical resection, and their metabolites collected via methanol extraction before LC/MS. Data is normalized to normal brain (NB). C Guanine ion abundance in NHA-conditioned media collected 24hrs after a single 4Gy dose of RT. D Endpoint neurosphere counts of DIPGXIII (left) and BT245 (right) treated with RT or MPA+RT in TSM media with or without supplemented guanine (+G). DIPGXIII cells were given 100µM guanine. BT245 cells were given 10µM guanine. E Spider plot of bioluminescent signal flux values for individual DIPGXIII-GFP/LUC-shHPRT1 mouse tumors. Blue box indicates the treatment period. F Kaplan-Meier survival analysis of Rag1-KO mice bearing DIPGXIII-GFP/LUC-shHPRT1 orthotopic xenograft tumors given vehicle control or RT (n=7 mice/group). Statistical analysis was performed by comparing individual survival curves using GraphPad Prism 10.0 software. G Representative bioluminescent imaging of an RT-treated mouse bearing an orthotopic DIPGXIII-GFP/LUC-shHPRT1 xenograft tumor that experienced a complete response following treatment. D# indicates the day at which the image was taken with respect to the end of the treatment regimen. The first image was taken two days prior to the completion of the treatment regimen (D0-2). H Kaplan-Meier survival analysis of patient H3K27M-expressing tumors based on HPRT1 expression where the 1^st quartile represents the highest HPRT1 expression, and the 4^th quartile represents the lowest HPRT1 expression