Abstract 3009: ATRX validated as tumor suppressor in a novel mouse model of pediatric and young adult GBM

Pediatric Glioblastoma (GBM) remains one of the most difficult childhood tumors to treat, and most children with this diagnosis will not survive longer than two years. ATRX is a histone chaperone protein that is mutated primarily in pediatric patients with GBM and younger adults with secondary GBM. No previous animal model has demonstrated the effect of ATRX loss on GBM formation. We cloned an ATRX knockdown sequence into a Sleeping Beauty (SB) transposase-responsive plasmid (shATRX) for insertion into host genomic DNA. Glioblastomas were induced in neonatal mice by injecting plasmids encoding SB transposase/ luciferase, shp53 and NRAS, with or without shATRX, into the ventricle of neonatal mice. Tumors in both groups (with or without shATRX) showed histological hallmarks of human glioblastoma. The loss of ATRX was specifically localized only within tumors generated with the shATRX plasmid and not in the adjacent cortex. Notably, loss of ATRX reduced median survival of mice by 43% (p = 0.012). ATRX-deficient tumors displayed evidence of telomeric lengthening using telomeric FISH assay for alternative lengthening of telomeres (ALT). ATRX-deficient tumors were significantly more likely to develop microsatellite instability (p = 0.014), a hallmark of impaired DNA-damage repair. Analysis of three human GBM sequencing datasets confirmed increased number of somatic nucleotide mutations in ATRX-deficient tumors. Treatment of primary cell cultures generated from mouse GBMs showed that ATRX-deficient tumor cells are significantly more sensitive to certain DNA damaging agents, with greater evidence of double-stranded DNA breakage, by gH2A.X. In addition, mice with ATRX-deficient GBM treated with whole brain irradiation showed reduced tumor growth by luminescence, with some long-term survivors. In summary, this mouse model prospectively validates ATRX as a tumor suppressor in human GBM for the first time in an animal model. In addition, loss of ATRX leads to increased genetic instability and response to DNA-damaging therapy. Based on these results, we have generated the hypothesis that ATRX loss leads to a genetically unstable tumor; which is more aggressive when untreated, but more responsive to DNA-damaging therapy, ultimately resulting in equivalent or improved overall survival. Supported by St. Baldrick's Fellowship and Alex's Lemonade Stand /Northwest Mutual Young Investigator Award to CK and NIH/NINDS grants to MGC and PRL. Citation Format: Carl Koschmann, Alex