DNA replication-associated mutations are repaired by two components: polymerase proofreading and mismatch repair. The mutation consequences of disruption to both repair components in humans are not well studied. We sequenced cancer genomes from children with inherited biallelic mismatch repair deficiency (bMMRD). High-grade bMMRD brain tumors exhibited massive numbers of substitution mutations (>250/Mb), which was greater than all childhood and most cancers (>7,000 analyzed). All ultra-hypermutated bMMRD cancers acquired early somatic driver mutations in DNA polymerase ɛ or δ. The ensuing mutation signatures and numbers are unique and diagnostic of childhood germ-line bMMRD (P < 10(-13)). Sequential tumor biopsy analysis revealed that bMMRD/polymerase-mutant cancers rapidly amass an excess of simultaneous mutations (∼600 mutations/cell division), reaching but not exceeding ∼20,000 exonic mutations in <6 months. This implies a threshold compatible with cancer-cell survival. We suggest a new mechanism of cancer progression in which mutations develop in a rapid burst after ablation of replication repair.

Purpose BRAF V600E is a potentially highly targetable mutation detected in a subset of pediatric low-grade gliomas (PLGGs). Its biologic and clinical effect within this diverse group of tumors remains unknown. Patients and MethodsA combined clinical and genetic institutional study of patients with PLGGs with long-term follow-up was performed (N = 510). Clinical and treatment data of patients with BRAF V600E mutated PLGG (n = 99) were compared with a large international independent cohort of patients with BRAF V600E mutated-PLGG (n = 180).Results BRAF V600E mutation was detected in 69 of 405 patients (17%) with PLGG across a broad spectrum of histologies and sites, including midline locations, which are not often routinely biopsied in clinical practice. Patients with BRAF V600E PLGG exhibited poor outcomes after chemotherapy and radiation therapies that resulted in a 10-year progression-free survival of 27% (95% CI, 12.1% to 41.9%) and 60.2% (95% CI, 53.3% to 67.1%) for BRAF V600E and wild-type PLGG, respectively (P , .001). Additional multivariable clinical and molecular stratification revealed that the extent of resection and CDKN2A deletion contributed independently to poor outcome in BRAF V600E PLGG. A similar independent role for CDKN2A and resection on outcome were observed in the independent cohort. Quantitative imaging analysis revealed progressive disease and a lack of response to conventional chemotherapy in most patients with BRAF V600E PLGG. ConclusionBRAF V600E PLGG constitutes a distinct entity with poor prognosis when treated with current adjuvant therapy. response to therapy and clinical outcome is still not known. As a result, as far as nonsurgical treatment is concerned, all patients with PLGGs receive similar treatment independent of their tumor's molecular alterations. 6 For deeply located tumors, such as hypothalamic/chiasmatic LGGs, the need for biopsy before treatment decisions are made for these children is still debated.The BRAF V600E mutation, which is observed in a variety of adult 7 and pediatric neoplasms, is thought to be present in only a small percentage of PLGGs.8 Controversy still exists as to whether BRAF V600E-mutant PLGG constitutes a unique subgroup with respect to natural history and outcome. 9,10 We have previously reported that PLGGs that transform to high-grade gliomas have a high incidence of BRAF V600E mutations in combination with CDKN2A deletion.11 CDKN2A is a tumor suppressor gene and a key regulator of the cell cycle. CDKN2A alterations act as a secondary hit, which allows for escape from cell cycle regulation and malignant behavior in multiple cancer types. 12,13 In PLGGs, CDKN2A loss has been reported to be associated with escape from oncogene-induced senescence, 14 especially when combined with BRAF mutations.To better define the clinical significance of BRAF V600E in these tumors, we performed a combined clinical and genetic analysis in an institutional discovery cohort of patients with PLGG who were diagnosed and treated in southern Ontario. 15 We then asse...

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Purpose To uncover the genetic events leading to transformation of pediatric low-grade glioma (PLGG) to secondary high-grade glioma (sHGG). Patients and Methods We retrospectively identified patients with sHGG from a population-based cohort of 886 patients with PLGG with long clinical follow-up. Exome sequencing and array CGH were performed on available samples followed by detailed genetic analysis of the entire sHGG cohort. Clinical and outcome data of genetically distinct subgroups were obtained. Results sHGG was observed in 2.9% of PLGGs (26 of 886 patients). Patients with sHGG had a high frequency of nonsilent somatic mutations compared with patients with primary pediatric high-grade glioma (HGG; median, 25 mutations per exome; P = .0042). Alterations in chromatin-modifying genes and telomere-maintenance pathways were commonly observed, whereas no sHGG harbored the BRAF-KIAA1549 fusion. The most recurrent alterations were BRAF V600E and CDKN2A deletion in 39% and 57% of sHGGs, respectively. Importantly, all BRAF V600E and 80% of CDKN2A alterations could be traced back to their PLGG counterparts. BRAF V600E distinguished sHGG from primary HGG (P = .0023), whereas BRAF and CDKN2A alterations were less commonly observed in PLGG that did not transform (P < .001 and P < .001 respectively). PLGGs with BRAF mutations had longer latency to transformation than wild-type PLGG (median, 6.65 years [range, 3.5 to 20.3 years] v 1.59 years [range, 0.32 to 15.9 years], respectively; P = .0389). Furthermore, 5-year overall survival was 75% ± 15% and 29% ± 12% for children with BRAF mutant and wild-type tumors, respectively (P = .024). Conclusion BRAF V600E mutations and CDKN2A deletions constitute a clinically distinct subtype of sHGG. The prolonged course to transformation for BRAF V600E PLGGs provides an opportunity for surgical interventions, surveillance, and targeted therapies to mitigate the outcome of sHGG.

Purpose Vinblastine monotherapy has shown promising activity and a low-toxicity profile in patients with pediatric low-grade glioma (PLGG) who experienced treatment failure after initial treatment with chemotherapy and/or radiation. The aim of this study was to assess the activity of vinblastine in therapy-naïve children. Patients and Methods Patients < 18 years old with unresectable and/or progressive therapy-naïve PLGG were eligible. Vinblastine was administered once per week at a dose of 6 mg/m intravenously over a period of 70 weeks. Vision, quality of life, neurofibromatosis type 1 (NF1) status, and BRAF mutation/fusion status were also determined and correlated with outcome. Results Fifty-four patients were enrolled onto the study, with a median age of 8 years (range, 0.7 to 17.2 years). Most patients had chiasmatic/hypothalamic tumors (55.5%), and 13 patients (24.1%) had NF1. The most common histology was pilocytic astrocytoma (46.3%). Seventeen patients were diagnosed using radiologic criteria alone. Best response to chemotherapy was centrally reviewed with a response rate (complete, partial, or minor response) of 25.9%. Disease stabilization (complete, partial, or minor response or stable disease) was achieved in 47 patients (87.0%). Visual improvement was observed in 20% of patients with optic pathway glioma. Five-year overall survival and progression-free survival (PFS) rates were 94.4% (95% CI, 88.5% to 100%) and 53.2% (95% CI, 41.3% to 68.5%), respectively, for the entire cohort. Patients with NF1 had a significantly better PFS (85.1%; 95% CI, 68.0% to 100%) when compared with patients without NF1 (42.0%; 95% CI, 29.1% to 60.7%; P = .012). Age< 3 years or > 10 years was not associated with poor outcome. Treatment was well tolerated, and quality of life was not affected during treatment. In this trial, there was no correlation between BRAF alterations and outcome. Conclusion Vinblastine administered once per week is well tolerated in children with treatment naïve PLGG. Overall survival and PFS are comparable to current therapies, with a favorable toxicity profile and a maintained quality of life.

BACKGROUND:The determinants of outcomes for adult survivors of pediatric low-grade glioma (PLGG) are largely unknown. METH-ODS: This study collected population-based follow-up information for all PLGG patients diagnosed in Ontario, Canada from 1985 to 2012 (n 5 1202) and determined factors affecting survival. The impact of upfront radiation treatment on overall survival (OS) was determined for a cohort of Ontario patients and an independent reference cohort from the Surveillance, Epidemiology, and End Results database. RESULTS: At a median follow-up of 12.73 years (range, 0.02-33 years), only 93 deaths (7.7%) were recorded, and the 20-year OS rate was 90.1% 6 1.1%. Children with neurofibromatosis type 1 had excellent survival and no tumor-related deaths during adulthood. Adverse risk factors included pleomorphic xanthoastrocytoma (P <.001) and a thalamic location (P <.001). For patients with unresectable tumors surviving more than 5 years after the diagnosis, upfront radiotherapy was associated with an approximately 3-fold increased risk of overall late deaths (hazard ratio [HR], 3.3; 95% confidence interval [CI], 1.6-6.6; P 5.001) and an approximately 4-fold increased risk of tumor-related deaths (HR, 4.4; 95% CI, 1.3-14.6; P 5.013). In a multivariate analysis, radiotherapy was the most significant factor associated with late all-cause deaths (HR, 3.0; 95% CI, 1.3-7.0; P 5.012) and tumor-related deaths (HR, 4.4; 95% CI, 1.3-14.6; P 5 0.014). A similar association between radiotherapy and late deaths was observed in the independent reference cohort (P <.001). In contrast to early deaths, late mortality was associated not with PLGG progression but rather with tumor transformation and non-oncological causes. CONCLUSIONS: The course of PLGG is associated with excellent long-term survival, but this is hampered by increased delayed mortality in patients receiving upfront radiotherapy. These observations should be considered when treatment options are being weighed for these patients. Cancer 2016;122:1261-9.

Paediatric brain tumours arising in the thalamus present significant diagnostic and therapeutic challenges to physicians due to their sensitive midline location. As such, genetic analysis for biomarkers to aid in the diagnosis, prognosis and treatment of these tumours is needed. Here, we identified 64 thalamic gliomas with clinical follow-up and characterized targeted genomic alterations using newly optimized droplet digital and NanoString-based assays. The median age at diagnosis was 9.25 years (range, 0.63–17.55) and median survival was 6.43 (range, 0.01–27.63) years. Our cohort contained 42 and 22 tumours reviewed as low and high grade gliomas, respectively. Five (12 %) low grade and 11 (50 %) high grade gliomas were positive for the H3F3A/HIST1H3B K27M (H3K27M) mutation. Kaplan-Meier survival analysis revealed significantly worse overall survival for patients harbouring the H3K27M mutation versus H3F3A/HIST1H3B wild type (H3WT) samples (log-rank p < 0.0001) with a median survival of 1.02 vs. 9.12 years. Mitogen-activated protein kinase (MAPK) pathway activation via BRAF or FGFR1 hotspot mutations or fusion events were detected in 44 % of patients, and was associated with long-term survival in the absence of H3K27M (log-rank p < 0.0001). Multivariate analysis demonstrated H3K27M status and high grade histology to be the most significant independent predictors of poor overall survival with hazard ratios of 6.945 and 7.721 (p < 0.0001), respectively. In contrast, MAPK pathway activation is a predictor of favourable patient outcome, although not independent of other clinical factors. Importantly, we show that low grade malignancies may harbour H3K27M mutations and that these tumours show a dismal survival compared to low grade H3WT cases. Our data strongly supports the inclusion of targeted genetic testing in childhood thalamic tumours to most accurately stratify patients into appropriate risk groups.Electronic supplementary materialThe online version of this article (doi:10.1186/s40478-016-0353-0) contains supplementary material, which is available to authorized users.

Although telomeres are maintained in most cancers by telomerase activation, a subset of tumors utilize alternative lengthening of telomeres (ALT) to sustain self-renewal capacity. In order to study the prevalence and significance of ALT in childhood brain tumors we screened 517 pediatric brain tumors using the novel C-circle assay. We examined the association of ALT with alterations in genes found to segregate with specific histological phenotypes and with clinical outcome. ALT was detected almost exclusively in malignant tumors (p = 0.001). ALT was highly enriched in primitive neuroectodermal tumors (12 %), choroid plexus carcinomas (23 %) and high-grade gliomas (22 %). Furthermore, in contrast to adult gliomas, pediatric low grade gliomas which progressed to high-grade tumors did not exhibit the ALT phenotype. Somatic but not germline TP53 mutations were highly associated with ALT (p = 1.01 × 10(-8)). Of the other alterations examined, only ATRX point mutations and reduced expression were associated with the ALT phenotype (p = 0.0005). Interestingly, ALT attenuated the poor outcome conferred by TP53 mutations in specific pediatric brain tumors. Due to very poor prognosis, one year overall survival was quantified in malignant gliomas, while in children with choroid plexus carcinoma, five year overall survival was investigated. For children with TP53 mutant malignant gliomas, one year overall survival was 63 ± 12 and 23 ± 10 % for ALT positive and negative tumors, respectively (p = 0.03), while for children with TP53 mutant choroid plexus carcinomas, 5 years overall survival was 67 ± 19 and 27 ± 13 % for ALT positive and negative tumors, respectively (p = 0.07). These observations suggest that the presence of ALT is limited to a specific group of childhood brain cancers which harbor somatic TP53 mutations and may influence the outcome of these patients. Analysis of ALT may contribute to risk stratification and targeted therapies to improve outcome for these children.