Alexander K. Diaz

The genomic landscape of diffuse intrinsic pontine glioma and pediatric non-brainstem high-grade glioma.

Pediatric high-grade glioma (HGG) is a devastating disease with a less than 20% survival rate 2 years after diagnosis. We analyzed 127 pediatric HGGs, including diffuse intrinsic pontine gliomas (DIPGs) and non-brainstem HGGs (NBS-HGGs), by whole-genome, whole-exome and/or transcriptome sequencing. We identified recurrent somatic mutations in ACVR1 exclusively in DIPGs (32%), in addition to previously reported frequent somatic mutations in histone H3 genes, TP53 and ATRX, in both DIPGs and NBS-HGGs. Structural variants generating fusion genes were found in 47% of DIPGs and NBS-HGGs, with recurrent fusions involving the neurotrophin receptor genes NTRK1, NTRK2 and NTRK3 in 40% of NBS-HGGs in infants. Mutations targeting receptor tyrosine kinase–RAS-PI3K signaling, histone modification or chromatin remodeling, and cell cycle regulation were found in 68%, 73% and 59% of pediatric HGGs, respectively, including in DIPGs and NBS-HGGs. This comprehensive analysis provides insights into the unique and shared pathways driving pediatric HGG within and outside the brainstem.

Novel Oncogenic PDGFRA Mutations in Pediatric High-Grade Gliomas

The outcome for children with high-grade gliomas (HGG) remains dismal, with a 2-year survival rate of only 10% to 30%. Diffuse intrinsic pontine glioma (DIPG) comprise a subset of HGG that arise in the brainstem almost exclusively in children. Genome-wide analyses of copy number imbalances previously showed that platelet-derived growth factor receptor α (PDGFRA) is the most frequent target of focal amplification in pediatric HGGs, including DIPGs. To determine whether PDGFRA is also targeted by more subtle mutations missed by copy number analysis, we sequenced all PDGFRA coding exons from a cohort of pediatric HGGs. Somatic-activating mutations were identified in 14.4% (13 of 90) of nonbrainstem pediatric HGGs and 4.7% (2 of 43) of DIPGs, including missense mutations and in-frame deletions and insertions not previously described. Forty percent of tumors with mutation showed concurrent amplification, whereas 60% carried heterozygous mutations. Six different mutations impacting different domains all resulted in ligand-independent receptor activation that was blocked by small molecule inhibitors of PDGFR. Expression of mutants in p53-null primary mouse astrocytes conferred a proliferative advantage in vitro and generated HGGs in vivo with complete penetrance when implanted into brain. The gene expression signatures of these murine HGGs reflected the spectrum of human diffuse HGGs. PDGFRA intragenic deletion of exons 8 and 9 were previously shown in adult HGG, but were not detected in 83 nonbrainstem pediatric HGG and 57 DIPGs. Thus, a distinct spectrum of mutations confers constitutive receptor activation and oncogenic activity to PDGFRα in childhood HGG.

The genetic signatures of pediatric high-grade glioma: no longer a one-act play.

Advances in understanding pediatric high-grade glioma (pHGG) genetics have revealed key differences between pHGG and adult HGG and have uncovered unique molecular drivers among subgroups within pHGG. The 3 core adult HGG pathways, the receptor tyrosine kinase-Ras-phosphatidylinositide 3-kinase, p53, and retinoblastoma networks, are also disrupted in pHGG, but they exhibit a different spectrum of effectors targeted by mutation. There are also similarities and differences in the genomic landscape of diffuse intrinsic pontine glioma (DIPG) and pediatric nonbrainstem (pNBS)-HGG. In 2012, histone H3 mutations were identified in nearly 80% of DIPGs and ~35% of pNBS-HGG. These were the first reports of histone mutations in human cancer, implicating novel biology in pediatric gliomagenesis. Additionally, DIPG and midline pNBS-HGG vary in the frequency and specific histone H3 amino acid substitution compared with pNBS-HGGs arising in the cerebral hemispheres, demonstrating a molecular difference among pHGG subgroups. The gene expression signatures as well as DNA methylation signatures of these tumors are also distinctive, reflecting a combination of the driving mutations and the developmental context from which they arise. These data collectively highlight unique selective pressures within the developing brainstem and solidify DIPG as a specific molecular and biological entity among pHGGs. Emerging studies continue to identify novel mutations that distinguish subgroups of pHGG. The molecular heterogeneity among pHGGs will undoubtedly have clinical implications moving forward. The discovery of unique oncogenic drivers is a critical first step in providing patients with appropriate, targeted therapies. Despite these insights, our vantage point has been largely limited to an in-depth analysis of protein coding sequences. Given the clear importance of histone mutations in pHGG, it will be interesting to see how aberrant epigenetic regulation contributes to tumorigenesis in the pediatric context. New mechanistic insights may allow for the identification of distinct vulnerabilities in this devastating spectrum of childhood tumors.

Abstract IA15: The unique genomic and epigenomic landscape of pediatric high-grade glioma

Diffuse high-grade gliomas (HGGs) of childhood are a spectrum of disease with devastatingly poor outcome. Despite similar histological features, pediatric HGGs arise from a broader distribution of anatomical locations when compared to adults, with approximately 50% arising in the brainstem as diffuse intrinsic pontine glioma (DIPG), a disease found almost exclusively in children. Recent genome-wide studies provided abundant evidence that unique selective pressures drive HGG in children compared to adults, identifying novel oncogenic mutations connecting tumorigenesis and chromatin regulation as well as developmental signaling pathways. The striking association between frequencies of specific mutations with spatiotemporal pattern of HGG formation in children further highlights context-dependent connections between developmental states and oncogenic drivers. Activating mutations in the BMP receptor ACVR1 are found exclusively in the youngest subgroup of DIPG patients, and not identified in HGGs outside the brainstem. Recurrent mutations in histone H3 establish a critical role for epigenetic regulation in disease pathogenesis of childhood HGG. H3K27M mutations occur in nearly 80% of DIPGs and more than half of HGGs arising in midline structures outside the brainstem such as the thalamus. In contrast, approximately 15% of HGGs arising in the cerebral cortex have an alternative H3 mutation, G34R/V, and are predominantly found in older adolescents and young adults. To study these mutations in the developing mammalian brain, we generated conditionally activated, epitope tagged H3f3a knock-in mice to express K27M, G34R or non-mutated H3.3 proteins from the endogenous H3f3a locus. We will present early analyses of these new oncohistone mouse models. Citation Format: Jon D. Larson, Andre B. Silveira, Lawryn H. Kasper, Alexander K. Diaz, Xiaoyan Zhu, Suzanne J. Baker. The unique genomic and epigenomic landscape of pediatric high-grade glioma. [abstract]. In: Proceedings of the AACR Special Conference: Developmental Biology and Cancer; Nov 30-Dec 3, 2015; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(4_Suppl):Abstract nr IA15.

Abstract PR03: The genomic landscape of diffuse intrinsic pontine glioma and pediatric non-brainstem high-grade glioma

Pediatric high-grade glioma (HGG) remains a tremendous clinical challenge, with a two-year survival of less than 20%. We analyzed 127 pediatric HGGs, including diffuse intrinsic pontine gliomas (DIPGs) and non-brainstem HGGs (NBS-HGGs) by whole-genome, whole-exome, and/or transcriptome sequencing. Somatic mutations in the bone morphogenetic protein (BMP) receptor ACVR1 occurred in 32% of DIPG, a finding exclusive to brainstem HGG. Structural variants generating fusion genes were found in 47% of pediatric HGG, with recurrent fusions involving the neurotrophin receptor genes NTRK1, 2, or 3 in 40% of infant NBS-HGGs and 5% of pediatric HGG overall. Multiple mutations targeted pathways involving histone modification or chromatin remodeling, cell cycle regulation and receptor tyrosine kinase/RAS/PI3K signaling, in both DIPG and NBS-HGGs at frequencies of greater than 39% in the entire cohort. The HGG mutation burden ranged from 2 non-silent mutations in an infant HGG to more than a million mutations in a tumor associated with germline mismatch repair deficiency. From these findings, we have established novel tumor models to better understand this devastating disease. This work provides new insight into the genetic events driving pediatric HGG tumorigenesis. This abstract is also presented as Poster B14. Citation Format: Alexander K. Diaz, Gang Wu, Barbara S. Paugh, Yongjin Li, Xiaoyan Zhu, Sherri Rankin, Chunxu Qu, Xiang Chen, Junyuan Zhang, John Easton, Michael Edmonson, Charles Lu, Panduka Nagahawatte, Erin Hedlund, Michael Rusch, Stanley Pounds, Tong Lin, Arzu Onar-Thomas, Robert Huether, Richard Kriwacki, Matthew Parker, Pankaj Gupta, Jared Becksfort, Lei Wei, Heather L. Mulder, Kristy Boggs, Bhavin Vadodaria, Donald Yergeau, Kerri Ochoa, Robert S. Fulton, Lucinda S. Fulton, Chris Jones, Alberto Broniscer, Cynthia Wetmore, Amar Gajjar, Li Ding, Elaine R. Mardis, Richard K. Wilson, James R. Downing, David W. Ellison, Jinghui Zhang, Suzanne J. Baker, For the St Jude Children9s Research Hospital – Washington University Pediatric Cancer Genome Project. The genomic landscape of diffuse intrinsic pontine glioma and pediatric non-brainstem high-grade glioma. [abstract]. In: Proceedings of the AACR Special Conference on Pediatric Cancer at the Crossroads: Translating Discovery into Improved Outcomes; Nov 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;74(20 Suppl):Abstract nr PR03.

Abstract A15: Variant histone H3 mutations associate with histone modification, DNA methylation, and gene expression changes in pediatric high-grade gliomas

High-grade gliomas (HGG) account for twenty percent of primary brain tumors in children and have a devastating outcome with a 2-year survival rate of less than 30%. While adult HGG arise frequently in the cerebral cortex, pediatric HGG can form in cerebral cortex as well as midline regions including thalamus and brainstem, and are genetically distinct from adult tumors. Brainstem HGGs, referred to as diffuse intrinsic pontine glioma (DIPG), are mostly exclusive to children and confer a dismal less than 10% 2-year survival rate. Recent genome-wide sequence analysis of pediatric DIPG and non-brainstem HGG revealed a predominance of mutually exclusive somatic heterozygous H3F3A or HIST1H3B mutations encoding a K27M variant of histone H3.3 or H3.1, respectively. These mutations were found in 78% of DIPG patients and 22% of non-brainstem pediatric HGG. An additional mutually exclusive somatic heterozygous H3F3A mutation encoding histone H3.3 G34R was identified in 14% of non-brainstem HGG. We established xenografts from human non-brainstem HGGs carrying wild-type H3 or the H3.3 G34R mutant, and DIPGs with H3.3 or H3.1 K27M mutations. H3 K27M mutant xenografts showed a marked reduction in H3K27 trimethylation (H3K27me3), while tumors with wild-type or H3.3 G34R mutations retain H3K27me3 as demonstrated by immunohistochemistry and Western blot analyses. To determine if this was a dominant effect of K27M mutation or a developmental signature of DIPG we transduced primary astrocyte cultures derived from p53-deficient neonatal mouse cortex or brainstem with retrovirus expressing wild-type, K27M, K27A or G34R H3 mutant variants. Low-level expression of exogenous H3.3 K27M compared to endogenous H3 was sufficient to recapitulate prominent H3K27me3 reduction observed in the xenografts. Furthermore, we evaluated correlation between H3 mutation and H3K27me3 status in a large cohort of primary pediatric HGG including DIPG. From this we identified a significant relationship between H3 K27M mutation and H3K27me3 loss in DIPG. However, we also observed rare wild-type H3 DIPG cases exhibiting reduced H3K27me3 indicating a potential alternative mechanism for this methylation state. In addition, we evaluated the association of H3K27me3 and K27M mutations with genome-wide DNA methylation patterns, and gene expression signatures. Together, these results implicate histone H3 K27M and G34R mutations as unique, dominant genetic drivers in distinct subtypes of pediatric HGG. Current efforts focus on delineating the divergent pathological roles of H3 K27M and G34R mutations during DIPG and non-brainstem HGG tumorigenesis. Citation Format: Jon D. Larson, Troy A. McEachron, Chunxu Qu, Xiaoyan Zhu, Alexander K. Diaz, David W. Ellison, Brent A. Orr, Suzanne J. Baker. Variant histone H3 mutations associate with histone modification, DNA methylation, and gene expression changes in pediatric high-grade gliomas. [abstract]. In: Proceedings of the AACR Special Conference on Chromatin and Epigenetics in Cancer; Jun 19-22, 2013; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2013;73(13 Suppl):Abstract nr A15.