A Novel Mouse Model of Diffuse Intrinsic Pontine Glioma Initiated in Pax3-Expressing Cells

Diffuse intrinsic pontine glioma (DIPG) is a rare and incurable brain tumor that arises predominately in children and involves the pons, a structure that along with the midbrain and medulla makes up the brainstem. We have previously developed genetically engineered mouse models of brainstem glioma using the RCAS/Tv-a system by targeting PDGF-B overexpression, p53 loss, and H3.3K27M mutation to Nestin-expressing brainstem progenitor cells of the neonatal mouse. Here we describe a novel mouse model targeting these same genetic alterations to Pax3-expressing cells, which in the neonatal mouse pons consist of a Pax3 +/Nestin +/Sox2 + population lining the fourth ventricle and a Pax3 +/NeuN + parenchymal population. Injection of RCAS-PDGF-B into the brainstem of Pax3-Tv-a mice at postnatal day 3 results in 40% of mice developing asymptomatic low-grade glioma. A mixture of low- and high-grade glioma results from injection of Pax3-Tv-a;p53^fl/fl mice with RCAS-PDGF-B and RCAS-Cre, with or without RCAS-H3.3K27M. These tumors are Ki67 +, Nestin +, Olig2 +, and largely GFAP − and can arise anywhere within the brainstem, including the classic DIPG location of the ventral pons. Expression of the H3.3K27M mutation reduces overall H3K27me3 as compared with tumors without the mutation, similar to what has been previously shown in human and mouse tumors. Thus, we have generated a novel genetically engineered mouse model of DIPG, which faithfully recapitulates the human disease and represents a novel platform with which to study the biology and treatment of this deadly disease.

Diffuse Intrinsic Pontine Glioma: Time for Cautious Optimism

Diffuse intrinsic pontine glioma is a lethal brain cancer that arises in the pons of children. The median survival for children with diffuse intrinsic pontine glioma is less than 1 year from diagnosis, and no improvement in survival has been realized in more than 30 years. Currently, the standard of care for diffuse intrinsic pontine glioma is focal radiation therapy, which provides only temporary relief. Recent genomic analysis of tumors from biopsies and autopsies, have resulted in the discovery of K27M H3.3/H3.1 mutations in 80% and ACVR1 mutations in 25% of diffuse intrinsic pontine gliomas, providing renewed hope for future success in identifying effective therapies. In addition, as stereotactic tumor biopsies at diagnosis at specialized centers have been demonstrated to be safe, biopsies have now been incorporated into several prospective clinical trials. This article summarizes the epidemiology, clinical presentation, diagnosis, prognosis, molecular genetics, current treatment, and future therapeutic directions for diffuse intrinsic pontine glioma.

A phase I/Ib trial targeting the Pi3k/Akt pathway using perifosine: Long-term progression-free survival of patients with resistant neuroblastoma

AKT plays a pivotal role in driving the malignant phenotype of many cancers, including high-risk neuroblastoma (HR-NB). AKT signaling, however, is active in normal tissues, raising concern about excessive toxicity from its suppression. The oral AKT inhibitor perifosine showed tolerable toxicity in adults and in our phase I trial in children with solid tumors (clinicaltrials.gov NCT00776867). We now report on the HR-NB experience. HR-NB patients received perifosine 50-75 mg m^-2  day^-1 after a loading dose of 100-200 mg m^-2 on day 1, and continued on study until progressive disease. The 27 HR-NB patients included three treated for primary refractory disease and 24 with disease resistant to salvage therapy after 1-5 (median 2) relapses; only one had MYCN-amplified HR-NB. Pharmacokinetic studies showed μM concentrations consistent with cytotoxic levels in preclinical models. Nine patients (all MYCN-non-amplified) remained progression-free through 43+ to 74+ (median 54+) months from study entry, including the sole patient to show a complete response and eight patients who had persistence of abnormal ¹²³ I-metaiodobenzylguanidine skeletal uptake but never developed progressive disease. Toxicity was negligible in all 27 patients, even with the prolonged treatment (11-62 months, median 38) in the nine long-term progression-free survivors. The clinical findings (i) confirm the safety of therapeutic serum levels of an AKT inhibitor in children; (ii) support perifosine for MYCN-non-amplified HR-NB as monotherapy after completion of standard treatment or combined with other agents (based on preclinical studies) to maximize antitumor effects; and (iii) highlight the welcome possibility that refractory or relapsed MYCN-non-amplified HR-NB is potentially curable.

Abstract 2688: A forward genetics screen of murine brain tumors identifies novel candidate genes involved in gliomagenesis

Glioma is the most frequent malignant brain tumor in adults. Platelet-derived growth factor (PDGF) signaling is commonly activated in glioma. We have used a retrovirus-driven PDGFB-induced murine glioma model that causes tumors that closely resemble human gliomas of various grades. Knowing that retroviruses have a capacity to induce insertional mutagenesis, we have employed whole genome sequencing to identify potential genes that, together with PDGFB, drive glioma development.

Gliomas were induced by RCAS virus injection into the brains of mice expressing the RCAS retroviral receptor from specific promoters. Genomic DNA from tumor cell lines was probed for retroviral tags and sequenced to identify genomic targets of the retrovirus. A streamlined analysis pipeline was developed for retrovirus integration detection and mapping to the reference mouse genome. Integration sites were analyzed and a common integration site (CIS) label was assigned to a gene, given that it was either tagged by a retrovirus more than once within a discovery set or found within the Retroviral Tagged Cancer Gene Database (RTCGD).

In a small discovery subset of 15 murine gliomas, we have identified 40 CIS, of which 37 were validated by Sanger sequencing. When compared with previously identified CIS in RTCGD, 5.5% of them were shared with our older screen, where we overexpressed PDGFB from another retrovirus in order to induce glioma. Less CIS genes were shared with other published tumor models induced by viruses driven by other cancer genes/viruses.

The majority of genes identified in our screen were tagged twice. However, Nfic, Cuecd1, Thra, Foxj1 and Nrxn1 were tagged three times, Ppfibp1 and Rhbg four times, and Mir29a/29b-1 seven times. As compared to control tumor lines, two top candidate genes, Mir29a and Ppfibp1, demonstrated significantly increased expression in tumor lines in were they were respectively tagged. Mir29a is often found downregulated in human tumors including gliomas, still high levels of Mir29a are sometimes found in certain aggressive cancers and in metastases.

Interestingly, we found that specific PDGFR inhibition negatively regulates Mir29a, indicating a possible role for PDGF signaling in Mir29a regulation. Ppfibp1 has not been extensively studied in cancer. However, Ppfibp1 seems to have a subgroup-specific expression in human glioblastoma, making it an interesting candidate for further analysis.

Here we present a new screening method that can be employed to identify genes involved in PDGFB-driven gliomagenesis. So far, we have identified 37 candidate genes by whole genome sequencing. Two of the most frequently tagged candidates, Mir29a and Ppfibp1 were upregulated as a consequence of retroviral mutagenesis. Their precise role in driving glioma formation in collaboration with PDGF is currently explored.

Citation Format: Matko Cancer, Holger Weishaupt, Gabriela Rosen, Ignas Bunikis, Yiwen Jiang, Smitha Sreedharan, Sara Bolin, Ulf Gyllensten, Oren J. Becher, Lene Uhrbom, Adam Ameur, Fredrik J. Swartling. A forward genetics screen of murine brain tumors identifies novel candidate genes involved in gliomagenesis. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2688.

The emerging role of NG2 in pediatric diffuse intrinsic pontine glioma

Diffuse intrinsic pontine gliomas (DIPGs) have a dismal prognosis and are poorly understood brain cancers. Receptor tyrosine kinases stabilized by neuron-glial antigen 2 (NG2) protein are known to induce gliomagenesis. Here, we investigated NG2 expression in a cohort of DIPG specimens (n= 50). We demonstrate NG2 expression in the majority of DIPG specimens tested and determine that tumors harboring histone 3.3 mutation express the highest NG2 levels. We further demonstrate that microRNA 129-2 (miR129-2) is downregulated and hypermethylated in human DIPGs, resulting in the increased expression of NG2. Treatment with 5-Azacytidine, a methyltransferase inhibitor, results in NG2 downregulation in DIPG primary tumor cells in vitro. NG2 expression is altered (symmetric segregation) in mitotic human DIPG and mouse tumor cells. These mitotic cells co-express oligodendrocyte (Olig2) and astrocyte (glial fibrillary acidic protein, GFAP) markers, indicating lack of terminal differentiation. NG2 knockdown retards cellular migration in vitro, while NG2 expressing neurospheres are highly tumorigenic in vivo, resulting in rapid growth of pontine tumors. NG2 expression is targetable in vivo using miR129-2 indicating a potential avenue for therapeutic interventions. This data implicates NG2 as a molecule of interest in DIPGs especially those with H3.3 mutation.

ABCG2 and ABCB1 Limit the Efficacy of Dasatinib in a PDGF-B-Driven Brainstem Glioma Model

Dasatinib is a multikinase inhibitor in clinical trials for glioma, and thus far has failed to demonstrate significant efficacy. We investigated whether the ABC efflux transporters ABCG2 and ABCB1 expressed in the blood-brain barrier (BBB), are limiting the efficacy of dasatinib in the treatment of glioma using genetic and pharmacologic approaches. We utilized a genetic brainstem glioma mouse model driven by platelet-derived growth factor-B and p53 loss using abcg2/abcb1 wild-type (ABC WT) or abcg2/abcb1 knockout mice (ABC KO). First, we observed that brainstem glioma tumor latency is significantly prolonged in ABC KO versus ABC WT mice (median survival of 47 vs. 34 days). Dasatinib treatment nearly doubles the survival of brainstem glioma-bearing ABC KO mice (44 vs. 80 days). Elacridar, an ABCG2 and ABCB1 inhibitor, significantly increases the efficacy of dasatinib in brainstem glioma-bearing ABC WT mice (42 vs. 59 days). Pharmacokinetic analysis demonstrates that dasatinib delivery into the normal brain, but not into the tumor core, is significantly increased in ABC KO mice compared with ABC WT mice. Surprisingly, elacridar did not significantly increase dasatinib delivery into the normal brain or the tumor core of ABC WT mice. Next, we demonstrate that the tight junctions of the BBB of this model are compromised as assessed by tissue permeability to Texas Red dextran. Finally, elacridar increases the cytotoxicity of dasatinib independent of ABCG2 and ABCB1 expression in vitro In conclusion, elacridar improves the efficacy of dasatinib in a brainstem glioma model without significantly increasing its delivery to the tumor core. Mol Cancer Ther; 15(5); 819-29. ©2016 AACR.

Glioma Stem-like Cells Keep Their H3.3 Variant Levels at Bay

Pediatric glioblastomas (GBMs) commonly harbor mutations in histone variant H3.3, while adult GBMs do not. In this issue of Cancer Cell, Gallo and colleagues demonstrate that adult GBM stem-like cells repress H3.3 expression to maintain self-renewal properties.

Pre-Clinical Models of Diffuse Intrinsic Pontine Glioma

Diffuse intrinsic pontine glioma (DIPG) is a rare and incurable brain tumor that arises in the brainstem of children predominantly between the ages of 6 and 8. Its intricate morphology and involvement of normal pons tissue precludes surgical resection, and the standard of care today remains fractionated radiation alone. In the past 30 years, there have been no significant advances made in the treatment of DIPG. This is largely because we lack good models of DIPG and therefore have little biological basis for treatment. In recent years, however, due to increased biopsy and acquisition of autopsy specimens, research is beginning to unravel the genetic and epigenetic drivers of DIPG. Insight gleaned from these studies has led to improvements in approaches to both model these tumors in the lab and to potentially treat them in the clinic. This review will detail the initial strides toward modeling DIPG in animals, which included allograft and xenograft rodent models using non-DIPG glioma cells. Important advances in the field came with the development of in vitro cell and in vivo xenograft models derived directly from autopsy material of DIPG patients or from human embryonic stem cells. Finally, we will summarize the progress made in the development of genetically engineered mouse models of DIPG. Cooperation of studies incorporating all of these modeling systems to both investigate the unique mechanisms of gliomagenesis in the brainstem and to test potential novel therapeutic agents in a preclinical setting will result in improvement in treatments for DIPG patients.

Orthogonal targeting of EGFRvIII expressing glioblastomas through simultaneous EGFR and PLK1 inhibition

We identified a synthetic lethality between PLK1 silencing and the expression of an oncogenic Epidermal Growth Factor Receptor, EGFRvIII. PLK1 promoted homologous recombination (HR), mitigating EGFRvIII induced oncogenic stress resulting from DNA damage accumulation. Accordingly, PLK1 inhibition enhanced the cytotoxic effects of the DNA damaging agent, temozolomide (TMZ). This effect was significantly more pronounced in an Ink4a/Arf(-/-) EGFRvIII glioblastoma model relative to an Ink4a/Arf(-/-) PDGF-β model. The tumoricidal and TMZ-sensitizing effects of BI2536 were uniformly observed across Ink4a/Arf(-/-) EGFRvIII glioblastoma clones that acquired independent resistance mechanisms to EGFR inhibitors, suggesting these resistant clones retain oncogenic stress that required PLK1 compensation. Although BI2536 significantly augmented the anti-neoplastic effect of EGFR inhibitors in the Ink4a/Arf(-/-) EGFRvIII model, durable response was not achieved until TMZ was added. Our results suggest that optimal therapeutic effect against glioblastomas requires a "multi-orthogonal" combination tailored to the molecular physiology associated with the target cancer genome.

Pax3 expression enhances PDGF-B-induced brainstem gliomagenesis and characterizes a subset of brainstem glioma

High-grade Brainstem Glioma (BSG), also known as Diffuse Intrinsic Pontine Glioma (DIPG), is an incurable pediatric brain cancer. Increasing evidence supports the existence of regional differences in gliomagenesis such that BSG is considered a distinct disease from glioma of the cerebral cortex (CG). In an effort to elucidate unique characteristics of BSG, we conducted expression analysis of mouse PDGF-B-driven BSG and CG initiated in Nestin progenitor cells and identified a short list of expression changes specific to the brainstem gliomagenesis process, including abnormal upregulation of paired box 3 (Pax3). In the neonatal mouse brain, Pax3 expression marks a subset of brainstem progenitor cells, while it is absent from the cerebral cortex, mirroring its regional expression in glioma. Ectopic expression of Pax3 in normal brainstem progenitors in vitro shows that Pax3 inhibits apoptosis. Pax3-induced inhibition of apoptosis is p53-dependent, however, and in the absence of p53, Pax3 promotes proliferation of brainstem progenitors. In vivo, Pax3 enhances PDGF-B-driven gliomagenesis by shortening tumor latency and increasing tumor penetrance and grade, in a region-specific manner, while loss of Pax3 function extends survival of PDGF-B-driven;p53-deficient BSG-bearing mice by 33%. Importantly, Pax3 is regionally expressed in human glioma as well, with high PAX3 mRNA characterizing 40% of human BSG, revealing a subset of tumors that significantly associates with PDGFRA alterations, amplifications of cell cycle regulatory genes, and is exclusive of ACVR1 mutations. Collectively, these data suggest that regional Pax3 expression not only marks a novel subset of BSG but also contributes to PDGF-B-induced brainstem gliomagenesis.

Abstract A32: A specific antibody for Histone H3 Lys27Met mutation for the characterization of pediatric glioblastomas

Pediatric gliomas such as glioblastoma multiforme (GBM) and diffuse intrinsic pontine glioma (DIPG) are aggressive brain tumors for which there is no effective therapy. The reason for the poor therapeutic outcome for children diagnosed with glioblastomas might be due to the therapeutic approach, which is not significantly different from the treatment of adult gliomas. Recent studies have shown that the gene expression profile of pediatric gliomas is distinct from histologically similar tumors in adults. These findings strongly suggest that the underlying mechanisms of tumor progression are different in these gliomas. Whole-genome sequencing of these pediatric tumors has revealed mutations in genes of H3F3A and HIST1H3B encoding histone H3.3 or H3.1, respectively. The mutations resulted in a substitution of lysine (K) to methionine (M) at position 27 of the protein (H3K27M). The lysine 27 residue of histone H3 is a posttranslational modification site, methylated by the polycomb repressive complex 2 (PRC2). A mutation like H3K27M might play a significant role in tumorigenesis and tumor progression since histones and their posttranslational modifications are key epigenetic regulators. In collaboration with the laboratory of David Allis, we raised antibodies against the K27M mutation of histone H3. Using this antibody, Lewis et al. corroborated the presence of H3 K27M protein in DIPG samples containing the mutant allele. Furthermore, they provided evidence that this mutation can alter the molecular processes in the nucleus through a gain-of-function mechanism in glioblastoma. Here we show that this Anti-Histone H3, K27M mutant antibody (Cat. No.: ABE419), together with other modified histone antibodies, are useful tools for the characterization and classification of different type of brain tumors and can be used in several applications including immunoblotting and chromatin immunoprecipitation (ChIP).

Citation Format: Anita Lozsa, Robin T. Clark, Peter W. Lewis, Matthew S. Koletsky, Oren J. Becher, John Rosenfeld, Sturges Michael, David C. Allis, Trinette Chuang, Wayne Speckmann, Alejandra Solache. A specific antibody for Histone H3 Lys27Met mutation for the characterization of pediatric glioblastomas. [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 A32.

Sox2, a marker for stem-like tumor cells in skin squamous cell carcinoma and hedgehog subgroup medulloblastoma

Heterogeneity within tumors is becoming increasingly recognized as an important cause of treatment failure in cancer. Two recent studies use fate-mapping and limiting dilution transplantation assays to identify SRY (sex determining region Y)-box 2 (Sox2) as cancer stem-cell marker and driver of cancer stemness.

Children are not just little adults: recent advances in understanding of diffuse intrinsic pontine glioma biology

Diffuse intrinsic pontine glioma (DIPG) is a high-grade glioma that originates in the pons and is seen exclusively in children. Despite numerous efforts to improve treatment, DIPG remains incurable with 90% of children dying within 2 y of diagnosis, making it one of the leading causes of death in children with brain tumors. With the advent of new genomic tools, the genetic landscape of DIPG is slowly being unraveled. The most common genetic alterations include a K27M mutation in H3.3 or H3.1, which are found in up to 78% of DIPGs, whereas p53 mutations are found in up to 77%. Other recently discovered alterations include amplification of components of the receptor tyrosine kinase/Ras/phosphatidylinositol 3-kinase signaling pathway, particularly platelet-derived growth factor receptor A. Recapitulating such alterations, genetically engineered DIPG preclinical models have been developed, and DIPG xenograft models have also been established. Both models have strengths and weaknesses but can help with the prioritization of novel agents for clinical trials for children with DIPG. As we move forward, it is important that we continue to study the complex and unique biology of DIPG and develop improved preclinical models to increase our understanding of DIPG pathogenesis, allowing translation into successful therapies in the not too distant future.

PD-0332991, a CDK4/6 inhibitor, significantly prolongs survival in a genetically engineered mouse model of brainstem glioma

Diffuse intrinsic pontine glioma (DIPG) is an incurable tumor that arises in the brainstem of children. To date there is not a single approved drug to effectively treat these tumors and thus novel therapies are desperately needed. Recent studies suggest that a significant fraction of these tumors contain alterations in cell cycle regulatory genes including amplification of the D-type cyclins and CDK4/6, and less commonly, loss of Ink4a-ARF leading to aberrant cell proliferation. In this study, we evaluated the therapeutic approach of targeting the cyclin-CDK-Retinoblastoma (Rb) pathway in a genetically engineered PDGF-B-driven brainstem glioma (BSG) mouse model. We found that PD-0332991 (PD), a CDK4/6 inhibitor, induces cell-cycle arrest in our PDGF-B; Ink4a-ARF deficient model both in vitro and in vivo. By contrast, the PDGF-B; p53 deficient model was mostly resistant to treatment with PD. We noted that a 7-day treatment course with PD significantly prolonged survival by 12% in the PDGF-B; Ink4a-ARF deficient BSG model. Furthermore, a single dose of 10 Gy radiation therapy (RT) followed by 7 days of treatment with PD increased the survival by 19% in comparison to RT alone. These findings provide the rationale for evaluating PD in children with Ink4a-ARF deficient gliomas.

Inhibition of PRC2 activity by a gain-of-function H3 mutation found in pediatric glioblastoma

Sequencing of pediatric gliomas has identified missense mutations Lys27Met (K27M) and Gly34Arg/Val (G34R/V) in genes encoding histone H3.3 (H3F3A) and H3.1 (HIST3H1B). We report that human diffuse intrinsic pontine gliomas (DIPGs) containing the K27M mutation display significantly lower overall amounts of H3 with trimethylated lysine 27 (H3K27me3) and that histone H3K27M transgenes are sufficient to reduce the amounts of H3K27me3 in vitro and in vivo. We find that H3K27M inhibits the enzymatic activity of the Polycomb repressive complex 2 through interaction with the EZH2 subunit. In addition, transgenes containing lysine-to-methionine substitutions at other known methylated lysines (H3K9 and H3K36) are sufficient to cause specific reduction in methylation through inhibition of SET-domain enzymes. We propose that K-to-M substitutions may represent a mechanism to alter epigenetic states in a variety of pathologies.

Abstract 5004: NG2 upregulation and its defective asymmetric distribution in pediatric brainstem glioma and diffuse intrinsic pontine glioma

Introduction:

Pediatric brainstem glioma (BSG) is one of the most difficult cancers to treat accounting for 10-20% of all pediatric central nervous system (CNS) tumors. BSGs may occur throughout the brainstem and are categorized into two main groups: diffuse intrinsic pontine gliomas (DIPGs) and focal brainstem gliomas. DIPGs represent about 80% of BSG and have a peak onset of six to nine years of age. DIPGs invade throughout the pons and may spread to other portions of brainstem. To better understand the pathophysiology of the disease, a genetically engineered (PDGFα-expressing) BSG mouse and a xenograft model have been established.

Results:

We recently reported proteome profiling of DIPG CSF and formalin fixed specimens. To expand our molecular studies of the disease, we generated complete protein profiles of specimens obtained from BSG mouse tumor and aged-matched healthy controls. Neuroglia 2 (NG2), also known as chondroitine sulfate proteoglycan 4 (CSPG4) was selected for further analysis based on its exclusive expression in BSG specimens. Although NG2 has been previously implicated in adult gliomas, its role in pediatric gliomas has not been investigated and currently there are no publications on the role of NG2 in DIPGs. Recently, NG2 expression has been shown to play a significant role in neoplastic transformation of glioma precursor cells. Gliomas are thought to originate from cells including astrocytes, stem cells, and glioma progenitor cells. Glial progenitor cells are specifically important since several groups have reported detection of oligodendrocyte markers including NG2, PDGFRα, and Olig-2 in gliomas.

Our preliminary data shows high expression of NG2 in murine model of brainstem glioma as well as 80% of pediatric DIPG specimens tested. We show that shRNA-mediated knockdown of NG2 reduces cellular migration in vitro. NG2 expression is defective (symmetric) in dividing cells in vitro and in vivo. The defective NG2 expression is consistent with a recent observation in adult high grade gliomas. Injection of NG2 expressing neurospheres (NS) into brainstems of 2 day old mice (P2) results in highly aggressive brainstem tumors resulting in death within 3-7 weeks post-injection. Therefore the NS injected mouse model of brainstem glioma provides a solid model for testing therapeutics and evaluating interventions. Furthermore, we show selective delivery of liposomal nanoparticles to brainstem of our robust BSG mouse model. We also show that nanoparticle-mediated delivery of doxorubicin will induce apoptosis in tumor and not the adjacent normal region.

Conclusion:

We introduce a robust murine model of brainstem glioma that is developed using NG2 expressing cells. High expression of NG2 in a subset of DIPGs and its defective expression may provide novel approaches for treating DIPGs and BSGs.

Citation Format: Sridevi Yadavilli, Madhuri Kambhampati, Oren J. Becher, Tobey MacDonald, Ravi Bellamkonda, Roger J. Paker, Javad Nazarian. NG2 upregulation and its defective asymmetric distribution in pediatric brainstem glioma and diffuse intrinsic pontine glioma. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5004. doi:10.1158/1538-7445.AM2013-5004

Disease control and ototoxicity using intensity-modulated radiation therapy tumor-bed boost for medulloblastoma

PURPOSE

We previously reported excellent local control for treating medulloblastoma with a limited boost to the tumor bed. In order to decrease ototoxicity, we subsequently implemented a tumor-bed boost using intensity-modulated radiation therapy (IMRT), the clinical results of which we report here.

PATIENTS AND METHODS

A total of 33 patients with newly diagnosed medulloblastoma, 25 with standard risk, and 8 with high risk, were treated on an IMRT tumor-bed boost following craniospinal irradiation (CSI). Six standard-risk patients were treated with an institutional protocol with 18 Gy CSI in conjunction with intrathecal iodine-131-labeled monoclonal antibody. The majority of patients received concurrent vincristine and standard adjuvant chemotherapy. Pure-tone audiograms were graded according to National Cancer Institute Common Terminology Criteria for Adverse Events version 3.0.

RESULTS

Median age was 9 years old (range, 4-46 years old). Median follow-up was 63 months. Kaplan-Meier estimates of progression-free survival (PFS) and overall survival (OS) rates for standard-risk patients who received 23.4 or 36 Gy CSI (not including those who received 18 Gy CSI with radioimmunotherapy) were 81.4% and 88.4%, respectively, at 5 years; 5-year PFS and OS rates for high-risk patients were both 87.5%. There were no isolated posterior fossa failures outside of the boost volume. Posttreatment audiograms were available for 31 patients, of whom 6%, at a median follow-up of 19 months, had developed Grade 3 hearing loss.

CONCLUSION

An IMRT tumor-bed boost results in excellent local control while delivering a low mean dose to the cochlea, resulting in a low rate of ototoxicity.

Perifosine and CCI 779 co-operate to induce cell death and decrease proliferation in PTEN-intact and PTEN-deficient PDGF-driven murine glioblastoma

Background

Platelet derived growth factor receptor (PDGFR) activity is deregulated in human GBM due to amplification and rearrangement of the PDGFR-alpha gene locus or overexpression of the PDGF ligand, resulting in the activation of downstream kinases such as phosphatidylinositol 3-kinase (PI3K), Akt, and mammalian target of rapamycin (mTOR). Aberrant PDGFR signaling is observed in approximately 25-30% of human GBMs, which are frequently molecularly classified as the proneural subclass. It would be valuable to understand how PDGFR driven GBMs respond to Akt and mTOR inhibition.

Methodology/Principal Findings

Using genetically engineered PTEN-intact and PTEN-deficient PDGF-driven mouse models of GBM that closely mimic the histology and genetics of the human PDGF subgroup, we investigated the effect of inhibiting Akt and mTOR alone or in combination in vitro and in vivo. We used perifosine and CCI-779 to inhibit Akt and mTOR, respectively. Here, we show in vitro data demonstrating that the most effective inhibition of Akt and mTOR activity in both PTEN-intact and PTEN-null primary glioma cell cultures is obtained when using both inhibitors in combination. We next investigated if the effects we observed in culture could be duplicated in vivo by treating mice with gliomas for 5 days. The in vivo treatments with the combination of CCI-779 and perifosine resulted in decreased Akt and mTOR signaling, which correlated to decreased proliferation and increased cell death independent of PTEN status, as monitored by immunoblot analysis, histology and MRI.

Conclusions/Significance

These findings underline the importance of simultaneously targeting Akt and mTOR to achieve significant down-regulation of the PI3K pathway and support the rationale for testing the perifosine and CCI-779 combination in the human PDGF-subgroup of GBM.

Evidence for and against regional differences in neural stem and progenitor cells of the CNS

Neural stem and progenitor cells (NSCs) give rise to the cellular diversity of the CNS. There is evidence both for and against differences in these cells based on the region of the brain in which they reside. Primary brain tumors mimic many aspects of NSC behavior. Recent data suggest that some of the variability in glioma biology may be, in part, a reflection of regional differences in the NSCs from which they arise. In this issue of Genes & Development, Lee and colleagues (pp. 2317-2329) examine how NF1 regulates NSC proliferation and glial differentiation in the brainstem and cortex of the postnatal mouse brain.

Preclinical evaluation of radiation and perifosine in a genetically and histologically accurate model of brainstem glioma

Brainstem gliomas (BSG) are a rare group of central nervous system tumors that arise mostly in children and usually portend a particularly poor prognosis. We report the development of a genetically engineered mouse model of BSG using the RCAS/tv-a system and its implementation in preclinical trials. Using immunohistochemistry, we found that platelet-derived growth factor (PDGF) receptor alpha is overexpressed in 67% of pediatric BSGs. Based on this observation, we induced low-grade BSGs by overexpressing PDGF-B in the posterior fossa of neonatal nestin tv-a mice. To generate high-grade BSGs, we overexpressed PDGF-B in combination with Ink4a-ARF loss, given that this locus is commonly lost in high-grade pediatric BSGs. We show that the likely cells of origin for these mouse BSGs exist on the floor of the fourth ventricle and cerebral aqueduct. Irradiation of these high-grade BSGs shows that although single doses of 2, 6, and 10 Gy significantly increased the percent of terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL)-positive nuclei, only 6 and 10 Gy significantly induce cell cycle arrest. Perifosine, an inhibitor of AKT signaling, significantly induced TUNEL-positive nuclei in this high-grade BSG model, but in combination with 10 Gy, it did not significantly increase the percent of TUNEL-positive nuclei relative to 10 Gy alone at 6, 24, and 72 hours. Survival analysis showed that a single dose of 10 Gy significantly prolonged survival by 27% (P = 0.0002) but perifosine did not (P = 0.92). Perifosine + 10 Gy did not result in a significantly increased survival relative to 10 Gy alone (P = 0.23). This PDGF-induced BSG model can serve as a preclinical tool for the testing of novel agents.

Large congenital melanotic nevi in an extremity with neurocutaneous melanocytosis

A 14-day-old boy presented with a large congenital melanocytic nevus over his left thigh with approximately 17 satellite nevi distributed over the rest of his skin surface. Six weeks later, he developed generalized tonic-clonic seizures and additional satellite nevi became apparent (n > 20). A subsequent brain magnetic resonance imaging demonstrated right temporal T1 hyperintense signal abnormality. At 4 months of age the patient underwent a lumbar puncture that was normal without evidence of melanocytes or tumor. Nevertheless, a few days later he underwent resection of his right medial temporal lesion which demonstrated melanocytosis in the temporal lobe as well as melanocytosis in subependymal areas in other parts of the brain and ventricles, confirming the suspected diagnosis of neurocutaneous melanocytosis. Our case supports previous studies that conclude that the number of satellite nevi is a greater predictor of neurocutaneous melanocytosis than is the location of large congenital melanocytic nevus. In our case, cerebrospinal fluid studies were not reliable even in the face of florid neurocutaneous melanocytosis involving the leptomeninges and ventricles.

IGFBP2 is overexpressed by pediatric malignant astrocytomas and induces the repair enzyme DNA-PK

To identify targets critical to malignant childhood astrocytoma, we compared the expression of receptor tyrosine kinase- associated genes between low-grade and high-grade pediatric astrocytomas. The highest differentially overexpressed gene in high-grade astrocytoma is insulin-like growth factor- binding protein-2 (P = .0006). Immunohistochemistry confirmed overexpression of insulin-like growth factor-binding protein-2 protein (P = .027). Insulin-like growth factor- binding protein-2 stimulation had no effect on astrocytoma cell growth and migration, and minimally inhibited insulin-like growth factor-1-mediated migration, but not insulin-like growth factor-2-mediated migration. However, insulin-like growth factor-binding protein-2 stimulation significantly upregulated the major DNA repair enzyme gene, DNA-PKcs, and induced DNA-dependent protein kinase catalytic subunit protein expression in a time-dependent and dose-dependent manner, whereas insulin-like growth factor-1 had no effect. DNA-PKcs is also highly overexpressed by high-grade astrocytomas. These findings suggest insulin-like growth factor-binding protein-2 plays a role in astrocytoma progression by promoting DNA-damage repair and therapeutic resistance.

Gli activity correlates with tumor grade in platelet-derived growth factor-induced gliomas

Gli signaling is critical for central nervous system development and is implicated in tumorigenesis. To monitor Gli signaling in gliomas in vivo, we created platelet-derived growth factor-induced gliomas in a Gli-luciferase reporter mouse. We find that Gli activation is found in gliomas and correlates with grade. In addition, we find that sonic hedgehog (SHH) is expressed in these tumors and also correlates with grade. We identify microvascular proliferation and pseudopalisades, elements that define high-grade gliomas as SHH-producing microenvironments. We describe two populations of SHH-producing stromal cells that reside in perivascular niche (PVN), namely low-cycling astrocytes and endothelial cells. Using the Ptc-LacZ knock-in mouse as a second Gli responsive reporter, we show beta-galactosidase activity in the PVN and in some tumors diffusely throughout the tumor. Lastly, we observe that SHH is similarly expressed in human gliomas and note that an intact tumor microenvironment or neurosphere conditions in vitro are required for Gli activity.

PI3K pathway regulates survival of cancer stem cells residing in the perivascular niche following radiation in medulloblastoma in vivo

Medulloblastomas are brain tumors that arise in the cerebellum of children and contain stem cells in a perivascular niche thought to give rise to recurrence following radiation. We used several mouse models of medulloblastomas in parallel to better understand how the critical cell types in these tumors respond to therapy. In our models, the proliferating cells in the tumor bulk undergo radiation-induced, p53-dependent apoptotic cell death. Activation of Akt signaling via PTEN loss transforms these cells to a nonproliferating extensive nodularity morphology. By contrast, the nestin-expressing perivascular stem cells survive radiation, activate PI3K/Akt pathway, undergo p53-dependent cell cycle arrest, and re-enter the cell cycle at 72 h. Furthermore, the ability of these cells to induce p53 is dependent on the presence of PTEN. These cellular characteristics are similar to human medulloblastomas. Finally, inhibition of Akt signaling sensitizes cells in the perivascular region to radiation-induced apoptosis.

Cancer stem cells and survival pathways

Gliomas and medulloblastomas are the most frequent malignant brain tumors in adult and children respectively. Although both tumors arise in the CNS, there is a significant difference in their therapeutic response. Medulloblastomas are relatively curable, while glioblastomas are basically incurable. During the last decade several reports have demonstrated the existence of cancer stem cells in brain tumors, their location and their response to treatment. We have recently described the therapeutic response of medulloblastomas to radiation in their native microenvironment, illustrating how p53 and Pi3K signaling pathways lead to the evasion of cell death by the nestin-expressing cells in the perivascular stem cell niche, even while the bulk of tumor succumbs to apoptosis.(1) It remains to be determined whether this mechanism of tumor resistance applies to the more complex stem-cell niche and tumor bulk of gliomas.

Genetically engineered models have advantages over xenografts for preclinical studies

Mouse models of human cancer are valuable tools for cancer research. Although xenografts and genetically engineered models (GEMs) possess limitations as well as advantages, each system plays a significant role in preclinical testing. Tumor xenografts are easy to use, relatively inexpensive, and reproducible. The main drawback of xenografts is that the genetics and histology of the tumors are frequently not representative of the respective human tumor and, thus far, these models have not been as predictive of therapeutic success as one would like. By contrast, GEMs are histologically and genetically accurate models of human cancer but have disadvantages of heterogeneity with regard to frequency, latency, and growth. These disadvantages are reminiscent of the variable behavior of actual human tumors. Recently, these shortcomings have been partly overcome with the development of anatomic and molecular in vivo imaging techniques such as magnetic resonance imaging and bioluminescence imaging. These new technologies will hopefully support the use of GEMs in preclinical trials and help determine if trials in GEMs are more predicative than xenografts of human responses.

Reasons for pediatrician nonadherence to asthma guidelines

BACKGROUND

The 1997 National Heart, Lung, and Blood Institute (NHLBI) asthma guidelines include recommendations on how to improve the quality of care for asthma.

OBJECTIVE

To identify barriers to physician adherence to the NHLBI guidelines.

DESIGN

Cross-sectional survey.

PARTICIPANTS

A national random sample of 829 primary care pediatricians.

MAIN OUTCOME MEASURES

Self-reported adherence to 4 components of the NHLBI guidelines (steroid prescription, instructing peak flow meter use, screening and counseling patients with asthma for smoking, and screening and counseling parents for smoking). We also collected information on physician demographics, practice characteristics, and possible barriers to adherence. We defined adherence as following a guideline component more than 90% of the time.

RESULTS

The response rate was 55% (456/829). Most of the responding pediatricians were aware of the guidelines (88%) and reported having access to a copy of the guidelines (81%). Self-reported rates of adherence were between 39% and 53% for the guideline components. After controlling for demographics and other barriers, we found that nonadherence was associated with specific barriers for each guideline component: for corticosteroid prescription, lack of agreement (odds ratio [OR], 6.8; 95% confidence interval [CI], 3.2-14.4); for peak flow meter use, lack of self-efficacy (OR, 3.4; 95% CI, 1.9-6.1) and lack of outcome expectancy (OR, 4.7; 95% CI, 2.5-8.9); and for screening and counseling of patients and parents for smoking, lack of self-efficacy (OR, 3.8; 95% CI, 1.7-6.2 and OR, 2.8; 95% CI, 1.3-5.9, respectively).

CONCLUSIONS

Although pediatricians in this sample were aware of the NHLBI guidelines, a variety of barriers precluded their successful use. To improve NHLBI guideline adherence, tailored interventions that address the barriers characteristic of a given guideline component need to be implemented.

Patellofemoral complications after total knee arthroplasty: a comparison of Modular Porous-Coated Anatomic with Duracon prostheses

Clinical and radiographic analyses were used to compare results obtained in 45 patients who underwent a Modular Porous-Coated Anatomic total knee arthroplasty with results in a similar group of 45 patients who underwent a Duracon total knee arthroplasty. The surgeries were consecutively performed over an 8-month period, and follow-up evaluation averaged 63 months. The two patient groups were similar with regard to the following variables: age, sex, diagnosis, height, weight, and preoperative and postoperative activity levels. Whereas both Duracon and Modular total knee arthroplasty prostheses yielded similar results approximately 5 years after surgery, the data suggest that Duracon appears to minimize patellofemoral complications.