Abstract
Pediatric high-grade gliomas of the subclass MYCN (HGG-MYCN) are highly aggressive tumors frequently carrying
MYCN
amplifications,
TP53
mutations, or both alterations. Due to their rarity, ...such tumors have only recently been identified as a distinct entity, and biological as well as clinical characteristics have not been addressed specifically. To gain insights into tumorigenesis and molecular profiles of these tumors, and to ultimately suggest alternative treatment options, we generated a genetically engineered mouse model by breeding
hGFAP-cre::Trp53
Fl/Fl
::lsl-MYCN
mice. All mice developed aggressive forebrain tumors early in their lifetime that mimic human HGG-MYCN regarding histology, DNA methylation, and gene expression. Single-cell RNA sequencing revealed a high intratumoral heterogeneity with neuronal and oligodendroglial lineage signatures. High-throughput drug screening using both mouse and human tumor cells finally indicated high efficacy of Doxorubicin, Irinotecan, and Etoposide as possible therapy options that children with HGG-MYCN might benefit from.
Recurrent mutations in chromatin modifiers are specifically prevalent in adolescent or adult patients with Sonic hedgehog-associated medulloblastoma (SHH MB). Here, we report that mutations in the ...acetyltransferase CREBBP have opposing effects during the development of the cerebellum, the primary site of origin of SHH MB. Our data reveal that loss of Crebbp in cerebellar granule neuron progenitors (GNPs) during embryonic development of mice compromises GNP development, in part by downregulation of brain-derived neurotrophic factor (Bdnf). Interestingly, concomitant cerebellar hypoplasia was also observed in patients with Rubinstein-Taybi syndrome, a congenital disorder caused by germline mutations of CREBBP. By contrast, loss of Crebbp in GNPs during postnatal development synergizes with oncogenic activation of SHH signaling to drive MB growth, thereby explaining the enrichment of somatic CREBBP mutations in SHH MB of adult patients. Together, our data provide insights into time-sensitive consequences of CREBBP mutations and corresponding associations with human diseases.
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•CREBBP mutations in human medulloblastoma deplete acetyltransferase activity•Crebbp mutations exert opposing effects during cerebellar development•Embryonal loss of Crebbp impairs normal cerebellar development•Postnatal loss of Crebbp synergizes with Shh to drive growth of medulloblastoma
Merk et al. show that the developmental time frame of CREBBP mutation acquisition in cerebellar granule neurons determines the pathogenic effect of these alterations in the cerebellum. These time-sensitive consequences explain phenotypic differences seen in patients with germline (Rubinstein-Taybi syndrome) or somatic mutations (adult SHH medulloblastoma) of CREBBP.
Group 3 medulloblastoma is one of the most aggressive types of childhood brain tumors. Roughly 30% of cases carry genetic alterations in MYC, SMARCA4, or both genes combined. While overexpression of ...MYC has previously been shown to drive medulloblastoma formation in mice, the functional significance of SMARCA4 mutations and their suitability as a therapeutic target remain largely unclear. To address this issue, we combined overexpression of MYC with a loss of SMARCA4 in granule cell precursors. Both alterations did not increase proliferation of granule cell precursors in vitro. However, combined MYC overexpression and SMARCA4 loss successfully induced tumor formation in vivo after orthotopic transplantation in recipient mice. Resulting tumors displayed anaplastic histology and exclusively consisted of SMARCA4-negative cells although a mixture of recombined and non-recombined cells was injected. These observations provide first evidence for a tumor-promoting role of a SMARCA4 deficiency in the development of medulloblastoma. In comparing the transcriptome of tumors to the cells of origin and an established Sonic Hedgehog medulloblastoma model, we gathered first hints on deregulated gene expression that could be specifically involved in SMARCA4/MYC driven tumorigenesis. Finally, an integration of RNA sequencing and DNA methylation data of murine tumors with human samples revealed a high resemblance to human Group 3 medulloblastoma on the molecular level. Altogether, the development of SMARCA4-deficient medulloblastomas in mice paves the way to deciphering the role of frequently occurring SMARCA4 alterations in Group 3 medulloblastoma with the perspective to explore targeted therapeutic options.
The gene of the Epidermal growth factor receptor (EGFR) is one of the most frequently altered genes in glioblastoma (GBM), with deletions of exons 2-7 (EGFRvIII) being amongst the most common genomic ...mutations. EGFRvIII is heterogeneously expressed in GBM. We already showed that EGFRvIII expression has an impact on chemosensitivity, replication stress, and the DNA damage response. Wee1 kinase is a major regulator of the DNA damage induced G2 checkpoint. It is highly expressed in GBM and its overexpression is associated with poor prognosis. Since Wee1 inhibition can lead to radiosensitization of EGFRvIII-negative (EGFRvIII-) GBM cells, we asked, if Wee1 inhibition is sufficient to radiosensitize also EGFRvIII-positive (EGFRvIII+) GBM cells.
We used the clinically relevant Wee1 inhibitor adavosertib and two pairs of isogenetic GBM cell lines with and without endogenous EGFRvIII expression exhibiting different TP53 status. Moreover, human GBM samples displaying heterogenous EGFRvIII expression were analyzed. Expression of Wee1 was assessed by Western blot and respectively immunohistochemistry. The impact of Wee1 inhibition in combination with irradiation on cell cycle and cell survival was analyzed by flow cytometry and colony formation assay.
Analysis of GBM cells and patient samples revealed a higher expression of Wee1 in EGFRvIII+ cells compared to their EGFRvIII- counterparts. Downregulation of EGFRvIII expression by siRNA resulted in a strong decrease in Wee1 expression. Wee1 inhibition efficiently abrogated radiation-induced G2-arrest and caused radiosensitization, without obvious differences between EGFRvIII- and EGFRvIII+ GBM cells.
We conclude that the inhibition of Wee1 is an effective targeting approach for the radiosensitization of both EGFRvIII- and EGFRvIII+ GBM cells and may therefore represent a promising new therapeutic option to increase response to radiotherapy.
The zinc-finger transcription factor GLI2 is frequently amplified in childhood medulloblastoma of the Sonic-hedgehog type (SHH-MB), with or without amplification of NMYC or deletion of TP53. Despite ...the aggressive tumor behavior, tumorigenesis is not well understood, and adequate mouse models are lacking. Therefore, we generated mice with a GLI2 overexpression under control of the hGFAP-promoter. These mice died within 150 days. The majority only survived until postnatal day 40. They displayed severe cerebellar hypoplasia, cortical malformations, but no brain tumors, except for one out of 23 animals with an undifferentiated hindbrain lesion. Additional loss of p53 did not result in cerebellar tumors, but partially rescued the cerebellar phenotype induced by GLI2 overexpression. Similarly, the combination of GLI2 and NMYC was neither sufficient for the development of SHH-MB. We therefore assume that the development of childhood SHH-MB in mice is either occurring in cellular origins outside the hGFAP-positive lineage or needs additional genetic drivers.
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•Childhood SHH-medulloblastoma shows amplification of Gli2 and/or NMYC or deletion of TP53•Mice with Gli2 overexpression show cerebellar malformations or rare brain tumors•Further p53Fl/Fl loss or NMYCFl/Fl expression affects cerebellar development and survival•Additional alterations did not lead to MB
Biological sciences; Molecular biology; Cancer
The tumor suppressor and chromatin modifier cAMP response element-binding protein binding protein (CREBBP) and v-myc avian myelocytomatosis viral oncogene neuroblastoma derived homolog (MYCN), a ...member of the MYC oncogene family, are critically involved in brain development. Both genes are frequently mutated in the same tumor entities, including high-grade glioma and medulloblastoma. Therefore, we hypothesized that alterations in both genes cooperate to induce brain tumor formation. For further investigation, hGFAP-cre::Crebbp
::lsl-MYCN mice were generated, which combine Crebbp deletion with overexpression of MYCN in neural stem cells (NSCs). Within eight months, these animals developed aggressive forebrain tumors. The first tumors were detectable in the olfactory bulbs of seven-day-old mice. This location raises the possibility that presumptive founder cells are derived from the ventricular-subventricular zone (V-SVZ). To examine the cellular biology of these tumors, single-cell RNA sequencing was performed, which revealed high intratumoral heterogeneity. Data comparison with reference CNS cell types indicated the highest similarity of tumor cells with transit-amplifying NSCs or activated NSCs of the V-SVZ. Consequently, we analyzed V-SVZ NSCs of our mouse model aiming to confirm that the tumors originate from this stem cell niche. Mutant V-SVZ NSCs showed significantly increased cell viability and proliferation as well as reduced glial and neural differentiation in vitro compared to control cells. In summary, we demonstrate the oncogenic potential of a combined loss of function of CREBBP and overexpression of MYCN in this cell population. hGFAP-cre::Crebbp
::lsl-MYCN mice thus provide a valuable tool to study tumor-driving mechanisms in a key neural stem/ progenitor cell niche.
CREB (cyclic AMP response element binding protein) binding protein (CBP, CREBBP) is a ubiquitously expressed transcription coactivator with intrinsic histone acetyltransferase (KAT) activity. ...Germline mutations within the CBP gene are known to cause Rubinstein-Taybi syndrome (RSTS), a developmental disorder characterized by intellectual disability, specific facial features and physical anomalies. Here, we investigate mechanisms of CBP function during brain development in order to elucidate morphological and functional mechanisms underlying the development of RSTS. Due to the embryonic lethality of conventional CBP knockout mice, we employed a tissue specific knockout mouse model (hGFAP-cre::CBP
, mutant mouse) to achieve a homozygous deletion of CBP in neural precursor cells of the central nervous system.Our findings suggest that CBP plays a central role in brain size regulation, correct neural cell differentiation and neural precursor cell migration. We provide evidence that CBP is both important for stem cell viability within the ventricular germinal zone during embryonic development and for unhindered establishment of adult neurogenesis. Prominent histological findings in adult animals include a significantly smaller hippocampus with fewer neural stem cells. In the subventricular zone, we observe large cell aggregations at the beginning of the rostral migratory stream due to a migration deficit caused by impaired attraction from the CBP-deficient olfactory bulb. The cerebral cortex of mutant mice is characterized by a shorter dendrite length, a diminished spine number, and a relatively decreased number of mature spines as well as a reduced number of synapses.In conclusion, we provide evidence that CBP is important for neurogenesis, shaping neuronal morphology, neural connectivity and that it is involved in neuronal cell migration. These findings may help to understand the molecular basis of intellectual disability in RSTS patients and may be employed to establish treatment options to improve patients' quality of life.
Almost all medulloblastomas (MB) of the Wingless/Int-1 (WNT) type are characterized by hotspot mutations in
, and mouse models have convincingly demonstrated the tumor-initiating role of these ...mutations. Additional alterations in
are detected in ∼20% of WNT MB, but their functional role is mostly unknown. We, therefore, amended previously described brain lipid binding protein (
)
mice by the introduction of floxed
alleles. Unexpectedly, mutated and thereby stabilized β-catenin on its own induced severe developmental phenotypes in male and female
mice in our hands, including a thinned cerebral cortex, hydrocephalus, missing cerebellar layering, and cell accumulations in the brainstem and cerebellum. An additional loss of SMARCA4 even resulted in prenatal death for most mice. Respective
mutants (male and female) developed large proliferative lesions in the cerebellum evolving from E13.5 to E16.5. Histological and molecular analysis of these lesions by DNA methylation profiling and single-cell RNA sequencing suggested an origin in early undifferentiated SOX2-positive cerebellar progenitors. Furthermore, upregulated WNT signaling, altered actin/cytoskeleton organization, and reduced neuronal differentiation were evident in mutant cells. In vitro, cells harboring alterations in both
and
were negatively selected and did not show tumorigenic potential after transplantation in adult female recipient mice. However, in cerebellar explant cultures, mutant cells displayed significantly increased proliferation, suggesting an important role of the embryonic microenvironment in the development of lesions. Altogether, these results represent an important first step toward the unraveling of tumorigenic mechanisms induced by aberrant WNT signaling and SMARCA4 deficiency.
Abstract
Group 3 medulloblastoma is one of the most aggressive types of childhood brain tumors. Roughly 30 % of cases carry genetic alterations in MYC, SMARCA4 or both genes combined. While ...overexpression of MYC has previously been shown to drive medulloblastoma formation in mice, the functional significance of SMARCA4 mutations and their suitability as a therapeutic target remain largely unclear. To address this issue, we combined an overexpression of MYC with a loss of SMARCA4 in cerebellar granule cell precursors. Cells were isolated from 7-day-old Math1-creERT2::Smarca4fl/fl pups after tamoxifen-induced loss of SMARCA4. Subsequently, MYC overexpression was achieved by lentiviral transduction, and transduced cells were transplanted into immunodeficient CD1nu/nu recipient mice. Preliminary results show tumor formation in 5/19 transplanted mice (26 %) after 6 months. SMARCA4 loss in all tumor cells was confirmed both immunohistochemically and on a genetic level and suggests a dependency of tumor growth on SMARCA4 loss. In a next step, additional cohorts will clarify if tumor development is accelerated by or even dependent on the loss of SMARCA4 in our model. Additionally, the neoplastic potential of tumor cells will be verified with the aid of secondary recipient mice. To evaluate to what extent the generated tumors are comparable to human Group 3 medulloblastomas, tumors will be extensively analyzed on a morphological, transcriptional, and epigenetic level. Altogether, we hope to establish a suitable mouse model for SMARCA4 mutated Group 3 medulloblastoma that will help to elucidate the role of SMARCA4 in tumor development and to identify new therapeutic targets.
Abstract
Almost all medulloblastomas (MB) of the WNT subgroup are characterized by hotspot mutations in CTNNB1, and mouse models have convincingly demonstrated the tumor-initiating role of these ...mutations as well as the tumor origin in the dorsal brain stem. Around 20 % of WNT MB additionally carry SMARCA4mutations, but the functional role of these alterations is mostly unknown. We therefore amended previously described Blbp-cre::Ctnnb1(ex3)Fl/+mice by the introduction of a floxed Smarca4 allele. In contrast to existing literature, Blbp-cre::Ctnnb1(ex3)Fl/+ mice had a maximum life span of only 17 days, even after breeding into two different genetic backgrounds (C57BL/6J and 129S2/Sv). The mice displayed a severe developmental phenotype including a thinned cerebral cortex, hydrocephalus, missing cerebellar foliation and layering as well as non-proliferative cell accumulations in brain stem and cerebellum. An additional homozygous loss of SMARCA4 even resulted in prenatal death for most mice and caused big proliferative lesions in the cerebellum at embryonal day 14.5. These lesions appear to originate from SOX2-positive progenitor cells in the cerebellar ventricular zone. In a next experiment, cells isolated from this region will be characterized in vitro and will be transplanted orthotopically to evaluate their neoplastic potential in vivo. Altogether, we hope to elucidate how a loss of SMARCA4 and mutations of Ctnnb1 cooperate during hindbrain development and tumor formation within this region.