Medulloblastoma: therapy and biologic considerations

Erbb4 Is Required for Cerebellar Developmentand Malignant Phenotype of Medulloblastoma

Medulloblastoma is the most common and malignant pediatric brain tumor in childhood. It originates from dysregulation of cerebellar development, due to an excessive proliferation of cerebellar granule neuron precursor cells (CGNPs). The underlying molecular mechanisms, except for the role of SHH and WNT pathways, remain largely unknown. ERBB4 is a tyrosine kinase receptor whose activity in cancer is tissue dependent. In this study, we characterized the role of ERBB4 during cerebellum development and medulloblastoma progression paying particular interests to its role in CGNPs and medulloblastoma stem cells (MBSCs). Our results show that ERBB4 is expressed in the CGNPs during cerebellum development where it plays a critical role in migration, apoptosis and differentiation. Similarly, it is enriched in the population of MBSCs, where also controls those critical processes, as well as self-renewal and tumor initiation for medulloblastoma progression. These results are translated to clinical samples where high levels of ERBB4 correlate with poor outcome in Group 4 and all medulloblastomas groups. Transcriptomic analysis identified critical processes and pathways altered in cells with knock-down of ERBB4. These results highlight the impact and underlying mechanisms of ERBB4 in critical processes during cerebellum development and medulloblastoma.

Bax deficiency prolongs cerebellar neurogenesis, accelerates medulloblastoma formation and paradoxically increases both malignancy and differentiation

Neurogenesis requires negative regulation through differentiation of progenitors or their programmed cell death (PCD). Growth regulation is particularly important in the postnatal cerebellum, where excessive progenitor proliferation promotes medulloblastoma, the most common malignant brain tumor in children. We present evidence that PCD operates alongside differentiation to regulate cerebellar granule neuron progenitors (CGNPs) and to prevent medulloblastoma.

Here we show that genetic deletion of pro-apoptotic Bax disrupts regulation of cerebellar neurogenesis and promotes medulloblastoma formation. In Bax^−/− mice, the period of neurogenesis was extended into the third week of postnatal life, and ectopic neurons and progenitors collected in the molecular layer of the cerebellum and adjacent tectum. Importantly, genetic deletion of Bax in medulloblastoma-prone ND2:SmoA1 transgenic mice greatly accelerated tumorigenesis. Bax-deficient medulloblastomas exhibited strikingly distinct pathology, with reduced apoptosis, increased neural differentiation and tectal migration. Comparing Bax^+/+ and Bax^−/− medulloblastomas, we were able to identify up-regulation of Bcl-2 and nuclear exclusion of p27 as tumorigenic changes that are required to mitigate the tumor suppressive effect of Bax. Studies on human tumors confirmed the importance of modulating Bax in medulloblastoma pathogenesis.

Our results demonstrate that Bax-dependent apoptosis regulates postnatal cerebellar neurogenesis, suppresses medulloblastoma formation, and imposes selective pressure on tumors that form. Functional resistance to Bax-mediated apoptosis, required for medulloblastoma tumorigenesis, may be a tumor-specific vulnerability to be exploited for therapeutic benefit.

Extracellular matrix metalloproteinase inducer is a negative prognostic factor of pediatric medulloblastoma

Medulloblastoma (MB) is the most common embryonal CNS tumor of childhood. Its survival rates have significantly improved over the years due to developments in diagnostic techniques and therapeutic strategies. However, it is still an important cause of cancer-related deaths in children. Extracellular matrix metalloproteinase inducer (EMMPRIN) is a member of the immunoglobulin family and a glycoprotein enriched on the surface of many types of tumor cells. Therefore, the aim of this study was to investigate whether the expression patterns of EMMPRIN may predict the clinical prognosis in MB. EMMPRIN expression in a series of 56 MB with various grades and pathological types was analyzed by immunohistochemical staining on paraffin-embedded sections. Then, the correlation of EMMPRIN expression patterns with clinical-pathological features of patients and its prognostic relevance were determined. Immunohistochemistry revealed that the positive expression rate of EMMPRIN in MB (75.0%, 42/56) was significantly higher than that in normal cerebellums (6.7%, 2/30, p < 0.001). In addition, EMMPRIN expression in MB was up-regulated in higher metastatic stage (p < 0.01), aggressive histopathological type (p < 0.005), necrosis (p < 0.01), as well as with undifferentiated tumor (p < 0.01). Furthermore, over-expression of EMMPRIN correlated significantly with poor prognosis (0.01 < p < 0.05) and represented an independent prognostic marker of overall survival on multivariate analysis (p = 0.01). Our study suggests that EMMPRIN expression was associated with the progression of MB and its over-activation may be an important predictor of poor survival in this patient cohort. Therefore, EMMPRIN may be regarded both as a prognostic factor and a therapeutic target for MB.

Chemokines in neuroectodermal cancers: the crucial growth signal from the soil

Although chemokines and their receptors were initially identified as regulators of cell trafficking during inflammation and immune response, they have emerged as crucial players in all stages of tumor development, primary growth, migration, angiogenesis, and establishment as metastases in distant target organs. Neuroectodermal tumors regroup neoplasms originating from the embryonic neural crest cells, which display clinical and biological similarities. These tumors are highly malignant and rapidly progressing diseases that disseminate to similar target organs such as bone marrow, bone, liver and lungs. There is increasing evidence that interaction of several chemokine receptors with corresponding chemokine ligands are implicated in the growth and invasive characteristics of these tumors. In this review we summarize the current knowledge on the role of CXCL12 chemokine and its CXCR4 and CXCR7 receptors in the progression and survival of neuroectodermal tumors, with particular emphasis on neuroblastoma, the most typical and enigmatic neuroectodermal childhood tumor.

Medulloblastoma in childhood: new biological advances

Medulloblastoma is the most common embryonal tumour in children. Patients with medulloblastoma are currently staged as average-risk or poor-risk on the basis of clinical findings. With current multimodality therapy, nearly 90% of children with average-risk, non-disseminated medulloblastoma have 5-year event-free survival, and those with high-risk disease have a 60-65% survival rate; however, the outcome for younger children, particularly infants, is worse. Children who survive medulloblastoma are at risk of long-term sequelae related to the neurological effects of the tumour, surgery, or radiotherapy, and the additive effects of chemotherapy. Molecular biology has changed our understanding of medulloblastoma and has implications for diagnostic stratification and treatment. As newer biological agents are translated from the lab to the bedside, clinicians need to understand the fundamental signalling pathways that are targeted during therapy. Greater understanding of the molecular biology of medulloblastoma is needed so that more children can be cured or have an improved quality of life.