MicroRNAs (miRNA) are crucial post‐transcriptional regulators of gene expression and control cell differentiation and proliferation. However, little is known about their targeting of specific developmental pathways. Hedgehog (Hh) signalling controls cerebellar granule cell progenitor development and a subversion of this pathway leads to neoplastic transformation into medulloblastoma (MB). Using a miRNA high‐throughput profile screening, we identify here a downregulated miRNA signature in human MBs with high Hh signalling. Specifically, we identify miR‐125b and miR‐326 as suppressors of the pathway activator Smoothened together with miR‐324‐5p, which also targets the downstream transcription factor Gli1. Downregulation of these miRNAs allows high levels of Hh‐dependent gene expression leading to tumour cell proliferation. Interestingly, the downregulation of miR‐324‐5p is genetically determined by MB‐associated deletion of chromosome 17p. We also report that whereas miRNA expression is downregulated in cerebellar neuronal progenitors, it increases alongside differentiation, thereby allowing cell maturation and growth inhibition. These findings identify a novel regulatory circuitry of the Hh signalling and suggest that misregulation of specific miRNAs, leading to its aberrant activation, sustain cancer development.

Medulloblastoma is an aggressive brain malignancy with high incidence in childhood. Current treatment approaches have limited efficacy and severe side effects. Therefore, new risk‐adapted therapeutic strategies based on molecular classification are required. MicroRNA expression analysis has emerged as a powerful tool to identify candidate molecules playing an important role in a large number of malignancies. However, no data are yet available on human primary medulloblastomas. A high throughput microRNA expression profiles was performed in human primary medulloblastoma specimens to investigate microRNA involvement in medulloblastoma carcinogenesis. We identified specific microRNA expression patterns which distinguish medulloblastoma differing in histotypes (anaplastic, classic and desmoplastic), in molecular features (ErbB2 or c‐Myc overexpressing tumors) and in disease‐risk stratification. MicroRNAs expression profile clearly differentiates medulloblastoma from either adult or fetal normal cerebellar tissues. Only a few microRNAs displayed upregulated expression, while most of them were downregulated in tumor samples, suggesting a tumor growth‐inhibitory function. This property has been addressed for miR‐9 and miR‐125a, whose rescued expression promoted medulloblastoma cell growth arrest and apoptosis while targeting the proproliferative truncated TrkC isoform. In conclusion, misregulated microRNA expression profiles characterize human medulloblastomas, and may provide potential targets for novel therapeutic strategies.

Hedgehog (Hh) pathway has a pivotal function in development and tumorigenesis, processes sustained by stem cells (SCs). The transcription factor Nanog controls stemness acting as a key determinant of both embryonic SC self‐renewal and differentiated somatic cells reprogramming to pluripotency, in concert with the loss of the oncosuppressor p53. How Nanog is regulated by microenvironmental signals in postnatal SC niches has been poorly investigated. Here, we show that Nanog is highly expressed in SCs from postnatal cerebellum and medulloblastoma, and acts as a critical mediator of Hh‐driven self‐renewal. Indeed, the downstream effectors of Hh activity, Gli1 and Gli2, bind to Nanog ‐specific cis ‐regulatory sequences both in mouse and human SCs. Loss of p53, a key event promoting cell stemness, activates Hh signalling, thereby contributing to Nanog upregulation. Conversely, Hh downregulates p53 but does not require p53 to control Nanog . Our data reveal a mechanism for the function of Hh in the control of stemness that represents a crucial component of an integrated circuitry determining cell fate decision and involved in the maintenance of cancer SCs. There is a [Have you seen ...?][1] (August 2010) associated with this Article. [1]: http://dx.doi.org/10.1038/emboj.2010.162

Hedgehog signaling is essential for tissue development and stemness, and its deregulation has been observed in many tumors. Aberrant activation of Hedgehog signaling is the result of genetic mutations of pathway components or other Smo‐dependent or independent mechanisms, all triggering the downstream effector Gli1. For this reason, understanding the poorly elucidated mechanism of Gli1‐mediated transcription allows to identify novel molecules blocking the pathway at a downstream level, representing a critical goal in tumor biology. Here, we clarify the structural requirements of the pathway effector Gli1 for binding to DNA and identify Glabrescione B as the first small molecule binding to Gli1 zinc finger and impairing Gli1 activity by interfering with its interaction with DNA. Remarkably, as a consequence of its robust inhibitory effect on Gli1 activity, Glabrescione B inhibited the growth of Hedgehog‐dependent tumor cells in vitro and in vivo as well as the self‐renewal ability and clonogenicity of tumor‐derived stem cells. The identification of the structural requirements of Gli1/DNA interaction highlights their relevance for pharmacologic interference of Gli signaling. ![][1] Structural features of the Gli1 zinc finger binding to DNA are explored to characterise Glabrescione B as novel and effective Hh‐inhibitor in Gli1‐dependent tumors. [1]: pending:yes