Summary Paragraph Medulloblastoma is a highly malignant paediatric brain tumour currently treated with a combination of surgery, radiation, and chemotherapy, posing a considerable burden of toxicity to the developing child. Genomics has illuminated the extensive intertumoural heterogeneity of medulloblastoma, identifying four distinct molecular subgroups. Group 3 and Group 4 subgroup medulloblastomas account for the majority of paediatric cases; yet, oncogenic drivers for these subtypes remain largely unidentified. Here we describe a series of prevalent, highly disparate genomic structural variants, restricted to Groups 3 and 4, resulting in specific and mutually exclusive activation of the growth factor independent 1 family protooncogenes, GFI1 and GFI1B. Somatic structural variants juxtapose GFI1/GFI1B coding sequences proximal to active enhancer elements, including super-enhancers, instigating oncogenic activity. Our results, supported by evidence from mouse models, identify GFI1 and GFI1B as prominent medulloblastoma oncogenes and implicate ‘enhancer hijacking’ as an efficient mechanism driving oncogene activation in a childhood cancer.

The interleukin-2 (IL-2) receptor (IL-2R) is composed of three subunits. Of these, IL-2R␣ is required for high-affinity IL-2 binding, while IL

Growth factors elicit their biological effects by activating a complex network of receptors and signaling pathways. Activation of transmembrane tyrosine kinases by serum or polypeptide growth factors results in the transit of cells through the G 1 phase of the cell cycle into S-phase. Several lines of evidence suggest that the D-type cyclins and their associated kinases (Cdks) 1 are among the targets of these growth signals (1). The D-type cyclins, D1, D2 and D3, are closely related proteins whose expression is induced by mitogens and growth factors (2-6) and down-regulated by growth factor deprivation or by antimitogens (7,8). The D-type cyclins associate with cyclindependent protein kinase Cdk4 or Cdk6 to form an active complex that phosphorylates and inactivates the retinoblastoma protein, pRb (9, 10). Inhibition of cyclin D1 expression either by antisense methodology or antibody microinjection lengthens the duration of the G 1 phase and causes a reduction in proliferation (11,12). Aberrant overexpression of D-type cyclins resulting from upstream growth factor receptor activation, gene amplification or rearrangement, or an increase in mRNA stability seems to be a common feature of a number of human cancers and may reduce the cell's dependence on physiologic growth stimuli (13-16).Changes in cyclin D expression integrate the proliferative effects of an array of extracellular factors, including cytokines, polypeptide growth factors, and steroid hormones (2-4, 7). Cellular stress results in the loss of cyclin D1 expression, with a concomitant arrest in the G 1 phase of the cell cycle (8, 17). The networks of pathways responsible for the transduction of these signals are complex and not completely understood. There is some evidence suggesting that a Ras-and MAP kinasedependent signaling pathway is involved. Expression of activated Ras is associated with the increased expression of cyclin D1 in both epithelial cells (12) and fibroblasts (11). Moreover, in the absence of growth factors, activation of the Raf1 3 MEK 3 MAP kinase pathway has been shown to be sufficient to induce cyclin D1 transcription (5). Herbimycin A is a natural product that binds to a specific site in Hsp90 and causes the degradation of transmembrane tyrosine protein kinases, Raf1, and steroid hormone receptors (18 -23). We found that treatment of tumor cells with this drug causes a decrease in the expression of D-type cyclins and an Rb-dependent G 1 block. 2 We report here that the reduction in the level of D-type cyclins induced by herbimycin A is due to inhibition of translation of cyclin D mRNAs. Furthermore, the increase in the level of D-type cyclins in cells treated with serum is due to an increase in the translation of their mRNAs. These effects are due to the regulation of a PI 3-kinase/Akt kinase-dependent, Raf1-and MAP kinase-independent pathway. This pathway is activated by serum and is blocked by the drug herbimycin A. EXPERIMENTAL PROCEDURESCells and Antibodies-Colo205, a human colon carcinoma cell line, and MCF7, a breast cancer c...

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Akt/PKB is a serine/threonine kinase that promotes tumor cell growth by phosphorylating transcription factors and cell cycle proteins. There is particular interest in finding tumor-specific substrates for Akt to understand how this protein functions in cancer and to provide new avenues for therapeutic targeting. Our laboratory sought to identify novel Akt substrates that are expressed in breast cancer. In this study, we determined that activated Akt is positively correlated with the protein expression of the transcription/translation factor Y-box binding protein-1 (YB-1) in primary breast cancer by screening tumor tissue microarrays. We therefore questioned whether Akt and YB-1 might be functionally linked. Herein, we illustrate that activated Akt binds to and phosphorylates the YB-1 cold shock domain at Ser102. We then addressed the functional significance of disrupting Ser102 by mutating it to Ala102. Following the stable expression of Flag:YB-1 and Flag:YB-1 (Ala102) in MCF-7 cells, we observed that disruption of the Akt phosphorylation site on YB-1 suppressed tumor cell growth in soft agar and in monolayer. This correlated with an inhibition of nuclear translocation by the YB-1(Ala102) mutant. In conclusion, YB-1 is a new Akt substrate and disruption of this specific site inhibits tumor cell growth.