Olivier Ayrault

Silencing of the miR-17∼92 Cluster Family Inhibits Medulloblastoma Progression

Medulloblastoma, originating in the cerebellum, is the most common malignant brain tumor in children. Medulloblastoma consists of four major groups where constitutive activation of the Sonic Hedgehog (SHH) signaling pathway is a hallmark of one group. Mouse and human SHH medulloblastomas exhibit increased expression of microRNAs encoded by the miR-17~92 and miR-106b~25 clusters compared with granule progenitors and postmitotic granule neurons. Here, we assessed the therapeutic potential of 8-mer seed-targeting locked nucleic acid (LNA)-modified anti-miR oligonucleotides, termed tiny LNAs, that inhibit microRNA seed families expressed by miR-17~92 and miR-106b~25 in two mouse models of SHH medulloblastomas. We found that tumor cells (medulloblastoma cells) passively took up 8-mer LNA-anti-miRs and specifically inhibited targeted microRNA seed-sharing family members. Inhibition of miR-17 and miR-19a seed families by anti-miR-17 and anti-miR-19, respectively, resulted in diminished tumor cell proliferation in vitro. Treatment of mice with systemic delivery of anti-miR-17 and anti-miR-19 reduced tumor growth in flank and brain allografts in vivo and prolonged the survival of mice with intracranial transplants, suggesting that inhibition of the miR-17~92 cluster family by 8-mer LNA-anti-miRs might be considered for the treatment of SHH medulloblastomas.

Pemetrexed and gemcitabine as combination therapy for the treatment of Group3 medulloblastoma.

We devised a high-throughput, cell-based assay to identify compounds to treat Group3 medulloblastoma (G3 MB). Mouse G3 MBs neurospheres were screened against a library of approximately 7,000 compounds including US Food and Drug Administration-approved drugs. We found that pemetrexed and gemcitabine preferentially inhibited G3 MB proliferation in vitro compared to control neurospheres and substantially inhibited G3 MB proliferation in vivo. When combined, these two drugs significantly increased survival of mice bearing cortical implants of mouse and human G3 MBs that overexpress MYC compared to each agent alone, while having little effect on mouse MBs of the sonic hedgehog subgroup. Our findings strongly suggest that combination therapy with pemetrexed and gemcitabine is a promising treatment for G3 MBs.

Shh Signaling Protects Atoh1 from Degradation Mediated by the E3 Ubiquitin Ligase Huwe1 in Neural Precursors

Signaling networks controlled by Sonic hedgehog (SHH) and the transcription factor Atoh1 regulate the proliferation and differentiation of cerebellar granule neuron progenitors (GNPs). Deregulations in those developmental processes lead to medulloblastoma formation, the most common malignant brain tumor in childhood. Although the protein Atoh1 is a key factor during both cerebellar development and medulloblastoma formation, up-to-date detailed mechanisms underlying its function and regulation have remained poorly understood. Here, we report that SHH regulates Atoh1 stability by preventing its phosphodependent degradation by the E3 ubiquitin ligase Huwe1. Our results reveal that SHH and Atoh1 contribute to a positive autoregulatory loop promoting neuronal precursor expansion. Consequently, Huwe1 loss in mouse SHH medulloblastoma illustrates the disruption of this developmental mechanism in cancer. Hence, the crosstalk between SHH signaling and Atoh1 during cerebellar development highlights a collaborative network that could be further targeted in medulloblastoma.

Hypermethylated in Cancer 1 (HIC1) Recruits Polycomb Repressive Complex 2 (PRC2) to a Subset of Its Target Genes through Interaction with Human Polycomb-like (hPCL) Proteins

Background: HIC1 is a transcriptional repressor recruiting CtBP and NuRD complexes. Results: HIC1 interacts with human Polycomb-like proteins. Conclusion: HIC1 recruits the Polycomb PRC2 on a subset of its target genes through interactions with Polycomb-like proteins. Significance: Our results implicate hPCL proteins in the recruitment of PRC2 by transcription factors in mammals. HIC1 (hypermethylated in cancer 1) is a tumor suppressor gene epigenetically silenced or deleted in many human cancers. HIC1 is involved in regulatory loops modulating p53- and E2F1-dependent cell survival, growth control, and stress responses. HIC1 is also essential for normal development because Hic1-deficient mice die perinatally and exhibit gross developmental defects throughout the second half of development. HIC1 encodes a transcriptional repressor with five C2H2 zinc fingers mediating sequence-specific DNA binding and two repression domains: an N-terminal BTB/POZ domain and a central region recruiting CtBP and NuRD complexes. By yeast two-hybrid screening, we identified the Polycomb-like protein hPCL3 as a novel co-repressor for HIC1. Using multiple biochemical strategies, we demonstrated that HIC1 interacts with hPCL3 and its paralog PHF1 to form a stable complex with the PRC2 members EZH2, EED, and Suz12. Confirming the implication of HIC1 in Polycomb recruitment, we showed that HIC1 shares some of its target genes with PRC2, including ATOH1. Depletion of HIC1 by siRNA interference leads to a partial displacement of EZH2 from the ATOH1 promoter. Furthermore, in vivo, ATOH1 repression by HIC1 is associated with Polycomb activity during mouse cerebellar development. Thus, our results identify HIC1 as the first transcription factor in mammals able to recruit PRC2 to some target promoters through its interaction with Polycomb-like proteins.

MRT-92 inhibits Hedgehog signaling by blocking overlapping binding sites in the transmembrane domain of the Smoothened receptor

The Smoothened (Smo) receptor, a member of class F G protein‐coupled receptors, is the main transducer of the Hedgehog (Hh) signaling pathway implicated in a wide range of developmental and adult processes. Smo is the target of anticancer drugs that bind to a long and narrow cavity in the 7‐transmembrane (7TM) domain. X‐ray structures of human Smo (hSmo) bound to several ligands have revealed 2 types of 7TM‐directed antagonists: those binding mostly to extracellular loops (site 1, e.g., LY2940680) and those penetrating deeply in the 7TM cavity (site 2, e.g., SANT‐1). Here we report the development of the acylguanidine MRT‐92, which displays subnanomolar antagonist activity against Smo in various Hh cell‐based assays. MRT‐92 inhibits rodent cerebellar granule cell proliferation induced by Hh pathway activation through pharmacologic (half maximal inhibitory concentration [IC50] = 0.4 nM) or genetic manipulation. Using [3H]MRT‐92 (Kd = 0.3 nM for hSmo), we created a comprehensive framework for the interaction of small molecule modulators with hSmo and for understanding chemoresistance linked to hSmo mutations. Guided by molecular docking and site‐directed mutagenesis data, our work convincingly confirms that MRT‐92 simultaneously recognized and occupied both sites 1 and 2. Our data demonstrate the existence of a third type of Smo antagonists, those entirely filling the Smo binding cavity from the upper extracellular part to the lower cytoplasmicproximal subpocket. Our studies should help design novel potent Smo antagonists and more effective therapeutic strategies for treating Hh‐linked cancers and associated chemoresistance.—Hoch, L., Faure, H., Roudaut, H., Schoenfelder, A., Mann, A., Girard, N., Bihannic, L., Ayrault, O. Petricci, E., Taddei, M., Rognan, D., Ruat, M. MRT‐92 inhibits Hedgehog signaling by blocking overlapping binding sites in the transmembrane domain of the Smoothened receptor. FASEB J. 29, 1817‐1829 (2015). www.fasebj.org

PML IV/ARF interaction enhances p53 SUMO-1 conjugation, activation, and senescence

Significance Promyelocytic leukemia protein (PML) nuclear bodies (NBs) are subnuclear domains proposed to facilitate posttranslational modifications. Among NB partners, proteins are p53 and are most of the regulators. Overexpression of a single PML splice variant, PML IV, triggers p53-driven senescence. We demonstrate an interaction between PML IV C terminus and the ARF tumor suppressor. This interaction is required to promote p53 SUMOylation and subsequent stabilization. These results unexpectedly bridge three key tumor suppressors and stress the key role of PML NBs as SUMOylation factories and regulators of senescence. Promyelocytic leukemia protein (PML) nuclear bodies (NBs) recruit multiple partners, including p53 and many of its regulators. NBs are believed to facilitate several posttranslational modifications and are key regulators of senescence. PML, the organizer of NBs, is expressed as a number of splice variants that all efficiently recruit p53 partners. However, overexpression of only one of them, PML IV, triggers p53-driven senescence. Here, we show that PML IV specifically binds ARF, a key p53 regulator. Similar to ARF, PML IV enhances global SUMO-1 conjugation, particularly that of p53, resulting in p53 stabilization and activation. ARF interacts with and stabilizes the NB-associated UBC9 SUMO-conjugating enzyme, possibly explaining PML IV-enhanced SUMOylation. These results unexpectedly link two key tumor suppressors, highlighting their convergence for global control of SUMO conjugation, p53 activation, and senescence induction.

The occurrence of intracranial rhabdoid tumours in mice depends on temporal control of Smarcb1 inactivation

Rhabdoid tumours (RTs) are highly aggressive tumours of infancy, frequently localized in the central nervous system (CNS) where they are termed atypical teratoid/rhabdoid tumours (AT/RTs) and characterized by bi-allelic inactivation of the SMARCB1 tumour suppressor gene. In this study, by temporal control of tamoxifen injection in Smarcb1flox/flox;Rosa26-CreERT2 mice, we explore the phenotypes associated with Smarcb1 inactivation at different developmental stages. Injection before E6, at birth or at 2 months of age recapitulates previously described phenotypes including embryonic lethality, hepatic toxicity or development of T-cell lymphomas, respectively. Injection between E6 and E10 leads to high penetrance tumours, mainly intra-cranial, with short delays (median: 3 months). These tumours demonstrate anatomical, morphological and gene expression profiles consistent with those of human AT/RTs. Moreover, intra- and inter-species comparisons of tumours reveal that human and mouse RTs can be split into different entities that may underline the variety of RT cells of origin.

Insights into cerebellar development and medulloblastoma.

Cerebellar development is an extensive process that begins during early embryonic stages and persists more than one year after birth in human. Therefore, the cerebellum is susceptible to acquire various developmental abnormalities leading to numerous diseases such as medulloblastoma, the most common pediatric malignant brain tumor. One third of the patients with medulloblastoma are incurable and survivors have a poor quality of life due to the aggressiveness of the broad-spectrum treatments. Within the past few years, it has been highlighted that medulloblastoma is a heterogeneous disease that is divided in four molecular subgroups. This recent advance in the field, combined with the development of associated preclinical models for each subgroup, should enable, in the future, the discovery and use of targeted therapy in clinical treatments for each subtype of medulloblastoma. In this review, we first aim to show how deregulation of cerebellar development can lead to medulloblastoma formation and then to present the advances in the molecular subgrouping of medulloblastoma and the associated preclinical models.

NRL and CRX Define Photoreceptor Identity and Reveal Subgroup-Specific Dependencies in Medulloblastoma

Cancer cells often express differentiation programs unrelated to their tissue of origin, although the contribution of these aberrant phenotypes to malignancy is poorly understood. An aggressive subgroup of medulloblastoma, a malignant pediatric brain tumor of the cerebellum, expresses a photoreceptor differentiation program normally expressed in the retina. We establish that two photoreceptor-specific transcription factors, NRL and CRX, are master regulators of this program and are required for tumor maintenance in this subgroup. Beyond photoreceptor lineage genes, we identify BCL-XL as a key transcriptional target of NRL and provide evidence substantiating anti-BCL therapy as a rational treatment opportunity for select MB patients. Our results highlight the utility of studying aberrant differentiation programs in cancer and their potential as selective therapeutic vulnerabilities.