Beat W. Schaefer

Podocyte Regeneration Driven by Renal Progenitors Determines Glomerular Disease Remission and Can Be Pharmacologically Enhanced

Summary Podocyte loss is a general mechanism of glomerular dysfunction that initiates and drives the progression of chronic kidney disease, which affects 10% of the world population. Here, we evaluate whether the regenerative response to podocyte injury influences chronic kidney disease outcome. In models of focal segmental glomerulosclerosis performed in inducible transgenic mice where podocytes are tagged, remission or progression of disease was determined by the amount of regenerated podocytes. When the same model was established in inducible transgenic mice where renal progenitors are tagged, the disease remitted if renal progenitors successfully differentiated into podocytes, while it persisted if differentiation was ineffective, resulting in glomerulosclerosis. Treatment with BIO, a GSK3s inhibitor, significantly increased disease remission by enhancing renal progenitor sensitivity to the differentiation effect of endogenous retinoic acid. These results establish renal progenitors as critical determinants of glomerular disease outcome and a pharmacological enhancement of their differentiation as a possible therapeutic strategy.

TLR9 triggering in Burkitt's lymphoma cell lines suppresses the EBV BZLF1 transcription via histone modification.

Endemic Burkitts lymphoma (BL) is considered to preferentially develop in equatorial Africa because of chronic co-infection with Epstein–Barr virus (EBV) and the malaria pathogen Plasmodium falciparum. The interaction and contribution of both pathogens in the oncogenic process are poorly understood. Earlier, we showed that immune activation with a synthetic Toll-like receptor 9 (TLR9) ligand suppresses the initiation of EBV lytic replication in primary human B cells. In this study we investigate the mechanism involved in the suppression of EBV lytic gene expression in BL cell lines. We show that this suppression is dependent on functional TLR9 and MyD88 signaling but independent of downstream signaling elements, including phosphatidylinositol-3 kinase, mitogen-activated protein kinases and nuclear factor-κB. We identified TLR9 triggering resulting in histone modifications to negatively affect the activation of the promoter of EBVs master regulatory lytic gene BZLF1. Finally, we show that P. falciparum hemozoin, a natural TLR9 ligand, suppresses induction of EBV lytic gene expression in a dose-dependent manner. Thus, we provide evidence for a possible interaction between P. falciparum and EBV at the B-cell level and the mechanism involved in suppressing lytic and thereby reinforcing latent EBV that has unique oncogenic potential.

PAX3–FOXO1 Establishes Myogenic Super Enhancers and Confers BET Bromodomain Vulnerability

Alveolar rhabdomyosarcoma is a life-threatening myogenic cancer of children and adolescent young adults, driven primarily by the chimeric transcription factor PAX3-FOXO1. The mechanisms by which PAX3-FOXO1 dysregulates chromatin are unknown. We find PAX3-FOXO1 reprograms the cis-regulatory landscape by inducing de novo super enhancers. PAX3-FOXO1 uses super enhancers to set up autoregulatory loops in collaboration with the master transcription factors MYOG, MYOD, and MYCN. This myogenic super enhancer circuitry is consistent across cell lines and primary tumors. Cells harboring the fusion gene are selectively sensitive to small-molecule inhibition of protein targets induced by, or bound to, PAX3-FOXO1-occupied super enhancers. Furthermore, PAX3-FOXO1 recruits and requires the BET bromodomain protein BRD4 to function at super enhancers, resulting in a complete dependence on BRD4 and a significant susceptibility to BRD inhibition. These results yield insights into the epigenetic functions of PAX3-FOXO1 and reveal a specific vulnerability that can be exploited for precision therapy.Significance: PAX3-FOXO1 drives pediatric fusion-positive rhabdomyosarcoma, and its chromatin-level functions are critical to understanding its oncogenic activity. We find that PAX3-FOXO1 establishes a myoblastic super enhancer landscape and creates a profound subtype-unique dependence on BET bromodomains, the inhibition of which ablates PAX3-FOXO1 function, providing a mechanistic rationale for exploring BET inhibitors for patients bearing PAX-fusion rhabdomyosarcoma. Cancer Discov; 7(8); 884-99. ©2017 AACR.This article is highlighted in the In This Issue feature, p. 783.

Abstract B01: Development of an in vitro drug-profiling platform for functional guidance of treatment decisions and identification of vulnerabilities in chemoresistant relapsed rhabdomyosarcoma tumors

Over the last decades, it has become increasingly clear that tumors are characterized by inter-individual heterogeneity, which can account as one of the prominent reasons for treatment failure. To address this problem, personalized precision medicine approaches need to be applied such as comprehensive genomic profiling to identify actionable driver mutations in individual tumors. Unfortunately, genomics alone is not sufficient in tumors that are driven by mutated but otherwise undruggable targets and a typically low mutational burden. This is a characteristic of many pediatric malignancies including rhabdomyosarcoma (RMS), the most common childhood soft-tissue sarcoma. Hence, we aim to develop an in vitro drug-profiling platform to identify and prioritize treatment strategies for RMS patients. To this end, we generated a panel of patient-derived xenografts (PDXs) including some diagnostic and relapse samples. We then screened 18 different culture conditions to identify suitable parameters to establish in vitro primary cultures of PDX tumors. Interestingly, addition of fetal calf serum to cell culture media has a detrimental effect on viability of most primary RMS cell cultures (PRCCs). In contrast, defined serum-free conditions allow to grow primary cultures for several passages that closely preserve the clonal composition and phenotypic characteristics of the parental tumor, as assessed by genomic and copy number analysis. Pharmacologic profiles of PRCCs using a targeted drug library of more than 200 compounds revealed patient-specific vulnerabilities, among them an unexpected sensitivity to AKT inhibitors in some fusion-positive RMS. Interestingly, hierarchical clustering of drug sensitivities clustered PAX3-FOXO1 fusion-positive tumors together and separated them from fusion-negative RMS. Moreover, a screen to establish effective drug combinations in a highly resistant high-risk relapse sample using standard chemotherapeutics (doxorubicin, etoposide, vincristine) together with our targeted compound library revealed the BH3-only mimetic ABT-263 as the top-scoring drug capable of resensitizing recurrent PRCCs to first-line treatment. Resensitizing with ABT-263 was not a patient-specific vulnerability as it was observed in several additional PRCCs. Mechanistically, genetic loss-of-function validation experiments revealed that this occurs via blockade of the BCL-XL-MCL-1 axis and is dependent on upregulation of the BH3-only protein NOXA. Taken together, our study provides an in vitro tool kit to prioritize actionable drug targets or combinatorial options for RMS patients for whom conventional therapies are failing. Citation Format: Beat W. Schaefer, Gabriele Manzella, Michaela Roemmele, Luduo Zhang, Joelle Tchinda, Felix Niggli, Marco Wachtel. Development of an in vitro drug-profiling platform for functional guidance of treatment decisions and identification of vulnerabilities in chemoresistant relapsed rhabdomyosarcoma tumors [abstract]. In: Proceedings of the AACR Special Conference: Pediatric Cancer Research: From Basic Science to the Clinic; 2017 Dec 3-6; Atlanta, Georgia. Philadelphia (PA): AACR; Cancer Res 2018;78(19 Suppl):Abstract nr B01.

Abstract 700: Characterization of the mode of action of Fenretinide treatment in alveolar rhabdomyosarcoma cells

Alveolar rhabdomyosarcoma (aRMS) is a highly malicious childhood malignancy characterized by a specific chromosomal translocation encoding the oncogenic transcription factor PAX3-FOXO1. As aRMS cells are addicted to the tumor-specific fusion protein, it may serve as an ideal therapeutic target. Previously, we have identified from a large drug library screen the compound Fenretinide (retinoic acid p-hydroxyanilide), which is already in clinical use, to affect both PAX3-FOXO1 expression as well as aRMS cell viability. The aim of this study was therefore to characterize the mode of action of Fenretinide in more detail. First, we were able to show that Fenretinide induced the generation of reactive oxygen species (ROS) in mitochondria. A more detailed characterization revealed that the Fenretinide-induced ROS derived from an interaction of Fenretinide around complex II of the mitochondrial respiratory chain, leading to the production of superoxides. ROS scavenging as well as complexing of iron ions completely abolished cell death. To identify the mode of cell death involved, we then used a range of pharmacological inhibitors of specific cell death pathways including Z-vad (pan -caspase inhibitor), Necrostatin-1 (necroptosis pathway inhibitor (RIP-1 kinase inhibitor)), 3-Methyadenine (3-MA) (autophagy pathway inhibitor (phosphatidylinositol 3-kinase inhibitor)) and Ferrostatin (ferroptosis pathway inhibitor) during Fenretinide treatment. Surprisingly, none of these inhibitors alone was able to prevent cell death and even different combinations were not sufficient to completely inhibit cell death. CRISPR/Cas9 mediated depletion of key players in the apoptotic and necroptotic pathway (Bak, Bax and RIPK1) confirmed the pharmacological data. We therefore conclude that other, less characterized cell death pathways or a combination of several pathways including apoptosis and necroptosis might be crucial. Interestingly, electron microscopic examination of cells pointed towards an excessive accumulation of vacuoles to be characteristic. Taken together, our data show that Fenretinide shows high potential for the treatment of aRMS, inducing several forms of cell death mediated through the production of ROS. These properties open the search for additional compounds acting in a combinatorial manner. Citation Format: Eva Brack, Marco Wachtel, Beat W. Schaefer. Characterization of the mode of action of Fenretinide treatment in alveolar rhabdomyosarcoma cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 700. doi:10.1158/1538-7445.AM2017-700

Abstract 4457: Chromatin remodeling as a potential new strategy for fusion positive rhabdomyosarcoma therapy

Fusion-positive rhabdomyosarcoma (FP-RMS) is a pediatric tumor driven by an oncogenic fusion protein, PAX3-FOXO1, which acts as a transcription factor. Conventional chemotherapy is effective for low risk patients who have a 5-year overall survival greater than 65%, while high risk patients, including those with metastatic disease, have less than 40% survival. Consequently, we hypothesize that targeting the fusion protein or its collaborators in transcription regulation will provide novel therapies for this aggressive subtype of RMS. To identify new druggable PAX3-FOXO1 interactors, we performed a combined proteomic and genetic screen which led to the discovery of the NuRD complex (Nucleosome Remodelling and Deacetylase) as a major PAX3-FOXO1 co-regulator. The NuRD complex is unique among the chromatin remodelling complexes due to its dual enzymatic activity. It can act by histone deacetylation through HDAC1/2 (histone deacetylases) or influence nucleosome positioning through CHD4 (chromodomain-DNA-binding protein 4). Intriguingly, it has been associated with both activating and repressive activities in gene expression and its role in cancer development is not fully understood yet. We found that in FP-RMS, silencing of CHD4 affected the expression of approximately 50% of PAX3-FOXO1 regulated target genes. These were mainly genes which are usually upregulated, suggesting an activating role for NuRD. Consistent with CHD4 activation activity, ChIP-seq experiments demonstrated that CHD4 and HDAC2 co-localize with the fusion protein in cis-regulatory sites of a subset of its target genes. Interestingly, gene expression analysis showed that both CHD4 and HDAC2 are highly expressed in tumor tissue and myoblasts when compared to normal skeletal muscle, inferring a potential role of the NuRD complex in maintaining the undifferentiated phenotype observed in FP-RMS. Importantly, CHD4 silencing had no effect on myoblasts proliferation whereas a profound growth reduction was seen in FP-RMS cell lines, suggesting a unique tumour dependency on this chromatin remodeler. In addition, depletion of CHD4 caused a complete regression of xenograft tumours in mice.In summary, we have identified the NuRD complex as an essential positive co-regulator of PAX3-FOXO1 transcriptional activity. Our data propose a critival role of one of the NuRD core component CHD4 in FP-RMS cell viability, making CHD4 an attractive new target for therapy. To our knowledge, CHD4 is the first chromatin remodeler identified to associate with PAX3-FOXO1 transcriptional activity, thus highlighting the relevance of epigenetic regulation in FP-RMS tumour development. Collectively, our findings suggest CHD4 as a potential novel therapeutic target in this childhood malignancy.Ongoing work is currently underway to identify first-in-class small molecules to inhibit CHD4 protein. Citation Format: Joana G. Marques, Berkley Gryder, Maria Boehm, Marco Wachtel, Young Song, Hsien-Chao Chou, Rajesh Patidar, Hongling Liao, Javed Khan, Beat W. Schaefer. Chromatin remodeling as a potential new strategy for fusion positive rhabdomyosarcoma therapy. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4457.

Abstract PR10: The chromatin remodeler CHD4 as a potential specific target for alveolar rhabdomyosarcoma therapy

Fusion-positive alveolar rhabdomyosarcoma (FP-RMS) is a paediatric tumour driven by an oncogenic fusion transcription factor, PAX3-FOXO1. Conventional chemotherapy is only effective for low risk patients which carry no metastasis, achieving a 5-year overall survival of 65%. The unique presence of this fusion protein in FP-RMS as well as the tumour cell survival dependency on PAX3-FOXO1 make this transcription factor a promising target for therapy. However, due to the difficulties associated with drug development targeting transcription factors, we performed a combined proteomic and genetic screen to identify new druggable co-regulators of PAX3-FOXO1 transcriptional activity. Interactor candidates were defined by mass spectrometry analysis of proteins co-purified with the fusion protein and individually validated for their relevance for PAX3-FOXO1 activity through siRNA silencing. The chromodomain-DNA-binding protein 4 (CHD4), a nucleosome remodeler and core member of the NuRD complex (Nucleosome Remodelling and Deacetylase), was identified as an essential positive co-regulator of PAX3-FOXO1 transcriptional activity. ChIP-qPCR experiments demonstrated that CHD4 not only co-localizes with PAX3-FOXO1 in the FP-RMS genome but also it is necessary for the binding of the fusion protein to cis-regulatory sites for a subset of its target genes. Consequently, CHD4 silencing affected the expression of more than 50% of PAX3-FOXO1 regulated target genes. Additionally, depletion of CHD4 impaired FP-RMS cell proliferation and caused a complete regression of xenograft tumours in mice. Moreover, CHD4 silencing had no effect in cell proliferation of human myoblasts or fibroblasts, suggesting a unique tumour dependency on this chromatin remodeler. In summary, our data propose that CHD4 has a crucial role as a co-regulator of PAX3-FOXO1 driven gene expression whose presence is required for FP-RMS cell viability. To our knowledge, CHD4 is the first identified chromatin remodeler associated with PAX3-FOXO1 transcriptional activity, thus highlighting the relevance of epigenetic regulation in FP-RMS tumour development. Collectively, our findings suggest CHD4 as a potential novel therapeutic target in this childhood malignancy, and are motivating ongoing work aimed at finding first-in-class small molecules to inhibit CHD4. This abstract is also presented as Poster A20. Citation Format: Joana Marques, Maria Boehm, Marco Wachtel, Beat Schaefer. The chromatin remodeler CHD4 as a potential specific target for alveolar rhabdomyosarcoma therapy. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Pediatric Cancer Research: From Mechanisms and Models to Treatment and Survivorship; 2015 Nov 9-12; Fort Lauderdale, FL. Philadelphia (PA): AACR; Cancer Res 2016;76(5 Suppl):Abstract nr PR10.

Abstract 478: EWS/FLI1 transcription is modulated by the PI3K pathway via SP1 in Ewing sarcoma

Ewing sarcoma (ES) is the second most frequent bone cancer in childhood and it is characterized by the presence of the balanced t(11;22)(q24;q12) translocation in more than 85% of cases, generating a dysregulated transcription factor EWS/FLI1. ES belongs to small-round-blue-cell tumors and it is a very aggressive osteolytic cancer with early tendency for development of metastasis. Mostly it affects bones such as pelvis, femour and ribs but can also arise in soft tissues, mainly in adults. EWS/FLI1 is an essential oncogenic component of ES development which is necessary for tumor cell maintenance, through inappropriate regulation of target genes that are crucial for the fully malignant phenotype. Therefore, EWS/FLI1 represents an attractive therapeutic target. Screening of a small library of 153 targeted compounds has identified inhibitors of the PI3K pathway as main modulators of EWS/FLI1 activity and surprisingly this was due to an effect of the compound on EWS/FLI1 transcription. Indeed, treatment of four different ES cell lines with BEZ235 (PI3K-mTOR inhibitor) resulted in downregulation of EWS/FLI1 by 50% expression with subsequent modulation of target gene expression. Analysis of the EWS/FLI1 promoter region (-2239/+67) using various deletion constructs identified two 14bp minimal elements, named Del2 and Del23 region, as being important for EWS/FLI1 transcription. Based on in silico prediction and on in vivo data, we were able to predict 4 transcription factor candidates to bind this region and being responsive to PI3K regulation. Among these, SP1 was identified as modulator of EWS/FLI1 gene expression by siRNA. Various approches such as qRT-PCR, Western Blot and Immunofluorescence confirmed a reduction of EWS/FLI1 in Ewing cells when PI3Kαγδ and SP1 were depleted; Electrophoretic Mobility Shift Assay (EMSA) and Chromatine Immuno Precipitation (ChIP) both confirmed that the transcription factor SP1 is indeed binding to Del23 region of the EWS/FLI1 promoter. In summary, our results provide the very first insights on the transcriptional regulation of EWS/FLI1, an area that has not been investigated so far, and offer a molecular explanation for the known sensitivity of ES cell lines to PI3K inhibition. Citation Format: Chiara Giorgi, Alexander Boro, Laura A. Lopez-Garcia, Beat W. Schaefer, Felix K. Niggli. EWS/FLI1 transcription is modulated by the PI3K pathway via SP1 in Ewing sarcoma. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 478. doi:10.1158/1538-7445.AM2015-478

Abstract 4332: Proprotein convertases as target for therapy of pediatric sarcomas

One of the biggest challenges of treating children with cancer is achieving complete remission, while ensuring that they will be able to lead normal, productive life in adulthood. To improve therapies for embryonal pediatric tumors such as rhabdomyosarcoma (RMS) the most common soft tissue sarcoma in children, we have selected by in vivo phage display for peptides that bind to RMS with high specificity. We have shown that coupling of Doxorubicin to RMS-P3, a RMS targeting peptide, considerably increases its therapeutic impact in a mouse model for RMS. We have identified proprotein convertases (PC), a family of ten serine proteases which convert inactive proprotein into their active form, as targets RMS binding peptides. Now we aimed at elucidating the contribution to RMS growth in vivo of the two proprotein convertases, furin and PC7, which are highly expressed in RMS tumors. To this end, we have generated a panel of RMS cell lines overexpressing furin, overexpressing a specific intrinsic PC inhibitor called α1-PDX, as well as cell lines were expression of furin or PC7 has been stably blocked by shRNA. These cells have been tested for their growth, migration and invasion potential in vitro as well as for their growth in vivo as subcutaneous xenografts. Histological analysis of tumor sections, in particular of microvessel density and angiogenesis has been performed to reveal if PC activity correlates with RMS progression. Our results confirm a strong correlation between PCs activity and RMS growth in vivo and suggest furin and PC7 as viable targets for therapeutic intervention and warrant further studies in the development of novel therapeutic approaches based both on specific inhibition of PCs protease activity and on targeted drug delivery by PCs targeting peptides. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4332. doi:1538-7445.AM2012-4332