Induction of apoptosis in rhabdomyosarcoma cells through down-regulation of PAX proteins

p160 nuclear receptor coactivator family members and their role in rare fusion‑driven neoplasms (Review)

Gene fusions with translocations involving nuclear receptor coactivators (NCoAs) are relatively common among fusion-driven malignancies. NCoAs are essential mediators of environmental cues and can modulate the transcription of downstream target genes upon binding to activated nuclear receptors. Therefore, fusion proteins containing NCoAs can become strong oncogenic drivers, affecting the cell transcriptional profile. These tumors show a strong dependency on the fusion oncogene; therefore, the direct pharmacological targeting of the fusion protein becomes an attractive strategy for therapy. Currently, different combinations of chemotherapy regimens are used to treat a variety of NCoA-fusion-driven tumors, but given the frequent tumor reoccurrence, more efficient treatment strategies are needed. Specific approaches directed towards inhibition or silencing of the fusion gene need to be developed while minimizing the interference with the original genes. This review highlights the relevant literature describing the normal function and structure of NCoAs and their oncogenic activity in NCoA-gene fusion-driven cancers, and explores potential strategies that could be effective in targeting these fusions.

Functional Classification of Fusion Proteins in Sarcoma

Sarcomas comprise a heterogeneous group of malignant tumors of mesenchymal origin. More than 80 entities are associated with different mesenchymal lineages. Sarcomas with fibroblastic, muscle, bone, vascular, adipocytic, and other characteristics are distinguished. Nearly half of all entities contain specific chromosomal translocations that give rise to fusion proteins. These are mostly pathognomonic, and their detection by various molecular techniques supports histopathologic classification. Moreover, the fusion proteins act as oncogenic drivers, and their blockade represents a promising therapeutic approach. This review summarizes the current knowledge on fusion proteins in sarcoma. We categorize the different fusion proteins into functional classes, including kinases, epigenetic regulators, and transcription factors, and describe their mechanisms of action. Interestingly, while fusion proteins acting as transcription factors are found in all mesenchymal lineages, the others have a more restricted pattern. Most kinase-driven sarcomas belong to the fibroblastic/myofibroblastic lineage. Fusion proteins with an epigenetic function are mainly associated with sarcomas of unclear differentiation, suggesting that epigenetic dysregulation leads to a major change in cell identity. Comparison of mechanisms of action reveals recurrent functional modes, including antagonism of Polycomb activity by fusion proteins with epigenetic activity and recruitment of histone acetyltransferases by fusion transcription factors of the myogenic lineage. Finally, based on their biology, we describe potential approaches to block the activity of fusion proteins for therapeutic intervention. Overall, our work highlights differences as well as similarities in the biology of fusion proteins from different sarcomas and provides the basis for a functional classification.

Piperacetazine Directly Binds to the PAX3::FOXO1 Fusion Protein and Inhibits Its Transcriptional Activity

The tumor-specific chromosomal translocation product, PAX3::FOXO1, is an aberrant fusion protein that plays a key role for oncogenesis in the alveolar subtype of rhabdomyosarcoma (RMS). PAX3::FOXO1 represents a validated molecular target for alveolar RMS and successful inhibition of its oncogenic activity is likely to have significant clinical applications. Even though several PAX3::FOXO1 function-based screening studies have been successfully completed, a directly binding small-molecule inhibitor of PAX3::FOXO1 has not been reported. Therefore, we screened small-molecule libraries to identify compounds that were capable of directly binding to PAX3::FOXO1 protein using surface plasmon resonance technology. Compounds that directly bound to PAX3::FOXO1 were further evaluated in secondary transcriptional activation assays. We discovered that piperacetazine can directly bind to PAX3::FOXO1 protein and inhibit fusion protein-derived transcription in multiple alveolar RMS cell lines. Piperacetazine inhibited anchorage-independent growth of fusion-positive alveolar RMS cells but not embryonal RMS cells. On the basis of our findings, piperacetazine is a molecular scaffold upon which derivatives could be developed as specific inhibitors of PAX3::FOXO1. These novel inhibitors could potentially be evaluated in future clinical trials for recurrent or metastatic alveolar RMS as novel targeted therapy options.

SIGNIFICANCE

RMS is a malignant soft-tissue tumor mainly affecting the pediatric population. A subgroup of RMS with worse prognosis harbors a unique chromosomal translocation creating an oncogenic fusion protein, PAX3::FOXO1. We identified piperacetazine as a direct inhibitor of PAX3::FOXO1, which may provide a scaffold for designing RMS-specific targeted therapy.

Myo-differentiation reporter screen reveals NF-Y as an activator of PAX3-FOXO1 in rhabdomyosarcoma

Recurrent chromosomal rearrangements found in rhabdomyosarcoma (RMS) produce the PAX3-FOXO1 fusion protein, which is an oncogenic driver and a dependency in this disease. One important function of PAX3-FOXO1 is to arrest myogenic differentiation, which is linked to the ability of RMS cells to gain an unlimited proliferation potential. Here, we developed a phenotypic screening strategy for identifying factors that collaborate with PAX3-FOXO1 to block myo-differentiation in RMS. Unlike most genes evaluated in our screen, we found that loss of any of the three subunits of the Nuclear Factor Y (NF-Y) complex leads to a myo-differentiation phenotype that resembles the effect of inactivating PAX3-FOXO1. While the transcriptomes of NF-Y- and PAX3-FOXO1-deficient RMS cells bear remarkable similarity to one another, we found that these two transcription factors occupy nonoverlapping sites along the genome: NF-Y preferentially occupies promoters, whereas PAX3-FOXO1 primarily binds to distal enhancers. By integrating multiple functional approaches, we map the PAX3 promoter as the point of intersection between these two regulators. We show that NF-Y occupies CCAAT motifs present upstream of PAX3 to function as a transcriptional activator of PAX3-FOXO1 expression in RMS. These findings reveal a critical upstream role of NF-Y in the oncogenic PAX3-FOXO1 pathway, highlighting how a broadly essential transcription factor can perform tumor-specific roles in governing cellular state.

EphA1 receptor tyrosine kinase is localized to the nucleus in rhabdomyosarcoma from multiple species

While the typical role of receptor tyrosine kinases is to receive and transmit signals at the cell surface, in some cellular contexts (particularly transformed cells) they may also act as nuclear proteins. Aberrant nuclear localization of receptor tyrosine kinases associated with transformation often enhances the transformed phenotype (i.e. nuclear ErbBs promote tumor progression in breast cancer). Rhabdomyosarcoma (RMS), the most common soft tissue tumor in children, develops to resemble immature skeletal muscle and has been proposed to derive from muscle stem/progenitor cells (satellite cells). It is an aggressive cancer with a 5-year survival rate of 33% if it has metastasized. Eph receptor tyrosine kinases have been implicated in the development and progression of many other tumor types, but there are only two published studies of Ephs localizing to the nucleus of any cell type and to date no nuclear RTKs have been identified in RMS. In a screen for protein expression of Ephs in canine RMS primary tumors as well as mouse and human RMS cell lines, we noted strong expression of EphA1 in the nucleus of interphase cells in tumors from all three species. This localization pattern changes in dividing cells, with EphA1 localizing to the nucleus or the cytoplasm depending on the phase of the cell cycle. These data represent the first case of a nuclear RTK in RMS, and the first time that EphA1 has been detected in the nucleus of any cell type.

Pax3 loss of function delays tumour progression in kRAS-induced zebrafish rhabdomyosarcoma models

Rhabdomyosarcoma is a soft tissue cancer that arises in skeletal muscle due to mutations in myogenic progenitors that lead to ineffective differentiation and malignant transformation. The transcription factors Pax3 and Pax7 and their downstream target genes are tightly linked with the fusion positive alveolar subtype, whereas the RAS pathway is usually involved in the embryonal, fusion negative variant. Here, we analyse the role of Pax3 in a fusion negative context, by linking alterations in gene expression in pax3a/pax3b double mutant zebrafish with tumour progression in kRAS-induced rhabdomyosarcoma tumours. Several genes in the RAS/MAPK signalling pathway were significantly down-regulated in pax3a/pax3b double mutant zebrafish. Progression of rhabdomyosarcoma tumours was also delayed in the pax3a/pax3b double mutant zebrafish indicating that Pax3 transcription factors have an unappreciated role in mediating malignancy in fusion negative rhabdomyosarcoma.

Synergistic Effects of Folic Acid and n-3 Polyunsaturated Fatty Acid in Preventing Neural Tube Defects in Diabetic Mice

The present study aimed to investigate whether a combination of folic acid (FA) and n-3 polyunsaturated fatty acids (PUFA) has a better preventive effect on maternal diabetes-induced neural tube defects (NTD) than FA alone. The experiment included five groups of pregnant mice: healthy control (HC), diabetes mellitus control (DMC), diabetes + n-3 PUFA (DMn-3), diabetes + FA (DMFA), and diabetes + FA + n-3 PUFA (DMFA + n-3). The incidence of NTD in DMFA + n-3 (1.04%) was significantly lower than that in DMFA (8.57%) and DMn-3 (7.82%). The incidence of NTD in DMFA and DMn-3 was significantly lower than that in DMC (19.41%). DMFA + n-3 had a lower apoptosis of neuroepithelial cells, a lower expression of P53 and Bax, and a higher expression of Pax3 and Bcl-2, compared with DMFA and DMn-3. Combination of FA and n-3 PUFA attenuated diabetes-induced hypermethylation of Pax3, overexpression and overactivity of Dnmt3b, abnormal expression of genes involved in one-carbon metabolism and elevation of homocysteine, and these improving effects were better than FA or n-3 PUFA alone. In conclusion, the combination of FA and n-3 PUFA has a synergistic effect on preventing maternal diabetes-induced NTD.

N-3 polyunsaturated fatty acids effectively protect against neural tube defects in diabetic mice induced by streptozotocin

Folate cannot prevent all neural tube defects (NTD), indicating that other pathogeneses still exist except for the folate deficiency. Maternal diabetes mellitus during pregnancy can increase the risk of offspring NTD. Our previous study showed that polyunsaturated fatty acids (PUFA) were lower in the placenta of human NTD cases than in healthy controls, and the supplementation of fish oil (rich in long-chain (LC) n-3 PUFA, mainly C20:5n-3 and C22:6n-3) had a better prevention effect against sodium valproate induced NTD than corn oil (rich in C18:2n-6) and flaxseed oil (rich in C18:3n-3). The aim of the present study was to investigate whether PUFA could prevent diabetes-induced NTD in mice. Streptozotocin (STZ)-induced diabetic pregnant mice were fed with a normal diet (DMC), a diet containing a low dose of fish oil (DMLn-3), a diet containing a high dose of fish oil (DMHn-3) or a diet rich in corn oil (DMn-6). Healthy pregnant mice were fed with a normal diet (HC). Compared with the DMC group, the rate of NTD was significantly lower in the DMHn-3 group (4.44% vs. 12.50%), but not in the DMLn-3 (11.11%) or DMn-6 group (12.03%). The NTD rate in the DMHn-3 group was comparable with that in the HC group (1.33%) (p = 0.246), and lower than that in the DMn-6 group (p = 0.052). The NTD rate in DMLn-3 and DMn-6 groups was significantly higher than that in the HC group. No significant difference was observed in NTD rate between DMLn-3 and DMHn-3 groups, and between DMLn-3 and DMn-6 groups. Compared with the HC group, the DMC group had a significantly lower C22:6n-3 in both serum and embryos. Fish oil supplementation ameliorated neuroepithelial cell apoptosis, and the apoptotic rate was comparable between DMHn-3 and HC groups. Although the apoptotic rate was significantly lower in the DMn-6 group than the DMC group, it was still much higher than that in the HC group. The proteins P53 and Bax in embryos were higher, while the proteins Bcl-2 and Pax3 were lower in the DMC group than in the HC group. The disturbance of Pax3, P53 and Bax induced by diabetes was abolished in DMLn-3, DMHn-3 and DMn-6 groups. Importantly, Bcl-2 in embryos was restored to the normal level only in the DMHn-3 group but not in the DMLn-3 or DMn-6 group. In conclusion, LC n-3 PUFA enriched fish oil has a protective effect against NTD in diabetes induced by STZ through improving neuroepithelial cell apoptosis, and the mechanism may be by increasing the anti-apoptosis protein Bcl-2 independently of Pax3 and P53.

High Frequency of Tumor Propagating Cells in Fusion-Positive Rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in children. Fusion-positive RMS (FPRMS), expressing the PAX3/7-FOXO1, has a worse prognosis compared to the more common fusion-negative RMS (FNRMS). Although several studies reported hierarchical organization for FNRMS with the identification of cancer stem cells, the cellular organization of FPRMS is not yet clear. In this study we investigated the expression of key stem cell markers, developed a sphere assay, and investigated the seven most common FPRMS cell lines for subpopulations of tumor propagating cancer stem-like cells, also called cancer stem cells (CSCs). Moreover, loss- and gain-of-functions of the stem cell genes SOX2, OCT4, and NANOG were investigated in the same cells. Single-cell clonal analysis was performed in vitro as well as in vivo. We found that no stable CSC subpopulation could be enriched in FPRMS. Unlike depletion of PAX3-FOXO1, neither overexpression nor siRNA-mediated downregulation of SOX2, OCT4, and NANOG affected physiology of RMS cells. Every single subclone-derived cell clone initiated tumor growth in mice, despite displaying considerable heterogeneity in gene expression. FPRMS appears to contain a high frequency of tumor propagating stem-like cells, which could explain their higher propensity for metastasis and relapse. Their dependency on PAX3-FOXO1 activity reinforces the importance of the fusion protein as the key therapeutic target.

PAX7 Balances the Cell Cycle Progression via Regulating Expression of Dnmt3b and Apobec2 in Differentiating PSCs

PAX7 transcription factor plays a crucial role in embryonic myogenesis and in adult muscles in which it secures proper function of satellite cells, including regulation of their self renewal. PAX7 downregulation is necessary for the myogenic differentiation of satellite cells induced after muscle damage, what is prerequisite step for regeneration. Using differentiating pluripotent stem cells we documented that the absence of functional PAX7 facilitates proliferation. Such action is executed by the modulation of the expression of two proteins involved in the DNA methylation, i.e., Dnmt3b and Apobec2. Increase in Dnmt3b expression led to the downregulation of the CDK inhibitors and facilitated cell cycle progression. Changes in Apobec2 expression, on the other hand, differently impacted proliferation/differentiation balance, depending on the experimental model used.

Molecular mechanisms underpinning sarcomas and implications for current and future therapy

Sarcomas are complex mesenchymal neoplasms with a poor prognosis. Their clinical management is highly challenging due to their heterogeneity and insensitivity to current treatments. Although there have been advances in understanding specific genomic alterations and genetic mutations driving sarcomagenesis, the underlying molecular mechanisms, which are likely to be unique for each sarcoma subtype, are not fully understood. This is in part due to a lack of consensus on the cells of origin, but there is now mounting evidence that they originate from mesenchymal stromal/stem cells (MSCs). To identify novel treatment strategies for sarcomas, research in recent years has adopted a mechanism-based search for molecular markers for targeted therapy which has included recapitulating sarcomagenesis using in vitro and in vivo MSC models. This review provides a comprehensive up to date overview of the molecular mechanisms that underpin sarcomagenesis, the contribution of MSCs to modelling sarcomagenesis in vivo, as well as novel topics such as the role of epithelial-to-mesenchymal-transition (EMT)/mesenchymal-to-epithelial-transition (MET) plasticity, exosomes, and microRNAs in sarcomagenesis. It also reviews current therapeutic options including ongoing pre-clinical and clinical studies for targeted sarcoma therapy and discusses new therapeutic avenues such as targeting recently identified molecular pathways and key transcription factors.

Negative correlation of single-cell PAX3:FOXO1 expression with tumorigenicity in rhabdomyosarcoma

Single-cell PAX3:FOXO1 expression in rhabdomyosarcoma is variable. PAX3:FOXO1 low cell states are characterized by more efficient adhesion, migration and tumor-propagating capacity.

Rhabdomyosarcomas (RMS) are phenotypically and functionally heterogeneous. Both primary human RMS cultures and low-passage Myf6Cre,Pax3:Foxo1,p53 mouse RMS cell lines, which express the fusion oncoprotein Pax3:Foxo1 and lack the tumor suppressor Tp53 (Myf6Cre,Pax3:Foxo1,p53), exhibit marked heterogeneity in PAX3:FOXO1 (P3F) expression at the single cell level. In mouse RMS cells, P3F expression is directed by the Pax3 promoter and coupled to eYFP. YFP^low/P3F^low mouse RMS cells included 87% G0/G1 cells and reorganized their actin cytoskeleton to produce a cellular phenotype characterized by more efficient adhesion and migration. This translated into higher tumor-propagating cell frequencies of YFP^low/P3F^low compared with YFP^high/P3F^high cells. Both YFP^low/P3F^low and YFP^high/P3F^high cells gave rise to mixed clones in vitro, consistent with fluctuations in P3F expression over time. Exposure to the anti-tropomyosin compound TR100 disrupted the cytoskeleton and reversed enhanced migration and adhesion of YFP^low/P3F^low RMS cells. Heterogeneous expression of PAX3:FOXO1 at the single cell level may provide a critical advantage during tumor progression.

Vincetoxicum arnottianum modulates motility features and metastatic marker expression in pediatric rhabdomyosarcoma by stabilizing the actin cytoskeleton

Background

Prevention of metastatic invasion is one of the main challenges in the treatment of alveolar rhabdomyosarcoma. Still the therapeutic options are limited. Therefore, an anti-tumor screening was initiated focusing on the anti-metastatic and anti-invasion properties of selected medicinal plant extracts and phytoestrogens, already known to be effective in the prevention and treatment of different cancer entities.

Methods

Treatment effects were first evaluated by cell viability, migration, invasion, and colony forming assays on the alveolar rhabdomyosarcoma cell line RH-30 in comparison with healthy primary cells.

Results

Initial anti-tumor screenings of all substances analyzed in this study, identified the plant extract of Vincetoxicum arnottianum (VSM) as the most promising candidate, harboring the highest anti-metastatic potential. Those significant anti-motility properties were proven by a reduced ability for migration (60%), invasion (99%) and colony formation (61%) under 48 h exposure to 25 μg/ml VSM. The restricted motility features were due to an induction of the stabilization of the cytoskeleton – actin fibers were 2.5-fold longer and were spanning the entire cell. Decreased proliferation (PCNA, AMT, GCSH) and altered metastasis (e. g. SGPL1, CXCR4, stathmin) marker expression on transcript and protein level confirmed the significant lowered tumorigenicity under VSM treatment. Finally, significant alterations in the cell metabolism were detected for 25 metabolites, with levels of uracil, N-acetyl serine and propanoyl phosphate harboring the greatest alterations. Compared to the conventional therapy with cisplatin, VSM treated cells demonstrated a similar metabolic shutdown of the primary cell metabolism. Primary control cells were not affected by the VSM treatment.

Conclusions

This study revealed the VSM root extract as a potential, new migrastatic drug candidate for the putative treatment of pediatric alveolar rhabdomyosarcoma with actin filament stabilizing properties and accompanied by a marginal effect on the vitality of primary cells.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12906-021-03299-x.

NuRD subunit CHD4 regulates super-enhancer accessibility in rhabdomyosarcoma and represents a general tumor dependency

The NuRD complex subunit CHD4 is essential for fusion-positive rhabdomyosarcoma (FP-RMS) survival, but the mechanisms underlying this dependency are not understood. Here, a NuRD-specific CRISPR screen demonstrates that FP-RMS is particularly sensitive to CHD4 amongst the NuRD members. Mechanistically, NuRD complex containing CHD4 localizes to super-enhancers where CHD4 generates a chromatin architecture permissive for the binding of the tumor driver and fusion protein PAX3-FOXO1, allowing downstream transcription of its oncogenic program. Moreover, CHD4 depletion removes HDAC2 from the chromatin, leading to an increase and spread of histone acetylation, and prevents the positioning of RNA Polymerase 2 at promoters impeding transcription initiation. Strikingly, analysis of genome-wide cancer dependency databases identifies CHD4 as a general cancer vulnerability. Our findings describe CHD4, a classically defined repressor, as positive regulator of transcription and super-enhancer accessibility as well as establish this remodeler as an unexpected broad tumor susceptibility and promising drug target for cancer therapy.

Fenretinide induces a new form of dynamin-dependent cell death in pediatric sarcoma

Alveolar rhabdomyosarcoma (aRMS) is a highly malicious childhood malignancy characterized by specific chromosomal translocations mostly encoding the oncogenic transcription factor PAX3-FOXO1 and therefore also referred to as fusion-positive RMS (FP-RMS). Previously, we have identified fenretinide (retinoic acid p-hydroxyanilide) to affect PAX3-FOXO1 expression levels as well as FP-RMS cell viability. Here, we characterize the mode of action of fenretinide in more detail. First, we demonstrate that fenretinide-induced generation of reactive oxygen species (ROS) depends on complex II of the mitochondrial respiratory chain, since ROS scavenging as well as complexing of iron completely abolished cell death. Second, we co-treated cells with a range of pharmacological inhibitors of specific cell death pathways including z-vad (apoptosis), necrostatin-1 (necroptosis), 3-methyladenine (3-MA) (autophagy), and ferrostatin-1 (ferroptosis) together with fenretinide. Surprisingly, none of these inhibitors was able to prevent cell death. Also genetic depletion of key players in the apoptotic and necroptotic pathway (BAK, BAX, and RIPK1) confirmed the pharmacological data. Interestingly however, electron microscopy of fenretinide-treated cells revealed an excessive accumulation of cytoplasmic vacuoles, which were distinct from autophagosomes. Further flow cytometry and fluorescence microscopy experiments suggested a hyperstimulation of macropinocytosis, leading to an accumulation of enlarged early and late endosomes. Surprisingly, pharmacological inhibition as well as genetic depletion of large dynamin GTPases completely abolished fenretinide-induced vesicle formation and subsequent cell death, suggesting a new form of dynamin-dependent programmed cell death. Taken together, our data identify a new form of cell death mediated through the production of ROS by fenretinide treatment, highlighting the value of this compound for treatment of sarcoma patients including FP-RMS.

Targeting the undruggable: exploiting neomorphic features of fusion oncoproteins in childhood sarcomas for innovative therapies

While sarcomas account for approximately 1% of malignant tumors of adults, they are particularly more common in children and adolescents affected by cancer. In contrast to malignancies that occur in later stages of life, childhood tumors, including sarcoma, are characterized by a striking paucity of somatic mutations. However, entity-defining fusion oncogenes acting as the main oncogenic driver mutations are frequently found in pediatric bone and soft-tissue sarcomas such as Ewing sarcoma (EWSR1-FLI1), alveolar rhabdomyosarcoma (PAX3/7-FOXO1), and synovial sarcoma (SS18-SSX1/2/4). Since strong oncogene-dependency has been demonstrated in these entities, direct pharmacological targeting of these fusion oncogenes has been excessively attempted, thus far, with limited success. Despite apparent challenges, our increasing understanding of the neomorphic features of these fusion oncogenes in conjunction with rapid technological advances will likely enable the development of new strategies to therapeutically exploit these neomorphic features and to ultimately turn the “undruggable” into first-line target structures. In this review, we provide a broad overview of the current literature on targeting neomorphic features of fusion oncogenes found in Ewing sarcoma, alveolar rhabdomyosarcoma, and synovial sarcoma, and give a perspective for future developments.

Graphical abstract

Aurora A Kinase Inhibition Destabilizes PAX3-FOXO1 and MYCN and Synergizes with Navitoclax to Induce Rhabdomyosarcoma Cell Death

The clinically aggressive alveolar rhabdomyosarcoma (RMS) subtype is characterized by expression of the oncogenic fusion protein PAX3-FOXO1, which is critical for tumorigenesis and cell survival. Here, we studied the mechanism of cell death induced by loss of PAX3-FOXO1 expression and identified a novel pharmacologic combination therapy that interferes with PAX3-FOXO1 biology at different levels. Depletion of PAX3-FOXO1 in fusion-positive (FP)-RMS cells induced intrinsic apoptosis in a NOXA-dependent manner. This was pharmacologically mimicked by the BH3 mimetic navitoclax, identified as top compound in a screen from 208 targeted compounds. In a parallel approach, and to identify drugs that alter the stability of PAX3-FOXO1 protein, the same drug library was screened and fusion protein levels were directly measured as a read-out. This revealed that inhibition of Aurora kinase A most efficiently negatively affected PAX3-FOXO1 protein levels. Interestingly, this occurred through a novel specific phosphorylation event in and binding to the fusion protein. Aurora kinase A inhibition also destabilized MYCN, which is both a functionally important oncogene and transcriptional target of PAX3-FOXO1. Combined treatment with an Aurora kinase A inhibitor and navitoclax in FP-RMS cell lines and patient-derived xenografts synergistically induced cell death and significantly slowed tumor growth. These studies identify a novel functional interaction of Aurora kinase A with both PAX3-FOXO1 and its effector MYCN, and reveal new opportunities for targeted combination treatment of FP-RMS. SIGNIFICANCE: These findings show that Aurora kinase A and Bcl-2 family proteins are potential targets for FP-RMS.

Transcription Factors That Govern Development and Disease: An Achilles Heel in Cancer

Development requires the careful orchestration of several biological events in order to create any structure and, eventually, to build an entire organism. On the other hand, the fate transformation of terminally differentiated cells is a consequence of erroneous development, and ultimately leads to cancer. In this review, we elaborate how development and cancer share several biological processes, including molecular controls. Transcription factors (TF) are at the helm of both these processes, among many others, and are evolutionarily conserved, ranging from yeast to humans. Here, we discuss four families of TFs that play a pivotal role and have been studied extensively in both embryonic development and cancer—high mobility group box (HMG), GATA, paired box (PAX) and basic helix-loop-helix (bHLH) in the context of their role in development, cancer, and their conservation across several species. Finally, we review TFs as possible therapeutic targets for cancer and reflect on the importance of natural resistance against cancer in certain organisms, yielding knowledge regarding TF function and cancer biology.

PAX7 nucleotide variants and the risk of non-syndromic orofacial clefts in the Polish population

OBJECTIVE

The etiology of non-syndromic cleft lip with or without cleft palate (nsCL/P) is multifactorial, heterogeneous, and still not completely understood. The aim of the present study was to examine the associations between common and rare PAX7 nucleotide variants and the risk of this common congenital anomaly in a Polish population.

SUBJECTS AND METHODS

Eight top nsCL/P-associated PAX7 variants identified in our cleft genome-wide association study (GWAS) were selected for replication analysis in an independent group of patients and controls (n = 247 and n = 445, respectively). In addition, mutation screening of the PAX7 protein-coding region was conducted.

RESULTS

Analysis of the pooled data from the GWAS and replication study confirmed that common PAX7 nucleotide variants are significantly associated with the increased risk of nsCL/P. The strongest individual variant was rs1339062 (c.586 + 15617T > C) with a p-value = 2.47E-05 (OR = 1.4, 95%CI: 1.20-1.64). Sequencing analysis identified a novel synonymous PAX7 substitution (c.87G > A, p.Val29Val) in a single patient with nsCLP. This transition located in the early exonic position was predicted to disrupt potential splice enhancer elements.

CONCLUSION

Our study confirmed that PAX7 is a strong candidate gene for nsCL/P. Nucleotide variants of this gene contribute to the etiology of nsCL/P in the homogenous Polish population.

Epigenetic Regulation of the PTEN-AKT-RAC1 Axis by G9a Is Critical for Tumor Growth in Alveolar Rhabdomyosarcoma

Alveolar rhabdomyosarcoma (ARMS) is an aggressive pediatric cancer with poor prognosis. As transient and stable modifications to chromatin have emerged as critical mechanisms in oncogenic signaling, efforts to target epigenetic modifiers as a therapeutic strategy have accelerated in recent years. To identify chromatin modifiers that sustain tumor growth, we performed an epigenetic screen and found that inhibition of lysine methyltransferase G9a significantly affected the viability of ARMS cell lines. Targeting expression or activity of G9a reduced cellular proliferation and motility in vitro and tumor growth in vivo. Transcriptome and chromatin immunoprecipitation-sequencing analysis provided mechanistic evidence that the tumor-suppressor PTEN was a direct target gene of G9a. G9a repressed PTEN expression in a methyltransferase activity-dependent manner, resulting in increased AKT and RAC1 activity. Re-expression of constitutively active RAC1 in G9a-deficient tumor cells restored oncogenic phenotypes, demonstrating its critical functions downstream of G9a. Collectively, our study provides evidence for a G9a-dependent epigenetic program that regulates tumor growth and suggests targeting G9a as a therapeutic strategy in ARMS. SIGNIFICANCE: These findings demonstrate that RAC1 is an effector of G9a oncogenic functions and highlight the potential of G9a inhibitors in the treatment of ARMS.

Therapeutic Approaches Targeting PAX3-FOXO1 and Its Regulatory and Transcriptional Pathways in Rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is a family of soft tissue cancers that are related to the skeletal muscle lineage and predominantly occur in children and young adults. A specific chromosomal translocation t(2;13)(q35;q14) that gives rise to the chimeric oncogenic transcription factor PAX3-FOXO1 has been identified as a hallmark of the aggressive alveolar subtype of RMS. PAX3-FOXO1 cooperates with additional molecular changes to promote oncogenic transformation and tumorigenesis in various human and murine models. Its expression is generally restricted to RMS tumor cells, thus providing a very specific target for therapeutic approaches for these RMS tumors. In this article, we review the recent understanding of PAX3-FOXO1 as a transcription factor in the pathogenesis of this cancer and discuss recent developments to target this oncoprotein for treatment of RMS.

PAX3: A Molecule with Oncogenic or Tumor Suppressor Function Is Involved in Cancer

Metastasis is the most deadly aspect of cancer and results from acquired gene regulation abnormalities in tumor cells. Transcriptional regulation is an essential component of controlling of gene function and its failure could contribute to tumor progression and metastasis. During cancer progression, deregulation of oncogenic or tumor suppressive transcription factors, as well as master cell fate regulators, collectively influences multiple steps of the metastasis cascade, including local invasion and dissemination of the tumor to distant organs. Transcription factor PAX3/Pax3, which contributes to diverse cell lineages during embryonic development, plays a major role in tumorigenesis. Mutations in this gene can cause neurodevelopmental disease and the existing literature supports that there is a potential link between aberrant expression of PAX3 genes in adult tissues and a wide variety of cancers. PAX3 function is tissue-specific and could contribute to tumorigenesis either directly as oncogene or as a tumor suppressor by losing its function. In this review, we discuss comprehensively the differential role played by PAX3 in various tissues and how its aberrant expression is implicated in disease development. This review particularly highlights the oncogenic and tumor suppressor role played by PAX3 in different cancers and underlines the importance of precisely identifying tissue-specific role of PAX3 in order to determine its exact role in development of cancer.

PAX3-FOXO1 drives miR-486-5p and represses miR-221 contributing to pathogenesis of alveolar rhabdomyosarcoma

Rhabdomyosarcoma is the most common soft-tissue sarcoma in childhood and histologically resembles developing skeletal muscle. Alveolar rhabdomyosarcoma (ARMS) is an aggressive subtype with a higher rate of metastasis and poorer prognosis. The majority of ARMS tumors (80%) harbor a PAX3-FOXO1 or less commonly a PAX7-FOXO1 fusion gene. The presence of either the PAX3-FOXO1 or PAX7-FOXO1 fusion gene foretells a poorer prognosis resulting in clinical re-classification as either fusion-positive (FP-RMS) or fusion-negative RMS (FN-RMS). The PAX3/7-FOXO1 fusion genes result in the production of a rogue transcription factors that drive FP-RMS pathogenesis and block myogenic differentiation. Despite knowing the molecular driver of FP-RMS, targeted therapies have yet to make an impact for patients, highlighting the need for a greater understanding of the molecular consequences of PAX3-FOXO1 and its target genes including microRNAs. Here we show FP-RMS patient-derived xenografts and cell lines display a distinct microRNA expression pattern. We utilized both loss- and gain-of function approaches in human cell lines with knockdown of PAX3-FOXO1 in FP-RMS cell lines and expression of PAX3-FOXO1 in human myoblasts and identified microRNAs both positively and negatively regulated by the PAX3-FOXO1 fusion protein. We demonstrate PAX3-FOXO1 represses miR-221/222 that functions as a tumor suppressing microRNA through the negative regulation of CCND2, CDK6, and ERBB3. In contrast, miR-486-5p is transcriptionally activated by PAX3-FOXO1 and promotes FP-RMS proliferation, invasion, and clonogenic growth. Inhibition of miR-486-5p in FP-RMS xenografts decreased tumor growth, illustrating a proof of principle for future therapeutic intervention. Therefore, PAX3-FOXO1 regulates key microRNAs that may represent novel therapeutic vulnerabilities in FP-RMS.

An Examination of the Role of Transcriptional and Posttranscriptional Regulation in Rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is an aggressive family of soft tissue tumors that most commonly manifests in children. RMS variants express several skeletal muscle markers, suggesting myogenic stem or progenitor cell origin of RMS. In this review, the roles of both recently identified and well-established microRNAs in RMS are discussed and summarized in a succinct, tabulated format. Additionally, the subtypes of RMS are reviewed along with the involvement of basic helix-loop-helix (bHLH) proteins, Pax proteins, and microRNAs in normal and pathologic myogenesis. Finally, the current and potential future treatment options for RMS are outlined.

Molecular diagnostics in the management of rhabdomyosarcoma

INTRODUCTION

A classification of rhabdomyosarcoma (RMS) with prognostic relevance has primarily relied on clinical features and histologic classification as either embryonal or alveolar RMS. The PAX3-FOXO1 and PAX7-FOXO1 gene fusions occur in 80% of cases with the alveolar subtype and are more predictive of outcome than histologic classification. Identifying additional molecular hallmarks that further subclassify RMS is an active area of research. Areas Covered: The authors review the current state of the PAX3-FOXO1 and PAX7-FOXO1 fusions as prognostic biomarkers. Emerging biomarkers, including mRNA expression profiling, MYOD1 mutations, RAS pathway mutations and gene fusions involving NCOA2 or VGLL2 are also reviewed. Expert commentary: Strategies for modifying RMS risk stratification based on molecular biomarkers are emerging with the potential to transform the clinical management of RMS, ultimately improving patient outcomes by tailoring therapy to predicted patient risk and identifying targets for novel therapies.

Myogenic potential of mouse embryonic stem cells lacking functional Pax7 tested in vitro by 5-azacitidine treatment and in vivo in regenerating skeletal muscle

Regeneration of skeletal muscle relies on the presence of satellite cells. Satellite cells deficiency accompanying some degenerative diseases is the reason for the search for the "replacement cells" that can be used in the muscle therapies. Due to their unique properties embryonic stem cells (ESCs), as well as myogenic cells derived from them, are considered as a promising source of therapeutic cells. Among the factors crucial for the specification of myogenic precursor cells is Pax7 that sustains proper function of satellite cells. In our previous studies we showed that ESCs lacking functional Pax7 are able to form myoblasts in vitro when differentiated within embryoid bodies and their outgrowths. In the current study we showed that ESCs lacking functional Pax7, cultured in vitro in monolayer in the medium supplemented with horse serum and 5azaC, expressed higher levels of factors associated with myogenesis, such as Pdgfra, Pax3, Myf5, and MyoD. Importantly, skeletal myosin immunolocalization confirmed that myogenic differentiation of ESCs was more effective in case of cells lacking Pax7. Our in vivo studies showed that ESCs transplanted into regenerating skeletal muscles were detectable at day 7 of regeneration and the number of Pax7-/- ESCs detected was significantly higher than of control cells. Our results support the concept that lack of functional Pax7 promotes proliferation of differentiating ESCs and for this reason more of them can turn into myogenic lineage.

Helicase CHD4 is an epigenetic coregulator of PAX3-FOXO1 in alveolar rhabdomyosarcoma

A vast number of cancer genes are transcription factors that drive tumorigenesis as oncogenic fusion proteins. Although the direct targeting of transcription factors remains challenging, therapies aimed at oncogenic fusion proteins are attractive as potential treatments for cancer. There is particular interest in targeting the oncogenic PAX3-FOXO1 fusion transcription factor, which induces alveolar rhabdomyosarcoma (aRMS), an aggressive cancer of skeletal muscle cells for which patient outcomes remain dismal. In this work, we have defined the interactome of PAX3-FOXO1 and screened 60 candidate interactors using siRNA-mediated depletion to identify candidates that affect fusion protein activity in aRMS cells. We report that chromodomain helicase DNA binding protein 4 (CHD4), an ATP-dependent chromatin remodeler, acts as crucial coregulator of PAX3-FOXO1 activity. CHD4 interacts with PAX3-FOXO1 via short DNA fragments. Together, they bind to regulatory regions of PAX3-FOXO1 target genes. Gene expression analysis suggested that CHD4 coregulatory activity is essential for a subset of PAX3-FOXO1 target genes. Depletion of CHD4 reduced cell viability of fusion-positive but not of fusion-negative RMS in vitro, which resembled loss of PAX3-FOXO1. It also caused specific regression of fusion-positive xenograft tumors in vivo. Therefore, this work identifies CHD4 as an epigenetic coregulator of PAX3-FOXO1 activity, providing rational evidence for CHD4 as a potential therapeutic target in aRMS.

P/CAF mediates PAX3-FOXO1-dependent oncogenesis in alveolar rhabdomyosarcoma

Alveolar rhabdomyosarcoma (ARMS) is an aggressive paediatric cancer of skeletal muscle with poor prognosis. A PAX3-FOXO1 fusion protein acts as a driver of malignancy in ARMS by disrupting tightly coupled but mutually exclusive pathways of proliferation and differentiation. While PAX3-FOXO1 is an attractive therapeutic target, no current treatments are designed to block its oncogenic activity. The present work shows that the histone acetyltransferase P/CAF (KAT2B) is overexpressed in primary tumours from ARMS patients. Interestingly, in fusion-positive ARMS cell lines, P/CAF acetylates and stabilizes PAX3-FOXO1 rather than MyoD, a master regulator of muscle differentiation. Silencing P/CAF, or pharmacological inhibition of its acetyltransferase activity, down-regulates PAX3-FOXO1 levels concomitant with reduced proliferation and tumour burden in xenograft mouse models. Our studies identify a P/CAF-PAX3-FOXO1 signalling node that promotes oncogenesis and may contribute to MyoD dysfunction in ARMS. This work exemplifies the therapeutic potential of targeting chromatin-modifying enzymes to inhibit fusion oncoproteins that are a frequent event in sarcomas. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Pediatric Rhabdomyosarcoma

Rhabdomyosarcoma is the most common soft-tissue sarcoma of childhood, and despite clinical advances, subsets of these patients continue to suffer high levels of morbidity and mortality associated with their disease. Recent genetic and molecular characterization of these tumors using sophisticated genomics techniques, including next-generation sequencing experiments, has revealed multiple areas that can be exploited for new molecularly targeted therapies for this disease.

PLK1 phosphorylates PAX3-FOXO1, the inhibition of which triggers regression of alveolar Rhabdomyosarcoma

Pediatric tumors harbor very low numbers of somatic mutations and therefore offer few targets to improve therapeutic management with targeted drugs. In particular, outcomes remain dismal for patients with metastatic alveolar rhabdomyosarcoma (aRMS), where the chimeric transcription factor PAX3/7-FOXO1 has been implicated but problematic to target. In this report, we addressed this challenge by developing a two-armed screen for druggable upstream regulatory kinases in the PAX3/7-FOXO1 pathway. Screening libraries of kinome siRNA and small molecules, we defined PLK1 as an upstream-acting regulator. Mechanistically, PLK1 interacted with and phosphorylated PAX3-FOXO1 at the novel site S503, leading to protein stabilization. Notably, PLK1 inhibition led to elevated ubiquitination and rapid proteasomal degradation of the PAX3-FOXO1 chimeric oncoprotein. On this basis, we embarked on a preclinical validation of PLK1 as a target in a xenograft mouse model of aRMS, where the PLK1 inhibitor BI 2536 reduced PAX3-FOXO1-mediated gene expression and elicited tumor regression. Clinically, analysis of human aRMS tumor biopsies documented high PLK1 expression to offer prognostic significance for both event-free survival and overall survival. Taken together, these preclinical studies validate the PLK1-PAX3-FOXO1 axis as a rational target to treat aRMS.

Pax3 expression enhances PDGF-B-induced brainstem gliomagenesis and characterizes a subset of brainstem glioma

High-grade Brainstem Glioma (BSG), also known as Diffuse Intrinsic Pontine Glioma (DIPG), is an incurable pediatric brain cancer. Increasing evidence supports the existence of regional differences in gliomagenesis such that BSG is considered a distinct disease from glioma of the cerebral cortex (CG). In an effort to elucidate unique characteristics of BSG, we conducted expression analysis of mouse PDGF-B-driven BSG and CG initiated in Nestin progenitor cells and identified a short list of expression changes specific to the brainstem gliomagenesis process, including abnormal upregulation of paired box 3 (Pax3). In the neonatal mouse brain, Pax3 expression marks a subset of brainstem progenitor cells, while it is absent from the cerebral cortex, mirroring its regional expression in glioma. Ectopic expression of Pax3 in normal brainstem progenitors in vitro shows that Pax3 inhibits apoptosis. Pax3-induced inhibition of apoptosis is p53-dependent, however, and in the absence of p53, Pax3 promotes proliferation of brainstem progenitors. In vivo, Pax3 enhances PDGF-B-driven gliomagenesis by shortening tumor latency and increasing tumor penetrance and grade, in a region-specific manner, while loss of Pax3 function extends survival of PDGF-B-driven;p53-deficient BSG-bearing mice by 33%. Importantly, Pax3 is regionally expressed in human glioma as well, with high PAX3 mRNA characterizing 40% of human BSG, revealing a subset of tumors that significantly associates with PDGFRA alterations, amplifications of cell cycle regulatory genes, and is exclusive of ACVR1 mutations. Collectively, these data suggest that regional Pax3 expression not only marks a novel subset of BSG but also contributes to PDGF-B-induced brainstem gliomagenesis.

PAX genes in childhood oncogenesis: developmental biology gone awry?

Childhood solid tumors often arise from embryonal-like cells, which are distinct from the epithelial cancers observed in adults, and etiologically can be considered as 'developmental patterning gone awry'. Paired-box (PAX) genes encode a family of evolutionarily conserved transcription factors that are important regulators of cell lineage specification, migration and tissue patterning. PAX loss-of-function mutations are well known to cause potent developmental phenotypes in animal models and underlie genetic disease in humans, whereas dysregulation and/or genetic modification of PAX genes have been shown to function as critical triggers for human tumorigenesis. Consequently, exploring PAX-related pathobiology generates insights into both normal developmental biology and key molecular mechanisms that underlie pediatric cancer, which are the topics of this review.

Molecular profiling of childhood cancer: Biomarkers and novel therapies

BACKGROUND

Technological advances including high-throughput sequencing have identified numerous tumor-specific genetic changes in pediatric and adolescent cancers that can be exploited as targets for novel therapies.

SCOPE OF REVIEW

This review provides a detailed overview of recent advances in the application of target-specific therapies for childhood cancers, either as single agents or in combination with other therapies. The review summarizes preclinical evidence on which clinical trials are based, early phase clinical trial results, and the incorporation of predictive biomarkers into clinical practice, according to cancer type.

MAJOR CONCLUSIONS

There is growing evidence that molecularly targeted therapies can valuably add to the arsenal available for treating childhood cancers, particularly when used in combination with other therapies. Nonetheless the introduction of molecularly targeted agents into practice remains challenging, due to the use of unselected populations in some clinical trials, inadequate methods to evaluate efficacy, and the need for improved preclinical models to both evaluate dosing and safety of combination therapies.

GENERAL SIGNIFICANCE

The increasing recognition of the heterogeneity of molecular causes of cancer favors the continued development of molecularly targeted agents, and their transfer to pediatric and adolescent populations.

Role of PAX8 in the regulation of MET and RON receptor tyrosine kinases in non-small cell lung cancer

BACKGROUND

Non-small cell lung cancers (NSCLC) are highly heterogeneous at the molecular level and comprise 75% of all lung tumors. We have previously shown that the receptor tyrosine kinase (RTK) MET frequently suffers gain-of-function mutations that significantly promote lung tumorigenesis. Subsequent studies from our lab also revealed that PAX5 transcription factor is preferentially expressed in small cell lung cancer (SCLC) and promotes MET transcription. PAX8, however, is also expressed in NSCLC cell lines. We therefore investigated the role of PAX8 in NSCLC.

METHODS

Using IHC analysis, PAX8 protein expression was determined in archival NSCLC tumor tissues (n = 254). In order to study the effects of PAX8 knockdown on NSCLC cellular functions such as apoptosis and motility, siRNA against PAX8 was used. Confocal fluorescence microscopy was used to monitor the localization of MET, RON and PAX8. The combinatorial effect of PAX8 knockdown and MET inhibition using SU11274 was investigated in NSCLC cell viability assay.

RESULTS

Relative levels of PAX8 protein were elevated (≥ + 2 on a scale of 0-3) in adenocarcinoma (58/94), large cell carcinoma (50/85), squamous cell carcinoma (28/47), and metastatic NSCLC (17/28; lymph node). Utilizing early progenitors isolated from NSCLC cell lines and fresh tumor tissues, we observed robust overexpression of PAX8, MET, and RON. PAX8 knockdown A549 cells revealed abrogated PAX8 expression with a concomitant loss in MET and the related RON kinase expression. A dramatic colocalization between the active form of MET (also RON) and PAX8 upon challenging A549 cells with HGF was visualized. A similar colocalization of MET and EGL5 (PAX8 ortholog) proteins was found in embryos of C. elegans. Most importantly, knockdown of PAX8 in A549 cells resulted in enhanced apoptosis (~6 fold) and decreased cell motility (~45%), thereby making PAX8 a potential therapeutic target. However, the combinatorial approach of PAX8 knockdown and treatment with MET inhibitor, SU11274, had marginal additive effect on loss of NSCLC cell viability.

CONCLUSION

PAX8 provides signals for growth and motility of NSCLC cells and is necessary for MET and RON expression. Further investigations are necessary to investigate the therapeutic potential of PA8 in NSCLC.

Increased paired box transcription factor 8 has a survival function in glioma

Background

The molecular basis to overcome therapeutic resistance to treat glioblastoma remains unclear. The anti-apoptotic b cell lymphoma 2 (BCL2) gene is associated with treatment resistance, and is transactivated by the paired box transcription factor 8 (PAX8). In earlier studies, we demonstrated that increased PAX8 expression in glioma cell lines was associated with the expression of telomerase. In this current study, we more extensively explored a role for PAX8 in gliomagenesis.

Methods

PAX8 expression was measured in 156 gliomas including telomerase-negative tumours, those with the alternative lengthening of telomeres (ALT) mechanism or with a non-defined telomere maintenance mechanism (NDTMM), using immunohistochemistry and quantitative PCR. We also tested the affect of PAX8 knockdown using siRNA in cell lines on cell survival and BCL2 expression.

Results

Seventy-two percent of glioblastomas were PAX8-positive (80% telomerase, 73% NDTMM, and 44% ALT). The majority of the low-grade gliomas and normal brain cells were PAX8-negative. The suppression of PAX8 was associated with a reduction in both cell growth and BCL2, suggesting that a reduction in PAX8 expression would sensitise tumours to cell death.

Conclusions

PAX8 is increased in the majority of glioblastomas and promoted cell survival. Because PAX8 is absent in normal brain tissue, it may be a promising therapeutic target pathway for treating aggressive gliomas.

Pax3 function is required specifically for inner ear structures with melanogenic fates

Pax3 mutations result in malformed inner ears in Splotch mutant mice and hearing loss in humans with Waardenburg's syndrome type I. In the inner ear, Pax3 is thought to be involved mainly in the development of neural crest. However, recent studies have shown that Pax3-expressing cells contribute extensively to multiple inner ear structures, some of which were considered to be derived from the otic epithelium. To examine the specific functions of Pax3 during inner ear development, fate mapping of Pax3 lineage was performed in the presence or absence of functional Pax3 proteins using Pax3(Cre) knock-in mice bred to Rosa26 reporter (R26R) line. β-gal-positive cells were widely distributed in Pax3(Cre/+); R26R inner ears at embryonic day (E) 15.5, including the endolymphatic duct, common crus, cristae, maculae, cochleovestibular ganglion, and stria vascularis. In the absence of Pax3 in Pax3(Cre/Cre); R26R inner ears, β-gal-positive cells disappeared from regions with melanocytes such as the stria vascularis of the cochlea and dark cells in the vestibule. Consistently, the expression of Dct, a melanoblast marker, was also absent in the mutant inner ears. However, when examined at E11.5, β-gal positive cells were present in Pax3(Cre/Cre) mutant otocysts, whereas Dct expression was absent, suggesting that Pax3 lineage with a melanogenic fate migrated to the inner ear, yet failed to differentiate and survive without Pax3 function. Gross inner ear morphology was generally normal in Pax3(Cre/Cre) mutants, unless neural tube defects extended to the cranial region. Taken together, these results suggest that despite the extensive contribution of Pax3-expressing cells to multiple inner ear tissues, Pax3 function is required specifically for inner ear components with melanogenic fates.

Targeted therapies for soft-tissue sarcomas

The past several decades have revealed certain challenges that are inherent to soft-tissue sarcomas with regards to devising, testing and setting treatment paradigms for such a rare and heterogeneous malignancy. Despite this, significant advances have been made through the efforts of scientists, clinicians and patients alike. We are now entering a molecular era of cancer and current biotechnology is beginning to unravel the pathogenic enigma of these often devastating tumors. As our understanding of these malignancies improves, so does our list of potential treatment options. The impetus now lies with the medical/scientific community to direct translational research and subsequently the development and clinical testing of novel compounds in a fashion that best serves this unique patient population. To do so, we must continue to integrate the lessons of the past with the resources and promise of the future. This review will outline current areas of therapeutic interest in soft-tissue sarcomas with regard to agents that have reached clinical testing.

PAX3 in neuroblastoma: oncogenic potential, chemosensitivity and signalling pathways

Transcription factor PAX3/Pax3 contributes to diverse cell lineages during embryonic development and is important in tumourigenesis. We found that PAX3 is re-expressed in neuroblastoma and malignant neuroblastic (N-type) neuroblastoma cells had significantly higher PAX3 protein expression than their benign substrate-adherent (S-type) counterparts. Knock-down of PAX3 expression by siRNA transfection resulted in persistent cell growth inhibition in both types of neuroblastoma cell, owing to G1 cell cycle arrest and progressive apoptosis. Inhibition of PAX3 expression significantly decreased the attachment of S-type SH-EP1 cells to extra-cellular matrix proteins, fibronectin, laminin and collagen IV. Migration and invasion of both neuroblastoma cell types were markedly reduced after PAX3 down-regulation. PAX3 knock-down significantly augmented the cytotoxic effect of chemotherapeutic agents, etoposide, vincristine and cisplatin, commonly used to treat neuroblastoma. Microarray analyses revealed that particularly signalling pathways involving cell cycle, apoptosis, cell adhesion, cytoskeletal remodelling and development were altered by PAX3 down-regulation. Changes in PAX3 downstream genes identified by microarray analyses were validated in 47 genes by quantitative PCR. These novel findings lead us to propose that PAX3 might contribute to oncogenic characteristics of neuroblastoma cells by regulating a variety of crucial signalling pathways.

Small molecule inhibition of PAX3-FOXO1 through AKT activation suppresses malignant phenotypes of alveolar rhabdomyosarcoma

Alveolar rhabdomyosarcoma comprises a rare highly malignant tumor presumed to be associated with skeletal muscle lineage in children. The hallmark of the majority of alveolar rhabdomyosarcoma is a chromosomal translocation that generates the PAX3-FOXO1 fusion protein, which is an oncogenic transcription factor responsible for the development of the malignant phenotype of this tumor. Alveolar rhabdomyosarcoma cells are dependent on the oncogenic activity of PAX3-FOXO1, and its expression status in alveolar rhabdomyosarcoma tumors correlates with worst patient outcome, suggesting that blocking this activity of PAX3-FOXO1 may be an attractive therapeutic strategy against this fusion-positive disease. In this study, we screened small molecule chemical libraries for inhibitors of PAX3-FOXO1 transcriptional activity using a cell-based readout system. We identified the Sarco/endoplasmic reticulum Ca(2+)-ATPases (SERCA) inhibitor thapsigargin as an effective inhibitor of PAX3-FOXO1. Subsequent experiments in alveolar rhabdomyosarcoma cells showed that activation of AKT by thapsigargin inhibited PAX3-FOXO1 activity via phosphorylation. Moreover, this AKT activation appears to be associated with the effects of thapsigargin on intracellular calcium levels. Furthermore, thapsigargin inhibited the binding of PAX3-FOXO1 to target genes and subsequently promoted its proteasomal degradation. In addition, thapsigargin treatment decreases the growth and invasive capacity of alveolar rhabdomyosarcoma cells while inducing apoptosis in vitro. Finally, thapsigargin can suppress the growth of an alveolar rhabdomyosarcoma xenograft tumor in vivo. These data reveal that thapsigargin-induced activation of AKT is an effective mechanism to inhibit PAX3-FOXO1 and a potential agent for targeted therapy against alveolar rhabdomyosarcoma.

Mechanisms of impaired differentiation in rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma of childhood, with presumed skeletal muscle origins, because of its myogenic phenotype. RMS is composed of two main subtypes, embryonal RMS (eRMS) and alveolar RMS (aRMS). Whereas eRMS histologically resembles embryonic skeletal muscle, the aRMS subtype is more aggressive and has a poorer prognosis. In addition, whereas the genetic profile of eRMS is not well established, aRMS is commonly associated with distinct chromosome translocations that fuse domains of the transcription factors Pax3 and Pax7 to the forkhead family member FOXO1A. Both eRMS and aRMS tumor cells express myogenic markers such as MyoD, but their ability to complete differentiation is impaired. How this impairment occurs is the subject of this review, which will focus on several themes, including signaling pathways that converge on Pax-forkhead gene targets, alterations in MyoD function, epigenetic modifications of myogenic promoters, and microRNAs whose expression patterns in RMS alter key regulatory circuits to help maintain tumor cells in an opportunistically less differentiated state.

Structural and functional studies of FKHR-PAX3, a reciprocal fusion gene of the t(2;13) chromosomal translocation in alveolar rhabdomyosarcoma

Alveolar rhabdomyosarcoma (ARMS) is an aggressive pediatric cancer of skeletal muscle. More than 70% of ARMS tumors carry balanced t(2;13) chromosomal translocation that leads to the production of two novel fusion genes, PAX3-FKHR and FKHR-PAX3. While the PAX3-FKHR gene has been intensely studied, the reciprocal FKHR-PAX3 gene has rarely been described. We report here the cloning and functional characterization of the FKHR-PAX3 gene as the first step towards a better understanding of its potential impact on ARMS biology. From RH30 ARMS cells, we detected and isolated three versions of FKHR-PAX3 cDNAs whose C-terminal sequences corresponded to PAX3c, PAX3d, and PAX3e isoforms. Unlike the nuclear-specific localization of PAX3-FKHR, the reciprocal FKHR-PAX3 proteins stayed predominantly in the cytoplasm. FKHR-PAX3 potently inhibited myogenesis in both non-transformed myoblast cells and ARMS cells. We showed that FKHR-PAX3 was not a classic oncogene but could act as a facilitator in oncogenic pathways by stabilizing PAX3-FKHR expression, enhancing cell proliferation, clonogenicity, anchorage-independent growth, and matrix adhesion in vitro, and accelerating the onset of tumor formation in xenograft mouse model in vivo. In addition to these pro-oncogenic behaviors, FKHR-PAX3 also negatively affected cell migration and invasion in vitro and lung metastasis in vivo. Taken together, these functional characteristics suggested that FKHR-PAX3 might have a critical role in the early stage of ARMS development.

Identification of target genes of PAX3-FOXO1 in alveolar rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is a soft tissue sarcoma categorized into two major subtypes: alveolar RMS (ARMS) and embryonal RMS (ERMS). Most ARMS express the PAX3-FOXO1 (P3F) fusion oncoprotein generated by the 2;13 chromosomal translocation. In the present study, the downstream target genes of P3F were identified by analyzing two independent sets of gene expression profiles: primary RMS tumors and RD ERMS cells transduced with inducible P3F constructs. We found 34 potential target genes (27 upregulated and 7 downregulated) that were significantly and differentially expressed between P3F-positive and P3F-negative categories, both in primary RMS tumors and in the inducible P3F cell culture system. Gene ontology analysis of microarray data of the inducible P3F cell culture system employed indicated apoptosis, cell death, development, and signal transduction as overrepresented significant functional categories found in both upregulated and downregulated genes. Therefore, among the 34 potential target genes, the expression of cell death-related [Gremlin1, cysteine knot superfamily 1, BMP antagonist 1 (GREM1) and death-associated protein kinase 1 (DAPK1)] and development-related [myogenic differentiation 1 (MYOD1) and hairy/enhancer-of-split related with YRPW motif 1 (HEY1)] genes were further investigated. The differential expression of GREM1, DAPK1, MYOD1 and HEY1 was confirmed in independent tumors and inducible cell culture systems. The expression of GREM1, DAPK1 and MYOD1 were significantly upregulated; HEY1 was significantly downregulated in independent P3F-positive ARMS tumors and transcriptionally active P3F cells, compared to those in ERMS tumors and transcriptionally inactive P3F cells. This study identified target genes of P3F and suggested that four downstream targets (GREM1, DAPK1, MYOD1 and HEY1) can contribute to the biological activities of P3F involved in growth suppression or cell death and myogenic differentiation.

A call to ARMS: targeting the PAX3-FOXO1 gene in alveolar rhabdomyosarcoma

INTRODUCTION

Expression of fusion oncoproteins generated by recurrent chromosomal translocations represents a major tumorigenic mechanism characteristic of multiple cancers, including one-third of all sarcomas. Oncogenic fusion genes provide novel targets for therapeutic intervention. The PAX3-FOXO1 oncoprotein in alveolar rhabdomyosarcoma (ARMS) is presented as a paradigm to examine therapeutic strategies for targeting sarcoma-associated fusion genes.

AREAS COVERED

This review discusses the role of PAX3-FOXO1 in ARMS tumors. Besides evaluating various approaches to molecularly target PAX3-FOXO1 itself, this review highlights therapeutically attractive downstream genes activated by PAX3-FOXO1.

EXPERT OPINION

Oncogenic fusion proteins represent desirable therapeutic targets because their expression is specific to tumor cells, but these fusions generally characterize rare malignancies. Full development and testing of potential drugs targeted to these fusions are complicated by the small numbers of patients in these disease categories. Although efforts to develop targeted therapies against fusion proteins should continue, molecular targets that are applicable to a broader tumor landscape should be pursued. A shift of the traditional paradigm to view therapeutic intervention as target-specific rather than tumor-specific will help to circumvent the challenges posed by rare tumors and maximize the possibility of developing successful new treatments for patients with these rare translocation-associated sarcomas.

Cell-based small-molecule compound screen identifies fenretinide as potential therapeutic for translocation-positive rhabdomyosarcoma

A subset of paediatric sarcomas are characterized by chromosomal translocations encoding specific oncogenic transcription factors. Such fusion proteins represent tumor specific therapeutic targets although so far it has not been possible to directly inhibit their activity by small-molecule compounds. In this study, we hypothesized that screening a small-molecule library might identify already existing drugs that are able to modulate the transcriptional activity of PAX3/FOXO1, the fusion protein specifically found in the pediatric tumor alveolar rhabdomyosarcoma (aRMS). Towards this end, we established a reporter cell line based on the well characterized PAX3/FOXO1 target gene AP2ß. A library enriched in mostly FDA approved drugs was screened using specific luciferase activity as read-out and normalized for cell viability. The most effective inhibitor identified from this screen was Fenretinide. Treatment with this compound resulted in down-regulation of PAX3/FOXO1 mRNA and protein levels as well as in reduced expression of several of its direct target genes, but not of wild-type FOXO1, in a dose- and time-dependent manner. Moreover, fenretinide induced reactive oxygen species and apoptosis as shown by caspase 9 and PARP cleavage and upregulated miR-9. Importantly, it demonstrated a significant anti-tumor effect in vivo. These results are similar to earlier reports for two other pediatric tumors, namely neuroblastoma and Ewing sarcoma, where fenretinide is under clinical development. Our results suggest that fenretinide might represent a novel treatment option also for translocation-positive rhabdomyosarcoma.

Caveolins in rhabdomyosarcoma

Caveolins are scaffolding proteins that play a pivotal role in numerous processes, including caveolae biogenesis, vesicular transport, cholesterol homeostasis and regulation of signal transduction. There are three different isoforms (Cav-1, -2 and -3) that form homo- and hetero-aggregates at the plasma membrane and modulate the activity of a number of intracellular binding proteins. Cav-1 and Cav-3, in particular, are respectively expressed in the reserve elements (e.g. satellite cells) and in mature myofibres of skeletal muscle and their expression interplay characterizes the switch from muscle precursors to differentiated elements. Recent findings have shown that caveolins are also expressed in rhabdomyosarcoma, a group of heterogeneous childhood soft-tissue sarcomas in which the cancer cells seem to derive from progenitors that resemble myogenic cells. In this review, we will focus on the role of caveolins in rhabdomyosarcomas and on their potential use as markers of the degree of differentiation in these paediatric tumours. Given that the function of Cav-1 as tumour conditional gene in cancer has been well-established, we will also discuss the relationship between Cav-1 and the progression of rhabdomyosarcoma.

Antitumor activity of sustained N-myc reduction in rhabdomyosarcomas and transcriptional block by antigene therapy

PURPOSE

Rhabdomyosarcomas are a major cause of cancer death in children, described with MYCN amplification and, in the alveolar subtype, transcription driven by the PAX3-FOXO1 fusion protein. Our aim was to determine the prevalence of N-Myc protein expression and the potential therapeutic effects of reducing expression in rhabdomyosarcomas, including use of an antigene strategy that inhibits transcription.

EXPERIMENTAL DESIGN

Protein expression was assessed by immunohistochemistry. MYCN expression was reduced in representative cell lines by RNA interference and an antigene peptide nucleic acid (PNA) oligonucleotide conjugated to a nuclear localization signal peptide. Associated gene expression changes, cell viability, and apoptosis were analyzed in vitro. As a paradigm for antigene therapy, the effects of systemic treatment of mice with rhabdomyosarcoma cell line xenografts were determined.

RESULTS

High N-Myc levels were significantly associated with genomic amplification, presence of the PAX3/7-FOXO1 fusion genes, and proliferative capacity. Sustained reduction of N-Myc levels in all rhabdomyosarcoma cell lines that express the protein decreased cell proliferation and increased apoptosis. Positive feedback was shown to regulate PAX3-FOXO1 and N-Myc levels in the alveolar subtype that critically decrease PAX3-FOXO1 levels on reducing N-Myc. Pharmacologic systemic administration of the antigene PNA can eliminate alveolar rhabdomyosarcoma xenografts in mice, without relapse or toxicity.

CONCLUSION

N-Myc, with its restricted expression in non-fetal tissues, is a therapeutic target to treat rhabdomyosarcomas, and blocking gene transcription using antigene oligonucleotide strategies has therapeutic potential in the treatment of cancer and other diseases that has not been previously realized in vivo.