Histone lysine demethylase 4 family proteins maintain the transcriptional program and adrenergic cellular state of MYCN-amplified neuroblastoma

Neuroblastoma with MYCN amplification (MNA) is a high-risk disease that has a poor survival rate. Neuroblastoma displays cellular heterogeneity, including more differentiated (adrenergic) and more primitive (mesenchymal) cellular states. Here, we demonstrate that MYCN oncoprotein promotes a cellular state switch in mesenchymal cells to an adrenergic state, accompanied by induction of histone lysine demethylase 4 family members (KDM4A-C) that act in concert to control the expression of MYCN and adrenergic core regulatory circulatory (CRC) transcription factors. Pharmacologic inhibition of KDM4 blocks expression of MYCN and the adrenergic CRC transcriptome with genome-wide induction of transcriptionally repressive H3K9me3, resulting in potent anticancer activity against neuroblastomas with MNA by inducing neuroblastic differentiation and apoptosis. Furthermore, a short-term KDM4 inhibition in combination with conventional, cytotoxic chemotherapy results in complete tumor responses of xenografts with MNA. Thus, KDM4 blockade may serve as a transformative strategy to target the adrenergic CRC dependencies in MNA neuroblastomas.

PAX3-FOXO1 dictates myogenic reprogramming and rhabdomyosarcoma identity in endothelial progenitors

Fusion-positive rhabdomyosarcoma (FP-RMS) driven by the expression of the PAX3-FOXO1 (P3F) fusion oncoprotein is an aggressive subtype of pediatric rhabdomyosarcoma. FP-RMS histologically resembles developing muscle yet occurs throughout the body in areas devoid of skeletal muscle highlighting that FP-RMS is not derived from an exclusively myogenic cell of origin. Here we demonstrate that P3F reprograms mouse and human endothelial progenitors to FP-RMS. We show that P3F expression in aP2-Cre expressing cells reprograms endothelial progenitors to functional myogenic stem cells capable of regenerating injured muscle fibers. Further, we describe a FP-RMS mouse model driven by P3F expression and Cdkn2a loss in endothelial cells. Additionally, we show that P3F expression in TP53-null human iPSCs blocks endothelial-directed differentiation and guides cells to become myogenic cells that form FP-RMS tumors in immunocompromised mice. Together these findings demonstrate that FP-RMS can originate from aberrant development of non-myogenic cells driven by P3F.

Proteasome inhibition targets the KMT2A transcriptional complex in acute lymphoblastic leukemia

Rearrangments in Histone-lysine-N-methyltransferase 2A (KMT2Ar) are associated with pediatric, adult and therapy-induced acute leukemias. Infants with KMT2Ar acute lymphoblastic leukemia (ALL) have a poor prognosis with an event-free-survival of 38%. Herein we evaluate 1116 FDA approved compounds in primary KMT2Ar infant ALL specimens and identify a sensitivity to proteasome inhibition. Upon exposure to this class of agents, cells demonstrate a depletion of histone H2B monoubiquitination (H2Bub1) and histone H3 lysine 79 dimethylation (H3K79me2) at KMT2A target genes in addition to a downregulation of the KMT2A gene expression signature, providing evidence that it targets the KMT2A transcriptional complex and alters the epigenome. A cohort of relapsed/refractory KMT2Ar patients treated with this approach on a compassionate basis had an overall response rate of 90%. In conclusion, we report on a high throughput drug screen in primary pediatric leukemia specimens whose results translate into clinically meaningful responses. This innovative treatment approach is now being evaluated in a multi-institutional upfront trial for infants with newly diagnosed ALL.

Abstract IA013: Rhabdomyosarcoma: visions through the looking glass

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in children. Despite aggressive treatment clinical outcomes for RMS have not improved for three decades, emphasizing the need to uncover the molecular underpinnings of the disease. RMS has been presumed to originate from derailed muscle progenitors based on the histologic appearance and gene expression pattern of the tumors resembling embryonic developing skeletal muscle. However, an origin restricted to skeletal muscle does not explain RMS occurring in tissues devoid of skeletal muscle such as the prostate, bladder, and salivary gland. Previously, we described that activation of Sonic Hedgehog signaling through expression of a conditional, constitutively active Smoothened allele, SmoM2, under control of a presumed adipocyte-restricted adipose protein 2 (aP2)-Cre recombinase transgene in mice gives rise to aggressive skeletal muscle tumors. These tumors display the histologic and molecular characteristics of human embryonal fusion-negative RMS (FN-RMS). This model suggested a potential non-myogenic origin of FN-RMS and an avenue to explain FN-RMS development in anatomic sites devoid of skeletal muscle. Lineage tracing showed that RMS can originate from cell reprogramming and transdifferentiation of endothelial progenitor cells. Hedgehog pathway activation in committed endothelial progenitors results in Tbx1 expression and subsequent Myod1 expression driving a partially myogenic program characteristic of FN-RMS. Our work identifies reprogramming cell fate as a mechanism of transformation in pediatric sarcoma and illustrates that it is dangerous to assume the cell of origin from the characteristics of the tumor cell. The cell-reprogramming mechanism that shifts endothelial progenitors to muscle-like cells provides a unique system to define the core regulatory circuitry controlling RMS cell fate and to determine in vivo if targeting this network is a therapeutic vulnerability. Genomic profiling of human FN-RMS failed to uncover a unique mutation that drives oncogenesis. However, the PTEN cis-regulatory region is hypermethylated in more than 90% of all human FN-ERMS tumors, resulting in decreased expression. However, inhibiting the PI3K/AKT/mTOR pathway has had varied efficacy in RMS. In our RMS mouse model, PTEN localizes to the cytoplasm and nucleus, suggesting that PTEN could have functions other than regulating the PI3K/AKT/mTOR pathway. We demonstrate that Pten loss cooperates in RMS tumorigenesis and results in tumors more reflective of human FN-RMS. We show that Pten loss drives expression of the transcription factor PAX7 and identified PAX7 as a dependency in human FN-RMS. Furthermore, Pax7 deletion completely rescues the deleterious effects of Pten loss but also alters tumor cell fate, giving rise to a smooth muscle tumor. Thus, PTEN loss drives the expression of PAX7, a key member of the RMS core regulatory circuitry dictating tumor cell fate. This highlights a synthetic essential relationship between PTEN and PAX7 in FN-RMS tumor maintenance and tumor-fate decisions.

Citation Format: Mark E. Hatley, Casey G. Langdon, Katherine E. Gadek, Matthew R. Garcia, Catherine J. Drummond, Jason A. Hanna, Hongjian E. Jin, Jerold E. Rehg. Rhabdomyosarcoma: visions through the looking glass [abstract]. In: Proceedings of the AACR Special Conference: Sarcomas; 2022 May 9-12; Montreal, QC, Canada. Philadelphia (PA): AACR; Clin Cancer Res 2022;28(18_Suppl):Abstract nr IA013.

Abstract 1667: Synthetic essentiality between PTEN and core dependency factor PAX7 dictates rhabdomyosarcoma identity

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in children with no improvements in treatment options for RMS patients over the past four decades. Therefore, it is critical to understand the fundamental processes underlying RMS tumorigenesis. RMS is divided into two major histologic subtypes - alveolar and embryonal RMS. Nearly all alveolar RMS express oncogenic fusions of PAX3-FOXO1 or PAX7-FOXO1 whereas embryonal RMS are not driven by these fusion proteins. Instead, embryonal or fusion-negative (FN-RMS) are molecularly heterogeneous. Approximately one-third of fusion-negative RMS (FN-RMS) patients have copy number loss of PTEN (phosphatase and tensin homolog), and approximately 90% of tumors are hypermethylated at the PTEN promoter leading to decreased PTEN expression. This indicates a near universal role for PTEN loss in FN-RMS, but the functional role of PTEN is still unclear. To determine PTEN’s function in FN-RMS, we bred Ptenflox alleles into our aP2-Cre;SmoM2 (ASPWT) FN-RMS mouse model to obtain aP2-Cre;SmoM2;Ptenflox/flox mice (ASPcKO). Conditional Pten deletion accelerated tumorigenesis and produced a tumor with a less differentiated histological appearance, much like human FN-RMS. Interestingly, in PtenWT tumors, we found predominant PTEN immunoreactivity within the nucleus suggesting a role for nuclear PTEN in FN-RMS. Transcriptome analyses revealed robust gene expression changes between the ASPWT and ASPcKO tumors. The top overexpressed gene in ASPcKO tumors was Dbx1 (Developing brain homeobox 1), a homeobox transcription factor with no known cancer function but involved in innate behavioral processes such as breathing. We found FN-RMS patient-derived xenografts are dependent on DBX1 expression, and that DBX1 expression is controlled by PAX7 (Paired Box 7). PAX7 is a transcription factor expressed in satellite cells and maintains a de-differentiated state in FN-RMS. PAX7 expression is also increased in our ASPcKO tumors, and we show that human FN-RMS cells are dependent on PAX7 expression for proliferation. This suggests PTEN loss in FN-RMS engages a new transcriptional program necessary for FN-RMS survival. To determine if Pax7 loss can rescue the deleterious effects of Pten loss in our murine FN-RMS model, we deleted both Pten and Pax7 in our aP2-Cre;SmoM2 mice (ASPcKOP7cKO). ASPcKOP7cKO tumor onset kinetics resembled tumors with wild-type PTEN and were negative for skeletal muscle markers MYOD1 and MYOGENIN. However, these tumors were positive for leiomyosarcoma markers smooth muscle actin and CALDESMON. Together, our data suggests PTEN and PAX7 have a synthetic essential relationship in FN-RMS and that PAX7 is a proliferative driver and lineage dependency for FN-RMS tumors. This work also illustrates the power of murine models to unravel the genetic dependencies underlying both tumor maintenance and identity.

Citation Format: Casey G. Langdon, Katherine E. Gadek, Matthew R. Garcia, Myron K. Evans, Kristin B. Reed, Madeline Bush, Jason A. Hanna, Catherine J. Drummond, Matthew C. Maguire, Patrick J. Leavey, David Finkelstein, Hongjian Jin, Patrick A. Schreiner, Jerold E. Rehg, Mark E. Hatley. Synthetic essentiality between PTEN and core dependency factor PAX7 dictates rhabdomyosarcoma identity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1667.

Pitfalls in the diagnosis of yolk sac tumor: Lessons from a clinical trial

Though outcomes for patients with recurrent/refractory malignant germ cell tumors (mGCTs) are poor, therapies targeting mTOR and EGFR inhibition have shown promise in vitro. We hypothesized that the combination of sirolimus and erlotinib will show activity in patients with recurrent/refractory mGCTs. Patients were enrolled in a prospective phase II clinical trial; central review of existing pathology specimens was performed. Of the five patients evaluated, two had their diagnoses revised to pancreatic acinar cell carcinoma and alpha-fetoprotein (AFP)-secreting gastric adenocarcinoma, respectively. Although mGCTs are common AFP-secreting neoplasms, recurrence or refractoriness to standard regimens should prompt histologic reevaluation for other diagnoses.

Genetic context of oncogenic drivers dictates vascular sarcoma development in aP2‐Cre mice

Angiosarcomas are aggressive vascular sarcomas that arise from endothelial cells and have an extremely poor prognosis. Because of the rarity of angiosarcomas, knowledge of molecular drivers and optimized treatment strategies is lacking, highlighting the need for in vivo models to study the disease. Previously, we generated genetically engineered mouse models of angiosarcoma driven by aP2‐Cre‐mediated biallelic loss of Dicer1 or conditional activation of Kras^G12D with Cdkn2a loss that histologically and genetically resemble human tumors. In the present study, we found that DICER1 functions as a potent tumor suppressor and its deletion, in combination with either KRAS^G12D expression or Cdkn2a loss, is associated with angiosarcoma development. Independent of the genetic driver, the mTOR pathway was activated in all murine angiosarcoma models. Direct activation of the mTOR pathway by conditional deletion of Tsc1 with aP2‐Cre resulted in tumors that resemble intermediate grade human kaposiform hemangioendotheliomas, indicating that mTOR activation was not sufficient to drive the malignant angiosarcoma phenotype. Genetic dissection of the spectrum of vascular tumors identified genes specifically regulated in the aggressive murine angiosarcomas that are also enriched in human angiosarcoma. The genetic dissection driving the transition across the malignant spectrum of endothelial sarcomas provides an opportunity to identify key determinants of the malignant phenotype, novel therapies for angiosarcoma, and novel in vivo models to further explore angiosarcoma pathogenesis. © 2022 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

The perfect PTEN – transcriptional regulation by PTEN dictates sarcoma identity

Fusion-negative rhabdomyosarcoma (FN-RMS) is molecularly heterogeneous with few universal alterations except for Phosphatase and tensin homolog (PTEN) promoter hypermethylation. We demonstrate that losing Pten in FN-RMS engages an aberrant transcriptional program key in tumor maintenance and cell identity. These results highlight the importance between transcriptional state, cell of origin, and genetic perturbation in tumorigenesis.

Synthetic essentiality between PTEN and core dependency factor PAX7 dictates rhabdomyosarcoma identity

PTEN promoter hypermethylation is nearly universal and PTEN copy number loss occurs in ~25% of fusion-negative rhabdomyosarcoma (FN-RMS). Here we show Pten deletion in a mouse model of FN-RMS results in less differentiated tumors more closely resembling human embryonal RMS. PTEN loss activated the PI3K pathway but did not increase mTOR activity. In wild-type tumors, PTEN was expressed in the nucleus suggesting loss of nuclear PTEN functions could account for these phenotypes. Pten deleted tumors had increased expression of transcription factors important in neural and skeletal muscle development including Dbx1 and Pax7. Pax7 deletion completely rescued the effects of Pten loss. Strikingly, these Pten;Pax7 deleted tumors were no longer FN-RMS but displayed smooth muscle differentiation similar to leiomyosarcoma. These data highlight how Pten loss in FN-RMS is connected to a PAX7 lineage-specific transcriptional output that creates a dependency or synthetic essentiality on the transcription factor PAX7 to maintain tumor identity.

Abstract 3022: Synthetic essentiality between PTEN and core dependency factor PAX7 dictates rhabdomyosarcoma indentity

Rhabdomyosarcoma (RMS) is an embryonal tumor resembling developing skeletal muscle and the most common pediatric soft-tissue sarcoma. RMS is molecularly defined by the presence or absence of a fusion oncoprotein corresponding with the histological subtypes alveolar and embryonal RMS, respectively. Embryonal, or fusion-negative, RMS (FN-RMS) is heterogeneous in its molecular alteration profile; the major exception is the near universal PTEN promoter hypermethylation found in FN-RMS corresponding with decreased PTEN expression. PTEN's functional role in FN-RMS remains unclear as does PTEN's role in defining tumor fate decisions. Organismal cancer models can help elucidate these decisions by defining the potential tumor fate landscape that can occur in transformed multipotent progenitor cells. Our laboratory leverages a highly penetrant, early onset model of FN-RMS driven by the transdifferentiation of endothelial progenitors into skeletal muscle-like RMS cells by Hedgehog pathway activation. Therefore, our model is uniquely primed to empirically determine the core regulatory factors critical in FN-RMS initiation. Here, we conditionally deleted Pten in these cells (ASPcKO). ASPcKO tumors presented earlier than wild-type tumors and more closely resemble human FN-RMS with a less differentiated skeletal muscle state. These were unique characteristics of ASPcKO tumors as mice with homozygous loss of other tumor suppressors - Cdkn2a, Trp53, and Rb1 - did not recapitulate these phenotypes. We further probed the downstream transcriptional outputs of ASPcKO tumors revealing a profound increase in expression of the neural developmental transcription factors Dbx1 and Pax7. These outputs are functionally important as human FN-RMS patient-derived xenografts are dependent on both DBX1 and PAX7. Subsequently, we also show that DBX1 is a downstream transcriptional target of PAX7 highlighting how Pten loss engages a unique transcriptional program for tumor maintenance. PAX7 is also a core FN-RMS regulatory circuit component. To further interrogate the role of PAX7 on tumor initiation, we concomitantly deleted Pten and Pax7 in our FN-RMS model and found not only that Pax7 loss rescues the survival kinetics observed when Pten is lost, but also alters the developmental trajectory of the tumors that do develop. Instead of Smoothened trans-differentiating our aP2-Cre expressing primordial endothelial cell into a skeletal-muscle like FN-RMS, Pten and Pax7 loss dictates these endothelial cells to give rise to tumors with smooth muscle-like differentiation, including human-like leiomyosarcoma. Together, this synthetic essential interaction between Pten and Pax7 in FN-RMS stresses the importance of the bifunctional role of PAX7 in tumor initiation and maintenance and how specific tumor suppressor loss can engage developmental transcriptional programs to alter tumor fate.

Citation Format: Casey G. Langdon, Katherine E. Gadek, Matthew R. Garcia, Kristin B. Reed, Madeline Bush, Jason A. Hanna, Catherine J. Drummond, Matthew C. Maguire, Patrick J. Leavey, David Finkelstein, Hongjian Jin, Jerold E. Rehg, Mark E. Hatley. Synthetic essentiality between PTEN and core dependency factor PAX7 dictates rhabdomyosarcoma indentity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 3022.

Abstract PR01: Developmental reprogramming via Hedgehog pathway activation in nonmyogenic endothelial progenitors drives fusion-negative rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in children. Despite aggressive chemotherapy, radiotherapy, and surgery, clinical outcomes for RMS have not improved for three decades, emphasizing the need to uncover the molecular underpinnings of the disease. RMS has been presumed to originate from derailed muscle progenitors based on the histologic appearance and gene expression pattern of the tumors, resembling embryonic developing skeletal muscle. However, an origin restricted to skeletal muscle does not explain RMS occurring in tissues devoid of skeletal muscle such as the prostate, bladder, salivary gland, biliary tree, and omentum. Previously, we described that activation of Sonic Hedgehog signaling through expression of a conditional, constitutively active Smoothened allele, SmoM2, under control of a presumed adipocyte-restricted adipose protein 2 (aP2)-Cre recombinase transgene in mice, gives rise to aggressive skeletal muscle tumors that display the histologic and molecular characteristics of human embryonal RMS (ERMS). With the short latency and anatomically restricted tumor location in the neck, we sought to leverage this model to explore the cell of origin of ERMS. Lineage tracing experiments identified aP2-Cre labeled cells are distinctly nonmyogenic and were identified as endothelial cells found in the interstitium between muscle fibers. We illustrate that aP2-Cre is not expressed in the quiescent or activated muscle stem cells or satellite cells. Expression of oncogenic SmoM2 with aP2-Cre results in proliferation and expansion of the aP2-Cre labeled muscle interstitial endothelial cells and myogenic transdifferentiation resulting in ERMS. Activation of the hedgehog pathway in aP2-Cre labeled endothelial progenitors results in expression of skeletal muscle specification factors specific in the head and neck development, including TBX1, PITX2, TCF21, and MSC. We illustrate that endothelium and skeletal muscle within the head and neck arise from KDR (VEFGR2)-expressing progenitors. Hedgehog pathway activation in committed KDR+ endothelial progenitors results in Tbx1 expression and subsequent MYOD1 expression driving a partially myogenic program characteristic of ERMS. Our work identifies reprogramming cell fate as a mechanism of transformation in pediatric sarcoma and illustrates that it is dangerous to assume the cell of origin from the characteristics of the tumor cell.

Citation Format: Catherine J. Drummond, Kathrine E. Gadek, Madeline Bush, Matthew R. Garcia, Mark E. Hatley. Developmental reprogramming via Hedgehog pathway activation in nonmyogenic endothelial progenitors drives fusion-negative rhabdomyosarcoma [abstract]. In: Proceedings of the AACR Special Conference on the Evolving Landscape of Cancer Modeling; 2020 Mar 2-5; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2020;80(11 Suppl):Abstract nr PR01.

Insights into pediatric rhabdomyosarcoma research: Challenges and goals

Overall survival rates for pediatric patients with high-risk or relapsed rhabdomyosarcoma (RMS) have not improved significantly since the 1980s. Recent studies have identified a number of targetable vulnerabilities in RMS, but these discoveries have infrequently translated into clinical trials. We propose streamlining the process by which agents are selected for clinical evaluation in RMS. We believe that strong consideration should be given to the development of combination therapies that add biologically targeted agents to conventional cytotoxic drugs. One example of this type of combination is the addition of the WEE1 inhibitor AZD1775 to the conventional cytotoxic chemotherapeutics, vincristine and irinotecan.

Abstract 4631: Modeling the vascular sarcoma spectrum with genetically engineered mice

Angiosarcomas are highly aggressive vascular sarcomas with an extremely poor prognosis. Angiosarcomas can develop spontaneously or are associated with prior radiation, chronic lymphedema, or exposure to toxic chemicals such as vinyl chloride. Sequencing efforts have identified a number of genetic alterations in angiosarcoma, however the genetic drivers and actionable targets remain unclear. Despite the poor outcome for patients, there are limited resources for studying angiosarcoma, highlighting the need for genetic, in vivo models to better elucidate the underlying biology of the disease. In studying the role of DICER1 and microRNAs in mouse models of tumorigenesis, we found that Dicer1 deletion with aP2-Cre leads to aggressive and metastatic angiosarcoma development with 100% penetrance. ERK and S6 hyperphosphorylation in the aP2-Cre;Dicer1cKO (AD) tumors suggest Dicer1 loss results in activation of the RAS-MEK-ERK and mTOR pathways. To determine if direct activation of the these pathways could similarly transform aP2-Cre expressing cells we first interrogated the combination of oncogenic KrasG12D with Cdkn2a inactivation. We found that aP2-Cre;LSL-KrasG12D;Cdkn2acKO (AKC) mice rapidly develop angiosarcomas providing an accelerated model for assessing cooperating alleles and therapies. To activate the mTOR pathway we tested conditional Tsc1 deletion with aP2-Cre and found that all aP2-Cre;Tsc1cKO (AT) animals develop vascular tumors in the paws. In contrast to the AD and AKC tumors, AT tumors express markers of lymphatic endothelial cells such as PROX1. In addition, the tumors display a distinct nodular spindle cell-like morphology that is consistent with a low grade angiosarcoma resembling human kaposiform hemangioendotheliomas, a vascular tumor occurring predominantly in the extremities of infants. The distinct onset, growth kinetics, anatomic locations, and histologic presentation of the tumors from AD, AT, and AKC mice provide a mechanism to interrogate the drivers of angiosarcoma in less aggressive AT paw tumors and more aggressive AD and AKC tumors. Furthermore, the aP2-Cre driven mouse models provide a platform to study angiosarcoma initiation, progression, and metastasis for the identification of novel therapeutics to improve outcomes for this understudied and devastating disease.

Citation Format: Jason A. Hanna, Casey G. Langdon, Matthew R. Garcia, David Finkelstein, Jerold E. Rehg, Mark E. Hatley. Modeling the vascular sarcoma spectrum with genetically engineered mice [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4631.

A Novel Oncolytic Herpes Simplex Virus Design based on the Common Overexpression of microRNA-21 in Tumors

Background

Recognition sequences for microRNAs (miRs) that are down-regulated in tumor cells have recently been used to render lytic viruses tumor-specific. Since different tumor types down-regulate different miRs, this strategy requires virus customization to the target tumor. We have explored a feature that is shared by many tumor types, the up-regulation of miR-21, as a means to generate an oncolytic herpes simplex virus (HSV) that is applicable to a broad range of cancers.

Methods

We assembled an expression construct for a dominant-negative (dn) form of the essential HSV replication factor U_L9 and inserted tandem copies of the miR-21 recognition sequence (T21) in the 3' untranslated region. Bacterial Artificial Chromosome (BAC) recombineering was used to introduce the dnU_L9 construct with or without T21 into the HSV genome. Virus was produced by transfection and replication was assessed in different tumor and control cell lines.

Results

Virus production was conditional on the presence of the T21 sequence. The dnU_L9-T21 virus replicated efficiently in tumor cell lines, less efficiently in cells that contained reduced miR-21 activity, and not at all in the absence of miR-21.

Conclusion

miR-21-sensitive expression of a dominant-negative inhibitor of HSV replication allows preferential destruction of tumor cells in vitro. This observation provides a basis for further development of a widely applicable oncolytic HSV.

Abstract PR12: Hedgehog pathway drives fusion-negative rhabdomyosarcoma initiated from nonmyogenic endothelial progenitors

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in children. Despite aggressive chemotherapy, radiotherapy, and surgery, clinical outcomes for RMS have not improved for three decades, emphasizing the need to uncover the molecular underpinnings of the disease. RMS has been presumed to originate from derailed muscle progenitors based on the histologic appearance and gene expression pattern of the tumors. However, an origin restricted to skeletal muscle does not explain RMS occurring in tissues devoid of skeletal muscle, such as the prostate, bladder, salivary gland, biliary tree, and the omentum. Previously, we described that activation of Sonic Hedgehog signaling through expression of a conditional, constitutively active Smoothened allele, SmoM2, under control of an adipocyte-restricted adipose protein 2 (aP2)-Cre recombinase transgene in mice gives rise to aggressive skeletal muscle tumors that display the histologic and molecular characteristics of human embryonal RMS (ERMS). With the short latency and anatomic restricted tumor location in the neck, we sought to leverage this model to explore the cell of origin of ERMS.

Lineage tracing experiments identified aP2-Cre labeled cells are distinctly nonmyogenic and were identified as endothelial cells found in the interstitium between muscle fibers. We illustrate that aP2-Cre is not expressed in the quiescent or activated muscle stem cells or satellite cells. Expression of oncogenic SmoM2 with aP2-Cre results in proliferation and expansion of the aP2-Cre labeled muscle interstitial cells and myogenic transdifferentiation resulting in ERMS. Activation of the Hedgehog pathway in aP2-Cre labeled endothelial progenitors results in Tbx1 expression, which is a skeletal muscle specification factor in the head and neck. We illustrate that endothelium and skeletal muscle within the head and neck arise from Kdr (Vegfr2) expressing progenitors. Hedgehog pathway activation in committed KDR+ endothelial progenitors results in Tbx1 expression and subsequent Myod1 expression driving a partially myogenic program characteristic of ERMS. Our work identifies reprogramming cell fate as a mechanism of transformation in pediatric sarcoma.

Citation Format: Catherine J. Drummond, Jason A. Hanna, Matthew R. Garcia, Daniel J. Devine, Alana J. Heyrana, David Finkelstein, Jerold E. Rehg, Mark E. Hatley. Hedgehog pathway drives fusion-negative rhabdomyosarcoma initiated from nonmyogenic endothelial progenitors [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 PR12.

Abstract 3014: Location specificity in fusion-negative rhabdomyosarcoma driven by cell of origin

Rhabdomyosarcoma (RMS) is the most common pediatric soft tissue sarcoma and despite aggressive treatment, clinical outcomes have not improved for three decades. There is a need to uncover the molecular underpinnings of RMS. Tumor location is a key prognostic indicator and although RMS occurs throughout the body, nearly 40% of all RMS occurs in the head and neck. It is unknown how the cell of origin affects location and therefore clinical outcome of RMS. Previously, we demonstrated that activation of Hedgehog signaling through expression of a conditional, constitutively active Smoothened allele, SmoM2, under the control of the adipose protein 2 (aP2)-Cre recombinase transgene gives rise to aggressive skeletal muscle tumors in mice that resemble human fusion negative RMS (FN-RMS). Interestingly, tumors were anatomically restricted to the neck in aP2-Cre;SmoM2 mice. In this study we leverage the aP2-Cre;SmoM2 model of FN-RMS to interrogate how cell of origin affects tumor localization.

By genetic fate mapping we determine that aP2-Cre labeled cells are non-myogenic and that aP2-Cre is not expressed in quiescent or activated muscle stem cells. Instead, we identify aP2-Cre expressing cells as endothelial cell progenitors within the muscle interstitium. Although aP2-Cre expressing endothelial cells were observed throughout the mouse, we observed that SmoM2 expression specifically drives embryonic expansion of aP2-Cre labeled cells only in the neck. SmoM2 expression reprograms endothelial progenitors resulting in a myogenic fate switch prior to terminal endothelial cell differentiation. We illustrate that endothelium and skeletal muscle within the head and neck arise from KDR expressing progenitors and that aP2-Cre is expressed after endothelial lineage commitment. Aberrant hedgehog activation in these aP2-Cre labeled endothelial progenitors results in TBX1 expression, a skeletal muscle specification factor in the head and neck, and subsequently MYOD1 expression, driving a partial myogenic program characteristic of FN-RMS. Further characterization of tumor cells isolated by flow cytometry revealed that FN-RMS in aP2-Cre;SmoM2 mice express TBX1, as well as PITX2, TCF21 and MSC, additional skeletal muscle specification factors in the head and neck. In contrast, PAX3, which specifies trunk and limb muscle, was not expressed in FN-RMS cells isolated from aP2-Cre;SmoM2 mice. Together, these results demonstrate that FN-RMS in the head and neck can arise from endothelial progenitor cells and suggest that aberrant activation of normal muscle development programs in non-myogenic cell types with developmental pliancy can drive location specific tumor formation.

Citation Format: Catherine J. Drummond, Jason A. Hanna, Matthew R. Garcia, Daniel J. Devine, Alana J. Heyrana, David Finkelstein, Mark E. Hatley. Location specificity in fusion-negative rhabdomyosarcoma driven by cell of origin [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3014.

A Case of mistaken identity: Rhabdomyosarcoma development from endothelial progenitor cells

Rhabdomyosarcoma (RMS) histologically resembles developing skeletal muscle and is thought to solely originate from a differentiation block in muscle progenitors. We demonstrate that RMS can arise from endothelial progenitor cells following reprogramming and myogenic transdifferentiation. These results highlight how tumors with identical morphological features can arise from different cell types and offer insight into RMS formation in non-myogenic tissue.

Abstract A31: Biallelic Dicer1 loss in endothelial cells drives angiosarcoma development

Angiosarcoma is an aggressive and highly metastatic vascular sarcoma with an extremely poor prognosis. Because of the rarity of the disease, the molecular drivers and optimal treatment strategies for patients remain unclear, highlighting the need for novel genetic and in vivo animal models. MicroRNAs are small RNAs that regulate gene expression and DICER1 is the RNase III endoribonuclease that processes pre-microRNAs into mature microRNAs. DICER1 and microRNAs are central to numerous cellular processes and are often dysregulated in many cancers, including sarcomas. Here we show that biallelic Dicer1 deletion with endothelial expression of aP2-Cre drives aggressive and metastatic angiosarcoma independent of other genetically engineered oncogenes or tumor suppressor loss. Angiosarcomas in aP2-Cre;Dicer1Flox/- mice histologically and genetically resemble human angiosarcoma, and we found microRNA-23 target genes enriched in mouse and human angiosarcoma. Because Ras mutations and Cdkn2a loss are common in angiosarcoma, we interrogated them as angiosarcoma modifiers. Interestingly, we found that both oncogenic LSL-KrasG12D and loss of conditional Cdkn2aFlox decreased tumor latency. In addition, aP2-Cre;LSL-KrasG12D;Cdkn2aFlox/Flox mice developed angiosarcomas, providing a model with Dicer1 expression intact to further study the function of microRNAs in angiosarcoma. Thus, these findings illustrate that Dicer1 can function as a traditional loss-of-function tumor-suppressor gene and provide simple and fully penetrant animal models for the study of angiosarcoma development and metastasis.

Citation Format: Jason A. Hanna, Catherine J. Drummond, Matthew R. Garcia, Jonathan C. Go, David Finkelstein, Jerold E. Rehg, Mark E. Hatley. Biallelic Dicer1 loss in endothelial cells drives angiosarcoma development [abstract]. In: Proceedings of the AACR Special Conference: Advances in Modeling Cancer in Mice: Technology, Biology, and Beyond; 2017 Sep 24-27; Orlando, Florida. Philadelphia (PA): AACR; Cancer Res 2018;78(10 Suppl):Abstract nr A31.

Abstract A16: Fusion-negative rhabdomyosarcoma originating from endothelial progenitors

Rhabdomyosarcoma (RMS) is the most common soft-tissue sarcoma in childhood, and despite rigorous clinical trials the survival for children with high-risk RMS has not changed for three decades. RMS is subdivided into two major classes, fusion-positive (FP) and fusion-negative RMS (FN-RMS), based on the presence or absence of the PAX3-FOXO1 or PAX7-FOXO1 gene fusions. RMS occurs at locations throughout the body with nearly 40% of tumors occurring in the head and neck. Tumor location and fusion status are key prognostic factors. RMS resembles developing skeletal muscle and has been speculated to originate from genetically compromised skeletal muscle progenitors. However, the genes that control RMS development and specify location remain elusive. RMS also occurs in tissues devoid of skeletal muscle such as the urinary bladder, prostate, and biliary tree, suggesting the possibility of origins outside of the skeletal muscle lineage. Currently, the cell of origin and the factors that specify RMS location and thus prognosis are unknown.

Previously, we showed that activation of Sonic Hedgehog signaling through expression of a conditional, constitutively active Smoothened allele, SmoM2, under control of an adipocyte-restricted adipose protein 2 (aP2)-Cre recombinase transgene in mice gives rise to aggressive skeletal muscle tumors that display the histologic and molecular characteristics of human FN-RMS. In this model, tumorigenesis occurs with high penetrance (~80%), is early onset (by 2 months of age), and is anatomically restricted to the head and neck. Also, unlike previous RMS models, this model requires no additional background mutations, such as inactivation of p53, and drives only FN-RMS neoplasia. We illustrated that the transcriptome of the aP2-Cre;SmoM2 tumors recapitulates both other mouse FN-RMS models as well as human FN-RMS. With the short latency and anatomic restricted tumor location, we sought to leverage this model to explore the cell of origin of FN-RMS.

Here we use genetic fate mapping with fluorescent reporter mice to interrogate the cell of origin of FN-RMS in the aP2-Cre;SmoM2 model. Tracing the aP2-Cre labeled cells with reporter mice illustrated labeled cells in both brown and white adipose tissue as well as a discrete population of cells lying between skeletal muscle fibers but not beneath the laminin sheath. These aP2-Cre labeled cells are distinct from Pax7-positive skeletal muscle stem cells or satellite cells and do not contribute to myotube formation in vitro or in vivo. Gene profiling of tomato positive cells isolated by FACS from the sternocleidomastoid (SCM) of aP2-Cre;R26-Tom and aP2-Cre;R26-Tom;SmoM2 revealed that these aP2-Cre labeled cells in muscle interstitium are endothelial cells. In SCM sections, aP2-Cre labeled cells colocalize with Pecam1. When compared to aP2-Cre;R26-Tom mice, the addition of oncogenic SmoM2 (aP2-Cre;R26-Tom;SmoM2) results in embryonic expansion of the aP2-labeled muscle interstitial cells and formation of FN-RMS. These expansions became positive for the myogenic regulatory factor MyoD1 and did not express Pecam1 at embryonic day 17.5. Subsequent gene expression analysis revealed that both myogenic regulatory factors and endothelial genes are upregulated in tumor cells. Skeletal muscle and endothelial cells share a common progenitor, and together these results suggest that aberrant Sonic Hedgehog signaling promotes a myogenic fate switch in aP2-Cre expressing endothelial progenitor cells that results in FN-RMS formation.

Citation Format: Catherine J. Drummond, Jason A. Hanna, Matthew R. Garcia, Daniel Devine, Jennifer Peters, Victoria Frohlich, David Finkelstein, Mark E. Hatley. Fusion-negative rhabdomyosarcoma originating from endothelial progenitors [abstract]. In: Proceedings of the AACR Conference on Advances in Sarcomas: From Basic Science to Clinical Translation; May 16-19, 2017; Philadelphia, PA. Philadelphia (PA): AACR; Clin Cancer Res 2018;24(2_Suppl):Abstract nr A16.

Hedgehog Pathway Drives Fusion-Negative Rhabdomyosarcoma Initiated From Non-myogenic Endothelial Progenitors

Rhabdomyosarcoma (RMS) is a pediatric soft tissue sarcoma that histologically resembles embryonic skeletal muscle. RMS occurs throughout the body and an exclusively myogenic origin does not account for RMS occurring in sites devoid of skeletal muscle. We previously described an RMS model activating a conditional constitutively active Smoothened mutant (SmoM2) with aP2-Cre. Using genetic fate mapping, we show SmoM2 expression in Cre-expressing endothelial progenitors results in myogenic transdifferentiation and RMS. We show that endothelium and skeletal muscle within the head and neck arise from Kdr-expressing progenitors, and that hedgehog pathway activation results in aberrant expression of myogenic specification factors as a potential mechanism driving RMS genesis. These findings suggest that RMS can originate from aberrant development of non-myogenic cells.

Biallelic Dicer1 Loss Mediated by aP2-Cre Drives Angiosarcoma

Angiosarcoma is an aggressive vascular sarcoma with an extremely poor prognosis. Because of the relative rarity of this disease, its molecular drivers and optimal treatment strategies are obscure. DICER1 is an RNase III endoribonuclease central to miRNA biogenesis, and germline DICER1 mutations result in a cancer predisposition syndrome, associated with an increased risk of many tumor types. Here, we show that biallelic Dicer1 deletion with aP2-Cre drives aggressive and metastatic angiosarcoma independent of other genetically engineered oncogenes or tumor suppressor loss. Angiosarcomas in aP2-Cre;Dicer1^Flox/- mice histologically and genetically resemble human angiosarcoma. miR-23 target genes, including the oncogenes Ccnd1 as well as Adam19, Plau, and Wsb1 that promote invasiveness and metastasis, were enriched in mouse and human angiosarcoma. These studies illustrate that Dicer1 can function as a traditional loss-of-function tumor suppressor gene, and they provide a fully penetrant animal model for the study of angiosarcoma development and metastasis. Cancer Res; 77(22); 6109-18. ©2017 AACR.

Abstract 461: Dichotomous roles of Dicer1 in rhabdomyosarcoma and angiosarcoma

Pediatric and adult angiosarcomas are rare and highly aggressive soft tissue sarcomas with an extremely poor prognosis. Due to the rarity of this disease especially in children, the molecular drivers and optimized treatment strategies for patients are lacking, highlighting the need for genetic and in vivo animal models. MicroRNAs are a class of small RNAs that regulate gene expression and are often dysregulated in cancers including sarcomas. DICER1 is required for microRNA biogenesis and germline DICER1 mutations result in a cancer predisposition syndrome associated with increased risk of benign and malignant tumors including rhabdomyosarcoma (RMS), a pediatric soft tissue sarcoma resembling developmentally arrested skeletal muscle. Here we show Dicer1 expression is required for tumorigenesis in a mouse model of RMS driven by activation of oncogenic Smoothened by Cre recombinase expressed from the adipose protein 2 (aP2) promoter (aP2-Cre). Unexpectedly, in studying the role of Dicer1 in RMS, we found that Dicer1 deletion with aP2-Cre leads to aggressive angiosarcoma. Angiosarcoma development was independent of the Smoothened oncogene and other genetically engineered oncogenes or tumor suppressor loss, providing the first in vivo mouse model of biallelic Dicer1 loss alone driving tumorigenesis. Angiosarcomas in aP2-Cre;Dicer1Flox/- mice histologically and genetically resemble human angiosarcoma and were enriched for microRNA-23 target genes including the oncogene Ccnd1 as well as Adam19, Plau, and Wsb1 that promote invasiveness and metastasis. The aP2-Cre;Dicer1Flox/- model provides a simple in vivo animal model to study angiosarcoma for novel therapeutics and the molecular mechanisms of cancer initiation, progression, and metastasis. In addition, our results demonstrate DICER1 and microRNAs play major and opposing roles in sarcomagenesis.

Citation Format: Jason A. Hanna, Matthew R. Garcia, Catherine J. Drummond, Jonathon C. Go, David Finkelstein, Jerold E. Rehg, Mark E. Hatley. Dichotomous roles of Dicer1 in rhabdomyosarcoma and angiosarcoma [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 461. doi:10.1158/1538-7445.AM2017-461

Abstract 1036: Non-myogenic origin of embryonal rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in children. Despite aggressive chemotherapy, radiotherapy and surgery, clinical outcomes for RMS have not improved for three decades, emphasizing the need to uncover the molecular underpinnings of the disease. RMS includes two histopathologic subtypes: alveolar RMS, driven by the fusion protein PAX3/7-FOXO1, and embryonal RMS (ERMS), which is genetically heterogeneous. RMS has been presumed to originate from derailed muscle progenitors based on the histologic appearance and gene expression pattern of the tumors. However, an origin restricted to skeletal muscle does not explain RMS occurring in tissues devoid of skeletal muscle such as the prostate, bladder, biliary tree and the omentum. Previously, we showed that activation of Sonic Hedgehog signaling through expression of a conditional, constitutively active Smoothened allele, SmoM2, under control of an adipocyte-restricted adipose protein 2 (aP2)-Cre recombinase transgene in mice gives rise to aggressive skeletal muscle tumors that display the histologic and molecular characteristics of human ERMS. In this model, tumorigenesis occurs with high penetrance (~80%), is early onset (by 2 months of age), and is restricted to the head and neck. Also, unlike previous RMS models, this model requires no additional background mutations, such as inactivation of p53, and results in only ERMS neoplasia. We illustrated that the gene expression signature of the aP2-Cre;SmoM2 tumors recapitulates both other mouse ERMS models as well as human ERMS. With the short latency and anatomic restricted tumor location, we sought to leverage this model to explore the cell of origin. Here, we use genetic fate mapping of aP2-cre with reporter mice to determine the effect of constitutive hedgehog activation on the identity and localization of aP2-Cre expressing cells. aP2-cre expressing cells are found in both brown and white adipose and to be localized within the muscle intersitium but not beneath the laminin sheath of the muscle fibers. These aP2-cre expressing cells are distinct from Pax7-positive skeletal muscle stem cells or satellite cells and do not contribute to myofiber formation. Instead these cells were confirmed to be endothelial by gene expression analysis. Colocalization between aP2-cre expressing cells and CD31 was also observed. When compared to aP2-Cre;R26-Tom mice, the addition of oncogenic SmoM2 (aP2-Cre;R26-Tom;SmoM2) resulted in embryonic expansion of these aP2-lineage interstitial muscle cells and formation of ERMS. Our findings suggest that non-skeletal muscle progenitors are a cell of origin for Sonic Hedgehog-driven ERMS.

Citation Format: Catherine J. Drummond, Matthew R. Garcia, Daniel J. Devine, Jennifer Peters, Victoria Frohlich, David Finkelstein, Mark E. Hatley. Non-myogenic origin of embryonal rhabdomyosarcoma [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 1036. doi:10.1158/1538-7445.AM2017-1036

PAX7 is a required target for microRNA-206-induced differentiation of fusion-negative rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma of childhood. RMS can be parsed based on clinical outcome into two subtypes, fusion-positive RMS (FP-RMS) or fusion-negative RMS (FN-RMS) based on the presence or absence of either PAX3-FOXO1 or PAX7-FOXO1 gene fusions. In both RMS subtypes, tumor cells show histology and a gene expression pattern resembling that of developmentally arrested skeletal muscle. Differentiation therapy is an attractive approach to embryonal tumors of childhood including RMS; however, agents to drive RMS differentiation have not entered the clinic and their mechanisms remain unclear. MicroRNA-206 (miR-206) expression increases through normal muscle development and has decreased levels in RMS compared with normal skeletal muscle. Increasing miR-206 expression drives differentiation of RMS, but the target genes responsible for the relief of the development arrest are largely unknown. Using a combinatorial approach with gene and proteomic profiling coupled with genetic rescue, we identified key miR-206 targets responsible for the FN-RMS differentiation blockade, PAX7, PAX3, NOTCH3, and CCND2. Specifically, we determined that PAX7 downregulation is necessary for miR-206-induced cell cycle exit and myogenic differentiation in FN-RMS but not in FP-RMS. Gene knockdown of targets necessary for miR-206-induced differentiation alone or in combination was not sufficient to phenocopy the differentiation phenotype from miR-206, thus illustrating that miR-206 replacement offers the ability to modulate a complex network of genes responsible for the developmental arrest in FN-RMS. Genetic deletion of miR-206 in a mouse model of FN-RMS accelerated and exacerbated tumor development, indicating that both in vitro and in vivo miR-206 acts as a tumor suppressor in FN-RMS at least partially through downregulation of PAX7. Collectively, our results illustrate that miR-206 relieves the differentiation arrest in FN-RMS and suggests that miR-206 replacement could be a potential therapeutic differentiation strategy.

Abstract B05: MicroRNA-206 drives rhabdomyosarcoma differentiation through downregulation of PAX7

The goal of this work is to understand the mechanism of microRNA-206 (miR-206) induced differentiation in rhabdomyosarcoma (RMS). RMS is the most common soft tissue sarcoma of childhood with both histology and gene expression suggestive that RMS tumor cells resemble a developmental arrest of skeletal muscle. miR-206 is a skeletal muscle specific miRNA expression increases through normal muscle development and has decreased levels in RMS compared to normal skeletal muscle. Exogneous miR-206 replacement drives differentiation and cell cycle exit of RMS. However, the target genes responsible for the relief of the development arrest are largely unknown. Using a combinatorial approach with mRNA microarrays and tandem mass tag (TMT) proteome profiling we identified 165 mRNAs and 272 proteins downregulated in miR-206 mimic transfected RMS cells. Potential targets were validated with 3'UTR reporter assays as well as real-time PCR and immunoblot from miR-206 mimic transfected cells. Further analysis with genetic rescue identified the key targets responsible for the RMS differentiation block as PAX7, PAX3, CCND2, and NOTCH3. However, siRNA knockdown of these targets alone and in a pooled combination failed to induce differentiation to the same extent as miR-206 despite knockdown levels comparable to miR-206. Thus, target identification collectively is key to understanding the mechanism of miR-206 function in RMS, suggesting the one miRNA-one target model to be insufficient for the complex phenotype of miR-206 induced differentiation. Genetic deletion of miR-206 in a mouse model of RMS accelerated and exacerbated tumor development indicating that both in vitro and in vivo miR-206 possesses tumor suppressive properties. In addition, Notch3 and Pax7 levels were increased in knockout tumors compared to wild type, providing further evidence for these genes as key targets of miR-206 in RMS. These results illustrate that miR-206 relieves the differentiation arrest in RMS and suggests that miR-206 replacement therapy could be a potential therapeutic strategy.

Citation Format: Jason A. Hanna, Matthew R. Garcia, Jonathan C. Go, David Finkelstein, Kiran Kodali, Vishwajeeth Pagala, Eric N. Olson, Junmin Peng, Mark E. Hatley. MicroRNA-206 drives rhabdomyosarcoma differentiation through downregulation of PAX7. [abstract]. In: Proceedings of the AACR Special Conference on Noncoding RNAs and Cancer: Mechanisms to Medicines ; 2015 Dec 4-7; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2016;76(6 Suppl):Abstract nr B05.

Abstract IA03: Insights into the origins and pathogenesis of embryonal rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in children. Despite aggressive chemotherapy, radiotherapy and surgery, clinical outcomes for RMS have not improved for three decades, emphasizing the need to uncover the molecular underpinnings of the disease. RMS is an aggressive skeletal muscle-lineage tumor composed of malignant myoblasts that fail to exit the cell cycle and are blocked from fusing into syncytial muscle. RMS includes two histolopathologic subtypes: alveolar RMS, driven by the fusion protein PAX3/7-FOXO1, and embryonal RMS (ERMS), which is genetically heterogeneous. RMS has been presumed to originate from derailed muscle progenitors based on the histologic appearance and gene expression pattern of the tumors. However, an origin restricted to skeletal muscle does not explain RMS occurring in tissues devoid of skeletal muscle such as the prostate, bladder, biliary tree and the omentum.

Previously, we showed that activation of Sonic Hedgehog signaling through expression of a conditional, constitutively active Smoothened allele, SmoM2, under control of an adipocyte-restricted adipose protein 2 (aP2)-Cre recombinase transgene in mice gives rise to aggressive skeletal muscle tumors that display the histologic and molecular characteristics of human ERMS. In this model, tumorigenesis occurs with high penetrance (~80%), is early onset (by 2 months of age), and is restricted to the head and neck. Also, unlike previous RMS models, this model requires no additional background mutations, such as inactivation of p53, and drives only ERMS neoplasia. We illustrated that the transcriptome of the aP2-Cre;SmoM2 tumors recapitulates both other mouse ERMS models as well as human ERMS.

With the short latency and anatomic restricted tumor location, we sought to leverage this model to explore the cell of origin. Lineage tracing the aP2-Cre with reporter mice illustrated aP2-Cre expression in both brown and white adipose tissue as well as a discrete population of cells lying between skeletal muscle fibers but not beneath the laminin sheath of the muscle fibers. These aP2-lineage cells are distinct from Pax7-positive skeletal muscle stem cells or satellite cells. The aP2-lineage cells do not contribute to myotube formation in vitro. When compared to aP2-Cre;R26-Tom mice, the addition of oncogenic SmoM2 (aP2-Cre;R26-Tom;SmoM2) results in embryonic expansion of the aP2-lineage interstitial muscle cells. Our findings suggest that non-skeletal muscle progenitors are a potential cell of origin for Sonic Hedgehog-driven ERMS.

Citation Format: Catherine J. Drummond, Daniel Devine, Matthew R. Garcia, Mark E. Hatley. Insights into the origins and pathogenesis of embryonal rhabdomyosarcoma. [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 IA03.

MicroRNA-21 Aggravates Cyst Growth in a Model of Polycystic Kidney Disease

Autosomal dominant polycystic kidney disease (ADPKD), one of the most common monogenetic disorders, is characterized by kidney failure caused by bilateral renal cyst growth. MicroRNAs (miRs) have been implicated in numerous diseases, but the role of these noncoding RNAs in ADPKD pathogenesis is still poorly defined. Here, we investigated the role of miR-21, an oncogenic miR, in kidney cyst growth. We found that transcriptional activation of miR-21 is a common feature of murine PKD. Furthermore, compared with renal tubules from kidney samples of normal controls, cysts in kidney samples from patients with ADPKD had increased levels of miR-21. cAMP signaling, a key pathogenic pathway in PKD, transactivated miR-21 promoter in kidney cells and promoted miR-21 expression in cystic kidneys of mice. Genetic deletion of miR-21 attenuated cyst burden, reduced kidney injury, and improved survival of an orthologous model of ADPKD. RNA sequencing analysis and additional in vivo assays showed that miR-21 inhibits apoptosis of cyst epithelial cells, likely through direct repression of its target gene programmed cell death 4 Thus, miR-21 functions downstream of the cAMP pathway and promotes disease progression in experimental PKD. Our results suggest that inhibiting miR-21 is a potential new therapeutic approach to slow cyst growth in PKD.

Probing for a deeper understanding of rhabdomyosarcoma: insights from complementary model systems

Rhabdomyosarcoma (RMS) is a mesenchymal malignancy composed of neoplastic primitive precursor cells that exhibit histological features of myogenic differentiation. Despite intensive conventional multimodal therapy, patients with high-risk RMS typically suffer from aggressive disease. The lack of directed therapies against RMS emphasizes the need to further uncover the molecular underpinnings of the disease. In this Review, we discuss the notable advances in the model systems now available to probe for new RMS-targetable pathogenetic mechanisms, and the possibilities for enhanced RMS therapeutics and improved clinical outcomes.

A mouse model of rhabdomyosarcoma originating from the adipocyte lineage

Rhabdomyosarcoma (RMS) is an aggressive skeletal muscle-lineage tumor composed of malignant myoblasts that fail to exit the cell cycle and are blocked from fusing into syncytial muscle. Rhabdomyosarcoma includes two histolopathologic subtypes: alveolar rhabdomyosarcoma, driven by the fusion protein PAX3-FOXO1 or PAX7-FOXO1, and embryonal rhabdomyosarcoma (ERMS), which is genetically heterogeneous. Here, we show that adipocyte-restricted activation of Sonic hedgehog signaling through expression of a constitutively active Smoothened allele in mice gives rise to aggressive skeletal muscle tumors that display the histologic and molecular characteristics of human ERMS with high penetrance. Our findings suggest that adipocyte progenitors can be a cell of origin for Sonic hedgehog-driven ERMS, showing that RMS can originate from nonskeletal muscle precursors.

Modulation of K-Ras-dependent lung tumorigenesis by MicroRNA-21

Lung cancer is the leading cause of cancer-related deaths in the world, and non-small-cell lung cancer (NSCLC) accounts for 80% of cases. MicroRNA-21 (miR-21) expression is increased and predicts poor survival in NSCLC. Although miR-21 function has been studied in vitro with cancer cell lines, the role of miR-21 in tumor development in vivo is unknown. We utilize transgenic mice with loss-of-function and gain-of-function miR-21 alleles combined with a model of NSCLC to determine the role of miR-21 in lung cancer. We show that overexpression of miR-21 enhances tumorigenesis and that genetic deletion of miR-21 partially protects against tumor formation. MiR-21 drives tumorigenesis through inhibition of negative regulators of the Ras/MEK/ERK pathway and inhibition of apoptosis.

Hyperglycaemia-induced superoxide production decreases eNOS expression via AP-1 activation in aortic endothelial cells

AIMS/HYPOTHESIS

Hyperglycaemia is a primary cause of vascular complications in diabetes. A hallmark of these vascular complications is endothelial cell dysfunction, which is partly due to the reduced production of nitric oxide. The aim of this study was to investigate the regulation of endothelial nitric oxide synthase (eNOS) activity by acute and chronic elevated glucose.

METHODS

Human aortic endothelial cells were cultured in 5.5 mmol/l (NG) or 25 mmol/l glucose (HG) for 4 h, 1 day, 3 days or 7 days. Mouse aortic endothelial cells were freshly isolated from C57BL/6J control and diabetic db/db mice. The expression and activity of eNOS were measured using quantitative PCR and nitrite measurements respectively. The binding of activator protein-1 (AP-1) to DNA in nuclear extracts was determined using electrophoretic mobility-shift assays.

RESULTS

Acute exposure (4 h) of human aortic endothelial cells to 25 mmol/l glucose moderately increased eNOS activity and eNOS mRNA and protein expression. In contrast, chronic exposure to elevated glucose (25 mmol/l for 7 days) reduced total nitrite levels (46% reduction), levels of eNOS mRNA (46% reduction) and eNOS protein (65% reduction). In addition, AP-1 DNA binding activity was increased in chronic HG-cultured human aortic endothelial cells, and this effect was reduced by the specific inhibition of reactive oxygen species production through the mitochondrial electron transport chain. Mutation of AP-1 sites in the human eNOS promoter reversed the effects of HG. Compared with C57BL/6J control mice, eNOS mRNA levels in diabetic db/db mouse aortic endothelial cells were reduced by 60%. This decrease was reversed by the overexpression of manganese superoxide dismutase using an adenoviral construct.

CONCLUSIONS/INTERPRETATION

In diabetes, the expression and activity of eNOS is regulated through glucose-mediated mitochondrial production of reactive oxygen species and activation of the oxidative stress transcription factor AP-1.

Allosteric determinants in guanine nucleotide-binding proteins

Members of the G protein superfamily contain nucleotide-dependent switches that dictate the specificity of their interactions with binding partners. Using a sequence-based method termed statistical coupling analysis (SCA), we have attempted to identify the allosteric core of these proteins, the network of amino acid residues that couples the domains responsible for nucleotide binding and protein-protein interactions. One-third of the 38 residues identified by SCA were mutated in the G protein Gs alpha, and the interactions of guanosine 5'-3-O-(thio)triphosphate- and GDP-bound mutant proteins were tested with both adenylyl cyclase (preferential binding to GTP-Gs alpha) and the G protein beta gamma subunit complex (preferential binding to GDP-Gs alpha). A two-state allosteric model predicts that mutation of residues that control the equilibrium between GDP- and GTP-bound conformations of the protein will cause the ratio of affinities of these species for adenylyl cyclase and G beta gamma to vary in a reciprocal fashion. Observed results were consistent with this prediction. The network of residues identified by the SCA appears to comprise a core allosteric mechanism conferring nucleotide-dependent switching; the specific features of different G protein family members are built on this core.

Expression, purification, and assay of cytosolic (catalytic) domains of membrane-bound mammalian adenylyl cyclases

The identification and isolation of the soluble catalytic domains of adenylyl cyclase have provided investigators with useful reagents for the study of these enzymes. They have permitted detailed mechanistic investigation of the actions of forskolin, Gs alpha, and the inhibitory G protein, Gi alpha. They have served as critical reagents for the development of plausible models of the catalytic mechanism of the enzyme. They have enabled X-ray crystallographic analysis of adenylyl cyclase; this technique was not approachable with the small quantities of the membrane-bound enzyme available previously. The information obtained by using the soluble domains of adenylyl cyclase has provided templates for description of the behavior of many forms of purine nucleotide cyclases from a variety of species. We now appreciate both adenylyl cyclases and guanylyl cyclases as dimeric enzymes with a 2-fold symmetrical domain arrangement (or pseudosymmetrical in the case of heterodimerization). The active sites are located at the interface between the two domains, both of which contribute binding surfaces.

Isolation and characterization of constitutively active mutants of mammalian adenylyl cyclase

A genetic screen in Saccharomyces cerevisiae identified mutations in mammalian adenylyl cyclase that activate the enzyme in the absence of G(s)alpha. Thirteen of these mutant proteins were characterized biochemically in an assay system that depends on a mixture of the two cytosolic domains (C(1) and C(2)) of mammalian adenylyl cyclases. Three mutations, I1010M, K1014N, and P1015Q located in the beta4-beta5 loop of the C(2) domain of type II adenylyl cyclase, increase enzymatic activity in the absence of activators. The K1014N mutation displays both increased maximal activity and apparent affinity for the C(1) domain of type V adenylyl cyclase in the absence of activators of the enzyme. The increased affinity of the mutant C(2) domain of adenylyl cyclase for the wild type C(1) domain was exploited to isolate a complex containing VC(1), IIC(2), and G(s)alpha-guanosine 5'-3-O-(thio)triphosphate (GTPgammaS) in the absence of forskolin and a complex of VC(1), IIC(2), forskolin, and P-site inhibitor in the absence of G(s)alpha-GTPgammaS. The isolation of these complexes should facilitate solution of crystal structures of low activity states of adenylyl cyclase and thus determination of the mechanism of activation of the enzyme by forskolin and G(s)alpha.