The BRAF pseudogene functions as a competitive endogenous RNA and induces lymphoma in vivo.

Research over the past decade has suggested important roles for pseudogenes in physiology and disease. In vitro experiments demonstrated that pseudogenes contribute to cell transformation through several mechanisms. However, in vivo evidence for a causal role of pseudogenes in cancer development is lacking. Here, we report that mice engineered to overexpress either the full-length murine B-Raf pseudogene Braf-rs1 or its pseudo "CDS" or "3' UTR" develop an aggressive malignancy resembling human diffuse large B cell lymphoma. We show that Braf-rs1 and its human ortholog, BRAFP1, elicit their oncogenic activity, at least in part, as competitive endogenous RNAs (ceRNAs) that elevate BRAF expression and MAPK activation in vitro and in vivo. Notably, we find that transcriptional or genomic aberrations of BRAFP1 occur frequently in multiple human cancers, including B cell lymphomas. Our engineered mouse models demonstrate the oncogenic potential of pseudogenes and indicate that ceRNA-mediated microRNA sequestration may contribute to the development of cancer.

Coding-independent regulation of the tumor suppressor PTEN by competing endogenous mRNAs.

Here, we demonstrate that protein-coding RNA transcripts can crosstalk by competing for common microRNAs, with microRNA response elements as the foundation of this interaction. We have termed such RNA transcripts as competing endogenous RNAs (ceRNAs). We tested this hypothesis in the context of PTEN, a key tumor suppressor whose abundance determines critical outcomes in tumorigenesis. By a combined computational and experimental approach, we identified and validated endogenous protein-coding transcripts that regulate PTEN, antagonize PI3K/AKT signaling, and possess growth- and tumor-suppressive properties. Notably, we also show that these genes display concordant expression patterns with PTEN and copy number loss in cancers. Our study presents a road map for the prediction and validation of ceRNA activity and networks and thus imparts a trans-regulatory function to protein-coding mRNAs.

In vivo identification of tumor- suppressive PTEN ceRNAs in an oncogenic BRAF-induced mouse model of melanoma.

We recently proposed that competitive endogenous RNAs (ceRNAs) sequester microRNAs to regulate mRNA transcripts containing common microRNA recognition elements (MREs). However, the functional role of ceRNAs in cancer remains unknown. Loss of PTEN, a tumor suppressor regulated by ceRNA activity, frequently occurs in melanoma. Here, we report the discovery of significant enrichment of putative PTEN ceRNAs among genes whose loss accelerates tumorigenesis following Sleeping Beauty insertional mutagenesis in a mouse model of melanoma. We validated several putative PTEN ceRNAs and further characterized one, the ZEB2 transcript. We show that ZEB2 modulates PTEN protein levels in a microRNA-dependent, protein coding-independent manner. Attenuation of ZEB2 expression activates the PI3K/AKT pathway, enhances cell transformation, and commonly occurs in human melanomas and other cancers expressing low PTEN levels. Our study genetically identifies multiple putative microRNA decoys for PTEN, validates ZEB2 mRNA as a bona fide PTEN ceRNA, and demonstrates that abrogated ZEB2 expression cooperates with BRAF(V600E) to promote melanomagenesis.

Functional analysis of the 1p34.3 risk locus implicates GNL2 in high-grade serous ovarian cancer.

The high-grade serous ovarian cancer (HGSOC) risk locus at chromosome 1p34.3 resides within a frequently amplified genomic region signifying the presence of an oncogene. Here, we integrate in silico variant-to-function analysis with functional studies to characterize the oncogenic potential of candidate genes in the 1p34.3 locus. Fine mapping of genome-wide association statistics identified candidate causal SNPs local to H3K27ac-demarcated enhancer regions that exhibit allele-specific binding for CTCF in HGSOC and normal fallopian tube secretory epithelium cells (FTSECs). SNP risk associations colocalized with eQTL for six genes (DNALI1, GNL2, RSPO1, SNIP1, MEAF6, and LINC01137) that are more highly expressed in carriers of the risk allele, and three (DNALI1, GNL2, and RSPO1) were upregulated in HGSOC compared to normal ovarian surface epithelium cells and/or FTSECs. Increased expression of GNL2 and MEAF6 was associated with shorter survival in HGSOC with 1p34.3 amplifications. Despite its activation of ß-catenin signaling, RSPO1 overexpression exerted no effects on proliferation or colony formation in our study of ovarian cancer and FTSECs. Instead, GNL2, MEAF6, and SNIP1 silencing impaired in vitro ovarian cancer cell growth. Additionally, GNL2 silencing diminished xenograft tumor formation, whereas overexpression stimulated proliferation and colony formation in FTSECs. GNL2 influences 60S ribosomal subunit maturation and global protein synthesis in ovarian cancer and FTSECs, providing a potential mechanism of how GNL2 upregulation might promote ovarian cancer development and mediate genetic susceptibility of HGSOC.

Focal Adhesion- and IGF1R-Dependent Survival and Migratory Pathways Mediate Tumor Resistance to mTORC1/2 Inhibition.

Aberrant signaling by the mammalian target of rapamycin (mTOR) contributes to the devastating features of cancer cells. Thus, mTOR is a critical therapeutic target and catalytic inhibitors are being investigated as anti-cancer drugs. Although mTOR inhibitors initially block cell proliferation, cell viability and migration in some cancer cells are quickly restored. Despite sustained inhibition of mTORC1/2 signaling, Akt, a kinase regulating cell survival and migration, regains phosphorylation at its regulatory sites. Mechanistically, mTORC1/2 inhibition promotes reorganization of integrin/focal adhesion kinase-mediated adhesomes, induction of IGFR/IR-dependent PI3K activation, and Akt phosphorylation via an integrin/FAK/IGFR-dependent process. This resistance mechanism contributes to xenograft tumor cell growth, which is prevented with mTOR plus IGFR inhibitors, supporting this combination as a therapeutic approach for cancers.

Genetic tools for the stable overexpression of circular RNAs.

Circular RNAs (circRNAs) are a class of non-coding RNAs featuring a covalently closed ring structure formed through backsplicing. circRNAs are broadly expressed and contribute to biological processes through a variety of functions. Standard gain-of-function and loss-of-function approaches to study gene functions have significant limitations when studying circRNAs. Overexpression studies in particular suffer from the lack of efficient genetic tools. While mammalian expression plasmids enable transient circRNA overexpression in cultured cells, most cell biological studies require long-term ectopic expression. Here we report the development and characterization of genetic tools enabling stable circRNA overexpression in vitro and in vivo. We demonstrated that circRNA expression constructs can be delivered to cultured cells via transposons, whereas lentiviral vectors have limited utility for the delivery of circRNA constructs due to viral RNA splicing in virus-producing cells. We further demonstrated ectopic circRNA expression in a hepatocellular carcinoma mouse model upon circRNA transposon delivery via hydrodynamic tail vein injection. Furthermore, we generated genetically engineered mice harbouring circRNA expression constructs. We demonstrated that this approach enables constitutive, global circRNA overexpression as well as inducible circRNA expression directed specifically to melanocytes in a melanoma mouse model. These tools expand the genetic toolkit available for the functional characterization of circRNAs.

Translational pathology, genomics and the development of systemic therapies for acral melanoma.

Acral melanomas arise on the non-hair bearing skin of the palms, soles and in the nail beds. These rare tumors comprise 2-3 % of all melanomas, are not linked to UV-exposure, and represent the most frequent subtype of melanomas in patients of Asian, African and Hispanic origin. Although recent work has revealed candidate molecular events that underlie acral melanoma development, this knowledge is not yet been translated into efficacious local, regional, or systemic therapies. In the current review, we describe the clinical characteristics of acral melanoma and outline the genetic basis of acral melanoma development. Further discussion is given to the current status of systemic therapy for acral melanoma with a focus on ongoing developments in both immunotherapy and targeted therapy for the treatment of advanced disease.

BRAF inhibitor resistance mediated by the AKT pathway in an oncogenic BRAF mouse melanoma model.

BRAF (v-raf murine sarcoma viral oncogene homolog B) inhibitors elicit a transient anti-tumor response in ∼ 80% of BRAF(V600)-mutant melanoma patients that almost uniformly precedes the emergence of resistance. Here we used a mouse model of melanoma in which melanocyte-specific expression of Braf(V618E) (analogous to the human BRAF(V600E) mutation) led to the development of skin hyperpigmentation and nevi, as well as melanoma formation with incomplete penetrance. Sleeping Beauty insertional mutagenesis in this model led to accelerated and fully penetrant melanomagenesis and synchronous tumor formation. Treatment of Braf(V618E) transposon mice with the BRAF inhibitor PLX4720 resulted in tumor regression followed by relapse. Analysis of transposon insertions identified eight genes including Braf, Mitf, and ERas (ES-cell expressed Ras) as candidate resistance genes. Expression of ERAS in human melanoma cell lines conferred resistance to PLX4720 and induced hyperphosphorylation of AKT (v-akt murine thymoma viral oncogene homolog 1), a phenotype reverted by combinatorial treatment with PLX4720 and the AKT inhibitor MK2206. We show that ERAS expression elicits a prosurvival signal associated with phosphorylation/inactivation of BAD, and that the resistance of hepatocyte growth factor-treated human melanoma cells to PLX4720 can be reverted by treatment with the BAD-like BH3 mimetic ABT-737. Thus, we define a role for the AKT/BAD pathway in resistance to BRAF inhibition and illustrate an in vivo approach for finding drug resistance genes.

Oncogene-induced Nrf2 transcription promotes ROS detoxification and tumorigenesis.

Reactive oxygen species (ROS) are mutagenic and may thereby promote cancer. Normally, ROS levels are tightly controlled by an inducible antioxidant program that responds to cellular stressors and is predominantly regulated by the transcription factor Nrf2 (also known as Nfe2l2) and its repressor protein Keap1 (refs 2-5). In contrast to the acute physiological regulation of Nrf2, in neoplasia there is evidence for increased basal activation of Nrf2. Indeed, somatic mutations that disrupt the Nrf2-Keap1 interaction to stabilize Nrf2 and increase the constitutive transcription of Nrf2 target genes were recently identified, indicating that enhanced ROS detoxification and additional Nrf2 functions may in fact be pro-tumorigenic. Here, we investigated ROS metabolism in primary murine cells following the expression of endogenous oncogenic alleles of Kras, Braf and Myc, and found that ROS are actively suppressed by these oncogenes. K-Ras(G12D), B-Raf(V619E) and Myc(ERT2) each increased the transcription of Nrf2 to stably elevate the basal Nrf2 antioxidant program and thereby lower intracellular ROS and confer a more reduced intracellular environment. Oncogene-directed increased expression of Nrf2 is a new mechanism for the activation of the Nrf2 antioxidant program, and is evident in primary cells and tissues of mice expressing K-Ras(G12D) and B-Raf(V619E), and in human pancreatic cancer. Furthermore, genetic targeting of the Nrf2 pathway impairs K-Ras(G12D)-induced proliferation and tumorigenesis in vivo. Thus, the Nrf2 antioxidant and cellular detoxification program represents a previously unappreciated mediator of oncogenesis.

C-Raf inhibits MAPK activation and transformation by B-Raf(V600E).

Activating B-Raf mutations that deregulate the MAPK pathway commonly occur in cancer. Whether additional proteins modulate the enzymatic activity of oncogenic B-Raf is unknown. Here we show that the proto-oncogene C-Raf paradoxically inhibits B-Raf(V600E) kinase activity through the formation of B-Raf(V600E)-C-Raf complexes. Although all Raf family members associate with oncogenic B-Raf, this inhibitory effect is specific to C-Raf. Indeed, a B-Raf(V600E) isoform with impaired ability to interact with C-Raf exhibits elevated oncogenic potential. Human melanoma cells expressing B-Raf(V600E) display a reduced C-Raf:B-Raf ratio, and further suppression of C-Raf increases MAPK activation and proliferation. Conversely, ectopic C-Raf expression lowers ERK phosphorylation and proliferation. Moreover, both oncogenic Ras and Sorafenib stabilize B-Raf(V600E)-C-Raf complexes, thereby impairing MAPK activation. This inhibitory function of C-Raf on B-Raf(V600E)-mediated MAPK activation may explain the lack of co-occurrence of B-Raf(V600E) and oncogenic Ras mutations, and influence the successful clinical development of small molecule inhibitors for B-Raf(V600E)-driven cancers.

Integrated transcriptional and competitive endogenous RNA networks are cross-regulated in permissive molecular environments.

Competitive endogenous (ce)RNAs cross-regulate each other through sequestration of shared microRNAs and form complex regulatory networks based on their microRNA signature. However, the molecular requirements for ceRNA cross-regulation and the extent of ceRNA networks remain unknown. Here, we present a mathematical mass-action model to determine the optimal conditions for ceRNA activity in silico. This model was validated using phosphatase and tensin homolog (PTEN) and its ceRNA VAMP (vesicle-associated membrane protein)-associated protein A (VAPA) as paradigmatic examples. A computational assessment of the complexity of ceRNA networks revealed that transcription factor and ceRNA networks are intimately intertwined. Notably, we found that ceRNA networks are responsive to transcription factor up-regulation or their aberrant expression in cancer. Thus, given optimal molecular conditions, alterations of one ceRNA can have striking effects on integrated ceRNA and transcriptional networks.

PTEN Lipid Phosphatase Activity Suppresses Melanoma Formation by Opposing an AKT/mTOR/FRA1 Signaling Axis.

Inactivating mutations in PTEN are prevalent in melanoma and are thought to support tumor development by hyperactivating the AKT/mTOR pathway. Conversely, activating mutations in AKT are relatively rare in melanoma, and therapies targeting AKT or mTOR have shown disappointing outcomes in preclinical models and clinical trials of melanoma. This has led to the speculation that PTEN suppresses melanoma by opposing AKT-independent pathways, potentially through noncanonical functions beyond its lipid phosphatase activity. In this study, we examined the mechanisms of PTEN-mediated suppression of melanoma formation through the restoration of various PTEN functions in PTEN-deficient cells or mouse models. PTEN lipid phosphatase activity predominantly inhibited melanoma cell proliferation, invasion, and tumor growth, with minimal contribution from its protein phosphatase and scaffold functions. A drug screen underscored the exquisite dependence of PTEN-deficient melanoma cells on the AKT/mTOR pathway. Furthermore, activation of AKT alone was sufficient to counteract several aspects of PTEN-mediated melanoma suppression, particularly invasion and the growth of allograft tumors. Phosphoproteomics analysis of the lipid phosphatase activity of PTEN validated its potent inhibition of AKT and many of its known targets, while also identifying the AP-1 transcription factor FRA1 as a downstream effector. The restoration of PTEN dampened FRA1 translation by inhibiting AKT/mTOR signaling, and FRA1 overexpression negated aspects of PTEN-mediated melanoma suppression akin to AKT. This study supports AKT as the key mediator of PTEN inactivation in melanoma and identifies an AKT/mTOR/FRA1 axis as a driver of melanomagenesis. SIGNIFICANCE: PTEN suppresses melanoma predominantly through its lipid phosphatase function, which when lost, elevates FRA1 levels through AKT/mTOR signaling to promote several aspects of melanomagenesis.

MAPK-mediated PHGDH induction is essential for melanoma formation and represents an actionable vulnerability.

Overexpression of PHGDH, the rate-limiting enzyme in the serine synthesis pathway, promotes melanomagenesis, melanoma cell proliferation, and survival of metastases in serine-low environments such as the brain. While PHGDH amplification explains PHGDH overexpression in a subset of melanomas, we find that PHGDH levels are universally increased in melanoma cells due to oncogenic BRAFV600E promoting PHGDH transcription through mTORC1-mediated translation of ATF4. Importantly, PHGDH expression was critical for melanomagenesis as depletion of PHGDH in genetic mouse models blocked melanoma formation. Despite BRAFV600E-mediated upregulation, PHGDH was further induced by exogenous serine restriction. Surprisingly, BRAFV600E inhibition diminished serine restriction-mediated PHGDH expression by preventing ATF4 induction, creating a potential vulnerability whereby melanoma cells could be specifically starved of serine by combining BRAFV600E inhibition with exogenous serine restriction. Indeed, we show that this combination promoted cell death in vitro and attenuated melanoma growth in vivo. This study identified a melanoma cell-specific PHGDH-dependent vulnerability.

Cathepsin B promotes the progression of pancreatic ductal adenocarcinoma in mice.

OBJECTIVE: The lysosomal protease cathepsin B is upregulated in human pancreatic ductal adenocarcinoma (PDA) and represents a potential therapeutic target. Loss of cathepsin B delays tumour progression in mouse models of islet, mammary and intestinal carcinoma and decreases invasion and metastasis. This study examines the role of cathepsin B in the initiation, progression and metastasis of PDA. METHODS: Cathepsin B germline knockout mice were crossed with animals expressing an endogenous Kras(G12D) allele in the pancreas, and mice were aged to evaluate the role of cathepsin B in pancreatic intraepithelial neoplasia (PanIN). A survival study was also performed with mice carrying an additional heterozygous conditional Trp53(R172H) allele. Cell lines derived from tumours were used to investigate the role of cathepsin B in vitro, and subcutaneous allografts investigated the cell autonomous and non-cell autonomous roles of cathepsin B in pancreatic cancer. RESULTS: Constitutive cathepsin B loss resulted in delayed progression of both PanIN and PDA and a significant survival advantage in mice. Cathepsin B-deficient PDA cells and PanIN showed decreased proliferation and mitogen-activated protein (MAP) kinase signalling. The reconstitution of deficient cells with cathepsin B reversed these findings, which correlated with decreased levels of the active forms of the related protease cathepsin L. Conversely, acute ablation of cathepsin L activated the MAP kinase cascade in PDA cells. CONCLUSIONS: These results confirm that cathepsin B plays an important cell autonomous role in the progression of PDA and suggest that the regulation of cathepsin L by cathepsin B may be a means of stimulating cell proliferation in neoplasia.

Genetically engineered mouse models of head and neck cancers.

The head and neck region is one of the anatomic sites commonly afflicted by cancer, with ~1.5 million new diagnoses reported worldwide in 2020 alone. Remarkable progress has been made in understanding the underlying disease mechanisms, personalizing care based on each tumor's individual molecular characteristics, and even therapeutically exploiting the inherent vulnerabilities of these neoplasms. In this regard, genetically engineered mouse models (GEMMs) have played an instrumental role. While progress in the development of GEMMs has been slower than in other major cancer types, several GEMMs are now available that recapitulate most of the heterogeneous characteristics of head and neck cancers such as the tumor microenvironment. Different approaches have been employed in GEMM development and implementation, though each can generally recapitulate only certain disease aspects. As a result, appropriate model selection is essential for addressing specific research questions. In this review, we present an overview of all currently available head and neck cancer GEMMs, encompassing models for head and neck squamous cell carcinoma, nasopharyngeal carcinoma, and salivary and thyroid gland carcinomas.

Distinct Nrf2 Signaling Thresholds Mediate Lung Tumor Initiation and Progression.

Mutations in the KEAP1-NRF2 (Kelch-like ECH-associated protein 1-nuclear factor-erythroid 2 p45-related factor 2) pathway occur in up to a third of non-small cell lung cancer (NSCLC) cases and often confer resistance to therapy and poor outcomes. Here, we developed murine alleles of the KEAP1 and NRF2 mutations found in human NSCLC and comprehensively interrogated their impact on tumor initiation and progression. Chronic NRF2 stabilization by Keap1 or Nrf2 mutation was not sufficient to induce tumorigenesis, even in the absence of tumor suppressors, p53 or LKB1. When combined with KrasG12D/+, constitutive NRF2 activation promoted lung tumor initiation and early progression of hyperplasia to low-grade tumors but impaired their progression to advanced-grade tumors, which was reversed by NRF2 deletion. Finally, NRF2 overexpression in KEAP1 mutant human NSCLC cell lines was detrimental to cell proliferation, viability, and anchorage-independent colony formation. Collectively, these results establish the context-dependence and activity threshold for NRF2 during the lung tumorigenic process. SIGNIFICANCE: Stabilization of the transcription factor NRF2 promotes oncogene-driven tumor initiation but blocks tumor progression, indicating distinct, threshold-dependent effects of the KEAP1/NRF2 pathway in different stages of lung tumorigenesis.

A Series of BRAF- and NRAS-Driven Murine Melanoma Cell Lines with Inducible Gene Modulation Capabilities.

Murine cancer cell lines are powerful research tools to complement studies in genetically engineered mouse models. We have established 21 melanoma cell lines from embryonic stem cell-genetically engineered mouse models driven by alleles that model the most frequent genetic alterations in human melanoma. In addition, these cell lines harbor regulatory alleles for the genomic integration of transgenes and the regulation of expression of such transgenes. In this study, we report a comprehensive characterization of these cell lines. Specifically, we validated melanocytic origin, driver allele recombination and expression, and activation of the oncogenic MAPK and protein kinase B pathways. We further tested tumor formation in syngeneic immunocompetent recipients as well as the functionality of the integrated Tet-ON system and recombination-mediated cassette exchange homing cassette. Finally, by deleting the transcription factor MAFG with an inducible CRISPR/Cas9 approach, we show the utility of the regulatory alleles for candidate gene modulation. These cell lines will be a valuable resource for studying melanoma biology and therapy.

Gain of Chromosome 1q Perturbs a Competitive Endogenous RNA Network to Promote Melanoma Metastasis.

Somatic copy-number alterations (CNA) promote cancer, but the underlying driver genes may not be comprehensively identified if only the functions of the encoded proteins are considered. mRNAs can act as competitive endogenous RNAs (ceRNA), which sponge miRNAs to posttranscriptionally regulate gene expression in a protein coding-independent manner. We investigated the contribution of ceRNAs to the oncogenic effects of CNAs. Chromosome 1q gains promoted melanoma progression and metastasis at least in part through overexpression of three mRNAs with ceRNA activity: CEP170, NUCKS1, and ZC3H11A. These ceRNAs enhanced melanoma metastasis by sequestering tumor suppressor miRNAs. Orthogonal genetic assays with miRNA inhibitors and target site blockers, along with rescue experiments, demonstrated that miRNA sequestration is critical for the oncogenic effects of CEP170, NUCKS1, and ZC3H11A mRNAs. Furthermore, chromosome 1q ceRNA-mediated miRNA sequestration alleviated the repression of several prometastatic target genes. This regulatory RNA network was evident in other cancer types, suggesting chromosome 1q ceRNA deregulation as a common driver of cancer progression. Taken together, this work demonstrates that ceRNAs mediate the oncogenicity of somatic CNAs. SIGNIFICANCE: The function of CEP170, NUCKS1, and ZC3H11A mRNAs as competitive endogenous RNAs that sequester tumor suppressor microRNAs underlies the oncogenic activity of chromosome 1q gains.

Single-cell Characterization of the Cellular Landscape of Acral Melanoma Identifies Novel Targets for Immunotherapy.

PURPOSE: Acral melanoma is a rare subtype of melanoma that arises on the non-hair-bearing skin of the palms, soles, and nail beds. In this study, we used single-cell RNA sequencing (scRNA-seq) to map the transcriptional landscape of acral melanoma and identify novel immunotherapeutic targets. EXPERIMENTAL DESIGN: We performed scRNA-seq on nine clinical specimens (five primary, four metastases) of acral melanoma. Detailed cell type curation was performed, the immune landscapes were mapped, and key results were validated by analysis of The Cancer Genome Atlas (TCGA) and single-cell datasets. Cell-cell interactions were inferred and compared with those in nonacral cutaneous melanoma. RESULTS: Multiple phenotypic subsets of T cells, natural killer (NK) cells, B cells, macrophages, and dendritic cells with varying levels of activation/exhaustion were identified. A comparison between primary and metastatic acral melanoma identified gene signatures associated with changes in immune responses and metabolism. Acral melanoma was characterized by a lower overall immune infiltrate, fewer effector CD8 T cells and NK cells, and a near-complete absence of γδ T cells compared with nonacral cutaneous melanomas. Immune cells associated with acral melanoma exhibited expression of multiple checkpoints including PD-1, LAG-3, CTLA-4, V-domain immunoglobin suppressor of T cell activation (VISTA), TIGIT, and the Adenosine A2A receptor (ADORA2). VISTA was expressed in 58.3% of myeloid cells and TIGIT was expressed in 22.3% of T/NK cells. CONCLUSIONS: Acral melanoma has a suppressed immune environment compared with that of cutaneous melanoma from nonacral skin. Expression of multiple, therapeutically tractable immune checkpoints were observed, offering new options for clinical translation.

Pseudogenes as Competitive Endogenous RNAs: Testing miRNA Dependency.

Pseudogenes may function as competitive endogenous RNAs (ceRNAs), where they regulate the expression of genes by sequestering shared miRNAs. ceRNAs are becoming more extensively identified and studied, and demonstrating the dependence of their effects on miRNA sequestration is critical to establish them as ceRNAs. Here, we outline an experimental approach to assess the miRNA dependency of a candidate pseudogene ceRNA.