Let-7 miRNA Biogenesis Research Articles

EGCG Disrupts the LIN28B/Let-7 Interaction and Reduces Neuroblastoma Aggressiveness.

Neuroblastoma (NB) is the most commonly diagnosed extracranial solid tumor in children, accounting for 15% of all childhood cancer deaths. Although the 5-year survival rate of patients with a high-risk disease has increased in recent decades, NB remains a challenge in pediatric oncology, and the identification of novel potential therapeutic targets and agents is an urgent clinical need. The RNA-binding protein LIN28B has been identified as an oncogene in NB and is associated with a poor prognosis. Given that LIN28B acts by negatively regulating the biogenesis of the tumor suppressor let-7 miRNAs, we reasoned that selective interference with the LIN28B/let-7 miRNA interaction would increase let-7 miRNA levels, ultimately leading to reduced NB aggressiveness. Here, we selected (-)-epigallocatechin 3-gallate (EGCG) out of 4959 molecules screened as the molecule with the best inhibitory activity on LIN28B/let-7 miRNA interaction and showed that treatment with PLC/PLGA-PEG nanoparticles containing EGCG (EGCG-NPs) led to an increase in mature let-7 miRNAs and a consequent inhibition of NB cell growth. In addition, EGCG-NP pretreatment reduced the tumorigenic potential of NB cells in vivo. These experiments suggest that the LIN28B/let-7 miRNA axis is a good therapeutic target in NB and that EGCG, which can interfere with this interaction, deserves further preclinical evaluation.

Development of a molecular glue-based Lin28 degrader to regulate cellular proliferation and stemness.

Let-7 microRNAs (miRNAs) regulate cellular processes including stemness and proliferation. Lin28, an RNA-binding protein, controls let-7 miRNA biogenesis and is a key factor in maintaining stem cell properties. We developed SB1349, a novel molecular glue-based degrader targeting Lin28. SB1349 induces Lin28 degradation through a proteasome-dependent pathway, enhances let-7 miRNA levels, and downregulates oncogenes c-Myc and IMP1. SB1349 also promotes the differentiation in neuroblastoma cells, highlighting its potential as a therapeutic agent for various diseases.

Growth-suppressor microRNAs mediate synaptic overgrowth and behavioral deficits in Fragile X mental retardation protein deficiency

Abnormal neuronal and synapse growth is a core pathology resulting from deficiency of the Fragile X mental retardation protein (FMRP), but molecular links underlying the excessive synthesis of key synaptic proteins remain incompletely defined. We find that basal brain levels of the growth suppressor let-7 microRNA (miRNA) family are selectively lowered in FMRP-deficient mice and activity-dependent let-7 downregulation is abrogated. Primary let-7 miRNA transcripts are not altered in FMRP-deficiency and posttranscriptional misregulation occurs downstream of MAPK pathway induction and elevation of Lin28a, a let-7 biogenesis inhibitor. Neonatal restoration of brain let-7 miRNAs corrects hallmarks of FMRP-deficiency, including dendritic spine overgrowth and social and cognitive behavioral deficits, in adult mice. Blockade of MAPK hyperactivation normalizes let-7 miRNA levels in both brain and peripheral blood plasma from Fmr1 KO mice. These results implicate dysregulated let-7 miRNA biogenesis in the pathogenesis of FMRP-deficiency, and highlight let-7 miRNA-based strategies for future biomarker and therapeutic development.

Retraction of 'DICER-dependent biogenesis of let-7 miRNAs affects human cell response to DNA damage via targeting p21/p27'.

Retraction of 'DICER-dependent biogenesis of let-7 miRNAs affects human cell response to DNA damage via targeting p21/p27'.

Expression of Concern on 'DICER-dependent biogenesis of let-7 miRNAs affects human cell response to DNA damage via targeting p21/p27'.

Expression of Concern on 'DICER-dependent biogenesis of let-7 miRNAs affects human cell response to DNA damage via targeting p21/p27'.

Non-microRNA binding competitively inhibits LIN28 regulation.

SUMMARYRNA binding protein (RBP) expression is finite. For RBPs that are vastly outnumbered by their potential target sites, a simple competition for binding can set the magnitude of post-transcriptional control. Here, we show that LIN28, best known for its direct regulation of let-7 miRNA biogenesis, is also indirectly regulated by its widespread binding of non-miRNA transcripts. Approximately 99% of LIN28 binding sites are found on non-miRNA transcripts, like protein coding and ribosomal RNAs. These sites are bound specifically and strongly, but they do not appear to mediate direct post-transcriptional regulation. Instead, non-miRNA sites act to sequester LIN28 protein and effectively change its functional availability, thus impeding the regulation of let-7 in cells. Together, these data show that the binding properties of the transcriptome broadly influence the ability of an RBP to mediate changes in RNA metabolism and gene expression.

Non-MicroRNA Binding Competitively Inhibits LIN28 Regulation

RNA binding protein (RBP) expression is finite. For RBPs that are vastly outnumbered by their potential target sites, a simple competition for binding can set the magnitude of post-transcriptional control. Here we show that LIN28, best known for its direct regulation of let-7 miRNA biogenesis, is also indirectly regulated by its widespread binding of non-miRNA transcripts. Approximately 99% of LIN28 binding sites are found on non-miRNA transcripts, like protein coding and ribosomal RNAs. These sites are bound specifically and strongly, but they do not appear to mediate direct post-transcriptional regulation. Instead, non-miRNA sites act to sequester LIN28 protein and effectively change its functional availability, thus impeding the regulation of let-7 in cells. Together, these data show that the binding properties of the transcriptome broadly influence the ability of an RBP to mediate changes in RNA metabolism and gene expression.

The Role of LIN28-let-7-ARID3B Pathway in Placental Development

Placental disorders are a major cause of pregnancy loss in humans, and 40–60% of embryos are lost between fertilization and birth. Successful embryo implantation and placental development requires rapid proliferation, invasion, and migration of trophoblast cells. In recent years, microRNAs (miRNAs) have emerged as key regulators of molecular pathways involved in trophoblast function. A miRNA binds its target mRNA in the 3ʹ-untranslated region (3ʹ-UTR), causing its degradation or translational repression. Lethal-7 (let-7) miRNAs induce cell differentiation and reduce cell proliferation by targeting proliferation-associated genes. The oncoprotein LIN28 represses the biogenesis of mature let-7 miRNAs. Proliferating cells have high LIN28 and low let-7 miRNAs, whereas differentiating cells have low LIN28 and high let-7 miRNAs. In placenta, low LIN28 and high let-7 miRNAs can lead to reduced proliferation of trophoblast cells, resulting in abnormal placental development. In trophoblast cells, let-7 miRNAs reduce the expression of proliferation factors either directly by binding their mRNA in 3ʹ-UTR or indirectly by targeting the AT-rich interaction domain (ARID)3B complex, a transcription-activating complex comprised of ARID3A, ARID3B, and histone demethylase 4C (KDM4C). In this review, we discuss regulation of trophoblast function by miRNAs, focusing on the role of LIN28-let-7-ARID3B pathway in placental development.

SYNCRIP, a new player in pri-let-7a processing.

microRNAs (miRNAs), a class of small and endogenous molecules that control gene expression, are broadly involved in biological processes. Although a number of cofactors that assist or antagonize let-7 miRNA biogenesis are well-established, more auxiliary factors remain to be investigated. Here, we identified SYNCRIP (Synaptotagmin Binding Cytoplasmic RNA Interacting Protein) as a new player for let-7a miRNA. SYNCRIP interacts with pri-let-7a both in vivo and in vitro. Knockdown of SYNCRIP impairs, while overexpression of SYNCRIP promotes, the expression of let-7a miRNA. A broad miRNA profiling analysis revealed that silencing of SYNCRIP regulates the expression of a set of mature miRNAs positively or negatively. In addition, SYNCRIP is associated with microprocessor complex and promotes the processing of pri-let-7a. Strikingly, the terminal loop of pri-let-7a was shown to be the main contributor for its interaction with SYNCRIP. Functional studies demonstrated that the SYNCRIP RRM2–3 domain can promote the processing of pri-let-7a. Structure-based alignment of RRM2–3 with other RNA binding proteins identified the residues likely to participate in protein–RNA interactions. Taken together, these findings suggest the promising role that SYNCRIP plays in miRNA regulation, thus providing insights into the function of SYNCRIP in eukaryotic development.

Lin28a expression protects against streptozotocin-induced β-cell destruction and prevents diabetes in mice.

Lin28, which is highly expressed during embryogenesis, has been shown to play an important role in cell growth and embryonic development. Meanwhile, Lin28 represses let-7 miRNA biogenesis and block pre-let-7 processing in the cytoplasm. The let-7 family of miRNAs is known to repress oncogenesis and cell cycle progression by targeting oncogenic genes and signalling pathways. Consequently, Lin28 acts as an oncogene by upregulating let-7 targets through the repression of let-7 biogenesis. A recent genome-wide association study (GWAS) showed that many genes related to Type 2 diabetes (T2D) are also oncogenes or cell cycle regulators. The role of Lin28 in mouse growth and glucose metabolism in metabolic-related tissues has also been studied. In these studies, whole-body Lin28 overexpression was found to promote glucose utilization and prevent weight gain by inhibiting let-7 biogenesis. Furthermore, Lin28 has been found to directly stimulate skeletal myogenesis and cell growth. Therefore, we determined whether similar effects mediated by Lin28a, which is essential for cell growth and proliferation, may also apply to pancreatic ß-cells. We found that overexpression of Lin28a protects pancreatic β-cells from streptozotocin (STZ)-induced β-cell destruction in vitro and in vivo. Furthermore, Lin28a-overexpressing transgenic (Tg) mice had higher insulin secretion in the presence of glucose than in control mice. Our findings suggest that the Lin28/let-7 axis is an important regulator of pancreatic β-cell functions and that precise modulation of this axis may be helpful in treating metabolic diseases such as diabetes. SIGNIFICANCE OF THE STUDY: We demonstrate that Lin28a prevents pancreatic β-cell death against streptozotocin (STZ)-induced β-cell destruction in vitro and in vivo. Furthermore, Lin28a promotes cell survival and proliferation by activating the PI3K-Akt signalling pathway, which may be dependent on let-7 regulation. Taken together, our results imply that the Lin28a/let-7 axis is an important regulator of pancreatic β-cell functions and that precise modulation of this axis may be helpful in treating metabolic diseases such as diabetes.

Abstract 299: RNA editing enzyme ADAR1 accelerates normal hematopoiesis cell cycle by regulation microRNA biogenesis

Abstract Compelling murine studies demonstrate that adenosine-to-inosine (A-to-I) RNA editing mediated by adenosine deaminase associated with RNA1 (ADAR1) is vital for both fetal and adult hematopoiesis. While genetic ablation of ADAR1 leads to murine embryonic lethality due to severe defects in erythropoiesis, conditional deletion in the hematopoietic system impairs maintenance indicative of cell type and context specific roles for ADAR1 in cell fate specification and self-renewal. By regulating mRNA and microRNA (miRNA) stability, ADAR1 exhibit wide-ranging effects on embryonic development and stem cell regulation. We have previously shown that inflammation-responsive ADAR1 plays important roles in both stem cell differentiation and self-renewal in CML (chronic myeloid leukemia) disease progression. Here, we describe a novel function of ADAR1 in cell cycle regulation of normal hematopoietic stem cell and progenitors (HSPC) by regulation of miRNA biogenesis. Our results demonstrated that ADAR1 induces G0 to G1 phase transition in normal cord blood HSPCs, as demonstrated by elevated expression of Ki67, reduced DiR signal, and enhanced in vivo cord blood engraftment. Cell cycle qRT-qPCR microarray of 84 cell cycle transcripts and whole transcriptome RNA-sequencing analysis of KEGG cell cycle pathway indicate that several cell cycle genes are differentially expression upon overexpression of ADAR1 WT or an A-to-I editing deficient ADAR1 mutant (ADAR1E912A). We previously demonstrated that impaired biogenesis of let-7 miRNAs by ADAR1 WT induces enhanced self-renewal in cord blood CD34+ HSPCs. To determine the miRNA targets of ADAR1-mediated RNA editing, we performed miRNome miScript PCR array of 1008 miRNA candidates in cord blood CD34+ HSPCs overexpressing ADAR1 WT or ADAR1E912A. Total of 263 miRNAs were differentially expressed (142 upregulated and 121 downregulated) by comparing ADAR1 WT to the backbone control. Interestingly, ADAR1E912A mutant also exhibit A-to-I editing independent regulation of miRNAs (307 upregulated and 59 downregulated). We found that the expression of miR-26a-5p, a miRNA frequently downregulated in leukemia, is inhibited by ADAR1-mediated RNA editing. ADAR1 directly binds and edits the DROSHA cleavage site of primary miR-26a transcript, thereby prevent miR26a-5p maturation. Moreover, lentiviral expression of mature miR26-5p reverses the effect of ADAR1 WT, including enhanced CDKN1A expression, inhibition of cord blood proliferation in vivo, as well as reduced HSC self-renewal as measured by colony-formation assay. Our finding suggests carefully regulated A-to-I editing by ADAR1 is essential for the maintenance of proper cell proliferation in HSC. For future study, it will be interesting to investigate if the elevated expression of ADAR1 in CML BC LSC contributes to false regulation of cell cycle that leads to the expansion of malignant leukemia stem cells. Citation Format: Qingfei Jiang, Maria Anna Zipeto, Nathan Delos Santos, Sheldon Morris, Catriona Jamieson. RNA editing enzyme ADAR1 accelerates normal hematopoiesis cell cycle by regulation microRNA biogenesis [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 299. doi:10.1158/1538-7445.AM2017-299

RNA Editing Enzyme ADAR1 Accelerates Normal Hematopoiesis Cell Cycle By Regulating microRNA Biogenesis

Compelling murine studies demonstrate that adenosine-to-inosine (A-to-I) RNA editing mediated by adenosine deaminase associated with RNA1 (ADAR1) is vital for both fetal and adult hematopoiesis. While genetic ablation of ADAR1 editase leads to murine embryonic lethality due to severe defects in erythropoiesis, conditional deletion in the hematopoietic system impairs maintenance indicative of cell type and context specific roles for ADAR1 in cell fate specification and self-renewal. By regulating mRNA and microRNA (miRNA) stability, ADAR1 exhibit wide-ranging effects on embryonic development and stem cell regulation. We have previously shown that inflammation-responsive ADAR1 plays important roles in both stem cell differentiation and self-renewal in CML (chronic myeloid leukemia) disease progression. Here, we describe a novel function of ADAR1 in cell cycle regulation of normal hematopoietic stem cell and progenitors (HSPC) by regulation of miRNA biogenesis.Our results demonstrated that ADAR1 induces G0 to G1 phase transition in normal cord blood HSPCs, as demonstrated by elevated expression of Ki67, reduced DiR signal, and enhanced in vivo cord blood engraftment. Cell cycle qRT-qPCR microarray of 84 cell cycle transcripts and whole transcriptome RNA-sequencing analysis of KEGG cell cycle pathway indicate that several cell cycle genes (CDKN1A, CDKN2A, CCNC, CCND1, BRCA2, etc) are differentially expression upon overexpression of ADAR1 WT or an A-to-I editing deficient ADAR1 mutant (ADAR1E912A). We previously demonstrated that impaired biogenesis of let-7 miRNAs by ADAR1 WT induces enhanced self-renewal in cord blood CD34+ HSPCs. To determine the miRNA targets of ADAR1-mediated RNA editing, we performed miRNome miScript PCR array of 1008 miRNA candidates in cord blood CD34+ HSPCs overexpressing ADAR1 WT or ADAR1E912A. Total of 263 miRNAs were differentially expressed (142 upregulated and 121 downregulated) by comparing ADAR1 WT to the backbone control. Interestingly, ADAR1E912A mutant also exhibit A-to-I editing independent regulation of miRNAs (307 upregulated and 59 downregulated). We found that the expression of miR-26a-5p, a miRNA frequently downregulated in leukemia, is inhibited by ADAR1-mediated RNA editing. ADAR1 directly binds and edits the DROSHA cleavage site of primary miR-26a transcript, thereby prevent miR26a-5p maturation. Moreover, lentiviral expression of mature miR26-5p reverses the effect of ADAR1 WT, including enhanced CDKN1A expression, inhibition of cord blood proliferation in vivo, as well as reduced HSC self-renewal as measured by colony-formation assay.Our finding suggests carefully regulated A-to-I editing by ADAR1 is essential for the maintenance of proper cell proliferation in HSC. For future study, it will be interesting to investigate if the elevated expression of ADAR1 in CML BC LSC contributes to false regulation of cell cycle that leads to the expansion of malignant leukemia stem cells. DisclosuresJamieson:CTI Biopharma: Research Funding; GlaxoSmithKline: Research Funding; Johnson & Johnson: Research Funding.

Lin28a uses distinct mechanisms of binding to RNA and affects miRNA levels positively and negatively.

Lin28a inhibits the biogenesis of let-7 miRNAs by triggering the polyuridylation and degradation of their precursors by terminal uridylyltransferases TUT4/7 and 3′-5′ exoribonuclease Dis3l2, respectively. Previously, we showed that Lin28a also controls the production of neuro-specific miRNA-9 via a polyuridylation-independent mechanism. Here we reveal that the sequences and structural characteristics of pre-let-7 and pre-miRNA-9 are eliciting two distinct modes of binding to Lin28a. We present evidence that Dis3l2 controls miRNA-9 production. Finally, we show that the constitutive expression of untagged Lin28a during neuronal differentiation in vitro positively and negatively affects numerous other miRNAs. Our findings shed light on the role of Lin28a in differentiating cells and on the ways in which one RNA-binding protein can perform multiple roles in the regulation of RNA processing.

Abstract LB-165: Multiple mechanisms disrupt let-7 miRNA biogenesis and function in neuroblastoma

Abstract The let-7 microRNA family are known tumor suppressors often deregulated in cancer, yet the underlying mechanisms of let-7 disruption remain poorly understood. Neuroblastoma, a neural crest derived tumor, is defined in part by poor prognosis associated with genetic amplification of MYCN, itself a let-7 target. The let-7 biogenesis inhibitor LIN28B has recently been implicated as a critical regulator of MYCN, but through CRISPR-mediated gene disruption we show that LIN28B is dispensable for both MYCN protein expression and growth of MYCN-amplified neuroblastoma cell lines despite robust de-repression of let-7, prompting us to explore additional mechanisms for let-7 disruption. Consequently, we have found a novel non-coding role for amplified MYCN mRNA as a potent let-7 sponge that through exceptionally high expression defines a sub-class of self-sponging amplified-competing-endogenous-RNA (aceRNA) and reconciling the dispensability of LIN28B in neuroblastoma cell lines. Furthermore, by analyzing a large cohort of tumor samples from patients, we observe frequent genomic loss of let-7 that inversely associates with MYCN-amplification, providing a functional explanation for the known MYCN-amplification-independent pattern of chromosome 3p and 11q loss, which harbor let-7g and let-7a2, respectively. We thus propose a model whereby let-7 disruption by genetic loss, LIN28B expression, or aceRNA sponging is a unifying mechanism of neuroblastoma pathogenesis. Indeed, our data show that the majority of neuroblastomas have at least one let-7 disruption event and that genetic loss in non-MYCN-amplified tumors marks decreased survival, further underscoring its importance. The inverse selective relationship between allelic loss and sponging of let-7 from highly expressed or amplified oncogenes may have broad implications for oncogenesis. Citation Format: John T. Powers, Kaloyan Tsanov, Frederik Roels, Catherine Spina, Richard Ebright, Marc Seligson, Yvanka de Soysa, Patrick Cahan, Daniel Pearson, Jessica Theißen, Ho-Chou Tu, Grace LaPier, Jihan Osborne, Samantha Ross, James Collins, Frank Berthold, George Daley. Multiple mechanisms disrupt let-7 miRNA biogenesis and function in neuroblastoma. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-165.

Lin28A Binds Active Promoters and Recruits Tet1 to Regulate Gene Expression

Lin28, a well-known RNA-binding protein, regulates diverse cellular properties. All physiological functions of Lin28A characterized so far have been attributed to its repression of let-7 miRNA biogenesis or modulation of mRNA translational efficiency. Here we show that Lin28A directly binds to a consensus DNA sequence in vitro and in mouse embryonic stem cells in vivo. ChIP-seq and RNA-seq reveal enrichment of Lin28A binding around transcription start sites and a positive correlation between its genomic occupancy and expression of many associated genes. Mechanistically, Lin28A recruits 5-methylcytosine-dioxygenase Tet1 to genomic binding sites to orchestrate 5-methylcytosine and 5-hydroxymethylcytosine dynamics. Either Lin28A or Tet1 knockdown leads to dysregulated DNA methylation and expression of common target genes. These results reveal a surprising role for Lin28A in transcriptional regulation via epigenetic DNA modifications and have implications for understanding mechanisms underlying versatile functions of Lin28A in mammalian systems.

Inflammatory Cytokine-Responsive ADAR1 Impairs Let-7 Biogenesis and Promotes Leukemia Stem Cell Generation

BackgroundIn advanced human malignancies, RNA sequencing (RNA-seq) has uncovered deregulation of adenosine deaminase acting on RNA (ADAR) editases that promote therapeutic resistance and leukemia stem cell (LSC) generation. Chronic myeloid leukemia (CML), an important paradigm for understanding LSC evolution, is initiated by BCR-ABL1 oncogene expression in hematopoietic stem cells (HSCs) but undergoes blast crisis (BC) transformation following aberrant self-renewal acquisition by myeloid progenitors harboring cytokine-responsive ADAR1 p150 overexpression. Emerging evidence suggests that adenosine to inosine editing at the level of primary (pri) or precursor (pre)-microRNA (miRNA), alters miRNA biogenesis and impairs biogenesis. However, relatively little is known about the role of inflammatory niche-driven ADAR1 miRNA editing in malignant reprogramming of progenitors into self-renewing LSCs.MethodsPrimary normal and CML progenitors were FACS-purified and RNA-Seq analysis as well as qRT-PCR validation were performed according to published methods (Jiang, 2013). MiRNAs were extracted from purified CD34+ cells derived from CP, BC CML and cord blood by RNeasy microKit (QIAGEN) and let-7 expression was evaluated by qRT-PCR using miScript Primer assay (QIAGEN). CD34+ cord blood (n=3) were transduced with lentiviral human JAK2, let-7a, wt-ADAR1 and mutant ADAR1, which lacks a functional deaminase domain. Because STAT signaling triggers ADAR1 transcriptional activation and both BCR-ABL1 and JAK2 activate STAT5a, nanoproteomics analysis of STAT5a levels was performed. Engrafted immunocompromised RAG2-/-γc-/- mice were treated with a JAK2 inhibitor, SAR302503, alone or in combination with a potent BCR-ABL1 TKI Dasatinib, for two weeks followed by FACS analysis of human progenitor engraftment in hematopoietic tissues and serial transplantation.ResultsRNA-seq and qRT-PCR analysis in FACS purified BC CML progenitors revealed an over-representation of inflammatory pathway activation and higher levels of JAK2-dependent inflammatory cytokine receptors, when compared to normal and chronic phase (CP) progenitors. Moreover, RNA-seq and qRT-PCR analysis showed decreased levels of mature let-7 family of stem cell regulatory miRNA in BC compared to normal and CP progenitors. Lentiviral human JAK2 transduction of CD34+ progenitors led to an increase of ADAR1 transcript levels and to a reduction in let-7 family members. Interestingly, lentiviral human JAK2 transduction of normal progenitors enhanced ADAR1 activity, as revealed by RNA editing-specific qRT-PCR and RNA-seq analysis. Moreover, qRT-PCR analysis of CD34+ progenitors transduced with wt-ADAR1, but not mutant ADAR1 lacking functional deaminase activity, reduced let-7 miRNA levels. These data suggested that ADAR1 impairs let-7 family biogenesis in a RNA editing dependent manner. Interestingly, RNA-seq analysis confirmed higher frequency of A-to-I editing events in pri- and pre-let-7 family members in CD34+ BC compared to CP progenitors, as well as normal progenitors transduced with human JAK2 and ADAR1-wt, but not mutant ADAR1. Lentiviral ADAR1 overexpression enhanced CP CML progenitor self-renewal and decreased levels of some members of the let-7 family. In contrast, lentiviral transduction of human let-7a significantly reduced self-renewal of progenitors. In vivo treatments with Dasatinib in combination with a JAK2 inhibitor, significantly reduced self-renewal of BCR-ABL1 expressing BC progenitors in the bone marrow thereby prolonging survival of serially transplanted mice. Finally, a reduction in ADAR1 p150 transcripts was also noted following combination treatment only suggesting a role for ADAR1 in CSC propagation.ConclusionThis is the first demonstration that intrinsic BCR-ABL oncogenic signaling and extrinsic cytokines signaling through JAK2 converge on activation of ADAR1 that drives LSC generation by impairing let-7 miRNA biogenesis. Targeted reversal of ADAR1-mediated miRNA editing may enhance eradication of inflammatory niche resident cancer stem cells in a broad array of malignancies, including JAK2-driven myeloproliferative neoplasms. DisclosuresJamieson:J&J: Research Funding; GSK: Research Funding.

Biogenesis and regulation of the let-7 miRNAs and their functional implications.

The let-7 miRNA was one of the first miRNAs discovered in the nematode, Caenorhabditis elegans, and its biological functions show a high level of evolutionary conservation from the nematode to the human. Unlike in C. elegans, higher animals have multiple isoforms of let-7 miRNAs; these isoforms share a consensus sequence called the ‘seed sequence’ and these isoforms are categorized into let-7 miRNA family. The expression of let-7 family is required for developmental timing and tumor suppressor function, but must be suppressed for the self-renewal of stem cells. Therefore, let-7 miRNA biogenesis must be carefully controlled. To generate a let-7 miRNA, a primary transcript is produced by RNA polymerase II and then subsequently processed by Drosha/DGCR8, TUTase, and Dicer. Because dysregulation of let-7 processing is deleterious, biogenesis of let-7 is tightly regulated by cellular factors, such as the RNA binding proteins, LIN28A/B and DIS3L2. In this review, we discuss the biological functions and biogenesis of let-7 miRNAs, focusing on the molecular mechanisms of regulation of let-7 biogenesis in vertebrates, such as the mouse and the human.

Abstract LB-290: Multiple distinct mechanisms disrupt let-7 miRNA biogenesis and function in neuroblastoma

Abstract The let-7 microRNA family are known tumor suppressors often deregulated in cancer, yet the underlying mechanisms of let-7 disruption remain poorly understood. Neuroblastoma, a neural crest derived tumor, is defined in part by poor prognosis associated with genetic amplification of MYCN, itself a let-7 target. The let-7 biogenesis inhibitor LIN28B has recently been implicated as a critical regulator of MYCN, but we have employed siRNA and CRISPR-mediated gene disruption to show that LIN28B is dispensable for both MYCN protein expression and growth of MYCN-amplified neuroblastoma cell lines despite robust de-repression of let-7, which prompted us to explore additional mechanisms for let-7 disruption. Consequently, we have found that amplified MYCN mRNA is a potent let-7 sponge that through exceptionally high expression defines a sub-class of self-sponging amplified-competing-endogenous-RNA (aceRNA), which reconciles the dispensability of LIN28B in NB cell lines. In addition, by analyzing a large cohort of tumor samples from patients, we observe frequent genomic loss of let-7 that inversely associates with MYCN-amplification, providing a functional explanation for the known MYCN-amplification-independent pattern of chromosome 3p and 11q loss, which harbor let-7g and let-7a2, respectively. We thus propose a model whereby let-7 disruption by genetic loss, LIN28B expression, or aceRNA sponging is a unifying mechanism of neuroblastoma pathogenesis. Indeed, our data show that the majority of neuroblastomas have at least one let-7 disruption event and that genetic loss in non-MYCN-amplified tumors marks decreased survival, further underscoring its importance. The inverse selective relationship between allelic loss and sponging of let-7 from highly expressed or amplified oncogenes may have broad implications for oncogenesis. Note: This abstract was not presented at the meeting. Citation Format: John T. Powers, Kaloyan M. Tsanov, Frederik Roles, Richard Ebright, Marc Seligson, Yvanka de Soysa, Patrick Cahan, Jessica Theissen, Grace S. LaPier, Dan S. Pearson, Frank Berthold, George Q. Daley. Multiple distinct mechanisms disrupt let-7 miRNA biogenesis and function in neuroblastoma. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr LB-290. doi:10.1158/1538-7445.AM2015-LB-290

A role of uridylation pathway for blockade of let-7 microRNA biogenesis by Lin28B.

The precise control of microRNA (miRNA) biosynthesis is crucial for gene regulation. Lin28A and Lin28B are selective inhibitors of biogenesis of let-7 miRNAs involved in development and tumorigenesis. Lin28A selectively inhibits let-7 biogenesis through cytoplasmic uridylation of precursor let-7 by TUT4 terminal uridyl transferase and subsequent degradation by Dis3l2 exonuclease. However, a role of this uridylation pathway remains unclear in let-7 blockade by Lin28B, a paralog of Lin28A, while Lin28B is reported to engage a distinct mechanism in the nucleus to suppress let-7. Here we revisit a functional link between Lin28B and the uridylation pathway with a focus on let-7 metabolism in cancer cells. Both Lin28A and Lin28B interacted with Dis3l2 in the cytoplasm, and silencing of Dis3l2 upregulated uridylated pre-let-7 in both Lin28A- and Lin28B-expressing cancer cell lines. In addition, we found that amounts of let-7 precursors influenced intracellular localization of Lin28B. Furthermore, we found that MCPIP1 (Zc3h12a) ribonuclease was also involved in degradation of both non-uridylated and uridylated pre-let-7. Cancer transcriptome analysis showed association of expression levels of Lin28B and uridylation pathway components, TUT4 and Dis3l2, in various human cancer cells and hepatocellular carcinoma. Collectively, these results suggest that cytoplasmic uridylation pathway actively participates in blockade of let-7 biogenesis by Lin28B.

Lin28B promotes melanoma growth by mediating a microRNA regulatory circuit.

It has been increasingly recognized that microRNAs (miRNAs) are often dysregulated in various human malignancies and can function as oncogenes or tumor-suppressors. However, the potential roles of miRNAs and components of the miRNA biogenesis pathway remain poorly defined in melanoma. Here, we systematically profiled miRNA expression in human melanocytes and melanoma cells, and identified a prominent function of miR-125a-5p in suppressing melanoma growth. Mechanistically, we discovered that Lin28B, a well-characterized inhibitor of let-7 miRNA biogenesis, was a direct target of miR-125a-5p in melanoma. We showed that the Lin28B was aberrantly expressed in a large proportion of melanoma patients and was functionally required for melanoma progression. We further demonstrated the involvement of let-7-dependent mechanism downstream of Lin28B, resulting in the activation of transforming growth factor-β signaling cascade. Collectively, our data implicate Lin28B as a novel oncogene in melanomagenesis by mediating a miRNA regulatory circuit.