Abstract

Lin28, a highly conserved RNA-binding protein, has emerged as a modulator of the processing of the let-7 microRNA. This role for Lin28 has important implications for our mechanistic understanding of pluripotency, the timing of development, and oncogenesis. Lin28, a highly conserved RNA-binding protein, has emerged as a modulator of the processing of the let-7 microRNA. This role for Lin28 has important implications for our mechanistic understanding of pluripotency, the timing of development, and oncogenesis. MicroRNAs (miRNAs) are a class of small RNAs with diverse regulatory roles. The discovery of miRNAs and closely related small-interfering RNAs (siRNAs) is a watershed moment in biology that has changed traditional views of genetic regulation and has placed small RNAs alongside transcription factors as critical regulators of gene expression. But what regulates these regulators? Recent studies indicate that miRNA biogenesis can be posttranscriptionally regulated by trans-acting factors. The Lin28/let-7 partnership is the best characterized example of the relationship between an miRNA and its posttranscriptional regulator and may shed light on the posttranscriptional regulation of other miRNAs. Here, we highlight recent advances in our understanding of the posttranscriptional regulation of the let-7 miRNA by the conserved RNA-binding protein Lin28 and show how this impacts our understanding of pluripotency, development, and cancer. Lin28 was first characterized in the nematode Caenorhabditis elegans as an important regulator of developmental timing (Ambros and Horvitz, 1984Ambros V. Horvitz H.R. Science. 1984; 226: 409-416Crossref PubMed Scopus (539) Google Scholar, Moss et al., 1997Moss E.G. Lee R.C. Ambros V. Cell. 1997; 88: 637-646Abstract Full Text Full Text PDF PubMed Scopus (655) Google Scholar). Mutations in the lin-28 gene and in other so-called "heterochronic genes" perturb developmental progression in the worm, such that characteristic events occur either earlier or later than normal. The mammalian homologs of lin-28, Lin28 and Lin28b, bind to the terminal loops of the precursors of let-7 family miRNAs and block their processing into mature miRNAs (Heo et al., 2008Heo I. Joo C. Cho J. Ha M. Han J. Kim V.N. Mol. Cell. 2008; 32: 276-284Abstract Full Text Full Text PDF PubMed Scopus (736) Google Scholar, Newman et al., 2008Newman M.A. Thomson J.M. Hammond S.M. RNA. 2008; 14: 1539-1549Crossref PubMed Scopus (568) Google Scholar, Rybak et al., 2008Rybak A. Fuchs H. Smirnova L. Brandt C. Pohl E.E. Nitsch R. Wulczyn F.G. Nat. Cell Biol. 2008; 10: 987-993Crossref PubMed Scopus (626) Google Scholar, Viswanathan et al., 2008Viswanathan S.R. Daley G.Q. Gregory R.I. Science. 2008; 320: 97-100Crossref PubMed Scopus (1133) Google Scholar, Piskounova et al., 2008Piskounova E. Viswanathan S.R. Janas M. LaPierre R.J. Daley G.Q. Sliz P. Gregory R.I. J. Biol. Chem. 2008; 283: 21310-21314Crossref PubMed Scopus (274) Google Scholar). Interestingly, let-7 is itself a heterochronic gene in the worm, suggesting that the interaction between Lin28 and let-7 is important for regulating the timing of development. Both lin-28 and let-7 are highly conserved across evolution (Moss et al., 1997Moss E.G. Lee R.C. Ambros V. Cell. 1997; 88: 637-646Abstract Full Text Full Text PDF PubMed Scopus (655) Google Scholar, Pasquinelli et al., 2000Pasquinelli A.E. Reinhart B.J. Slack F. Martindale M.Q. Kuroda M.I. Maller B. Hayward D.C. Ball E.E. Degnan B. Muller P. et al.Nature. 2000; 408: 86-89Crossref PubMed Scopus (1735) Google Scholar), and recent data are beginning to uncover roles for Lin28 in mammalian development. Several groups have reported the reprogramming of adult human fibroblasts to induced pluripotent stem (iPS) cells using the same combination of factors (OCT4, SOX2, KLF4, and c-MYC) originally discovered and used by Shinya Yamanaka to reprogram mouse fibroblasts back to a pluripotent state. However, James Thomson's group created iPS cells from adult human fibroblasts using a slightly different cocktail of factors: OCT4, SOX2, NANOG, and LIN28 (reviewed in Yamanaka, 2008Yamanaka S. Philos. Trans. R. Soc. Lond. B Biol. Sci. 2008; 363: 2079-2087Crossref PubMed Scopus (115) Google Scholar). Together with recent evidence that Lin28 represses the processing of a let-7 precursor into the mature let-7 miRNA, this raises the tantalizing possibility that repression of let-7 is important in establishing the pluripotent state. In support of this notion, the let-7 miRNA is present in low amounts or is absent in a variety of different stem and progenitor cell populations. Furthermore, as organisms age, the self-renewal capacity of their neural stem cells declines, apparently due to an increase in let-7 and a corresponding decrease in the expression of HMGA2, a target gene that is silenced by let-7 (Nishino et al., 2008Nishino J. Kim I. Chada K. Morrison S.J. Cell. 2008; 135: 227-239Abstract Full Text Full Text PDF PubMed Scopus (468) Google Scholar). Mature miRNAs of the let-7 family are poorly expressed in undifferentiated embryonic stem cells (ESCs) but their primary let-7 precursor transcripts are readily detected (Thomson et al., 2006Thomson J.M. Newman M. Parker J.S. Morin-Kensicki E.M. Wright T. Hammond S.M. Genes Dev. 2006; 20: 2202-2207Crossref PubMed Scopus (716) Google Scholar).This suggests that mature let-7 miRNAs are maintained at low levels posttranscriptionally in ESCs and their target genes are not repressed. Recent evidence suggests that let-7 opposes the actions of a family of ESC cell-cycle-regulating miRNAs that maintain self-renewal (Melton et al., 2010Melton C. Judson R.L. Blelloch R. Nature. 2010; 463: 621-626Crossref PubMed Scopus (531) Google Scholar). In the reprogramming cocktail used by Yu et al., 2007Yu F. Yao H. Zhu P. Zhang X. Pan Q. Gong C. Huang Y. Hu X. Su F. Lieberman J. Song E. Cell. 2007; 131: 1109-1123Abstract Full Text Full Text PDF PubMed Scopus (1578) Google Scholar, LIN28 functionally replaces c-MYC, and c-MYC is a known target of let-7 (Kumar et al., 2007Kumar M.S. Lu J. Mercer K.L. Golub T.R. Jacks T. Nat. Genet. 2007; 39: 673-677Crossref PubMed Scopus (1195) Google Scholar), which supports the notion that LIN28 promotes reprogramming by preventing production of mature let-7 miRNAs. Several reports also indicate that Lin28 can affect protein levels by regulating mRNA stability (Polesskaya et al., 2007Polesskaya A. Cuvellier S. Naguibneva I. Duquet A. Moss E.G. Harel-Bellan A. Genes Dev. 2007; 21: 1125-1138Crossref PubMed Scopus (225) Google Scholar, Xu et al., 2009Xu B. Zhang K. Huang Y. RNA. 2009; 15: 357-361Crossref PubMed Scopus (151) Google Scholar, Qiu et al., 2009Qiu C. Ma Y. Wang J. Peng S. Huang Y. Nucleic Acids Res. 2009; (Published online December 4, 2009)https://doi.org/10.1093/nar/gkp1071Crossref Scopus (166) Google Scholar), and that Lin28 directly promotes the translation of OCT4 mRNA (Qiu et al., 2009Qiu C. Ma Y. Wang J. Peng S. Huang Y. Nucleic Acids Res. 2009; (Published online December 4, 2009)https://doi.org/10.1093/nar/gkp1071Crossref Scopus (166) Google Scholar) and may modulate cell proliferation by enhancing the translation of various cell-cycle regulators in mouse ESCs (Xu et al., 2009Xu B. Zhang K. Huang Y. RNA. 2009; 15: 357-361Crossref PubMed Scopus (151) Google Scholar). Therefore, Lin28 may promote reprogramming through both miRNA-dependent and miRNA-independent pathways. In some cases, it is also possible that regulation of mRNA stability may be a secondary consequence of Lin28-mediated modulation of miRNA processing. In addition to c-MYC, several other oncogenes are known to be targets of let-7, including K-Ras and cyclin D1 (reviewed in Roush and Slack, 2008Roush S. Slack F.J. Trends Cell Biol. 2008; 18: 505-516Abstract Full Text Full Text PDF PubMed Scopus (909) Google Scholar). Therefore, let-7 may act as a tumor suppressor miRNA, and its loss has been linked to oncogenesis (Johnson et al., 2005Johnson S.M. Grosshans H. Shingara J. Byrom M. Jarvis R. Cheng A. Labourier E. Reinert K.L. Brown D. Slack F.J. Cell. 2005; 120: 635-647Abstract Full Text Full Text PDF PubMed Scopus (2984) Google Scholar, Takamizawa et al., 2004Takamizawa J. Konishi H. Yanagisawa K. Tomida S. Osada H. Endoh H. Harano T. Yatabe Y. Nagino M. Nimura Y. et al.Cancer Res. 2004; 64: 3753-3756Crossref PubMed Scopus (2071) Google Scholar). Mammalian genomes contain multiple let-7 family members processed from distinct precursor transcripts. In settings where let-7 family members are coexpressed and show functional redundancy, genetic events that inactivate individual let-7 members may not have dramatic phenotypic consequences. However, given that LIN28/LIN28B regulates all let-7 family miRNAs, could LIN28/LIN28B contribute to oncogenesis by coordinately inactivating let-7 family miRNAs? We have found that LIN28/LIN28B can promote transformation by repressing let-7 miRNAs, and that activation of LIN28/LIN28B occurs in many different human tumors with a frequency of ∼15% (Viswanathan et al., 2009Viswanathan S.R. Powers J.T. Einhorn W. Hoshida Y. Ng T.L. Toffanin S. O'Sullivan M. Lu J. Phillips L.A. Lockhart V.L. et al.Nat. Genet. 2009; 41: 843-848Crossref PubMed Scopus (616) Google Scholar). It is indeed intriguing, but perhaps no coincidence, that all currently described reprogramming factors—OCT4, SOX2, KLF4, c-MYC, NANOG, and LIN28—have been linked to oncogenesis. This underscores the notion that the complex genomic reprogramming that accompanies induced pluripotency shares many commonalities with the complex, albeit aberrant, genomic reprogramming that accompanies neoplastic transformation. Strikingly, LIN28 and LIN28B are specifically activated in the subset of tumors that are poorly differentiated and carry the worst prognosis (Viswanathan et al., 2009Viswanathan S.R. Powers J.T. Einhorn W. Hoshida Y. Ng T.L. Toffanin S. O'Sullivan M. Lu J. Phillips L.A. Lockhart V.L. et al.Nat. Genet. 2009; 41: 843-848Crossref PubMed Scopus (616) Google Scholar). Aggressive, high-grade tumors are frequently characterized by impaired differentiation, and pathologists often use the differentiation status of a tumor as a metric for scoring cancer severity. When expressed in the proper genetic and epigenetic context, LIN28 together with OCT4, SOX2, and NANOG can promote the reprogramming of a terminally differentiated cell to a pluripotent ESC-like cell. But when aberrantly expressed, LIN28/LIN28B may contribute to the development of an aggressive, poorly differentiated tumor. Indeed, a recent report suggests that several different types of aggressive poorly differentiated tumors, including basal-like breast cancers and high-grade bladder carcinomas, express an ESC-like gene signature (Ben Porath et al., 2008Ben Porath I. Thomson M.W. Carey V.J. Ge R. Bell G.W. Regev A. Weinberg R.A. Nat. Genet. 2008; 40: 499-507Crossref PubMed Scopus (1800) Google Scholar). Poorly differentiated tumors might arise either from stem or progenitor cells that acquire additional malignant hits or from somatic cells that dedifferentiate by activating components of the pluripotency machinery (Figure 1). In the case of LIN28B, rare amplification or translocation events might explain activation in some cases (Viswanathan et al., 2009Viswanathan S.R. Powers J.T. Einhorn W. Hoshida Y. Ng T.L. Toffanin S. O'Sullivan M. Lu J. Phillips L.A. Lockhart V.L. et al.Nat. Genet. 2009; 41: 843-848Crossref PubMed Scopus (616) Google Scholar). A more common mechanism, however, might be transcriptional activation by an upstream factor or factors during tumor progression. In support of this notion, c-Myc binds to both the Lin28 (Dangi-Garimella et al., 2009Dangi-Garimella S. Yun J. Eves E.M. Newman M. Erkeland S.J. Hammond S.M. Minn A.J. Rosner M.R. EMBO J. 2009; 28: 347-358Crossref PubMed Scopus (297) Google Scholar) and LIN28B (Chang et al., 2009Chang T.C. Zeitels L.R. Hwang H.W. Chivukula R.R. Wentzel E.A. Dews M. Jung J. Gao P. Dang C.V. Beer M.A. et al.Proc. Natl. Acad. Sci. USA. 2009; 106: 3384-3389Crossref PubMed Scopus (309) Google Scholar) loci and activates expression of these genes. However, c-Myc activation in tumors is far more common than LIN28/LIN28B activation—the need for transcriptional coactivators or epigenetic inaccessibility of the LIN28/LIN28B loci may explain this. Finally, in some cases, LIN28/LIN28B may be expressed in a rare somatic progenitor cell of origin that becomes transformed (Figure 1). Regardless of the mechanism of activation, which may vary from tumor to tumor, cancers expressing LIN28/LIN28B appear to be dependent on these proteins for growth (Chang et al., 2009Chang T.C. Zeitels L.R. Hwang H.W. Chivukula R.R. Wentzel E.A. Dews M. Jung J. Gao P. Dang C.V. Beer M.A. et al.Proc. Natl. Acad. Sci. USA. 2009; 106: 3384-3389Crossref PubMed Scopus (309) Google Scholar, Dangi-Garimella et al., 2009Dangi-Garimella S. Yun J. Eves E.M. Newman M. Erkeland S.J. Hammond S.M. Minn A.J. Rosner M.R. EMBO J. 2009; 28: 347-358Crossref PubMed Scopus (297) Google Scholar, Viswanathan et al., 2009Viswanathan S.R. Powers J.T. Einhorn W. Hoshida Y. Ng T.L. Toffanin S. O'Sullivan M. Lu J. Phillips L.A. Lockhart V.L. et al.Nat. Genet. 2009; 41: 843-848Crossref PubMed Scopus (616) Google Scholar). The precise manner by which LIN28 contributes to tumorigenesis presents a compelling avenue for future investigation, and there are several intriguing possibilities. Rare breast tumor-initiating cells express low levels of all let-7 family miRNAs (Yu et al., 2007Yu F. Yao H. Zhu P. Zhang X. Pan Q. Gong C. Huang Y. Hu X. Su F. Lieberman J. Song E. Cell. 2007; 131: 1109-1123Abstract Full Text Full Text PDF PubMed Scopus (1578) Google Scholar). Meanwhile reduced let-7 levels have been shown to confer radioresistance, whereas increased let-7 levels confer radiosensitivity (Weidhaas et al., 2007Weidhaas J.B. Babar I. Nallur S.M. Trang P. Roush S. Boehm M. Gillespie E. Slack F.J. Cancer Res. 2007; 67: 11111-11116Crossref PubMed Scopus (337) Google Scholar). Knocking down LIN28B expression in a radioresistant lung cancer cell line increased let-7 levels and decreased expression of K-RAS, a let-7 target, resulting in enhanced radiosensitivity (Jeong et al., 2009Jeong S.H. Wu H.G. Park W.Y. Exp. Mol. Med. 2009; 41: 912-918Crossref PubMed Scopus (45) Google Scholar). Therefore, tumor-initiating cells expressing LIN28/LIN28B may persist after chemotherapy and may seed relapse in some patients. Emerging evidence suggests that HMGA2, another target of let-7, actively promotes the epithelial-to-mesenchymal transition (EMT), a key event in development and metastasis (Dangi-Garimella et al., 2009Dangi-Garimella S. Yun J. Eves E.M. Newman M. Erkeland S.J. Hammond S.M. Minn A.J. Rosner M.R. EMBO J. 2009; 28: 347-358Crossref PubMed Scopus (297) Google Scholar, Shell et al., 2007Shell S. Park S.M. Radjabi A.R. Schickel R. Kistner E.O. Jewell D.A. Feig C. Lengyel E. Peter M.E. Proc. Natl. Acad. Sci. USA. 2007; 104: 11400-11405Crossref PubMed Scopus (388) Google Scholar). The observation that LIN28 and LIN28B are specifically activated in advanced stage and high-grade tumors suggests that they play a role later during tumorigenesis. Furthermore, Lin28 and let-7 have been placed directly within a metastasis signaling cascade (Dangi-Garimella et al., 2009Dangi-Garimella S. Yun J. Eves E.M. Newman M. Erkeland S.J. Hammond S.M. Minn A.J. Rosner M.R. EMBO J. 2009; 28: 347-358Crossref PubMed Scopus (297) Google Scholar). Notably, Lin28 shares many parallels with Twist, a factor recently identified as an inducer of EMT and a key mediator of metastasis (Dangi-Garimella et al., 2009Dangi-Garimella S. Yun J. Eves E.M. Newman M. Erkeland S.J. Hammond S.M. Minn A.J. Rosner M.R. EMBO J. 2009; 28: 347-358Crossref PubMed Scopus (297) Google Scholar, Mani et al., 2008Mani S.A. Guo W. Liao M.J. Eaton E.N. Ayyanan A. Zhou A.Y. Brooks M. Reinhard F. Zhang C.C. Shipitsin M. et al.Cell. 2008; 133: 704-715Abstract Full Text Full Text PDF PubMed Scopus (6231) Google Scholar). Both Lin28 and Twist are encoded by developmentally regulated genes, are associated with advanced malignancy and regulatory circuits involving c-Myc, and both strongly cooperate with other oncogenes in assays of cell transformation (Mani et al., 2008Mani S.A. Guo W. Liao M.J. Eaton E.N. Ayyanan A. Zhou A.Y. Brooks M. Reinhard F. Zhang C.C. Shipitsin M. et al.Cell. 2008; 133: 704-715Abstract Full Text Full Text PDF PubMed Scopus (6231) Google Scholar, Valsesia-Wittmann et al., 2004Valsesia-Wittmann S. Magdeleine M. Dupasquier S. Garin E. Jallas A.C. Combaret V. Krause A. Leissner P. Puisieux A. Cancer Cell. 2004; 6: 625-630Abstract Full Text Full Text PDF PubMed Scopus (202) Google Scholar). Elucidating whether LIN28 and LIN28B have a direct role in promoting metastasis is a potentially rewarding avenue for future work. Several studies place Lin28/Lin28b within an important regulatory network involving c-Myc and let-7 (Chang et al., 2009Chang T.C. Zeitels L.R. Hwang H.W. Chivukula R.R. Wentzel E.A. Dews M. Jung J. Gao P. Dang C.V. Beer M.A. et al.Proc. Natl. Acad. Sci. USA. 2009; 106: 3384-3389Crossref PubMed Scopus (309) Google Scholar, Dangi-Garimella et al., 2009Dangi-Garimella S. Yun J. Eves E.M. Newman M. Erkeland S.J. Hammond S.M. Minn A.J. Rosner M.R. EMBO J. 2009; 28: 347-358Crossref PubMed Scopus (297) Google Scholar). Lin28/Lin28b represses let-7, which is itself able to repress Lin28/Lin28b by binding to the 3′UTR of Lin28/Lin28b transcripts, thus forming a double-negative-feedback loop. A second feedback loop exists between Lin28 and c-Myc: Lin28/Lin28b derepresses c-Myc by repressing let-7, and c-Myc transcriptionally activates both Lin28 and Lin28b (Chang et al., 2009Chang T.C. Zeitels L.R. Hwang H.W. Chivukula R.R. Wentzel E.A. Dews M. Jung J. Gao P. Dang C.V. Beer M.A. et al.Proc. Natl. Acad. Sci. USA. 2009; 106: 3384-3389Crossref PubMed Scopus (309) Google Scholar, Dangi-Garimella et al., 2009Dangi-Garimella S. Yun J. Eves E.M. Newman M. Erkeland S.J. Hammond S.M. Minn A.J. Rosner M.R. EMBO J. 2009; 28: 347-358Crossref PubMed Scopus (297) Google Scholar). Notably, derepression of c-Myc might also contribute to the transcriptional repression of diverse miRNAs and transcriptional activation of certain oncogenic miRNAs (Chang et al., 2008Chang T.C. Yu D. Lee Y.S. Wentzel E.A. Arking D.E. West K.M. Dang C.V. Thomas-Tikhonenko A. Mendell J.T. Nat. Genet. 2008; 40: 43-50Crossref PubMed Scopus (1051) Google Scholar). This Lin28/let-7/c-Myc loop may partially explain the widespread deregulation of miRNAs observed in many human malignancies (Lu et al., 2005Lu J. Getz G. Miska E.A. Alvarez-Saavedra E. Lamb J. Peck D. Sweet-Cordero A. Ebert B.L. Mak R.H. Ferrando A.A. et al.Nature. 2005; 435: 834-838Crossref PubMed Scopus (7741) Google Scholar). There is also a positive-feedback loop involving NF-κB, Lin28b, let-7, and IL-6: NF-κB induces expression of Lin28b, leading to repression of let-7 and expression of the gene encoding IL-6 (a let-7 target). IL-6 can itself activate NF-κB, completing a positive-feedback loop. In an experimental system, transient induction of the Src tyrosine kinase activates NF-κB and initiates this positive-feedback loop, resulting in neoplastic transformation. This loop operates in cancer cell lines and breast tumor-initiating cells and has been suggested to link the inflammatory response, of which IL-6 is a component, to cancer (Iliopoulos et al., 2009Iliopoulos D. Hirsch H.A. Struhl K. Cell. 2009; 139: 693-706Abstract Full Text Full Text PDF PubMed Scopus (1067) Google Scholar). Both of these loops may function in normal development: double negative-feedback loops have been postulated to play a central role in gene regulatory networks by forming "bistable switches" that reinforce binary cell fate decisions (Martinez et al., 2008Martinez N.J. Ow M.C. Barrasa M.I. Hammell M. Sequerra R. Doucette-Stamm L. Roth F.P. Ambros V.R. Walhout A.J.M. Genes Dev. 2008; 22: 2535-2549Crossref PubMed Scopus (178) Google Scholar). Several genome-wide association studies (GWAS) have implicated LIN28B as a regulator of human development. Hirschhorn and colleagues identified markers near 12 loci that account for 2% of the population variation in human height; 1 of the 12 loci identified was LIN28B, and several other loci encode let-7 targets (Lettre et al., 2008Lettre G. Jackson A.U. Gieger C. Schumacher F.R. Berndt S.I. Sanna S. Eyheramendy S. Voight B.F. Butler J.L. Guiducci C. et al.Nat. Genet. 2008; 40: 584-591Crossref PubMed Scopus (436) Google Scholar). More recently, several independent GWAS have identified common polymorphisms near LIN28B as being associated with a woman's age at menarche (reviewed in Hartge, 2009Hartge P. Nat. Genet. 2009; 41: 637-638Crossref PubMed Scopus (59) Google Scholar). Together with established data in the worm, the identification of relevant human genetic variations suggests that the Lin28/let-7 axis may be an important regulator of cell growth and development in evolutionarily distant organisms. Subtle variations in the activity of this pathway may result in height or size differences among individuals of the same species, and more dramatic perturbations of this pathway may drive oncogenesis. Thus, Lin28/Lin28b may be similar to PI3-kinase, mTOR, and components of the Hippo pathway, which contribute to the regulation of organ growth and size and may be deregulated in some cancers. Several recent reports suggest that Lin28 may cooperate with a terminal uridylyl transferase (TUTase) to regulate blockade of let-7 production (Figure 2). Lin28 and Lin28b are associated with a TUTase activity that results in the addition of a poly-U tail to the let-7 precursor, pre-let-7 (Heo et al., 2008Heo I. Joo C. Cho J. Ha M. Han J. Kim V.N. Mol. Cell. 2008; 32: 276-284Abstract Full Text Full Text PDF PubMed Scopus (736) Google Scholar). Uridylated pre-let-7 cannot be efficiently processed by the enzyme Dicer and is targeted for degradation. Three groups have now identified the enzyme responsible for this activity in both mammalian cells (TUT4/zcchc11) (Heo et al., 2009Heo I. Joo C. Kim Y.K. Ha M. Yoon M.J. Cho J. Yeom K.H. Han J. Kim V.N. Cell. 2009; 138: 696-708Abstract Full Text Full Text PDF PubMed Scopus (599) Google Scholar, Hagan et al., 2009Hagan J.P. Piskounova E. Gregory R.I. Nat. Struct. Mol. Biol. 2009; 16: 1021-1025Crossref PubMed Scopus (390) Google Scholar) and in C. elegans (PUP-2) (Lehrbach et al., 2009Lehrbach N.J. Armisen J. Lightfoot H.L. Murfitt K.J. Bugaut A. Balasubramanian S. Miska E.A. Nat. Struct. Mol. Biol. 2009; 16: 1016-1020Crossref PubMed Scopus (190) Google Scholar). Together, these reports outline an evolutionarily ancient pathway in which Lin28 recognizes pre-let-7 via a motif in its terminal loop and recruits a TUTase to mark pre-let-7 for degradation. Lin28/TUT4-mediated uridylation of other pre-miRNAs with similar loop motifs has also been reported (Heo et al., 2009Heo I. Joo C. Kim Y.K. Ha M. Yoon M.J. Cho J. Yeom K.H. Han J. Kim V.N. Cell. 2009; 138: 696-708Abstract Full Text Full Text PDF PubMed Scopus (599) Google Scholar). Given that polymerases are facile targets for pharmacological inhibition by small molecules, TUT4 may prove to be an attractive pharmaceutical target for manipulating the LIN28/let-7 axis in cancer cells. The terminal loops of miRNA precursors serve essential regulatory roles (Trabucchi et al., 2009Trabucchi M. Briata P. Garcia-Mayoral M. Haase A.D. Filipowicz W. Ramos A. Gherzi R. Rosenfeld M.G. Nature. 2009; 459: 1010-1014Crossref PubMed Scopus (491) Google Scholar, Michlewski et al., 2008Michlewski G. Guil S. Semple C.A. Cßceres J.F. Mol. Cell. 2008; 32: 383-393Abstract Full Text Full Text PDF PubMed Scopus (264) Google Scholar, Guil and Caceres, 2007Guil S. Caceres J.F. Nat. Struct. Mol. Biol. 2007; 14: 591-596Crossref PubMed Scopus (415) Google Scholar, Piskounova et al., 2008Piskounova E. Viswanathan S.R. Janas M. LaPierre R.J. Daley G.Q. Sliz P. Gregory R.I. J. Biol. Chem. 2008; 283: 21310-21314Crossref PubMed Scopus (274) Google Scholar); for example, hnRNPA1 binds to the terminal loop of pri-miR-18a and enhances Drosha-mediated processing of this precursor into the mature miRNA (Guil and Caceres, 2007Guil S. Caceres J.F. Nat. Struct. Mol. Biol. 2007; 14: 591-596Crossref PubMed Scopus (415) Google Scholar). The RNA-binding protein KSRP binds to the terminal loop of a subset of miRNAs, including let-7, and enhances their processing (Trabucchi et al., 2009Trabucchi M. Briata P. Garcia-Mayoral M. Haase A.D. Filipowicz W. Ramos A. Gherzi R. Rosenfeld M.G. Nature. 2009; 459: 1010-1014Crossref PubMed Scopus (491) Google Scholar). Indeed, it is quite likely that the expression of many miRNAs is controlled at the level of processing, with the terminal loop serving as a point of regulation (Michlewski et al., 2008Michlewski G. Guil S. Semple C.A. Cßceres J.F. Mol. Cell. 2008; 32: 383-393Abstract Full Text Full Text PDF PubMed Scopus (264) Google Scholar). The factors that control miRNA biogenesis are likely to confer an additional layer of precision, complexity, and versatility to gene regulation. The therapeutic targeting of such regulatory factors may be a viable strategy to reduce or enhance the expression of specific miRNAs in the setting of human disease. Recent studies of Lin28 and let-7 have highlighted the fact that normal development, somatic cell reprogramming, and oncogenesis share common pathways. Going forward, it will be critical to define precise roles for Lin28/Lin28b in each of these settings. This will allow us to understand how the Lin28/let-7 pathway functions in physiological development, how it is deregulated during tumorigenesis, and how it can be experimentally exploited to enhance the production of pluripotent cells. More broadly, further investigation of the Lin28/let-7 partnership may lend insight into the mechanisms and consequences of the regulation of miRNA expression by trans-acting factors. GQD is a member of the Scientific Advisory Board of MPM Capital, iPierian, and Epizyme.