Nuclear Bodies: Random Aggregates of Sticky Proteins or Crucibles of Macromolecular Assembly?
2009; Elsevier BV; Volume: 17; Issue: 5 Linguagem: Inglês
10.1016/j.devcel.2009.10.017
ISSN1878-1551
AutoresA. Gregory Matera, Mario Izaguire-Sierra, Kavita Praveen, T. K. Rajendra,
Tópico(s)RNA modifications and cancer
ResumoThe principles of self-assembly and self-organization are major tenets of molecular and cellular biology. Governed by these principles, the eukaryotic nucleus is composed of numerous subdomains and compartments, collectively described as nuclear bodies. Emerging evidence reveals that associations within and between various nuclear bodies and genomic loci are dynamic and can change in response to cellular signals. This review will discuss recent progress in our understanding of how nuclear body components come together, what happens when they form, and what benefit these subcellular structures may provide to the tissues or organisms in which they are found. The principles of self-assembly and self-organization are major tenets of molecular and cellular biology. Governed by these principles, the eukaryotic nucleus is composed of numerous subdomains and compartments, collectively described as nuclear bodies. Emerging evidence reveals that associations within and between various nuclear bodies and genomic loci are dynamic and can change in response to cellular signals. This review will discuss recent progress in our understanding of how nuclear body components come together, what happens when they form, and what benefit these subcellular structures may provide to the tissues or organisms in which they are found. The spatial arrangement of chromatin within the nuclear volume entails a complex interplay between factors involved in chromosome maintenance and those involved in gene expression. Understanding how genomes actually function in vivo has been termed the “Holy Grail” of genome biology and a logical next step after the sequencing projects (Misteli, 2007Misteli T. Beyond the sequence: cellular organization of genome function.Cell. 2007; 128: 787-800Abstract Full Text Full Text PDF PubMed Scopus (473) Google Scholar). To accomplish this lofty goal, we must learn in detail how the Central Dogma is applied in three dimensions over developmental time. Fundamental to this understanding will be knowledge of the relationship between the chromatin and the interchromatin space, i.e., the genome and its immediate environment. The cell nucleus is a complex organelle whose dynamic architecture consists of numerous subcellular compartments, collectively referred to as nuclear bodies (Figure 1; Matera, 1999Matera A.G. Nuclear bodies: multifaceted subdomains of the interchromatin space.Trends Cell Biol. 1999; 9: 302-309Abstract Full Text Full Text PDF PubMed Scopus (282) Google Scholar). These structures include nucleoli, Cajal bodies (CBs), histone locus bodies (HLBs), splicing factor compartments (a.k.a. speckles or interchromatin granule clusters), paraspeckles, promyelocytic leukemia (PML) bodies, Gemini bodies (gems), perinucleolar compartments (PNCs), polycomb group (PcG) bodies, heat shock factor 1 (HSF1) foci, SAM-68 bodies, GATA-1 foci, and many more. Important nuclear processes, such as DNA replication and repair (Hozak et al., 1993Hozak P. Hassan A.B. Jackson D.A. Cook P.R. Visualization of replication factories attached to nucleoskeleton.Cell. 1993; 73: 361-373Abstract Full Text PDF PubMed Google Scholar, Jackson et al., 1994Jackson D.A. Hassan A.B. Errington R.J. Cook P.R. Sites in human nuclei where damage induced by ultraviolet light is repaired: localization relative to transcription sites and concentrations of proliferating cell nuclear antigen and the tumour suppressor protein, p53.J. 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One of the emergent principles of nuclear organization is that certain subnuclear domains are associated with specific gene loci. Another important rule is that associations between these subdomains and loci are dynamic and can change in response to cellular signals. As suggested in the title of this review, nuclear bodies might simply be a reflection of a propensity for certain proteins to form macromolecular aggregates. Indeed, many of the signature proteins of nuclear bodies are known to self-interact (Hebert and Matera, 2000Hebert M.D. Matera A.G. Self-association of coilin reveals a common theme in nuclear body localization.Mol. Biol. Cell. 2000; 11: 4159-4171Crossref PubMed Google Scholar and references therein). Protein aggregation and misfolding are cardinal features of numerous devastating diseases, including Alzheimer's, Huntington's, cystic fibrosis, Creutzfeldt-Jakob syndrome, and type II diabetes. However, overexpression of nuclear body signature proteins does not typically induce the formation of aberrant nuclear foci or result in an increase in the number or the size of their respective nuclear bodies. Nuclear body proteins are not known to be associated with protein folding diseases, and, in fact, there may even be a negative correlation. Comparative genome analyses have shown that natural selection acts against the aggregation of essential or self-interacting proteins (Chen and Dokholyan, 2008Chen Y. Dokholyan N.V. Natural selection against protein aggregation on self-interacting and essential proteins in yeast, fly, and worm.Mol. Biol. Evol. 2008; 25: 1530-1533Crossref PubMed Scopus (26) Google Scholar). Thus, if nuclear bodies are not simply aggregates of sticky proteins, what functional roles do they play? This review will focus on studies that are beginning to elucidate the molecular mechanisms underlying nuclear body assembly and function, using nucleoli, Cajal bodies, and histone locus bodies as paradigms. Historically, the term “nuclear bodies” has been reserved for structures that were characterized morphologically in the electron microscope. More recently, however, nuclear foci observed in the light microscope by immunocytochemistry have often been termed “bodies” without prior morphological evidence at the ultrastructural level. Although we are still far from understanding why most nuclear bodies form, recent progress has been made in elucidating how they are assembled in the cell. Two distinct assembly models have been considered, both of which involve recruitment of individual subunits (or small subcomplexes thereof) from a soluble nucleoplasmic pool (Cook, 2002Cook P.R. Predicting three-dimensional genome structure from transcriptional activity.Nat. 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Cell Biol. 2007; 9: 675-682Crossref PubMed Scopus (199) Google Scholar), Dundr and colleagues showed that essentially any Cajal body protein can nucleate formation of the entire CB structure de novo (Kaiser et al., 2008Kaiser T.E. Intine R.V. Dundr M. De novo formation of a subnuclear body.Science. 2008; 322: 1713-1717Crossref PubMed Scopus (91) Google Scholar). By tethering a given CB component to a specific site in the genome using the lac repressor/operator system, the investigators showed that the tethered protein or RNA was able to recruit most, if not all, of the other CB components (Kaiser et al., 2008Kaiser T.E. Intine R.V. Dundr M. De novo formation of a subnuclear body.Science. 2008; 322: 1713-1717Crossref PubMed Scopus (91) Google Scholar). The structures formed de novo had similar size to their endogenous counterparts, and the components of the tethered structures had similar dissociation kinetics to those of endogenous CBs (Dundr et al., 2004Dundr M. Hebert M.D. Karpova T.S. Stanek D. Xu H. Shpargel K.B. Meier U.T. Neugebauer K.M. Matera A.G. Misteli T. In vivo kinetics of Cajal body components.J. Cell Biol. 2004; 164: 831-842Crossref PubMed Scopus (101) Google Scholar, Kaiser et al., 2008Kaiser T.E. Intine R.V. Dundr M. De novo formation of a subnuclear body.Science. 2008; 322: 1713-1717Crossref PubMed Scopus (91) Google Scholar). Moreover, the tethered CBs could be disassembled (or reassembled) by interfering with (or restoring) the lac repressor's ability to bind to the operator (Kaiser et al., 2008Kaiser T.E. Intine R.V. Dundr M. De novo formation of a subnuclear body.Science. 2008; 322: 1713-1717Crossref PubMed Scopus (91) Google Scholar). Finally, tethering non-CB components to the lac operator array failed to nucleate CB formation, whereas tethering of PML body components resulted in formation of de novo PML bodies (Kaiser et al., 2008Kaiser T.E. Intine R.V. Dundr M. De novo formation of a subnuclear body.Science. 2008; 322: 1713-1717Crossref PubMed Scopus (91) Google Scholar). Taken together, these data strongly support a stochastic assembly model and argue against an ordered or hierarchical nuclear body assembly pathway (Figure 2). The Kaiser et al., 2008Kaiser T.E. Intine R.V. Dundr M. De novo formation of a subnuclear body.Science. 2008; 322: 1713-1717Crossref PubMed Scopus (91) Google Scholar study represents a kind of cellular “Field of Dreams” experiment—if you tether it, will they come? Though certainly a major step forward, the lac repressor tethering system essentially creates an artificial scaffold, raising the question of whether or not the system faithfully reflects the formation of nuclear bodies in vivo. One argument to the positive is that the de novo CBs formed by tethering are of a similar size and shape as the endogenous CBs (Kaiser et al., 2008Kaiser T.E. Intine R.V. Dundr M. De novo formation of a subnuclear body.Science. 2008; 322: 1713-1717Crossref PubMed Scopus (91) Google Scholar). However, we do not know whether the size is a function of the number of lac operator repeats. What happens if you change the length of the tethering chromatin? Does it change the size of the resultant CB? Does a single component truly seed the formation of a nuclear body on its own, or must it assemble some kind of subcomplex prior to its arrival at the lac operator targeting site? A more basic question is whether or not CBs require a tether in the first place. In other words, do CBs require specific DNA or RNA sequences in order to nucleate, or can they form independently (Matera, 1998Matera A.G. Of coiled bodies, gems, and salmon.J. Cell. Biochem. 1998; 70: 181-192Crossref PubMed Scopus (63) Google Scholar)? Previous studies can shed some light here. In amphibian oocytes, CBs (a.k.a. “spheres”) are known to associate with the histone gene clusters at sites termed “sphere organizers” (Callan et al., 1991Callan H.G. Gall J.G. Murphy C. Histone genes are located at the sphere loci of Xenopus lampbrush chromosomes.Chromosoma. 1991; 101: 245-251Crossref PubMed Google Scholar, Gall et al., 1981Gall J.G. Stephenson E.C. Erba H.P. Diaz M.O. Barsacchi-Pilone G. Histone genes are located at the sphere loci of newt lampbrush chromosomes.Chromosoma. 1981; 84: 159-171Crossref PubMed Scopus (55) Google Scholar). Similarly, in interphase human cells, histone and small nuclear (sn)RNA genes associate nonrandomly with CBs (Frey and Matera, 1995Frey M.R. Matera A.G. Coiled bodies contain U7 small nuclear RNA and associate with specific DNA sequences in interphase human cells.Proc. Natl. Acad. Sci. USA. 1995; 92: 5915-5919Crossref PubMed Scopus (184) Google Scholar, Gao et al., 1997Gao L. Frey M.R. Matera A.G. Human genes encoding U3 snRNA associate with coiled bodies in interphase cells and are clustered on chromosome 17p11.2 in a complex inverted repeat structure.Nucleic Acids Res. 1997; 25: 4740-4747Crossref PubMed Google Scholar, Jacobs et al., 1999Jacobs E.Y. Frey M.R. Wu W. Ingledue T.C. Gebuhr T.C. Gao L. Marzluff W.F. Matera A.G. Coiled bodies preferentially associate with U4, U11, and U12 small nuclear RNA genes in interphase HeLa cells but not with U6 and U7 genes.Mol. Biol. Cell. 1999; 10: 1653-1663Crossref PubMed Google Scholar, Shopland et al., 2001Shopland L.S. Byron M. Stein J.L. Lian J.B. Stein G.S. Lawrence J.B. Replication-dependent histone gene expression is related to Cajal body (CB) association but does not require sustained CB contact.Mol. Biol. Cell. 2001; 12: 565-576Crossref PubMed Google Scholar, Smith et al., 1995Smith K.P. Carter K.C. Johnson C.V. Lawrence J.B. U2 and U1 snRNA gene loci associate with coiled bodies.J. Cell. Biochem. 1995; 59: 473-485Crossref PubMed Scopus (74) Google Scholar), and these sites have been termed “CB organizers” (Frey and Matera, 1995Frey M.R. Matera A.G. Coiled bodies contain U7 small nuclear RNA and associate with specific DNA sequences in interphase human cells.Proc. Natl. Acad. Sci. USA. 1995; 92: 5915-5919Crossref PubMed Scopus (184) Google Scholar, Gao et al., 1997Gao L. Frey M.R. Matera A.G. Human genes encoding U3 snRNA associate with coiled bodies in interphase cells and are clustered on chromosome 17p11.2 in a complex inverted repeat structure.Nucleic Acids Res. 1997; 25: 4740-4747Crossref PubMed Google Scholar). The association of CBs and snRNA genes is not coincidental, as ectopically expressed snRNA genes can function as CB organizers (Frey et al., 1999Frey M.R. Bailey A.D. Weiner A.M. Matera A.G. Association of snRNA genes with coiled bodies is mediated by nascent snRNA transcripts.Curr. Biol. 1999; 9: 126-135Abstract Full Text Full Text PDF PubMed Scopus (72) Google Scholar, Frey and Matera, 2001Frey M.R. Matera A.G. RNA-mediated interaction of Cajal bodies and U2 snRNA genes.J. Cell Biol. 2001; 154: 499-509Crossref PubMed Scopus (51) Google Scholar). However, unlike the well-known rRNA genes that act as nucleolus organizers, CBs are not nucleated at snRNA gene loci following induction of transcription; rather, the snRNA genes are recruited to extant CBs (Dundr et al., 2007Dundr M. Ospina J.K. Sung M.H. John S. Upender M. Ried T. Hager G.L. Matera A.G. Actin-dependent intranuclear repositioning of an active gene locus in vivo.J. Cell Biol. 2007; 179: 1095-1103Crossref PubMed Scopus (124) Google Scholar). Other lines of evidence against a requirement for tethering at a specific genomic locus are the findings that CBs can be assembled in vitro using Xenopus egg extracts that are completely devoid of genomic frog DNA (Bauer et al., 1994Bauer D.W. Murphy C. Wu Z. Wu C.H. Gall J.G. In vitro assembly of coiled bodies in Xenopus egg extract.Mol. Biol. Cell. 1994; 5: 633-644Crossref PubMed Google Scholar) or that microinjection of U7 snRNA can nucleate formation of mini-CBs in frog oocytes (Tuma and Roth, 1999Tuma R.S. Roth M.B. Induction of coiled body-like structures in Xenopus oocytes by U7 snRNA.Chromosoma. 1999; 108: 337-344Crossref PubMed Scopus (15) Google Scholar). Thus, at least in certain circumstances, CBs can be formed independently. Immobilization of components to a specific site in the genome can nucleate formation of a body. Alternatively, it is possible that the clustering of factors at their normal sites of action might also lead to formation of a nuclear body. We assume that the downstream, postnucleation assembly events will proceed by the same molecular mechanisms (e.g., stochastic self-organization) regardless of whether or not nuclear body formation was initiated by immobilizing a given component. However, in the absence of an appropriate assay, we cannot say for sure that the tethered bodies are functional. Experiments on another kind of nuclear body, DNA repair foci, suggest that the tethered, de novo structures are functional. Soutoglou and Misteli, 2008Soutoglou E. Misteli T. Activation of the cellular DNA damage response in the absence of DNA lesions.Science. 2008; 320: 1507-1510Crossref PubMed Scopus (144) Google Scholar showed that DNA repair factors could be tethered to specific sites that could not only nucleate formation of DNA repair foci, but could also elicit the cellular DNA damage response, even in the absence of DNA lesions. These data argue strongly that the formation of subcellular compartments is governed by stochastic self-organization and that, once a nuclear body is formed, it is functional. Unlike most of the nuclear bodies shown in Figure 1, which are not constitutively associated with a specific chromosomal locus, the nucleolus is intimately associated with the genes that encode the 35S preribosomal RNA. More than two decades ago, elegant work in Drosophila showed that RNA polymerase I-mediated transcription of rRNA transgenes directed formation of ectopic nucleoli, whereas expression of transgenes lacking pol I promoters did not (Karpen et al., 1988Karpen G.H. Schaefer J.E. Laird C.D. A Drosophila rRNA gene located in euchromatin is active in transcription and nucleolus formation.Genes Dev. 1988; 2: 1745-1763Crossref PubMed Google Scholar). Given that nucleolar morphology has long been shown to correlate with the relative transcriptional activity of the endogenous rRNA genes (Haaf et al., 1991Haaf T. Hayman D.L. Schmid M. Quantitative determination of rDNA transcription units in vertebrate cells.Exp. Cell Res. 1991; 193: 78-86Crossref PubMed Google Scholar, Scheer et al., 1984Scheer U. Hugle B. Hazan R. Rose K.M. Drug-induced dispersal of transcribed rRNA genes and transcriptional products: immunolocalization and silver staining of different nucleolar components in rat cells treated with 5,6-dichloro-beta-D-ribofuranosylbenzimidazole.J. Cell Biol. 1984; 99: 672-679Crossref PubMed Google Scholar), it is clear that nucleoli form as a consequence of rRNA transcription and the downstream processing and ribosomal subunit assembly steps. Notably, ectopic insertion of an array of upstream binding factor (UBF, a pol I transcription factor) binding sites results in sequestration of UBF and other pol I transcription factors to the ectopic sites, although a full-blown nucleolus is not formed (Mais et al., 2005Mais C. Wright J.E. Prieto J.L. Raggett S.L. McStay B. UBF-binding site arrays form pseudo-NORs and sequester the RNA polymerase I transcription machinery.Genes Dev. 2005; 19: 50-64Crossref PubMed Scopus (87) Google Scholar). It would be interesting to see whether lac repressor fusions of other nucleolar components might generate nucleolus-like subcompartments on lac operator arrays. Self-organization notwithstanding, given the complex nature of the pol I transcription and rRNA-processing machineries, it seems doubtful that tethering any given nucleolar protein would nucleate assembly of an entire nucleolus. The HLB is another example of a chromatin-associated nuclear body (Figure 3). Metazoan genomes typically contain a set of histone genes that are expressed only during DNA replication (S phase) and another set of “replacement” histone genes that are constitutively expressed (reviewed in Marzluff et al., 2002Marzluff W.F. Gongidi P. Woods K.R. Jin J. Maltais L.J. The human and mouse replication-dependent histone genes.Genomics. 2002; 80: 487-498Crossref PubMed Google Scholar). The genes encoding the replication-dependent histones are typically clustered, whereas the replacement histone genes are interspersed (Marzluff et al., 2002Marzluff W.F. Gongidi P. Woods K.R. Jin J. Maltais L.J. The human and mouse replication-dependent histone genes.Genomics. 2002; 80: 487-498Crossref PubMed Google Scholar). HLBs associate specifically with the replication-dependent histone gene clusters and are thought to coordinate the transcription and 3′ end processing of histone pre-mRNAs (for details, see Figure 3 legend). Factors required for histone gene expression, including NPAT, HiNF-P, FLASH, and the U7 snRNP, all are concentrated within the structure we now refer to as the HLB (Barcaroli et al., 2006Barcaroli D. Bongiorno-Borbone L. Terrinoni A. Hofmann T.G. Rossi M. Knight R.A. Matera A.G. Melino G. De Laurenzi V. FLASH is required for histone transcription and S-phase progression.Proc. Natl. Acad. Sci. USA. 2006; 103: 14808-14812Crossref PubMed Scopus (46) Google Scholar, Bongiorno-Borbone et al., 2008Bongiorno-Borbone L. De Cola A. Vernole P. Finos L. Barcaroli D. Knight R.A. Melino G. De Laurenzi V. FLASH and NPAT positive but not Coilin positive Cajal Bodies correlate with cell ploidy.Cell Cycle. 2008; 7: 2357-2367PubMed Google Scholar, Ghule et al., 2009Ghule P.N. Dominski Z. Lian J.B. Stein J.L. van Wijnen A.J. Stein G.S. The subnuclear organization of histone gene regulatory proteins and 3′ end processing factors of normal somatic and embryonic stem cells is compromised in selected human cancer cell types.J. Cell. Physiol. 2009; 220: 129-135Crossref PubMed Scopus (14) Google Scholar, Liu et al., 2006Liu J.-L. Murphy C. Buszczak M. Clatterbuck S. Goodman R. Gall J.G. The Drosophila melanogaster Cajal body.J. 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USA. 1995; 92: 5915-5919Crossref PubMed Scopus (184) Google Scholar, Pillai et al., 2001Pillai R.S. Will C.L. Luhrmann R. Schumperli D. Muller B. Purified U7 snRNPs lack the Sm proteins D1 and D2 but contain Lsm10, a new 14 kDa Sm D1-like protein.EMBO J. 2001; 20: 5470-5479Crossref PubMed Scopus (93) Google Scholar, Shopland et al., 2001Shopland L.S. Byron M. Stein J.L. Lian J.B. Stein G.S. Lawrence J.B. Replication-dependent histone gene expression is related to Cajal body (CB) association but does not require sustained CB contact.Mol. Biol. Cell. 2001; 12: 565-576Crossref PubMed Google Scholar). However, in Drosophila, the U7 snRNP typically colocalizes with the histone gene cluster in HLBs, structures that are distinct from but often adjacent to CBs (Liu et al., 2006Liu J.-L. Murphy C. Buszczak M. Clatterbuck S. Goodman R. Gall J.G. The Drosophila melanogaster Cajal body.J. Cell Biol. 2006; 172: 875-884Crossref PubMed Scopus (91) Google Scholar, Liu et al., 2009Liu J.L. Wu Z. Nizami Z. Deryusheva S. Rajendra T.K. Beumer K.J. Gao H. Matera A.G. Carroll D. Gall J.G. Coilin is essential for Cajal body organization in Drosophila melanogaster.Mol. Biol. Cell. 2009; 20: 1661-1670Crossref PubMed Scopus (48) Google Scholar). The peculiar localization of the U7 snRNP to CBs in most human cancer cell lines (Figure 3) has therefore caused some confusion. The recent availability of monospecific antibodies targeting Lsm10 and Lsm11 (components of U7 snRNP) has allowed investigators to reconcile work in the mammalian and invertebrate systems. The emerging picture is that, in human primary cells, U7 snRNP components colocalize precisely with the HLB marker proteins NPAT and FLASH (Ghule et al., 2009Ghule P.N. Dominski Z. Lian J.B. Stein J.L. van Wijnen A.J. Stein G.S. The subnuclear organization of histone gene regulatory proteins and 3′ end processing factors of normal somatic and embryonic stem cells is compromised in selected human cancer cell types.J. Cell. 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Thus, it remains an open question as to why U7 snRNP becomes delocalized from HLBs in cancer cell lines. Interestingly, Duronio and coworkers have shown that at least some of the components of HLBs can form nuclear foci in the absence of the histone gene cluster (i.e., in a strain carrying an appropriate deletion; White et al., 2007White A.E. Leslie M.E. Calvi B.R. Marzluff W.F. Duronio R.J. Developmental and cell cycle regulation of the Drosophila histone locus body.Mol. Biol. Cell. 2007; 18: 2491-2502Crossref PubMed Scopus (29) Google Scholar). These findings are somewhat reminiscent of the “residual” Cajal bodies that form in coilin knockout cells (Tucker et al., 2001Tucker K.E. Berciano M.T. Jacobs E.Y. LePage D.F. Shpargel K.B. Rossire J.J. Chan E.K. Lafarga M. Conlon R.A. Matera A.G. Residual Cajal bodies in coilin knockout mice fail to recruit Sm snRNPs and SMN, the spinal muscular atrophy gene product.J. Cell Biol. 2001; 154: 293-307Crossref PubMed Scopus (124) Google Scholar, Jady et al., 2003Jady B.E. Darzacq X. Tucker K.E. Matera A.G. Bertrand E. Kiss T. Modification of Sm small nuclear RNAs occurs in the nucleoplasmic Cajal body following import from the cytoplasm.EMBO J. 2003; 22: 1878-1888Crossref PubMed Scopus (125) Google Scholar) and suggest that stochastic self-organization is also an important factor in the assembly of HLBs. However, unlike CBs, HLBs can be viewed as nuclear subdomains that are dedicated to the expression of replication-dependent histone genes. Although they are not thought to be constitutively bound to particular chromosomal regions, two prominent nuclear subdomains (Cajal and PML bodies) are known to associate transiently with specific genomic loci. As discussed above, CBs have been shown to associate with histone, snRNA, and small nucleolar (sno)RNA genes in various human cancer cell lines (Frey and Matera, 1995Frey M.R. Matera A.G. Coiled bodies contain U7 small nuclear RNA and associate with specific DNA sequences in interphase human cells.Proc. Natl. Acad. Sci. USA. 1995; 92: 5915-5919Crossref PubMed Scopus (184) Google Scholar, Gao et al., 1997Gao L. Frey M.R. Matera A.G. Human genes encoding U3 snRNA associate with coiled bodies in interphase cells and are clustered on chromosome 17p11.2 in a complex inverted repeat structure.Nucleic Acids Res. 1997; 25: 4740-4747Crossref PubMed Google Scholar, Jacobs et al., 1999Jacobs E.Y. Frey M.R. Wu W. Ingledue T.C. Gebuhr T.C. Gao L. Marzluff W.F. Matera A.G. Coiled bodies preferentially associate with U4, U11, and U12 small nuclear RNA genes in interphase HeLa cells but not with U6 and U7 genes.Mol. Biol. Cell. 1999; 10: 1653-1663Crossref PubMed Google Scholar, Schul et al., 1998Schul W. van Driel R. de Jong L. Coiled bodies and U2 snRNA genes adjacent to coiled bodies are enriched in factors required for snRNA transcription.Mol. Biol. 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