Rethinking Unconventional Translation in Neurodegeneration
2017; Cell Press; Volume: 171; Issue: 5 Linguagem: Inglês
10.1016/j.cell.2017.10.042
ISSN1097-4172
AutoresFen‐Biao Gao, Joel D. Richter, Don W. Cleveland,
Tópico(s)Mitochondrial Function and Pathology
ResumoEukaryotic translation is tightly regulated to ensure that protein production occurs at the right time and place. Recent studies on abnormal repeat proteins, especially in age-dependent neurodegenerative diseases caused by nucleotide repeat expansion, have highlighted or identified two forms of unconventional translation initiation: usage of AUG-like sites (near cognates) or repeat-associated non-AUG (RAN) translation. We discuss how repeat proteins may differ due to not just unconventional initiation, but also ribosomal frameshifting and/or imperfect repeat DNA replication, expansion, and repair, and we highlight how research on translation of repeats may uncover insights into the biology of translation and its contribution to disease. Eukaryotic translation is tightly regulated to ensure that protein production occurs at the right time and place. Recent studies on abnormal repeat proteins, especially in age-dependent neurodegenerative diseases caused by nucleotide repeat expansion, have highlighted or identified two forms of unconventional translation initiation: usage of AUG-like sites (near cognates) or repeat-associated non-AUG (RAN) translation. We discuss how repeat proteins may differ due to not just unconventional initiation, but also ribosomal frameshifting and/or imperfect repeat DNA replication, expansion, and repair, and we highlight how research on translation of repeats may uncover insights into the biology of translation and its contribution to disease. In 1961, Marshall W. Nirenberg and his postdoctoral fellow Johann H. Matthaei at the National Institutes of Health found that mixing polyuridylic acid (poly-U) with cell-free extracts from E. coli resulted in the production of proteins made entirely of phenylalanine (Nirenberg and Matthaei, 1961Nirenberg M.W. Matthaei J.H. The dependence of cell-free protein synthesis in E. coli upon naturally occurring or synthetic polyribonucleotides.Proc. Natl. Acad. Sci. USA. 1961; 47: 1588-1602Crossref PubMed Scopus (797) Google Scholar). Using additional synthetic RNAs, including those with repeating triplets, Nirenberg and H. Gobind Khorana produced different mono- and dipeptide proteins. Together, they deciphered the genetic code (Khorana, 1968Khorana H.G. Synthetic nucleic acids and the genetic code.JAMA. 1968; 206: 1978-1982Crossref PubMed Scopus (7) Google Scholar) and, for this work, shared the 1968 Nobel Prize in Physiology or Medicine, along with Robert W. Holley, who first isolated tRNA. Today, translation in eukaryote cells is recognized to be a highly regulated process. Initiation is the rate-limiting step and thus is the phase of protein synthesis most subject to regulation (Richter and Sonenberg, 2005Richter J.D. Sonenberg N. Regulation of cap-dependent translation by eIF4E inhibitory proteins.Nature. 2005; 433: 477-480Crossref PubMed Scopus (752) Google Scholar). Although initiation itself is a multi-step process, it culminates in 5′ to 3′ "scanning" of a complex containing the 40S ribosome subunit—often referred to as a 43S preinitiation complex that includes a ternary complex (TC) of eIF2, GTP, and initiator methionine tRNA. Usually upon encountering the first AUG codon, which more often than not resides in a favorable context (the 5′ A/GCCAUGG 3′ Kozak sequence that promotes initiation [Kozak, 1986Kozak M. Point mutations define a sequence flanking the AUG initiator codon that modulates translation by eukaryotic ribosomes.Cell. 1986; 44: 283-292Abstract Full Text PDF PubMed Scopus (3583) Google Scholar, Hinnebusch et al., 2016Hinnebusch A.G. Ivanov I.P. Sonenberg N. Translational control by 5′-untranslated regions of eukaryotic mRNAs.Science. 2016; 352: 1413-1416Crossref PubMed Scopus (546) Google Scholar]), the preinitiation complex stops and recruits the 60S subunit, and polypeptide elongation begins (Figure 1A ). 5′ to the classic AUG initiation codon, however, it is now clear that many RNAs have upstream open reading frames (uORFs), which can serve several different roles in translation. Some uORFs lack a stop codon and begin with a canonical AUG initiator codon in-frame with the downstream AUG. In these cases, "leaky" scanning may occur where peptide synthesis occurs at both the uORF and downstream open reading frames (ORFs), resulting in two peptides distinct only at the amino terminus (Hinnebusch et al., 2016Hinnebusch A.G. Ivanov I.P. Sonenberg N. Translational control by 5′-untranslated regions of eukaryotic mRNAs.Science. 2016; 352: 1413-1416Crossref PubMed Scopus (546) Google Scholar). Other uORFs can suppress or prevent translation at the downstream ORF when a stop codon is present between them because ribosome re-initiation is not robust (Figure 1B). Moreover, scanning 40S ribosomal subunits can sometimes bypass the uORF altogether and initiate translation at the downstream AUG codon, which can be particularly robust when the uORF acts as an enhancer of downstream initiation (Ivanov et al., 2010Ivanov I.P. Atkins J.F. Michael A.J. A profusion of upstream open reading frame mechanisms in polyamine-responsive translational regulation.Nucleic Acids Res. 2010; 38: 353-359Crossref PubMed Scopus (67) Google Scholar). With the advent of next-generation sequencing and ribosome profiling, which together can map the positions and relative amounts of ribosomes on mRNA (Ingolia et al., 2009Ingolia N.T. Ghaemmaghami S. Newman J.R. Weissman J.S. Genome-wide analysis in vivo of translation with nucleotide resolution using ribosome profiling.Science. 2009; 324: 218-223Crossref PubMed Scopus (2395) Google Scholar), the following has become clear: (1) Most mRNAs have uORFs; and (2) These uORFs frequently use alternative (non-AUG) start codons. For example, Ingolia et al., 2011Ingolia N.T. Lareau L.F. Weissman J.S. Ribosome profiling of mouse embryonic stem cells reveals the complexity and dynamics of mammalian proteomes.Cell. 2011; 147: 789-802Abstract Full Text Full Text PDF PubMed Scopus (1417) Google Scholar and Lee et al., 2012Lee S. Liu B. Lee S. Huang S.-X. Shen B. Qian S.-B. Global mapping of translation initiation sites in mammalian cells at single-nucleotide resolution.Proc. Natl. Acad. Sci. USA. 2012; 109: E2424-E2432Crossref PubMed Scopus (402) Google Scholar performed ribosome profiling in cultured cells in the presence of the translation inhibitor cycloheximide, which binds both initiating and elongating ribosomes, or of other drugs such as harringtonine and lactimidomycin, which bind initiating ribosomes with an exit (E) site that is devoid of tRNA. These drugs allow elongating ribosomes to run off the mRNA while maintaining the initiating ribosome in place, thereby enabling start codon identification on a global basis following sequencing of the RNAs protected by this initiating ribosome (an approach called global translation initiation sequencing, or GTI-seq). These and related analyses (e.g., Bazzini et al., 2014Bazzini A.A. Johnstone T.G. Christiano R. Mackowiak S.D. Obermayer B. Fleming E.S. Vejnar C.E. Lee M.T. Rajewsky N. Walther T.C. Giraldez A.J. Identification of small ORFs in vertebrates using ribosome footprinting and evolutionary conservation.EMBO J. 2014; 33: 981-993Crossref PubMed Scopus (418) Google Scholar, Fields et al., 2015Fields A.P. Rodriguez E.H. Jovanovic M. Stern-Ginossar N. Haas B.J. Mertins P. Raychowdhury R. Hacohen N. Carr S.A. Ingolia N.T. et al.A Regression-based analysis of ribosome-profiling data reveals a conserved complexity to mammalian translation.Mol. Cell. 2015; 60: 816-827Abstract Full Text Full Text PDF PubMed Scopus (138) Google Scholar, Ji et al., 2015Ji Z. Song R. Regev A. Struhl K. Many lncRNAs, 5'UTRs, and pseudogenes are translated and some are likely to express functional proteins.eLife. 2015; 4: e08890Crossref PubMed Scopus (294) Google Scholar) have revealed that > 50% of mRNAs contain uORFs that are occupied by ribosomes, in close agreement with a similar proportion estimated by an earlier RNA-seq effort (Calvo et al., 2009Calvo S.E. Pagliarini D.J. Mootha V.K. Upstream open reading frames cause widespread reduction of protein expression and are polymorphic among humans.Proc. Natl. Acad. Sci. USA. 2009; 106: 7507-7512Crossref PubMed Scopus (563) Google Scholar). Use of GTI-seq has also demonstrated remarkable variation in translation initiation sites (uORF and ORF), with only 51% beginning with AUG, 16% with CUG, and 24% with other sequences (UUG, CUG, AGG, ACG, AAG, AUC, AUA, AUU). For uORFs in particular, 75% begin with non-AUG codons, with CUG (encoding leucine) being the most prevalent. The preponderance of uORFs implies widespread regulation of downstream ORF translation (Morris and Geballe, 2000Morris D.R. Geballe A.P. Upstream open reading frames as regulators of mRNA translation.Mol. Cell. Biol. 2000; 20: 8635-8642Crossref PubMed Scopus (566) Google Scholar), and it is the TC component eIF2α that is the key factor controlling uORF utilization. Phosphorylation of eIF2α serine 51 inhibits the activity of the guanine nucleotide exchange factor eIF2B, which in turn impedes formation of the 43S pre-initiation complex (PIC) and consequently reduces general translation. However, some mRNAs escape this inhibition and are translated even though they have uORFs. This is caused by stress-induced eIF2α phosphorylation and resulting TC limitation, which render uORF initiation unfavorable; it is thus bypassed, and initiation occurs at the downstream AUG. Examples of mRNAs under this type of regulation encode such proteins as the transcription factors CHOP (CCAAT-enhancer-binding protein homologous protein) and ATF4 and ATF5, the growth arrest and DNA damage-inducible protein GADD34, among many others (Baird et al., 2014Baird T.D. Palam L.R. Fusakio M.E. Willy J.A. Davis C.M. McClintick J.N. Anthony T.G. Wek R.C. Selective mRNA translation during eIF2 phosphorylation induces expression of IBTKα.Mol. Biol. Cell. 2014; 25: 1686-1697Crossref PubMed Scopus (76) Google Scholar, Young and Wek, 2016Young S.K. Wek R.C. Upstream open reading frames differentially regulate gene-specific translation in the integrated stress response.J. Biol. Chem. 2016; 291: 16927-16935Crossref PubMed Scopus (184) Google Scholar). Added to this, a peptide encoded by the uORF in CHOP mRNA regulates translation of the downstream ORF (Jousse et al., 2001Jousse C. Bruhat A. Carraro V. Urano F. Ferrara M. Ron D. Fafournoux P. Inhibition of CHOP translation by a peptide encoded by an open reading frame localized in the chop 5'UTR.Nucleic Acids Res. 2001; 29: 4341-4351Crossref PubMed Scopus (106) Google Scholar). uORF regulation of translation and eIF2α phosphorylation are central components of an integrated stress response (Young and Wek, 2016Young S.K. Wek R.C. Upstream open reading frames differentially regulate gene-specific translation in the integrated stress response.J. Biol. Chem. 2016; 291: 16927-16935Crossref PubMed Scopus (184) Google Scholar). Four kinases respond to different environmental stresses and phosphorylate eIF2α serine 51: PERK responds to ER stress; HRI responds to oxidative stress and heat shock; PKR responds to viral infection; and GCN2 is a nutrient sensor. Correspondingly, cellular stress, through any one of these kinases, can modulate uORF and downstream ORF translation, which may result in unique biological responses depending on the cell type. One highly specialized cell type is the neuron, which can live for decades without undergoing cell division. Although these cells can endure even when encountering various stresses, in some neurological diseases, they undergo a remarkable molecular reorganization. Repeat expansion disorders are a set of more than 30 genetic diseases, mostly of the nervous system, that are caused by expansion of short repeat sequences of 3–6 nucleotides in a host gene's coding sequences, introns, or 5′ and 3′ untranslated regions (UTRs) (Zhang and Ashizawa, 2017Zhang N. Ashizawa T. RNA toxicity and foci formation in microsatellite expansion diseases.Curr. Opin. Genet. Dev. 2017; 44: 17-29Crossref PubMed Scopus (66) Google Scholar). During the last two decades, unconventional translation products from various expanded repeat sequences have been repeatedly detected in the nervous system of affected individuals. Three mechanisms have been proposed to account for the generation of these disease-specific proteins: (1) ribosomal frameshifting (Gaspar et al., 2000Gaspar C. Jannatipour M. Dion P. Laganière J. Sequeiros J. Brais B. Rouleau G.A. CAG tract of MJD-1 may be prone to frameshifts causing polyalanine accumulation.Hum. Mol. Genet. 2000; 9: 1957-1966Crossref PubMed Scopus (48) Google Scholar), (2) repeat-associated non-AUG (RAN) translation (Zu et al., 2011Zu T. Gibbens B. Doty N.S. Gomes-Pereira M. Huguet A. Stone M.D. Margolis J. Peterson M. Markowski T.W. Ingram M.A. et al.Non-ATG-initiated translation directed by microsatellite expansions.Proc. Natl. Acad. Sci. USA. 2011; 108: 260-265Crossref PubMed Scopus (626) Google Scholar), and (3) near-cognate start codon initiated translation (Sellier et al., 2017Sellier C. Buijsen R.A. He F. Natla S. Jung L. Tropel P. Gaucherot A. Jacobs H. Meziane H. Vincent A. et al.Translation of expanded CGG repeats into FMRpolyG is pathogenic and may contribute to Fragile X Tremor Ataxia Syndrome.Neuron. 2017; 93: 331-347Abstract Full Text Full Text PDF PubMed Scopus (137) Google Scholar). Beginning with the work of Jacks and Varmus (Mardon and Varmus, 1983Mardon G. Varmus H.E. Frameshift and intragenic suppressor mutations in a Rous sarcoma provirus suggest src encodes two proteins.Cell. 1983; 32: 871-879Abstract Full Text PDF PubMed Scopus (24) Google Scholar, Jacks and Varmus, 1985Jacks T. Varmus H.E. Expression of the Rous sarcoma virus pol gene by ribosomal frameshifting.Science. 1985; 230: 1237-1242Crossref PubMed Scopus (285) Google Scholar, Jacks et al., 1988Jacks T. Power M.D. Masiarz F.R. Luciw P.A. Barr P.J. Varmus H.E. Characterization of ribosomal frameshifting in HIV-1 gag-pol expression.Nature. 1988; 331: 280-283Crossref PubMed Scopus (678) Google Scholar), it has become clear that most retroviruses, including HIV, utilize ribosomal frameshifting as an obligatory component of their life cycle. Frameshifting can be highly efficient and driven by a combination of a 7-base "slippery" RNA sequence and an RNA pseudoknot 3′ to the site of frameshifting (Chamorro et al., 1992Chamorro M. Parkin N. Varmus H.E. An RNA pseudoknot and an optimal heptameric shift site are required for highly efficient ribosomal frameshifting on a retroviral messenger RNA.Proc. Natl. Acad. Sci. USA. 1992; 89: 713-717Crossref PubMed Scopus (142) Google Scholar). Beyond viruses, frameshifting is also found frequently in bacteria and known for some eukaryotic mRNAs (Caliskan et al., 2015Caliskan N. Peske F. Rodnina M.V. Changed in translation: mRNA recoding by -1 programmed ribosomal frameshifting.Trends Biochem. Sci. 2015; 40: 265-274Abstract Full Text Full Text PDF PubMed Scopus (74) Google Scholar). Similar frameshifting appears to be a component in some polyglutamine diseases in which polyglutamine tracts are synthesized from expanded CAG repeats in frame with the coding regions of respective host genes (Orr and Zoghbi, 2007Orr H.T. Zoghbi H.Y. Trinucleotide repeat disorders.Annu. Rev. Neurosci. 2007; 30: 575-621Crossref PubMed Scopus (1096) Google Scholar). For example, in spinocerebellar ataxia type 3 (SCA3), ataxin 3 contains CAG repeats whose in-frame translation produces an expanded polyglutamine track; however, polyalanine-containing proteins are also generated in patient neurons (Gaspar et al., 2000Gaspar C. Jannatipour M. Dion P. Laganière J. Sequeiros J. Brais B. Rouleau G.A. CAG tract of MJD-1 may be prone to frameshifts causing polyalanine accumulation.Hum. Mol. Genet. 2000; 9: 1957-1966Crossref PubMed Scopus (48) Google Scholar). Using reporter constructs in which different tags were engineered into separate reading frames, polyalanine synthesis was initially proposed to be caused by ribosomal slippage into the GCA frame in a repeat-length-dependent manner, although the slippage site was not identified (Toulouse et al., 2005Toulouse A. Au-Yeung F. Gaspar C. Roussel J. Dion P. Rouleau G.A. Ribosomal frameshifting on MJD-1 transcripts with long CAG tracts.Hum. Mol. Genet. 2005; 14: 2649-2660Crossref PubMed Scopus (47) Google Scholar). In both Drosophila and mammalian neurons, production of polyalanine may be a key component of CAG repeat toxicity (Stochmanski et al., 2012Stochmanski S.J. Therrien M. Laganière J. Rochefort D. Laurent S. Karemera L. Gaudet R. Vyboh K. Van Meyel D.J. Di Cristo G. et al.Expanded ATXN3 frameshifting events are toxic in Drosophila and mammalian neuron models.Hum. Mol. Genet. 2012; 21: 2211-2218Crossref PubMed Scopus (35) Google Scholar). In Huntington's disease (HD), the most common CAG repeat disease, the repeats are in-frame with the huntingtin ORF, producing polyglutamine after codon number 17 in huntingtin (HTT). Nevertheless, both polyserine and polyalanine—encoded by the +1 and −1 frames of expanded CAG repeats, respectively—have been detected in autopsy samples of HD patients and in a transgenic mouse model of HD (Davies and Rubinsztein, 2006Davies J.E. Rubinsztein D.C. Polyalanine and polyserine frameshift products in Huntington's disease.J. Med. Genet. 2006; 43: 893-896Crossref PubMed Scopus (36) Google Scholar, Bañez-Coronel et al., 2015Bañez-Coronel M. Ayhan F. Tarabochia A.D. Zu T. Perez B.A. Tusi S.K. Pletnikova O. Borchelt D.R. Ross C.A. Margolis R.L. et al.RAN Translation in Huntington Disease.Neuron. 2015; 88: 667-677Abstract Full Text Full Text PDF PubMed Scopus (217) Google Scholar). Frameshifting has been established to account for production of polyserine or polyalanine linked to the amino terminal portion of huntingtin. This frameshifting can occur within the expanded CAG repeats and is enhanced by depletion of the charged glutaminyl-tRNA that pairs to the CAG codon (Girstmair et al., 2013Girstmair H. Saffert P. Rode S. Czech A. Holland G. Bannert N. Ignatova Z. Depletion of cognate charged transfer RNA causes translational frameshifting within the expanded CAG stretch in huntingtin.Cell Rep. 2013; 3: 148-159Abstract Full Text Full Text PDF PubMed Scopus (46) Google Scholar). Detailed mass spectrometry and molecular analysis revealed that a UUCC sequence at the 5′ end of CAG repeats in the Huntingtin coding region initiates the +1 frameshifting, and the effect of the UUCC sequence is enhanced by increased formation of stem-loop structures by expanded CAG repeats, providing a specific molecular mechanism for potential ribosomal frameshifting in HD patient cells (Saffert et al., 2016Saffert P. Adamla F. Schieweck R. Atkins J.F. Ignatova Z. An expanded CAG repeat in Huntingtin causes +1 frameshifting.J. Biol. Chem. 2016; 291: 18505-18513Crossref PubMed Scopus (15) Google Scholar). Polyalanine has also been detected in tissues of patients with another CAG repeat disease (SCA8) and in mouse models of the disease (Zu et al., 2011Zu T. Gibbens B. Doty N.S. Gomes-Pereira M. Huguet A. Stone M.D. Margolis J. Peterson M. Markowski T.W. Ingram M.A. et al.Non-ATG-initiated translation directed by microsatellite expansions.Proc. Natl. Acad. Sci. USA. 2011; 108: 260-265Crossref PubMed Scopus (626) Google Scholar). Here, CAG repeats are in-frame with the coding sequence of ataxin 8 (ATXN8). Rather than frameshifting, it has been proposed that polyalanine is produced by a different, unconventional translation mechanism, repeat associated non-AUG (RAN) translation, in which initiation in any of the three reading frames within expanded CAG (or other) repeats is thought to occur internally (Zu et al., 2011Zu T. Gibbens B. Doty N.S. Gomes-Pereira M. Huguet A. Stone M.D. Margolis J. Peterson M. Markowski T.W. Ingram M.A. et al.Non-ATG-initiated translation directed by microsatellite expansions.Proc. Natl. Acad. Sci. USA. 2011; 108: 260-265Crossref PubMed Scopus (626) Google Scholar; Figure 1C), seemingly consistent with the internal ribosome binding of mRNA observed by Nirenberg and Khorana decades ago using cell-free extracts primed with synthetic polyribonucleotides (Nirenberg and Matthaei, 1961Nirenberg M.W. Matthaei J.H. The dependence of cell-free protein synthesis in E. coli upon naturally occurring or synthetic polyribonucleotides.Proc. Natl. Acad. Sci. USA. 1961; 47: 1588-1602Crossref PubMed Scopus (797) Google Scholar, Nishimura et al., 1964Nishimura S. Jacob T.M. Khorana H.G. Synthetic Deoxyribopolynucleotides as Templates for Ribonucleic Acid Polymerase: The Formation and characterization of a ribopolynucleotide with a repeating trinucleotide sequence.Proc. Natl. Acad. Sci. USA. 1964; 52: 1494-1501Crossref PubMed Scopus (18) Google Scholar). Expression of engineered repeat-containing ATXN8 or HTT mRNA with multiple stop codons 5′ to the CAG repeats in the +1 and +2 frames has shown that RAN translation certainly occurs under certain experimental conditions (Zu et al., 2011Zu T. Gibbens B. Doty N.S. Gomes-Pereira M. Huguet A. Stone M.D. Margolis J. Peterson M. Markowski T.W. Ingram M.A. et al.Non-ATG-initiated translation directed by microsatellite expansions.Proc. Natl. Acad. Sci. USA. 2011; 108: 260-265Crossref PubMed Scopus (626) Google Scholar, Bañez-Coronel et al., 2015Bañez-Coronel M. Ayhan F. Tarabochia A.D. Zu T. Perez B.A. Tusi S.K. Pletnikova O. Borchelt D.R. Ross C.A. Margolis R.L. et al.RAN Translation in Huntington Disease.Neuron. 2015; 88: 667-677Abstract Full Text Full Text PDF PubMed Scopus (217) Google Scholar). Whether this process requires poly(A) tail or other cis-elements remains unknown. Abnormal disease-specific repeat proteins proposed to be synthesized from both sense and antisense transcripts through RAN translation have also been detected in brain tissues of patients with CTG repeat expansion in the 3′UTR of DMPK in myotonic dystrophy type 1 (DM1) (Zu et al., 2011Zu T. Gibbens B. Doty N.S. Gomes-Pereira M. Huguet A. Stone M.D. Margolis J. Peterson M. Markowski T.W. Ingram M.A. et al.Non-ATG-initiated translation directed by microsatellite expansions.Proc. Natl. Acad. Sci. USA. 2011; 108: 260-265Crossref PubMed Scopus (626) Google Scholar), patients with GGGGCC repeat expansion in the first intron of C9ORF72 that causes both amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) (Ash et al., 2013Ash P.E.A. Bieniek K.F. Gendron T.F. Caulfield T. Lin W.-L. Dejesus-Hernandez M. van Blitterswijk M.M. Jansen-West K. Paul 3rd, J.W. Rademakers R. et al.Unconventional translation of C9ORF72 GGGGCC expansion generates insoluble polypeptides specific to c9FTD/ALS.Neuron. 2013; 77: 639-646Abstract Full Text Full Text PDF PubMed Scopus (773) Google Scholar, Mori et al., 2013Mori K. Weng S.-M. Arzberger T. May S. Rentzsch K. Kremmer E. Schmid B. Kretzschmar H.A. Cruts M. Van Broeckhoven C. et al.The C9orf72 GGGGCC repeat is translated into aggregating dipeptide-repeat proteins in FTLD/ALS.Science. 2013; 339: 1335-1338Crossref PubMed Scopus (864) Google Scholar, Zu et al., 2013Zu T. Liu Y. Bañez-Coronel M. Reid T. Pletnikova O. Lewis J. Miller T.M. Harms M.B. Falchook A.E. Subramony S.H. et al.RAN proteins and RNA foci from antisense transcripts in C9ORF72 ALS and frontotemporal dementia.Proc. Natl. Acad. Sci. USA. 2013; 110: E4968-E4977Crossref PubMed Scopus (534) Google Scholar), and patients with CCTG repeat expansion in the first intron of ZNF9 in myotonic dystrophy type 2 (DM2) (Zu et al., 2017Zu T. Cleary J.D. Liu Y. Bañez-Coronel M. Bubenik J.L. Ayhan F. Ashizawa T. Xia G. Clark H.B. Yachnis A.T. et al.RAN translation regulated by Muscleblind proteins in myotonic dystrophy type 2.Neuron. 2017; 95: 1292-1305.e5Abstract Full Text Full Text PDF PubMed Scopus (81) Google Scholar). Thus, RAN translation is posited to be a general phenomenon in various repeat expansion disorders (Cleary and Ranum, 2017Cleary J.D. Ranum L.P. New developments in RAN translation: insights from multiple diseases.Curr. Opin. Genet. Dev. 2017; 44: 125-134Crossref PubMed Scopus (61) Google Scholar). Near-cognate start codon-initiated translation is a third mechanism that can yield mono- or di-peptide-containing proteins from translation of nucleotide repeats. The founding example of this came from fragile X-associated tremor/ataxia syndrome (FXTAS), which is caused by CGG expansion (55–200 repeats) in the 5′UTR of the fragile X mental retardation 1 (FMR1) gene (Hagerman and Hagerman, 2016Hagerman R.J. Hagerman P. Fragile X-associated tremor/ataxia syndrome - features, mechanisms and management.Nat. Rev. Neurol. 2016; 12: 403-412Crossref PubMed Scopus (163) Google Scholar). The GGC frame of these repeats encodes polyglycine (FMRpolyG), which is indeed detected in FXTAS patient brains (Todd et al., 2013Todd P.K. Oh S.Y. Krans A. He F. Sellier C. Frazer M. Renoux A.J. Chen K.C. Scaglione K.M. Basrur V. et al.CGG repeat-associated translation mediates neurodegeneration in fragile X tremor ataxia syndrome.Neuron. 2013; 78: 440-455Abstract Full Text Full Text PDF PubMed Scopus (340) Google Scholar, Sellier et al., 2017Sellier C. Buijsen R.A. He F. Natla S. Jung L. Tropel P. Gaucherot A. Jacobs H. Meziane H. Vincent A. et al.Translation of expanded CGG repeats into FMRpolyG is pathogenic and may contribute to Fragile X Tremor Ataxia Syndrome.Neuron. 2017; 93: 331-347Abstract Full Text Full Text PDF PubMed Scopus (137) Google Scholar). The production of polyglycine does not seem to be mediated by RAN translation; rather, it requires a cap-dependent scanning mechanism for translation initiation (Kearse et al., 2016Kearse M.G. Green K.M. Krans A. Rodriguez C.M. Linsalata A.E. Goldstrohm A.C. Todd P.K. CGG Repeat-Associated Non-AUG Translation Utilizes a Cap-Dependent Scanning Mechanism of Initiation to Produce Toxic Proteins.Mol. Cell. 2016; 62: 314-322Abstract Full Text Full Text PDF PubMed Scopus (100) Google Scholar) and depends on an upstream, in-frame ACG near-cognate start codon embedded in a Kozak consensus sequence (Sellier et al., 2017Sellier C. Buijsen R.A. He F. Natla S. Jung L. Tropel P. Gaucherot A. Jacobs H. Meziane H. Vincent A. et al.Translation of expanded CGG repeats into FMRpolyG is pathogenic and may contribute to Fragile X Tremor Ataxia Syndrome.Neuron. 2017; 93: 331-347Abstract Full Text Full Text PDF PubMed Scopus (137) Google Scholar). The expanded CGG repeats create a large uORF 5′ to the FMR1 open reading frame. Cap-dependent ribosome scanning that initiates at the ACG codon produces a protein with an N-terminal methionine and a short polypeptide, followed by polyglycine, and then a 42-amino-acid C terminus, all encoded by the sequences in what is typically annotated as the FMR1 5′UTR (Kearse et al., 2016Kearse M.G. Green K.M. Krans A. Rodriguez C.M. Linsalata A.E. Goldstrohm A.C. Todd P.K. CGG Repeat-Associated Non-AUG Translation Utilizes a Cap-Dependent Scanning Mechanism of Initiation to Produce Toxic Proteins.Mol. Cell. 2016; 62: 314-322Abstract Full Text Full Text PDF PubMed Scopus (100) Google Scholar, Sellier et al., 2017Sellier C. Buijsen R.A. He F. Natla S. Jung L. Tropel P. Gaucherot A. Jacobs H. Meziane H. Vincent A. et al.Translation of expanded CGG repeats into FMRpolyG is pathogenic and may contribute to Fragile X Tremor Ataxia Syndrome.Neuron. 2017; 93: 331-347Abstract Full Text Full Text PDF PubMed Scopus (137) Google Scholar). Both polyglycine and the 42-amino-acid C terminus that follows it appear to be key components driving disease, with deletion of the ACG near-cognate start codon abolishing polyglycine production and neurotoxicity (Sellier et al., 2017Sellier C. Buijsen R.A. He F. Natla S. Jung L. Tropel P. Gaucherot A. Jacobs H. Meziane H. Vincent A. et al.Translation of expanded CGG repeats into FMRpolyG is pathogenic and may contribute to Fragile X Tremor Ataxia Syndrome.Neuron. 2017; 93: 331-347Abstract Full Text Full Text PDF PubMed Scopus (137) Google Scholar). The complexity of atypical translation in repeat expansion diseases is most apparent in ALS, a fatal paralytic disease, and in FTD, the second-most common form of pre-senile dementia. The most frequent genetic cause of these diseases is GGGGCC repeat expansion in the first intron of C9ORF72, with up to thousands of copies in the affected areas of the human nervous system (DeJesus-Hernandez et al., 2011DeJesus-Hernandez M. Mackenzie I.R. Boeve B.F. Boxer A.L. Baker M. Rutherford N.J. Nicholson A.M. Finch N.A. Flynn H. Adamson J. et al.Expanded GGGGCC hexanucleotide repeat in noncoding region of C9ORF72 causes chromosome 9p-linked FTD and ALS.Neuron. 2011; 72: 245-256Abstract Full Text Full Text PDF PubMed Scopus (3382) Google Scholar, Renton et al., 2011Renton A.E. Majounie E. Waite A. Simón-Sánchez J. Rollinson S. Gibbs J.R. Schymick J.C. Laaksovirta H. van Swieten J.C. Myllykangas L. et al.ITALSGEN ConsortiumA hexanucleotide repeat expansion in C9ORF72 is the cause of chromosome 9p21-linked ALS-FTD.Neuron. 2011; 72: 257-268Abstract Full Text Full Text PDF PubMed Scopus (3081) Google Scholar; Figure 2A ). Since its discovery in 2011, rapid progress has been made in identifying dysregulated molecular pathways in C9ORF72-ALS/FTD (Gao et al., 2017Gao F.B. Almeida S. Lopez-Gonzalez R. Dysregulated molecular pathways in amyotrophic lateral sclerosis-frontotemporal dementia spectrum disorder.EMBO J. 2017; 36: 2931-2950Crossref PubMed Scopus (111) Google Scholar). Although repeat RNAs themselves may be partially responsible for neurotoxicity, accumulating evidence suggests that dipeptide repeat (DPR) proteins are pathogenic (Gitler and Tsuiji, 2016Gitler A.D. Tsuiji H. There has been an awakening: Emerging mechanisms of C9orf72 mutations in FTD/ALS.Brain Res. 2016; 1647: 19-29Crossref PubMed Scopus (103) Google Scholar). At least five DPR proteins (poly[GA], poly[GR], poly[GP], poly[PR], and poly[PA]) encoded in
Referência(s)