Recessive Mutations in POLR3B, Encoding the Second Largest Subunit of Pol III, Cause a Rare Hypomyelinating Leukodystrophy
2011; Elsevier BV; Volume: 89; Issue: 5 Linguagem: Inglês
10.1016/j.ajhg.2011.10.006
ISSN1537-6605
AutoresMartine Tétreault, Karine Choquet, Simona Orcesi, Davide Tonduti, Umberto Balottin, Martin Teichmann, Sébastien Fribourg, Raphael Schiffmann, Bernard Brais, Adeline Vanderver, Geneviève Bernard,
Tópico(s)Neutrophil, Myeloperoxidase and Oxidative Mechanisms
ResumoMutations in POLR3A encoding the largest subunit of RNA polymerase III (Pol III) were found to be responsible for the majority of cases presenting with three clinically overlapping hypomyelinating leukodystrophy phenotypes. We uncovered in three cases without POLR3A mutation recessive mutations in POLR3B, which codes for the second largest subunit of Pol III. Mutations in genes coding for Pol III subunits are a major cause of childhood-onset hypomyelinating leukodystrophies with prominent cerebellar dysfunction, oligodontia, and hypogonadotropic hypogonadism. Mutations in POLR3A encoding the largest subunit of RNA polymerase III (Pol III) were found to be responsible for the majority of cases presenting with three clinically overlapping hypomyelinating leukodystrophy phenotypes. We uncovered in three cases without POLR3A mutation recessive mutations in POLR3B, which codes for the second largest subunit of Pol III. Mutations in genes coding for Pol III subunits are a major cause of childhood-onset hypomyelinating leukodystrophies with prominent cerebellar dysfunction, oligodontia, and hypogonadotropic hypogonadism. Leukodystrophies are a heterogeneous group of neurodegenerative disorders characterized by abnormal central nervous system white matter.1Schiffmann R. van der Knaap M.S. The latest on leukodystrophies.Curr. Opin. Neurol. 2004; 17: 187-192Crossref PubMed Scopus (75) Google Scholar It is estimated that at least 30% to 40% of patients with leukodystrophies remain without a precise diagnosis despite extensive investigations.2Schiffmann R. van der Knaap M.S. Invited article: An MRI-based approach to the diagnosis of white matter disorders.Neurology. 2009; 72: 750-759Crossref PubMed Scopus (346) Google Scholar We recently demonstrated that the majority of cases affected by tremor-ataxia with central hypomyelination (TACH),3Bernard G. Thiffault I. Tetreault M. Putorti M.L. Bouchard I. Sylvain M. Melançon S. Laframboise R. Langevin P. Bouchard J.P. et al.Tremor-ataxia with central hypomyelination (TACH) leukodystrophy maps to chromosome 10q22.3-10q23.31.Neurogenetics. 2010; 11: 457-464Crossref PubMed Scopus (30) Google Scholar, 4Tetreault, M., Putorti, M.L., Thiffault, I., Sylvain, M., Vanderver, A., Schiffmann, R., Brais, B., and Bernard, G. Refinement of the locus responsible for Tremor-Ataxia with Central Hypomyelination (TACH) leukodystrophy to chromosome 10q22.3-23.1. Can. J. Neurol. Sci., in press.Google Scholar hypomyelination, hypodontia, and hypogonadotropic hypogonadism (4H syndrome [MIM 612440]),5Timmons M. Tsokos M. Asab M.A. Seminara S.B. Zirzow G.C. Kaneski C.R. Heiss J.D. van der Knaap M.S. Vanier M.T. Schiffmann R. Wong K. Peripheral and central hypomyelination with hypogonadotropic hypogonadism and hypodontia.Neurology. 2006; 67: 2066-2069Crossref PubMed Scopus (69) Google Scholar, 6Vázquez-López M. Ruiz-Martín Y. de Castro-Castro P. 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Hypomyelination, hypogonadotropic hypogonadism, hypodontia - First Polish patient.Brain Dev. 2010; 32: 574-578Abstract Full Text Full Text PDF PubMed Scopus (15) Google Scholar and leukodystrophy with oligodontia (LO [MIM 607694])10Atrouni S. Darazé A. Tamraz J. Cassia A. Caillaud C. Mégarbané A. Leukodystrophy associated with oligodontia in a large inbred family: Fortuitous association or new entity?.Am. J. Med. Genet. A. 2003; 118A: 76-81Crossref PubMed Scopus (28) Google Scholar, 11Chouery E. Delague V. Jalkh N. Salem N. Kfoury J. Rodriguez D. Chabrol B. Boespflug-Tanguy O. Lévy N. Serre J.L. Mégarbané A. A whole-genome scan in a large family with leukodystrophy and oligodontia reveals linkage to 10q22.Neurogenetics. 2011; 12: 73-78Crossref PubMed Scopus (12) Google Scholar are caused by mutations in POLR3A (MIM 610210).12Bernard G. Chouery E. Putorti M.L. Tétreault M. Takanohashi A. Carosso G. Clément I. Boespflug-Tanguy O. Rodriguez D. Delague V. et al.Mutations of POLR3A encoding a catalytic subunit of RNA polymerase Pol III cause a recessive hypomyelinating leukodystrophy.Am. J. Hum. Genet. 2011; 89: 415-423Abstract Full Text Full Text PDF PubMed Scopus (150) Google Scholar POLR3A is the largest of the 17 subunits that constitute RNA polymerase III (Pol III). It forms, together with the second largest subunit (POLR3B), the catalytic center of the enzyme. Pol III transcribes small untranslated RNAs (e.g., tRNAs, 5S RNA, 7SK RNA, and U6 RNA) involved in the regulation of essential cellular processes, including transcription, RNA processing, and translation.13Dumay-Odelot H. Durrieu-Gaillard S. Da Silva D. Roeder R.G. Teichmann M. Cell growth- and differentiation-dependent regulation of RNA polymerase III transcription.Cell Cycle. 2010; 9: 3687-3699Crossref PubMed Scopus (46) Google Scholar We suggested that these allelic conditions be referred to as Pol III-related hypomyelinating leukodystrophies. Having not identified POLR3A mutations in four of nine 4H syndrome cases,12Bernard G. Chouery E. Putorti M.L. Tétreault M. Takanohashi A. Carosso G. Clément I. Boespflug-Tanguy O. Rodriguez D. Delague V. et al.Mutations of POLR3A encoding a catalytic subunit of RNA polymerase Pol III cause a recessive hypomyelinating leukodystrophy.Am. J. Hum. Genet. 2011; 89: 415-423Abstract Full Text Full Text PDF PubMed Scopus (150) Google Scholar we searched for mutations in POLR3B, which codes for the other subunit that forms Pol III's catalytic site. The research project was approved by the institutional ethics committee of the Centre de Recherche du CHUM (CRCHUM), Montreal, Canada; the institutional review board of the National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland, USA; and the Children's National Medical Center, Washington D.C., USA. Informed consent was obtained from all participants. We sequenced all exons, exon-intron boundaries and 3′ and 5′ UTR of POLR3B (NM_018082, hg19) on available genomic DNA from two of the four 4H cases not found previously to carry POLR3A mutations (individuals 1 and 2)12Bernard G. Chouery E. Putorti M.L. Tétreault M. Takanohashi A. Carosso G. Clément I. Boespflug-Tanguy O. Rodriguez D. Delague V. et al.Mutations of POLR3A encoding a catalytic subunit of RNA polymerase Pol III cause a recessive hypomyelinating leukodystrophy.Am. J. Hum. Genet. 2011; 89: 415-423Abstract Full Text Full Text PDF PubMed Scopus (150) Google Scholar and one other case of 4H syndrome (individual 3) (Table S1, available online). All three cases were found to be compound heterozygote for mutations in POLR3B (Figure 1A ). One missense mutation in exon 15, c.1568T>A (p.Val523Glu), is common to all three individuals, who are from different ethnic backgrounds; two of the individuals reside in the USA and are of mixed European descent (individuals 1 and 2), and one individual is Italian (individual 3). This common mutation could result from an ancestral haplotype shared by all three individuals. However, we do not have access to genotyping data on these individuals to validate this hypothesis. Individual 1 carries another missense mutation, also in exon 15, that changes a threonine for a lysine at position 503 (c.1508C>A [p.Thr503Lys]). The impact of these two missense mutations on POLR3B was assessed with AlignGVGD, PhD-SNP, PolyPhen and SIFT. All bioinformatics programs predicted the mutations to be pathogenic. In addition to the common mutation, individual 2 was carrying a deletion of one base pair (c.1533delT [p.Ile511MetfsX513]) in exon 15 causing a frameshift and leading to a premature stop codon at position 513. The second mutation in individual 3 is a nonsense variant in exon 23 (c.2686A>T) creating a stop codon at position 896 (p.Lys896X). As seen in POLR3A mutations, none of the participants is homozygous for null mutations.12Bernard G. Chouery E. Putorti M.L. Tétreault M. Takanohashi A. Carosso G. Clément I. Boespflug-Tanguy O. Rodriguez D. Delague V. et al.Mutations of POLR3A encoding a catalytic subunit of RNA polymerase Pol III cause a recessive hypomyelinating leukodystrophy.Am. J. Hum. Genet. 2011; 89: 415-423Abstract Full Text Full Text PDF PubMed Scopus (150) Google Scholar All parents were unaffected and found to carry one of their affected child's mutations. All variants appear to be conserved across species (Figure 1A) and were absent in more than 340 control chromosomes except for the common mutation, c.1568T>A, which was found in 2/374 (0.5%) of control chromosomes. All control individuals were of European descent. This variant has never been reported in the SNPs database (dbSNP132). The locations of the different mutations are shown in Figure 1B. POLR3B (RPC2) and POLR2B (RPB2) subunits of RNA polymerase III and RNA polymerase II, respectively, are highly conserved proteins with a similar structure.14Jasiak A.J. Armache K.J. Martens B. Jansen R.P. Cramer P. Structural biology of RNA polymerase III: Subcomplex C17/25 X-ray structure and 11 subunit enzyme model.Mol. Cell. 2006; 23: 71-81Abstract Full Text Full Text PDF PubMed Scopus (67) Google Scholar Based on the extrapolation of POLR3B (RPC2) from the yeast Pol II structure, the common mutation c.1568T>A (p.Val523Glu) corresponds to Leu566 in POLR2B (RPB2) that is, across species, always an aliphatic or aromatic residue. The mutation c.1508C>A (p.Thr503lys) corresponds to Ser546 in RPB2, which is highly conserved across species. The null mutation c.2686A>T (p.Lys896X), which corresponds to Lys979 in POLR2B (RPB2), and the deletion mutation leading to a frameshift and to a premature stop codon, c.1533delT (p.Ile511MetX513), which corresponds to Ile554 in POLR2B (RPB2), are predicted to generate, if the mRNA is stable enough, a truncated POLR3B subunit leading to a nonfunctional polymerase because the active site is expected to be largely affected in its function. Based on the Pol III electron microscopy structure and photocross-linking experiments,15Fernández-Tornero C. Böttcher B. Rashid U.J. Steuerwald U. Flörchinger B. Devos D.P. Lindner D. Müller C.W. Conformational flexibility of RNA polymerase III during transcriptional elongation.EMBO J. 2010; 29: 3762-3772Crossref PubMed Scopus (55) Google Scholar, 16Wu C.C. Lin Y.C. Chen H.T. The TFIIF-like Rpc37/53 dimer lies at the center of a protein network to connect TFIIIC, Bdp1, and the RNA polymerase III active center.Mol. Cell. Biol. 2011; 31: 2715-2728Crossref PubMed Scopus (56) Google Scholar the p.Val523Glu and p.Thr503Lys residues are located near the “jaw,” an area of Pol III where POLR3D (RPC4) and POLR3E (RPC5) are localized (counterpart of yeast RPC53 and RPC37). These two point mutations are predicted to affect the POLR3B structure locally and thus impair the proper function of RPC4 and RPC5 in transcription. All three cases had characteristic clinical phenotypes of 4H syndrome.5Timmons M. Tsokos M. Asab M.A. Seminara S.B. Zirzow G.C. Kaneski C.R. Heiss J.D. van der Knaap M.S. Vanier M.T. Schiffmann R. Wong K. Peripheral and central hypomyelination with hypogonadotropic hypogonadism and hypodontia.Neurology. 2006; 67: 2066-2069Crossref PubMed Scopus (69) Google Scholar, 8Wolf N.I. Harting I. Innes A.M. Patzer S. Zeitler P. Schneider A. Wolff A. Baier K. Zschocke J. Ebinger F. et al.Ataxia, delayed dentition and hypomyelination: A novel leukoencephalopathy.Neuropediatrics. 2007; 38: 64-70Crossref PubMed Scopus (46) Google Scholar, 17Wolff A. Koch M.J. Benzinger S. van Waes H. Wolf N.I. Boltshauser E. Luder H.U. Rare dental peculiarities associated with the hypomyelinating leukoencephalopathy 4H syndrome/ADDH.Pediatr. Dent. 2010; 32: 386-392PubMed Google Scholar They all presented in early childhood with developmental delays and developed dysarthria as well as progressive motor difficulties, including cerebellar ataxia and, in individuals 1 and 2, progressive spasticity. Individuals 1 and 3 both developed hypogonadotropic hypogonadism, whereas individual 2 was too young to evaluate for endocrine dysfunction. All three individuals had teeth abnormalities, which are presented in Figure 2 for individuals 1 and 2 and consisted of neonatal upper incisors, delayed eruption of deciduous teeth and permanent teeth, abnormal sequence of eruption, and malposition in individual 3. The clinical features of individual 3 have been published previously.18Orcesi S. Tonduti D. Uggetti C. Larizza D. Fazzi E. Balottin U. New case of 4H syndrome and a review of the literature.Pediatr. Neurol. 2010; 42: 359-364Abstract Full Text Full Text PDF PubMed Scopus (19) Google Scholar MRI features of the three individuals were also characteristic of previous descriptions of 4H syndrome19Steenweg M.E. Vanderver A. Blaser S. Bizzi A. de Koning T.J. Mancini G.M. van Wieringen W.N. Barkhof F. Wolf N.I. van der Knaap M.S. Magnetic resonance imaging pattern recognition in hypomyelinating disorders.Brain. 2010; 133: 2971-2982Crossref PubMed Scopus (187) Google Scholar (Figure 3).Figure 3MRI CharacteristicsShow full caption(A, D, and G) Sagittal T1-weighted images of the brain for individuals 1 (A), 2 (D), and 3 (G) at the level of the midline demonstrating the thin corpus callosum and vermian cerebellar atrophy (thick white arrows).(B, E, and H) Axial T2-weighted images of the brain for individuals 1 (B), 2 (E), and 3 (H) at the level of the pons and the dentate nucleus demonstrating a mild T2 hyperintensity of the cerebellar white matter associated with the typical hypointensity of the dentate nucleus (small white arrows).19(C, F, and I) Axial T2-weighted images of the brain for individuals 1 (C), 2 (F), and 3 (I) at the level of the basal ganglia showing the mild hyperintense white matter characteristic of hypomyelination; there is relative sparing of the optic radiations and anterolateral nucleus of the thalamus (small black arrows), which appear hypointense.19Steenweg M.E. Vanderver A. Blaser S. Bizzi A. de Koning T.J. Mancini G.M. van Wieringen W.N. Barkhof F. Wolf N.I. van der Knaap M.S. Magnetic resonance imaging pattern recognition in hypomyelinating disorders.Brain. 2010; 133: 2971-2982Crossref PubMed Scopus (187) Google ScholarView Large Image Figure ViewerDownload Hi-res image Download (PPT) (A, D, and G) Sagittal T1-weighted images of the brain for individuals 1 (A), 2 (D), and 3 (G) at the level of the midline demonstrating the thin corpus callosum and vermian cerebellar atrophy (thick white arrows). (B, E, and H) Axial T2-weighted images of the brain for individuals 1 (B), 2 (E), and 3 (H) at the level of the pons and the dentate nucleus demonstrating a mild T2 hyperintensity of the cerebellar white matter associated with the typical hypointensity of the dentate nucleus (small white arrows).19 (C, F, and I) Axial T2-weighted images of the brain for individuals 1 (C), 2 (F), and 3 (I) at the level of the basal ganglia showing the mild hyperintense white matter characteristic of hypomyelination; there is relative sparing of the optic radiations and anterolateral nucleus of the thalamus (small black arrows), which appear hypointense.19Steenweg M.E. Vanderver A. Blaser S. Bizzi A. de Koning T.J. Mancini G.M. van Wieringen W.N. Barkhof F. Wolf N.I. van der Knaap M.S. Magnetic resonance imaging pattern recognition in hypomyelinating disorders.Brain. 2010; 133: 2971-2982Crossref PubMed Scopus (187) Google Scholar Pol III-related hypomyelinating leukodystrophies are a genetically and clinically heterogeneous group of disorders. We demonstrated previously that POLR3A mutations account for the majority of our relatively small group of affected individuals and are reporting that POLR3B mutations account for the remaining cases for which DNA was available (2/4), as well as one other case of 4H syndrome (individual 3). It is, however, likely that genes encoding for the other Pol III subunits are mutated in cases of hypomyelinating leukodystrophies. As previously suggested, because Pol III is responsible for the transcription of transfer RNAs (tRNAs) and other essential small RNAs, it is possible that mutations in POLR3A and POLR3B lead to an abnormal Pol III function that might affect the levels of certain tRNAs important for the development of the central nervous system white matter,20Dittmar K.A. Goodenbour J.M. Pan T. Tissue-specific differences in human transfer RNA expression.PLoS Genet. 2006; 2: e221Crossref PubMed Scopus (355) Google Scholar leading to abnormal protein production. This pathophysiological mechanism is also attributed to leukoencephalopathy with brainstem and spinal cord involvement and lactate elevation (LBSL [MIM 611105]) caused by mutations in the nuclear-coded mitochondrial aspartyl-tRNA synthetase (DARS2 [MIM 610956]) and hypomyelinating leukodystrophy-3 (MIM 260600) caused by a mutation in aminoacyl-tRNA synthetase complex-interacting-multifunctional protein 1 (AIMP1/p43 [MIM 603605]).21Feinstein M. 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Genet. 2011; 88 (author reply 393–395): 392-393Abstract Full Text Full Text PDF PubMed Scopus (14) Google Scholar We believe that genes coding for other Pol III subunits and possibly genes coding for proteins interacting with Pol III could be mutated in cases of genetically uncharacterized hypomyelinating leukodystrophies. We would like to thank all participants and the clinicians who referred patients to us. We would like to thank the “Fondation sur les Leucodystrophies” and the European Leukodystrophy Association for financing this research project. G.B. received fellowship scholarships from the Réseau de Médecine Génétique Appliquée and Fonds de Recherche en Santé du Québec. M. Tétreault received the Frederick Banting and Charles Best Doctoral scholarship from the Canadian Institute of Health Research. A.V.'s contribution is supported in part by the Intramural Research Program of the National Human Genome Research Institute and by the Myelin Disorders Bioregistry Project. M. Teichmann has received grants from the Agence Nationale de Recherche, the National Cancer Institute, the regional government of Aquitaine, and the Ligue Contre le Cancer (équipe labellisée). S.F. has received grants from La Ligue Contre le Cancer (Comité Dordogne), the regional government of Aquitaine, and the Inserm. The authors wish to thank all collaborators (including William McClintock) and patients participating in the Myelin Bioregistry Project. Download .pdf (.02 MB) Help with pdf files Document S1. One Table The URLs for the data presented herein are as follows:AlignGVGD, http://agvgd.iarc.fr/agvgd_input.phpOnline Mendelian Inheritance in Man (OMIM), http://www.omim.orgPhD-SNP, http://gpcr.biocomp.unibo.it/∼emidio/PhD-SNP/PhD-SNP.htmPolyPhen, http://genetics.bwh.harvard.edu/pph/SIFT, http://sift.jcvi.org/
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