Haploinsufficiency of KMT2B, Encoding the Lysine-Specific Histone Methyltransferase 2B, Results in Early-Onset Generalized Dystonia
2016; Elsevier BV; Volume: 99; Issue: 6 Linguagem: Inglês
10.1016/j.ajhg.2016.10.010
ISSN1537-6605
AutoresMichael Zech, Sylvia Boesch, Esther M. Maier, Ingo Borggraefe, Katharina Vill, Franco Laccone, Veronika Pilshofer, Andrés Ceballos-Baumann, Bader Alhaddad, Riccardo Berutti, Werner Poewe, Tobias B. Haack, Bernhard Haslinger, Tim M. Strom, Juliane Winkelmann,
Tópico(s)Genomics and Rare Diseases
ResumoEarly-onset generalized dystonia represents the severest form of dystonia, a hyperkinetic movement disorder defined by involuntary twisting postures. Although frequently transmitted as a single-gene trait, the molecular basis of dystonia remains largely obscure. By whole-exome sequencing a parent-offspring trio in an Austrian kindred affected by non-familial early-onset generalized dystonia, we identified a dominant de novo frameshift mutation, c.6406delC (p.Leu2136Serfs∗17), in KMT2B, encoding a lysine-specific methyltransferase involved in transcriptional regulation via post-translational modification of histones. Whole-exome-sequencing-based exploration of a further 30 German-Austrian individuals with early-onset generalized dystonia uncovered another three deleterious mutations in KMT2B—one de novo nonsense mutation (c.1633C>T [p.Arg545∗]), one de novo essential splice-site mutation (c.7050−2A>G [p.Phe2321Serfs∗93]), and one inherited nonsense mutation (c.2428C>T [p.Gln810∗]) co-segregating with dystonia in a three-generation kindred. Each of the four mutations was predicted to mediate a loss-of-function effect by introducing a premature termination codon. Suggestive of haploinsufficiency, we found significantly decreased total mRNA levels of KMT2B in mutant fibroblasts. The phenotype of individuals with KMT2B loss-of-function mutations was dominated by childhood lower-limb-onset generalized dystonia, and the family harboring c.2428C>T (p.Gln810∗) showed variable expressivity. In most cases, dystonic symptoms were accompanied by heterogeneous non-motor features. Independent support for pathogenicity of the mutations comes from the observation of high rates of dystonic presentations in KMT2B-involving microdeletion syndromes. Our findings thus establish generalized dystonia as the human phenotype associated with haploinsufficiency of KMT2B. Moreover, we provide evidence for a causative role of disordered histone modification, chromatin states, and transcriptional deregulation in dystonia pathogenesis. Early-onset generalized dystonia represents the severest form of dystonia, a hyperkinetic movement disorder defined by involuntary twisting postures. Although frequently transmitted as a single-gene trait, the molecular basis of dystonia remains largely obscure. By whole-exome sequencing a parent-offspring trio in an Austrian kindred affected by non-familial early-onset generalized dystonia, we identified a dominant de novo frameshift mutation, c.6406delC (p.Leu2136Serfs∗17), in KMT2B, encoding a lysine-specific methyltransferase involved in transcriptional regulation via post-translational modification of histones. Whole-exome-sequencing-based exploration of a further 30 German-Austrian individuals with early-onset generalized dystonia uncovered another three deleterious mutations in KMT2B—one de novo nonsense mutation (c.1633C>T [p.Arg545∗]), one de novo essential splice-site mutation (c.7050−2A>G [p.Phe2321Serfs∗93]), and one inherited nonsense mutation (c.2428C>T [p.Gln810∗]) co-segregating with dystonia in a three-generation kindred. Each of the four mutations was predicted to mediate a loss-of-function effect by introducing a premature termination codon. Suggestive of haploinsufficiency, we found significantly decreased total mRNA levels of KMT2B in mutant fibroblasts. The phenotype of individuals with KMT2B loss-of-function mutations was dominated by childhood lower-limb-onset generalized dystonia, and the family harboring c.2428C>T (p.Gln810∗) showed variable expressivity. In most cases, dystonic symptoms were accompanied by heterogeneous non-motor features. Independent support for pathogenicity of the mutations comes from the observation of high rates of dystonic presentations in KMT2B-involving microdeletion syndromes. Our findings thus establish generalized dystonia as the human phenotype associated with haploinsufficiency of KMT2B. Moreover, we provide evidence for a causative role of disordered histone modification, chromatin states, and transcriptional deregulation in dystonia pathogenesis. Dystonia is a motor disorder characterized by uncontrolled hyperkinetic muscle contractions resulting in twisting movements and disabling postures.1Albanese A. Bhatia K. Bressman S.B. Delong M.R. Fahn S. Fung V.S. Hallett M. Jankovic J. Jinnah H.A. Klein C. et al.Phenomenology and classification of dystonia: a consensus update.Mov. Disord. 2013; 28: 863-873Crossref PubMed Scopus (1374) Google Scholar, 2Balint B. Bhatia K.P. Dystonia: an update on phenomenology, classification, pathogenesis and treatment.Curr. Opin. Neurol. 2014; 27: 468-476Crossref PubMed Scopus (73) Google Scholar In generalized manifestations, dystonic symptoms affect the trunk and at least two additional body regions and can evolve either in isolation (referred to as generalized isolated dystonia) or in combination with other movement abnormalities (referred to as generalized combined dystonia).1Albanese A. Bhatia K. Bressman S.B. Delong M.R. Fahn S. Fung V.S. Hallett M. Jankovic J. Jinnah H.A. Klein C. et al.Phenomenology and classification of dystonia: a consensus update.Mov. Disord. 2013; 28: 863-873Crossref PubMed Scopus (1374) Google Scholar, 2Balint B. Bhatia K.P. Dystonia: an update on phenomenology, classification, pathogenesis and treatment.Curr. Opin. Neurol. 2014; 27: 468-476Crossref PubMed Scopus (73) Google Scholar Still, no mechanism-based treatment options are available for the vast majority of dystonic syndromes, mostly because of the fragmentary knowledge about their underlying pathophysiology.1Albanese A. Bhatia K. Bressman S.B. Delong M.R. Fahn S. Fung V.S. Hallett M. Jankovic J. Jinnah H.A. Klein C. et al.Phenomenology and classification of dystonia: a consensus update.Mov. Disord. 2013; 28: 863-873Crossref PubMed Scopus (1374) Google Scholar, 2Balint B. Bhatia K.P. Dystonia: an update on phenomenology, classification, pathogenesis and treatment.Curr. Opin. Neurol. 2014; 27: 468-476Crossref PubMed Scopus (73) Google Scholar For generalized dystonia, only a few genes harboring causative mutations have been described, explaining a small proportion of cases.1Albanese A. Bhatia K. Bressman S.B. Delong M.R. Fahn S. Fung V.S. Hallett M. Jankovic J. Jinnah H.A. Klein C. et al.Phenomenology and classification of dystonia: a consensus update.Mov. Disord. 2013; 28: 863-873Crossref PubMed Scopus (1374) Google Scholar, 3Klein C. Genetics in dystonia.Parkinsonism Relat. Disord. 2014; 20: S137-S142Abstract Full Text PDF PubMed Scopus (76) Google Scholar, 4Domingo A. Erro R. Lohmann K. Novel Dystonia Genes: Clues on Disease Mechanisms and the Complexities of High-Throughput Sequencing.Mov. Disord. 2016; 31: 471-477Crossref PubMed Scopus (52) Google Scholar In this study, we employed whole-exome trio analysis in a family affected by early-onset generalized dystonia and subsequent whole-exome sequencing (WES)-based interrogation of candidate genes in a replication cohort. We reveal four unique loss-of-function (LoF) mutations in KMT2B (MIM: 606834; originally named MLL2, MLL4, or WBP7), encoding a lysine-specific histone methyltransferase in chromosomal region 19q13.12. By combining our WES results with KMT2B expression studies in LoF mutation carriers and manual review of phenotypic effects in 19q13.12 contiguous gene-deletion syndromes, we have identified KMT2B haploinsufficiency as a highly penetrant risk factor for early-onset generalized dystonia in humans and define a role for defective histone modification in dystonia pathogenesis. Probands of the study were enrolled from multiple recruitment sites including Innsbruck (11 individuals, including the index subject of family F1), Munich (18 individuals, including the probands of families F2 and F3), and Vienna (four individuals, including the probands of family F4). Biological samples were collected after written informed consent was given, and the study was approved by the local ethics committees. We revisited a simplex Austrian index subject with severe early-onset generalized dystonia (F1-II-5 in family F1; Figure 1; Table 1), in whom previous WES had not identified mutations of known dystonia-causing genes.5Zech M. Boesch S. Jochim A. Weber S. Meindl T. Schormair B. Wieland T. Lunetta C. Sansone V. Messner M. et al.Clinical exome sequencing in early-onset generalized dystonia and large-scale resequencing follow-up.Mov. Disord. 2016; https://doi.org/10.1002/mds.26808Crossref Scopus (77) Google Scholar To allow the detection of recessive or de novo dominant variants in a yet-undiscovered etiologically involved gene, we chose to complement the initial proband-only exome approach by performing full proband-parent trio WES. As described before for the index subject,5Zech M. Boesch S. Jochim A. Weber S. Meindl T. Schormair B. Wieland T. Lunetta C. Sansone V. Messner M. et al.Clinical exome sequencing in early-onset generalized dystonia and large-scale resequencing follow-up.Mov. Disord. 2016; https://doi.org/10.1002/mds.26808Crossref Scopus (77) Google Scholar exomic sequences of both unaffected parents were captured and processed at the Helmholtz Center Munich in Germany according to fully validated protocols. In short, blood-cell-derived genomic DNA libraries were enriched with the SureSelect Human All Exon 50 Mb Kit v.5 (Agilent Technologies) and sequenced on a HiSeq 2500 machine (Illumina) to an average read depth of 181× (Table S1). Reads were mapped to the human reference genome (UCSC Genome Browser hg19) with the Burrows-Wheeler Aligner (v.0.6.2.). SAMtools (v.0.1.18), PINDEL (v.0.2.4t), ExomeDepth (v.1.0.0), and Custom Perl scripts were employed for variant detection and annotation on all sequenced family members simultaneously. For inclusion in downstream analyses, variant calls were required to display a minimum read depth of 10× and a minimum quality score of 30 (defined as high-confidence calls). Considering the single occurrence of disease in family F1, bioinformatics filtering of variants was based on recessive and de novo dominant inheritance patterns. All retained candidate variants were verified by Sanger sequencing and tested for co-segregation in F1. Prioritization of recessive protein-altering variants with a minor allele frequency ≤ 0.001 in the Exome Aggregation Consortium (ExAC) Browser (v.0.3.1) or an internal database containing 7,900 control exomes yielded two co-segregating alterations that were deemed unlikely to be causative for the observed dystonia phenotype (Table S2). Regarding a de novo dominant effect, we next searched the index subject's WES data for protein-altering sequence variations that were absent from (1) the ExAC Browser, (2) our internal control exome database, and (3) the exome variant profiles of each parent. After this process and confirmatory Sanger evaluation, three heterozygous de novo events were left: two missense substitutions (c.32G>T [p.Gly11Val] in TDRD6 [MIM: 611200] and c.2483G>A [p.Gly828Glu] in NYNRIN) and one LoF variant (c.6406delC [p.Leu2136Serfs∗17] in KMT2B [GenBank: NM_014727.2 and NP_055542.1]) (Table S3). By introducing a frameshift and a premature translation stop at amino acid position 2,152, the KMT2B single-nucleotide deletion was the only identified de novo change predicted to have a severe impact on protein structure (Figure 1). Further, although we found evidence of large numbers of singleton missense variants in TDRD6 and NYNRIN among population-based control individuals (ExAC-derived missense Z scores of −3.35 and 0.43, respectively),6Lek M. Karczewski K.J. Minikel E.V. Samocha K.E. Banks E. Fennell T. O’Donnell-Luria A.H. Ware J.S. Hill A.J. Cummings B.B. et al.Exome Aggregation ConsortiumAnalysis of protein-coding genetic variation in 60,706 humans.Nature. 2016; 536: 285-291Crossref PubMed Scopus (6555) Google Scholar we noted a highly restricted repertoire of KMT2B LoF variants in the ExAC Browser (probability of being LoF intolerant [pLI] score of 1.0)6Lek M. Karczewski K.J. Minikel E.V. Samocha K.E. Banks E. Fennell T. O’Donnell-Luria A.H. Ware J.S. Hill A.J. Cummings B.B. et al.Exome Aggregation ConsortiumAnalysis of protein-coding genetic variation in 60,706 humans.Nature. 2016; 536: 285-291Crossref PubMed Scopus (6555) Google Scholar (Table S4). Similarly, we observed no single high-confidence KMT2B LoF variant call in 7,900 in-house control exomes, rendering KMT2B the most promising candidate for further evaluation. To determine whether mutations in TDRD6, NYNRIN, or KMT2B were present in other individuals with early-onset generalized dystonia, we extended our genetic investigation to a German-Austrian replication cohort of 30 individuals with unsolved disease (67% female, mean age at onset = 9.9 ± 9.2 years), including 26 simplex cases (Table S5). According to the dystonia consensus definition,1Albanese A. Bhatia K. Bressman S.B. Delong M.R. Fahn S. Fung V.S. Hallett M. Jankovic J. Jinnah H.A. Klein C. et al.Phenomenology and classification of dystonia: a consensus update.Mov. Disord. 2013; 28: 863-873Crossref PubMed Scopus (1374) Google Scholar 23 of these individuals were classified as having generalized isolated dystonia, and seven were classified as having generalized combined dystonia (Table S5). In eight individuals, dystonic symptoms were accompanied by variable non-motor features (Table S5). Cerebral MRI was unremarkable across the replication cohort, and medical interviews did not indicate any acquired etiologies. The entire replication cohort had undergone WES at the Helmholtz Center Munich in Germany via the same sequencing methodology and variant-annotation techniques as detailed above (Table S6). Complementary trio WES had been performed in six individuals. In all cases, the WES data were used to exclude pathogenic mutations of known dystonia-related genes, as previously described.5Zech M. Boesch S. Jochim A. Weber S. Meindl T. Schormair B. Wieland T. Lunetta C. Sansone V. Messner M. et al.Clinical exome sequencing in early-onset generalized dystonia and large-scale resequencing follow-up.Mov. Disord. 2016; https://doi.org/10.1002/mds.26808Crossref Scopus (77) Google Scholar Despite a high depth of coverage over the target regions (Table S7), interrogation of the 30 replication-cohort exomes uncovered no additional rare protein-altering variants in TDRD6 or NYNRIN. By contrast, three separate dystonia-affected individuals were found to harbor heterozygous high-confidence variant calls in KMT2B, none of which were registered in the ExAC Browser or our in-house control exome database (collectively encompassing > 135,000 sequenced alleles). Strikingly, each of the three unique sequence alterations was predicted to engender a LoF effect (Figure 1; Table 1). In individual RC-16 (simplex case F2-II-1 in family F2; Figure 1; Table 1), we encountered c.1633C>T resulting in a nonsense mutation at amino acid position 545 (p.Arg545∗). Individual F2-II-1 had undergone trio WES, directly indicating that c.1633C>T (p.Arg545∗) had arisen de novo. Sanger sequencing validated the de novo status of the mutation (Figure 1). This mutation was the only observed de novo change in F2-II-1 with an expected deleterious impact on protein structure (Table S8). Individual RC-26 (simplex case F3-II-3 in family F3; Figure 1; Table 1) was found to possess an A-to-G transition at the conserved AG splice-acceptor dinucleotide of intron 29 (c.7050−2A>G). Direct sequencing of cDNA fragments amplified by RT-PCR from F3-II-3’s whole-blood-derived RNA revealed that the mutation produced a complex pattern of incorrect splicing, including exon 29 skipping, cryptic intron 29 splice-acceptor utilization, and partial intron 29 retention (Figure 2). In terms of structure, the misspliced transcript was predicted to contain a shift in the reading frame and a premature stop codon within exon 32 (p.Phe2321Serfs∗93) (Figure 2). The c.7050−2A>G mutation was confirmed and proven to be de novo by Sanger sequencing of DNA samples from the subject and his biological parents (Figure 1). Finally, in RC-29 (familial case F4-III-2 in family F4; Figure 1; Table 1), we discovered c.2428C>T leading to a premature termination of the protein product at amino acid position 810 (p.Gln810∗). Sanger sequencing verified the variant and showed its co-segregation with dystonia in the proband’s father (F4-II-4) and paternal grandfather (F4-I-3) (Figure 1). On the protein level, all identified LoF variants were expected to deleteriously affect the functionally important C-terminal SET domain, conferring methyltransferase activity (Figure 1).Table 1Synopsis of KMT2B LoF Genotypes Identified in This Study and Associated Clinical FeaturesFamily F1Family F2Family F3Family F4F1-II-5 (Index Subject)F2-II-1 (RC-16)F3-II-3 (RC-26)F4-III-2 (RC-29)F4-II-4 (Father)F4-I-3 (Grandfather)SexfemalefemalemalefemalemalemaleOriginAustrianGermanGermanAustrianAustrianAustrianAge at last examination (years)31111563661KMT2B LoF variant Genomic position (hg19)chr19: 36,223,856chr19: 36,211,882chr19: 36,224,662chr19: 36,212,677chr19: 36,212,677chr19: 36,212,677 ExonaNumbering is according to GenBank: NM_014727.2 and NP_055542.1 (Ensemble: ENST00000222270 and ENSP00000222270).283IVS29bIVS, intervening sequence (intron).333 Variation nucleotideaNumbering is according to GenBank: NM_014727.2 and NP_055542.1 (Ensemble: ENST00000222270 and ENSP00000222270).c.6406delCc.1633C>Tc.7050−2A>Gc.2428C>Tc.2428C>Tc.2428C>T Variation amino acidaNumbering is according to GenBank: NM_014727.2 and NP_055542.1 (Ensemble: ENST00000222270 and ENSP00000222270).p.Leu2136Serfs∗17p.Arg545∗p.Phe2321Serfs∗93p.Gln810∗p.Gln810∗p.Gln810∗ Variant typeframeshiftnonsensecanonical splicenonsensenonsensenonsense Inheritancede novode novode novopaternalpaternalunknownPregnancy and deliverynormalnormalnormalnormal (gemini)normalnormalPostnatal development Head circumferencenormalmicrocephaly(<3rd percentile)microcephaly(<1st percentile)microcephaly(<1st percentile)microcephaly(<3rd percentile)normal Heightnormalshort stature(<3rd percentile)normalshort stature(<1st percentile)short stature(<3rd percentile)normal Weightnormalshort weight(<3rd percentile)normalshort weight( 5,000 non-dystonia parent-offspring trios sequenced as part of diverse disease-related projects7Rauch A. Wieczorek D. Graf E. Wieland T. Endele S. Schwarzmayr T. Albrecht B. Bartholdi D. Beygo J. Di Donato N. et al.Range of genetic mutations associated with severe non-syndromic sporadic intellectual disability: an exome sequencing study.Lancet. 2012; 380: 1674-1682Abstract Full Text Full Text PDF PubMed Scopus (764) Google Scholar, 8Fromer M. Pocklington A.J. Kavanagh D.H. Williams H.J. Dwyer S. Gormley P. Georgieva L. Rees E. Palta P. Ruderfer D.M. et al.De novo mutations in schizophrenia implicate synaptic networks.Nature. 2014; 506: 179-184Crossref PubMed Scopus (1127) Google Scholar, 9Gulsuner S. Walsh T. Watts A.C. Lee M.K. Thornton A.M. Casadei S. Rippey C. Shahin H. Nimgaonkar V.L. Go R.C. et al.Consortium on the Genetics of Schizophrenia (COGS)PAARTNERS Study GroupSpatial and temporal mapping of de novo mutations in schizophrenia to a fetal prefrontal cortical network.Cell. 2013; 154: 518-529Abstract Full Text Full Text PDF PubMed Scopus (385) Google Scholar, 10Xu B. Ionita-Laza I. Roos J.L. Boone B. Woodrick S. Sun Y. Levy S. Gogos J.A. Karayiorgou M. De novo gene mutations highlight patterns of genetic and neural complexity in schizophrenia.Nat. Genet. 2012; 44: 1365-1369Crossref PubMed Scopus (328) Google Scholar, 11Allen A.S. Berkovic S.F. Cossette P. Delanty N. Dlugos D. Eichler E.E. Epstein M.P. Glauser T. Goldstein D.B. Han Y. et al.Epi4K ConsortiumEpilepsy Phenome/Genome ProjectDe novo mutations in epileptic encephalopathies.Nature. 2013; 501: 217-221Crossref PubMed Scopus (1086) Google Scholar, 12Chesi A. Staahl B.T. Jovičić A. Couthouis J. Fasolino M. Raphael A.R. Yamazaki T. Elias L. Polak M. Kelly C. et al.Exome sequencing to identify de novo mutations in sporadic ALS trios.Nat. Neurosci. 2013; 16: 851-855Crossref PubMed Scopus (115) Google Scholar, 13Iossifov I. O’Roak B.J. Sanders S.J. Ronemus M. Krumm N. Levy D. Stessman H.A. Witherspoon K.T. Vives L. Patterson K.E. et al.The contribution of de novo coding mutations to autism spectrum disorder.Nature. 2014; 515: 216-221Crossref PubMed Scopus (1498) Google Scholar, 14Homsy J. Zaidi S. Shen Y. Ware J.S. Samocha K.E. Karczewski K.J. DePalma S.R. McKean D. Wakimoto H. Gorham J. et al.De novo mutations in congenital heart disease with neurodevelopmental and other congenital anomalies.Science. 2015; 350: 1262-1266Crossref PubMed Scopus (436) Google Scholar and (2) KMT2B is severely depleted of disruptive variation in the human population.6Lek M. Karczewski K.J. Minikel E.V. Samocha K.E. Banks E. Fennell T. O’Donnell-Luria A.H. Ware J.S. Hill A.J. Cummings B.B. et al.Exome Aggregation ConsortiumAnalysis of protein-coding genetic variation in 60,706 humans.Nature. 2016; 536: 285-291Crossref PubMed Scopus (6555) Google Scholar, 15Samocha K.E. Robinson E.B. Sanders S.J. Stevens C. Sabo A. McGrath L.M. Kosmicki J.A. Rehnström K. Mallick S. Kirby A. et al.A framework for the interpretation of de novo mutation in human disease.Nat. Genet. 2014; 46: 944-950Crossref PubMed Scopus (611) Google Scholar, 16Petrovski S. Wang Q. Heinzen E.L. Allen A.S. Goldstein D.B. Genic intolerance to functional variation and the interpretation of personal genomes.PLoS Genet. 2013; 9: e1003709Crossref PubMed Scopus (640) Google Scholar KMT2B has a residual variation-intolerance score of −2.37 (1.12th percentile)16Petrovski S. Wang Q. Heinzen E.L. Allen A.S. Goldstein D.B. Genic intolerance to functional variation and the interpretation of personal genomes.PLoS Genet. 2013; 9: e1003709Crossref PubMed Scopus (640) Google Scholar and ranks among the top 1,003 constraint genes released by the de novo excess algorithm,15Samocha K.E. Robinson E.B. Sanders S.J. Stevens C. Sabo A. McGrath L.M. Kosmicki J.A. Rehnström K. Mallick S. Kirby A. et al.A framework for the interpretation of de novo mutation in human disease.Nat. Genet. 2014; 46: 944-950Crossref PubMed Scopus (611) Google Scholar indicating that it is extremely sensitive to mutational changes. Moreover, application of the ExAC-derived pLI score (1.0) defines KMT2B as a gene that is (1) particularly intolerant to LoF variants, (2) dosage sensitive, and (3) predicted to have a high probability of being relevant to haploinsufficient disease.6Lek M. Karczewski K.J. Minikel E.V. Samocha K.E. Banks E. Fennell T. O’Donnell-Luria A.H. Ware J.S. Hill A.J. Cummings B.B. et al.Exome Aggregation ConsortiumAnalysis of protein-coding genetic variation in 60,706 humans.Nature. 2016; 536: 285-291Crossref PubMed Scopus (6555) Google Scholar In support of KMT2B haploinsufficiency, each of the identified dystonia-related mutations created a premature termination codon, predictably resulting in mRNA products that are vulnerable to nonsense-mediated decay.17Kervestin S. Jacobson A. NMD: a multifaceted response to premature translational termination.Nat. Rev. Mol. Cell Biol. 2012; 13: 700-712Crossref PubMed Scopus (390) Google Scholar In agreement, cDNA from the aberrant transcript associated with c.7050−2A>G (F3-II-3) was detectable at significantly lower levels than the corresponding wild-type species (Figure 2). To test whether KMT2B LoF alleles are subject to nonsense-mediated decay, we investigated the c.6406delC (F1-II-5) and c.7050−2A>G (F3-II-3) variants by qRT-PCR in fibroblast cell lines acquired from proband skin biopsies. As diagrammed in Figure 3, the mutations showed a reduction of KMT2B mRNA expression to approximately 55%–70% of that of control individuals, compatible with relevant degradation of the mutant transcripts. To gain independent evidence for a causal role of KMT2B mutations in dystonia and establish haploinsufficiency as the underlying pathogenic mechanism, we assessed the phenotypic consequences of DNA structural variations resulting in the loss of one KMT2B copy. Systematic analysis of the medical literature led to identification of n
Referência(s)