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Zech, Michael; Boesch, Sylvia; Maier, Esther M.; Borggraefe, Ingo; Vill, Katharina; Laccone, Franco; Pilshofer, Veronika; Ceballos-Baumann, Andres; Alhaddad, Bader; Berutti, Riccardo; Poewe, Werner; Haack, Tobias B.; Haslinger, Bernhard; Strom, Tim M.; Winkelmann, Juliane (2016): Haploinsufficiency of KMT2B, Encoding the Lysine-Specific Histone Methyltransferase 2B, Results in Early-Onset Generalized Dystonia. In: American Journal of Human Genetics, Vol. 99, No. 6: pp. 1377-1387
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Abstract

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.