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.
Dysfunction of mitochondrial respiration is an increasingly recognized cause of isolated hypertrophic cardiomyopathy. To gain insight into the genetic origin of this condition, we used ...next-generation exome sequencing to identify mutations in MTO1, which encodes mitochondrial translation optimization 1. Two affected siblings carried a maternal c.1858dup (p.Arg620Lysfs∗8) frameshift and a paternal c.1282G>A (p.Ala428Thr) missense mutation. A third unrelated individual was homozygous for the latter change. In both humans and yeast, MTO1 increases the accuracy and efficiency of mtDNA translation by catalyzing the 5-carboxymethylaminomethylation of the wobble uridine base in three mitochondrial tRNAs (mt-tRNAs). Accordingly, mutant muscle and fibroblasts showed variably combined reduction in mtDNA-dependent respiratory chain activities. Reduced respiration in mutant cells was corrected by expressing a wild-type MTO1 cDNA. Conversely, defective respiration of a yeast mto1Δ strain failed to be corrected by an Mto1Pro622∗ variant, equivalent to human MTO1Arg620Lysfs∗8, whereas incomplete correction was achieved by an Mto1Ala431Thr variant, corresponding to human MTO1Ala428Thr. The respiratory yeast phenotype was dramatically worsened in stress conditions and in the presence of a paromomycin-resistant (PR) mitochondrial rRNA mutation. Lastly, in vivo mtDNA translation was impaired in the mutant yeast strains.
Neurodegeneration with brain iron accumulation (NBIA) comprises a clinically and genetically heterogeneous group of disorders with progressive extrapyramidal signs and neurological deterioration, ...characterized by iron accumulation in the basal ganglia. Exome sequencing revealed the presence of recessive missense mutations in COASY, encoding coenzyme A (CoA) synthase in one NBIA-affected subject. A second unrelated individual carrying mutations in COASY was identified by Sanger sequence analysis. CoA synthase is a bifunctional enzyme catalyzing the final steps of CoA biosynthesis by coupling phosphopantetheine with ATP to form dephospho-CoA and its subsequent phosphorylation to generate CoA. We demonstrate alterations in RNA and protein expression levels of CoA synthase, as well as CoA amount, in fibroblasts derived from the two clinical cases and in yeast. This is the second inborn error of coenzyme A biosynthesis to be implicated in NBIA.
To safeguard the cell from the accumulation of potentially harmful metabolic intermediates, specific repair mechanisms have evolved. APOA1BP, now renamed NAXE, encodes an epimerase essential in the ...cellular metabolite repair for NADHX and NADPHX. The enzyme catalyzes the epimerization of NAD(P)HX, thereby avoiding the accumulation of toxic metabolites. The clinical importance of the NAD(P)HX repair system has been unknown. Exome sequencing revealed pathogenic biallelic mutations in NAXE in children from four families with (sub-) acute-onset ataxia, cerebellar edema, spinal myelopathy, and skin lesions. Lactate was elevated in cerebrospinal fluid of all affected individuals. Disease onset was during the second year of life and clinical signs as well as episodes of deterioration were triggered by febrile infections. Disease course was rapidly progressive, leading to coma, global brain atrophy, and finally to death in all affected individuals. NAXE levels were undetectable in fibroblasts from affected individuals of two families. In these fibroblasts we measured highly elevated concentrations of the toxic metabolite cyclic-NADHX, confirming a deficiency of the mitochondrial NAD(P)HX repair system. Finally, NAD or nicotinic acid (vitamin B3) supplementation might have therapeutic implications for this fatal disorder.
Exome sequencing of an individual with congenital cataracts, hypertrophic cardiomyopathy, skeletal myopathy, and lactic acidosis, all typical symptoms of Sengers syndrome, discovered two nonsense ...mutations in the gene encoding mitochondrial acylglycerol kinase (AGK). Mutation screening of AGK in further individuals with congenital cataracts and cardiomyopathy identified numerous loss-of-function mutations in an additional eight families, confirming the causal nature of AGK deficiency in Sengers syndrome. The loss of AGK led to a decrease of the adenine nucleotide translocator in the inner mitochondrial membrane in muscle, consistent with a role of AGK in driving the assembly of the translocator as a result of its effects on phospholipid metabolism in mitochondria.
Diabetes mellitus and neurodegeneration are common diseases for which shared genetic factors are still only partly known. Here, we show that loss of the BiP (immunoglobulin heavy-chain binding ...protein) co-chaperone DNAJC3 leads to diabetes mellitus and widespread neurodegeneration. We investigated three siblings with juvenile-onset diabetes and central and peripheral neurodegeneration, including ataxia, upper-motor-neuron damage, peripheral neuropathy, hearing loss, and cerebral atrophy. Exome sequencing identified a homozygous stop mutation in DNAJC3. Screening of a diabetes database with 226,194 individuals yielded eight phenotypically similar individuals and one family carrying a homozygous DNAJC3 deletion. DNAJC3 was absent in fibroblasts from all affected subjects in both families. To delineate the phenotypic and mutational spectrum and the genetic variability of DNAJC3, we analyzed 8,603 exomes, including 506 from families affected by diabetes, ataxia, upper-motor-neuron damage, peripheral neuropathy, or hearing loss. This analysis revealed only one further loss-of-function allele in DNAJC3 and no further associations in subjects with only a subset of the features of the main phenotype. Our findings demonstrate that loss-of-function DNAJC3 mutations lead to a monogenic, recessive form of diabetes mellitus in humans. Moreover, they present a common denominator for diabetes and widespread neurodegeneration. This complements findings from mice in which knockout of Dnajc3 leads to diabetes and modifies disease in a neurodegenerative model of Marinesco-Sjögren syndrome.
Biallelic pathogenic variants in
are mostly associated with severe rod-driven inherited retinal degeneration. In this study, we analyzed clinical heterogeneity in 17 patients and characterized the ...underlying biallelic variants in
. All patients underwent thorough ophthalmological examinations. Minigene assays and structural analyses were performed to assess the consequences of splice variants and missense variants. Three patients were diagnosed with Leber congenital amaurosis, nine with early onset retinitis pigmentosa, two with retinitis pigmentosa with an onset in adulthood, one with cone dystrophy, and two with cone-rod dystrophy. Seventeen different alleles were identified, namely eight missense variants, six nonsense variants, one in-frame deletion variant, and two splice site variants. For the latter two, minigene assays revealed aberrant transcripts containing frameshifts and premature termination codons. Structural analysis and molecular modeling suggested different degrees of structural destabilization for the missense variants. In conclusion, we report the largest cohort of patients with
-associated IRD published to date. Most of the patients exhibited rod-driven disease, yet a fraction of the patients exhibited cone-driven disease. Our data support the hypothesis that
variants do not fold properly and thus trigger unfolded protein response, resulting in photoreceptor death.
Mitochondria are dynamic organelles which undergo continuous fission and fusion to maintain their diverse cellular functions. Components of the fission machinery are partly shared between ...mitochondria and peroxisomes, and inherited defects in two such components (dynamin-related protein (DRP1) and ganglioside-induced differentiation-associated protein 1 (GDAP1)) have been associated with human disease. Deficiency of a third component (mitochondrial fission factor, MFF) was recently reported in one index patient, rendering MFF another candidate disease gene within the expanding field of mitochondrial and peroxisomal dynamics. Here we investigated three new patients from two families with pathogenic mutations in MFF.
The patients underwent clinical examination, brain MRI, and biochemical, cytological and molecular analyses, including exome sequencing.
The patients became symptomatic within the first year of life, exhibiting seizures, developmental delay and acquired microcephaly. Dysphagia, spasticity and optic and peripheral neuropathy developed subsequently. Brain MRI showed Leigh-like patterns with bilateral changes of the basal ganglia and subthalamic nucleus, suggestive of impaired mitochondrial energy metabolism. However, activities of mitochondrial respiratory chain complexes were found to be normal in skeletal muscle. Exome sequencing revealed three different biallelic loss-of-function variants in MFF in both index cases. Western blot studies of patient-derived fibroblasts indicated normal content of mitochondria and peroxisomes, whereas immunofluorescence staining revealed elongated mitochondria and peroxisomes. Furthermore, increased mitochondrial branching and an abnormal distribution of fission-mediating DRP1 were observed.
Our findings establish MFF loss of function as a cause of disturbed mitochondrial and peroxisomal dynamics associated with early-onset Leigh-like basal ganglia disease. We suggest that, even if laboratory findings are not indicative of mitochondrial or peroxisomal dysfunction, the co-occurrence of optic and/or peripheral neuropathy with seizures warrants genetic testing for MFF mutations.
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