Dominant GDAP1 mutations cause predominantly mild CMT phenotypes

OBJECTIVE

Ganglioside-induced differentiation associated-protein 1 (GDAP1) mutations are commonly associated with autosomal recessive Charcot-Marie-Tooth (ARCMT) neuropathy; however, in rare instances, they also lead to autosomal dominant Charcot-Marie-Tooth (ADCMT). We aimed to investigate the frequency of disease-causing heterozygous GDAP1 mutations in ADCMT and their associated phenotype.

METHODS

We performed mutation analysis in a large cohort of ADCMT patients by means of bidirectional sequencing of coding regions and exon-intron boundaries of GDAP1. Intragenic GDAP1 deletions were excluded using an allele quantification assay. We confirmed the pathogenic character of one sequence variant by in vitro experiments assaying mitochondrial morphology and function.

RESULTS

In 8 Charcot-Marie-Tooth disease (CMT) families we identified 4 pathogenic heterozygous GDAP1 mutations, 3 of which are novel. Three of the mutations displayed reduced disease penetrance. Disease onset in the affected individuals was variable, ranging from early childhood to adulthood. Disease progression was slow in most patients and overall severity milder than typically seen in autosomal recessive GDAP1 mutations. Electrophysiologic changes are heterogeneous but compatible with axonal neuropathy in the majority of patients.

CONCLUSIONS

With this study, we broaden the phenotypic and genetic spectrum of autosomal dominant GDAP1-associated neuropathies. We show that patients with dominant GDAP1 mutations may display clear axonal CMT, but may also have only minimal clinical and electrophysiologic abnormalities. We demonstrate that cell-based functional assays can be reliably used to test the pathogenicity of unknown variants. We discuss the implications of phenotypic variability and the reduced penetrance of autosomal dominant GDAP1 mutations for CMT diagnostic testing and counseling.

Increasing expression and decreasing degradation of SMN ameliorate the spinal muscular atrophy phenotype in mice

Spinal muscular atrophy (SMA) is a neuromuscular disorder caused by reduced levels of the survival motor neuron (SMN) protein. Here we show that the proteasome inhibitor, bortezomib, increases SMN in cultured cells and in peripheral tissues of SMA model mice. Bortezomib-treated animals had improved motor function, which was associated with reduced spinal cord and muscle pathology and improved neuromuscular junction size, but no change in survival. Combining bortezomib with the histone deacetylase inhibitor trichostatin A (TSA) resulted in a synergistic increase in SMN protein levels in mouse tissue and extended survival of SMA mice more than TSA alone. Our results demonstrate that a combined regimen of drugs that decrease SMN protein degradation and increase SMN gene transcription synergistically increases SMN levels and improves the lifespan of SMA model mice. Moreover, this study indicates that while increasing SMN levels in the central nervous system may help extend survival, peripheral tissues can also be targeted to improve the SMA disease phenotype.

Clinical and genetic analysis of spinocerebellar ataxia in Mali

BACKGROUND

Autosomal dominant cerebellar ataxia, currently denominated spinocerebellar ataxia (SCAs), represents a heterogeneous group of neurodegenerative disorders affecting the cerebellum and its connections. We describe the clinical and molecular findings in 16 patients originating from Malian families, who suffer from progressive cerebellar ataxia syndrome.

METHODS AND RESULTS

Molecular analysis allows genetic profiles of SCA to be distinguished. In seven patients, SCA type 2 (CAG) mutation was expanded from 39 to 43 repeats. SCA type 7 (CAG) mutation was confirmed in six patients. Mutations were expanded from 49 to 59 repeats. In three patients, SCA type3 was diagnosed and CAG mutation was expanded to 73 repeats.

CONCLUSIONS

Our data suggest that the most frequent types of SCA are SCA2 and SCA7. However, further studies are needed to confirm these preliminary results.

Therapeutic approaches to spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy is a hereditary motor neuron disease caused by trinucleotide repeat expansion in the androgen receptor gene. The disease mechanism probably involves a toxic gain of function in the mutant protein, because other mutations that cause a loss of androgen receptor function result in a different phenotype and the mutant protein is toxic in mouse models. In these models, the toxicity is ligand-dependent and is associated with protein aggregation, as well as altered transcriptional regulation, axonal transport and mitochondrial function. Various therapeutic approaches have shown efficacy in mouse models, including androgen reduction, heat shock protein 90 (HSP90) inhibition and insulin-like growth factor (IGF)-1 overexpression. Clinical trials of androgen-reducing agents have had mixed results, with indications of efficacy but no proof of clinically meaningful benefit to date. These clinical studies have established outcome measures for future trials of other agents that have been beneficial in animal studies.

B2 attenuates polyglutamine-expanded androgen receptor toxicity in cell and fly models of spinal and bulbar muscular atrophy

Expanded polyglutamine tracts cause neurodegeneration through a toxic gain-of-function mechanism. Generation of inclusions is a common feature of polyglutamine diseases and other protein misfolding disorders. Inclusion formation is likely to be a defensive response of the cell to the presence of unfolded protein. Recently, the compound B2 has been shown to increase inclusion formation and decrease toxicity of polyglutamine-expanded huntingtin in cultured cells. We explored the effect of B2 on spinal and bulbar muscular atrophy (SBMA). SBMA is caused by expansion of polyglutamine in the androgen receptor (AR) and is characterized by the loss of motor neurons in the brainstem and spinal cord. We found that B2 increases the deposition of mutant AR into nuclear inclusions, without altering the ligand-induced aggregation, expression, or subcellular distribution of the mutant protein. The effect of B2 on inclusions was associated with a decrease in AR transactivation function. We show that B2 reduces mutant AR toxicity in cell and fly models of SBMA, further supporting the idea that accumulation of polyglutamine-expanded protein into inclusions is protective. Our findings suggest B2 as a novel approach to therapy for SBMA.

Dominant mutations in the cation channel gene transient receptor potential vanilloid 4 cause an unusual spectrum of neuropathies

Hereditary neuropathies form a heterogeneous group of disorders for which over 40 causal genes have been identified to date. Recently, dominant mutations in the transient receptor potential vanilloid 4 gene were found to be associated with three distinct neuromuscular phenotypes: hereditary motor and sensory neuropathy 2C, scapuloperoneal spinal muscular atrophy and congenital distal spinal muscular atrophy. Transient receptor potential vanilloid 4 encodes a cation channel previously implicated in several types of dominantly inherited bone dysplasia syndromes. We performed DNA sequencing of the coding regions of transient receptor potential vanilloid 4 in a cohort of 145 patients with various types of hereditary neuropathy and identified five different heterozygous missense mutations in eight unrelated families. One mutation arose de novo in an isolated patient, and the remainder segregated in families. Two of the mutations were recurrent in unrelated families. Four mutations in transient receptor potential vanilloid 4 targeted conserved arginine residues in the ankyrin repeat domain, which is believed to be important in protein-protein interactions. Striking phenotypic variability between and within families was observed. The majority of patients displayed a predominantly, or pure, motor neuropathy with axonal characteristics observed on electrophysiological testing. The age of onset varied widely, ranging from congenital to late adulthood onset. Various combinations of additional features were present in most patients including vocal fold paralysis, scapular weakness, contractures and hearing loss. We identified six asymptomatic mutation carriers, indicating reduced penetrance of the transient receptor potential vanilloid 4 defects. This finding is relatively unusual in the context of hereditary neuropathies and has important implications for diagnostic testing and genetic counselling.

GARS axonopathy: not every neuron's cup of tRNA

Charcot-Marie-Tooth disease type 2D, a hereditary axonal neuropathy, is caused by mutations in glycyl-tRNA synthetase (GARS). The mutations are distributed throughout the protein in multiple functional domains. In biochemical and cell culture experiments, some mutant forms of GARS have been indistinguishable from wild-type protein, suggesting that these in vitro tests might not adequately assess the aberrant activity responsible for axonal degeneration. Recently, mouse and fly models have offered new insights into the disease mechanism. There are still gaps in our understanding of how mutations in a ubiquitously expressed component of the translation machinery result in axonal neuropathy. Here, we review recent reports, weigh the evidence for and against possible mechanisms and suggest areas of focus for future work.

Clinical features of spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy is an X-linked motor neuron disease caused by a CAG repeat expansion in the androgen receptor gene. To characterize the natural history and define outcome measures for clinical trials, we assessed the clinical history, laboratory findings and muscle strength and function in 57 patients with genetically confirmed disease. We also administered self-assessment questionnaires for activities of daily living, quality of life and erectile function. We found an average delay of over 5 years from onset of weakness to diagnosis. Muscle strength and function correlated directly with serum testosterone levels and inversely with CAG repeat length, age and duration of weakness. Motor unit number estimation was decreased by about half compared to healthy controls. Sensory nerve action potentials were reduced in nearly all subjects. Quantitative muscle assessment and timed 2 min walk may be useful as meaningful indicators of disease status. The direct correlation of testosterone levels with muscle strength indicates that androgens may have a positive effect on muscle function in spinal and bulbar muscular atrophy patients, in addition to the toxic effects described in animal models.

Hereditary spastic paraplegia and amyotrophy associated with a novel locus on chromosome 19

We identified a family in Mali with two sisters affected by spastic paraplegia. In addition to spasticity and weakness of the lower limbs, the patients had marked atrophy of the distal upper extremities. Homozygosity mapping using single nucleotide polymorphism arrays showed that the sisters shared a region of extended homozygosity at chromosome 19p13.11-q12 that was not shared by controls. These findings indicate a clinically and genetically distinct form of hereditary spastic paraplegia with amyotrophy, designated SPG43.

WITHDRAWN: Familial Degenerative Encephalopathy with Intracranial Calcification and Metaphyseal Dysplasia

Ahead of Print abstract has been withdrawn by the Publisher. This abstract has not been published by the Journal of Bone and Mineral Research and was placed online due to an error.

Mitochondrial abnormalities in spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) is a motor neuron disease caused by polyglutamine expansion mutation in the androgen receptor (AR). We investigated whether the mutant protein alters mitochondrial function. We found that constitutive and doxycycline-induced expression of the mutant AR in MN-1 and PC12 cells, respectively, are associated with depolarization of the mitochondrial membrane. This was mitigated by cyclosporine A, which inhibits opening of the mitochondrial permeability transition pore. We also found that the expression of the mutant protein in the presence of ligand results in an elevated level of reactive oxygen species, which is blocked by the treatment with the antioxidants co-enzyme Q10 and idebenone. The mutant protein in MN-1 cells also resulted in increased Bax, caspase 9 and caspase 3. We assessed the effects of mutant AR on the transcription of mitochondrial proteins and found altered expression of the peroxisome proliferator-activated receptor γ coactivator 1 and the mitochondrial specific antioxidant superoxide dismutase-2 in affected tissues of SBMA knock-in mice. In addition, we found that the AR associates with mitochondria in cultured cells. This study thus provides evidence for mitochondrial dysfunction in SBMA cell and animal models, either through indirect effects on the transcription of nuclear-encoded mitochondrial genes or through direct effects of the mutant protein on mitochondria or both. These findings indicate possible benefit from mitochondrial therapy for SBMA.

A duplication at chromosome 11q12.2-11q12.3 is associated with spinocerebellar ataxia type 20

Spinocerebellar ataxia type 20 (SCA20) has been linked to chromosome 11q12, but the underlying genetic defect has yet to be identified. We applied single-nucleotide polymorphism genotyping to detect structural alterations in the genomic DNA of patients with SCA20. We found a 260 kb duplication within the previously linked SCA20 region, which was confirmed by quantitative polymerase chain reaction and fiber fluorescence in situ hybridization, the latter also showing its direct orientation. The duplication spans 10 known and 2 unknown genes, and is present in all affected individuals in the single reported SCA20 pedigree. While the mechanism whereby this duplication may be pathogenic remains to be established, we speculate that the critical gene within the duplicated segment may be DAGLA, the product of which is normally present at the base of Purkinje cell dendritic spines and contributes to the modulation of parallel fiber-Purkinje cell synapses.

Expression of expanded polyglutamine targets profilin for degradation and alters actin dynamics

Huntington's disease is caused by polyglutamine expansion in the huntingtin protein. Huntingtin directly interacts with profilin, a major actin monomer sequestering protein and a key integrator of signals leading to actin polymerization. We observed a progressive loss of profilin in the cerebral cortex of Huntington's disease patients, and in cell culture and Drosophila models of polyglutamine disease. This loss of profilin is likely due to increased degradation through the ubiquitin proteasome system. Profilin loss reduces the F/G actin ratio, indicating a shift in actin polymerization. Overexpression of profilin abolishes mutant huntingtin toxicity in cells and partially ameliorates the morphological and functional eye phenotype and extends lifespan in a transgenic polyglutamine Drosophila model. These results indicate a link between huntingtin and profilin and implicate profilin in Huntington's disease pathogenesis.

Splice mutation in the iron-sulfur cluster scaffold protein ISCU causes myopathy with exercise intolerance

A myopathy with severe exercise intolerance and myoglobinuria has been described in patients from northern Sweden, with associated deficiencies of succinate dehydrogenase and aconitase in skeletal muscle. We identified the gene for the iron-sulfur cluster scaffold protein ISCU as a candidate within a region of shared homozygosity among patients with this disease. We found a single mutation in ISCU that likely strengthens a weak splice acceptor site, with consequent exon retention. A marked reduction of ISCU mRNA and mitochondrial ISCU protein in patient muscle was associated with a decrease in the iron regulatory protein IRP1 and intracellular iron overload in skeletal muscle, consistent with a muscle-specific alteration of iron homeostasis in this disease. ISCU interacts with the Friedreich ataxia gene product frataxin in iron-sulfur cluster biosynthesis. Our results therefore extend the range of known human diseases that are caused by defects in iron-sulfur cluster biogenesis.

Neurological effects of high-dose idebenone in patients with Friedreich's ataxia: a randomised, placebo-controlled trial

BACKGROUND

Friedreich's ataxia (FA) is a progressive, multisystem, degenerative disorder caused by a reduction in frataxin. Loss of frataxin results in mitochondrial dysfunction and oxidative damage in patients and model systems. Previous studies have indicated that the antioxidant idebenone (5 mg/kg daily) reduces cardiac hypertrophy, but definite improvement in neurological function has not been shown.

METHODS

48 genetically confirmed FA patients, aged 9-17 years, were enrolled in a 6-month, randomised, double-blind, placebo-controlled study. The patients received placebo or one of three doses of idebenone (approximately 5 mg/kg, 15 mg/kg, and 45 mg/kg), stratified by body weight. The primary endpoint was change from baseline in urinary 8-hydroxy-2'-deoxyguanosine (8OH2'dG), a marker of oxidative DNA damage. Secondary endpoints included changes in the international cooperative ataxia rating scale (ICARS), the FA rating scale (FARS), and a survey of activities of daily living (ADL). This study is registered with ClinicalTrials.gov, number NCT00229632.

FINDINGS

Idebenone was generally well tolerated with similar numbers of adverse events in each group. One child receiving high-dose idebenone developed neutropenia after 6 months, which resolved after discontinuation of treatment. 8OH2'dG concentrations were not increased, and did not significantly change with idebenone treatment. Whereas an overall analysis did not show a significant difference in ICARS, FARS, or ADL total scores, there were indications of a dose-dependent response in the ICARS score. A second, pre-specified analysis, excluding patients who required wheelchair assistance, showed a significant improvement in ICARS (Bonferroni p=0.03) and suggested a dose-related response in ICARS, FARS, and ADL scores.

INTERPRETATION

Treatment with higher doses of idebenone was generally well tolerated and associated with improvement in neurological function and ADL in patients with FA. The degree of improvement correlated with the dose of idebenone, suggesting that higher doses may be necessary to have a beneficial effect on neurological function.

Safety, Tolerability, and Pharmacokinetics of High-Dose Idebenone in Patients With Friedreich Ataxia

BACKGROUND

Friedreich ataxia (FA) is a progressive, multisystem degenerative disorder in which oxidative stress is believed to have a role. Recent clinical studies indicate that the antioxidant idebenone, administered at 5 mg/kg per day, reduces the cardiac hypertrophy that occurs in FA, but improvement in neurologic measures is unclear. Some studies suggest that higher doses of idebenone may be more effective, but pharmacology and toxicology at higher doses have not been investigated in human beings.

OBJECTIVE

To determine the safety, tolerability, and pharmacokinetics of increasing doses of idebenone in subjects with FA.

DESIGN

Open-label, phase 1A dose-escalation trial followed by an open-label, 1-month phase 1B trial.

SETTING

National Institutes of Health Clinical Center, Bethesda, Md.

PATIENTS

Phase 1A included 78 subjects with FA (24 adults, 27 adolescents, and 27 children), and phase 1B included 15 subjects with FA (5 adults, 5 adolescents, and 5 children).

INTERVENTIONS

Oral idebenone was administered to groups of 3 subjects in each age cohort during day 1. In phase 1A, the dose was increased in 10-mg/kg increments in each successive dose group to a maximum of 75 mg/kg. In phase 1B, oral idebenone was administered at 60 mg/kg divided in 3 doses per day for 1 month.

MAIN OUTCOME MEASURES

We studied the type, number, and frequency of adverse events, and pharmacokinetic parameters including maximum drug concentration, time to maximum drug concentration, area under the curve, and half-life.

RESULTS

In the first phase of the study, no dose-limiting toxicity was observed and the maximum allowed dose of 75 mg/kg was achieved in all cohorts. Plasma levels of total idebenone were found to increase proportional to drug dose up to 55 mg/kg. Variability in absorption of the drug was observed, but drug half-life was relatively consistent across dose levels. In the second phase of the study, 14 of 15 subjects with FA tolerated idebenone at a dose of 60 mg/kg per day for 1 month. All adverse events were mild, and pharmacokinetic parameters including maximum drug concentration, time to maximum drug concentration, and half-life did not differ significantly across age cohorts.

CONCLUSIONS

These findings indicate that higher doses of idebenone lead to a proportional increase in plasma levels up to 55 mg/kg per day and that high-dose idebenone is well-tolerated in patients with FA. These findings are essential to planning efficacy trials of high-dose idebenone in FA and other degenerative diseases in which oxidative damage has been implicated.

Akt blocks ligand binding and protects against expanded polyglutamine androgen receptor toxicity

Spinal and bulbar muscular atrophy (SBMA) is a progressive neurodegenerative disease caused by an expansion of the polyglutamine tract in the androgen receptor (AR). Here, we investigated the regulation of AR phosphorylation in order to understand factors that may modify SBMA disease progression. We show that expanded polyglutamine AR is phosphorylated by Akt. Substitution of the AR at two Akt consensus sites, S215 and S792, with aspartate, which mimics phosphorylation, reduces ligand binding, ligand-dependent nuclear translocation, transcriptional activation and toxicity of expanded polyglutamine AR. Co-expression of constitutively active Akt and the AR has similar consequences, which are blocked by alanine substitutions at residues 215 and 792. Furthermore, in motor neuron-derived MN-1 cells toxicity associated with polyglutamine-expanded AR is rescued by co-expression with Akt. Insulin-like growth factor-1 (IGF-1) stimulation, which activates several cell survival promoting pathways, also reduces toxicity of the expanded polyglutamine AR in MN-1 cells, in a manner dependent upon phospho-inositol-3-kinase. IGF-1 rescue of AR toxicity is diminished by alanine substitutions at the Akt consensus sites. These results highlight potential targets for therapeutic intervention in SBMA.

Trichostatin A increases SMN expression and survival in a mouse model of spinal muscular atrophy

The inherited motor neuron disease spinal muscular atrophy (SMA) is caused by mutation of the telomeric survival motor neuron 1 (SMN1) gene with retention of the centromeric SMN2 gene. We sought to establish whether the potent and specific hydroxamic acid class of histone deacetylase (HDAC) inhibitors activates SMN2 gene expression in vivo and modulates the SMA disease phenotype when delivered after disease onset. Single intraperitoneal doses of 10 mg/kg trichostatin A (TSA) in nontransgenic and SMA model mice resulted in increased levels of acetylated H3 and H4 histones and modest increases in SMN gene expression. Repeated daily doses of TSA caused increases in both SMN2-derived transcript and SMN protein levels in neural tissues and muscle, which were associated with an improvement in small nuclear ribonucleoprotein (snRNP) assembly. When TSA was delivered daily beginning on P5, after the onset of weight loss and motor deficit, there was improved survival, attenuated weight loss, and enhanced motor behavior. Pathological analysis showed increased myofiber size and number and increased anterior horn cell size. These results indicate that the hydroxamic acid class of HDAC inhibitors activates SMN2 gene expression in vivo and has an ameliorating effect on the SMA disease phenotype when administered after disease onset.

Functional analyses of glycyl-tRNA synthetase mutations suggest a key role for tRNA-charging enzymes in peripheral axons

Charcot-Marie-Tooth disease type 2D (CMT2D) and distal spinal muscular atrophy type V (dSMA-V) are axonal neuropathies characterized by a phenotype that is more severe in the upper extremities. We previously implicated mutations in the gene encoding glycyl-tRNA synthetase (GARS) as the cause of CMT2D and dSMA-V. GARS is a member of the family of aminoacyl-tRNA synthetases responsible for charging tRNA with cognate amino acids; GARS ligates glycine to tRNA(Gly). Here, we present functional analyses of disease-associated GARS mutations and show that there are not any significant mutation-associated changes in GARS expression levels; that the majority of identified GARS mutations modeled in yeast severely impair viability; and that, in most cases, mutant GARS protein mislocalizes in neuronal cells. Indeed, four of the five mutations studied show loss-of-function features in at least one assay, suggesting that tRNA-charging deficits play a role in disease pathogenesis. Finally, we detected endogenous GARS-associated granules in the neurite projections of cultured neurons and in the peripheral nerve axons of normal human tissue. These data are particularly important in light of the recent identification of CMT-associated mutations in another tRNA synthetase gene [YARS (tyrosyl-tRNA synthetase gene)]. Together, these findings suggest that tRNA-charging enzymes play a key role in maintaining peripheral axons.

The effects of a dominant connexin32 mutant in myelinating Schwann cells

Mutations in GJB1, the gene encoding the gap junction protein connexin32 (Cx32), cause X-linked Charcot-Marie-Tooth disease, an inherited demyelinating peripheral neuropathy. We generated transgenic mice that express the R142W mutation in myelinating Schwann cells. The R142W mutant protein was aberrantly localized to the Golgi, indicating that it does not traffic properly, but the molecular organization of the myelin sheath, including the localization of Cx29, another connexin expressed by myelinating Schwann cells, was not disrupted. In a wild type background, this mutation dramatically decreased the level of wild type mouse Cx32 in immunoblots of sciatic nerve and caused demyelination. The expression of wild type human Cx32 with the same transgenic construct had different effects-increased amounts of Cx32, normal localization of Cx32 at nodes and incisures, and split myelin sheaths. Thus, the R142W mutant protein has dominant effects that are distinct from overexpression.

A motor neuron disease-associated mutation in p150Glued perturbs dynactin function and induces protein aggregation

The microtubule motor cytoplasmic dynein and its activator dynactin drive vesicular transport and mitotic spindle organization. Dynactin is ubiquitously expressed in eukaryotes, but a G59S mutation in the p150^Glued subunit of dynactin results in the specific degeneration of motor neurons. This mutation in the conserved cytoskeleton-associated protein, glycine-rich (CAP-Gly) domain lowers the affinity of p150^Glued for microtubules and EB1. Cell lines from patients are morphologically normal but show delayed recovery after nocodazole treatment, consistent with a subtle disruption of dynein/dynactin function. The G59S mutation disrupts the folding of the CAP-Gly domain, resulting in aggregation of the p150^Glued protein both in vitro and in vivo, which is accompanied by an increase in cell death in a motor neuron cell line. Overexpression of the chaperone Hsp70 inhibits aggregate formation and prevents cell death. These data support a model in which a point mutation in p150^Glued causes both loss of dynein/dynactin function and gain of toxic function, which together lead to motor neuron cell death.