Novel PRKCG/SCA14 mutation in a Dutch spinocerebellar ataxia family: expanding the phenotype

Tremor in Spinocerebellar Ataxia: A Scoping Review

Background

Spinocerebellar ataxia (SCA) denotes an expanding list of autosomal dominant cerebellar ataxias. Although tremor is an important aspect of the clinical spectrum of the SCAs, its prevalence, phenomenology, and pathophysiology are unknown.

Objectives

This review aims to describe the various types of tremors seen in the different SCAs, with a discussion on the pathophysiology of the tremors, and the possible treatment modalities.

Methods

The authors conducted a literature search on PubMed using search terms including tremor and the various SCAs. Relevant articles were included in the review after excluding duplicate publications.

Results

While action (postural and intention) tremors are most frequently associated with SCA, rest and other rare tremors have also been documented. The prevalence and types of tremors vary among the different SCAs. SCA12, common in certain ethnic populations, presents a unique situation, where the tremor is typically the principal manifestation. Clinical manifestations of SCAs may be confused with essential tremor or Parkinson's disease. The pathophysiology of tremors in SCAs predominantly involves the cerebellum and its networks, especially the cerebello-thalamo-cortical circuit. Additionally, connections with the basal ganglia, and striatal dopaminergic dysfunction may have a role. Medical management of tremor is usually guided by the phenomenology and associated clinical features. Deep brain stimulation surgery may be helpful in treatment-resistant tremors.

Conclusions

Tremor is an elemental component of SCAs, with diverse phenomenology, and emphasizes the role of the cerebellum in tremor. Further studies will be useful to delineate the clinical, pathophysiological, and therapeutic aspects of tremor in SCAs.

Mutations in protein kinase Cγ promote spinocerebellar ataxia type 14 by impairing kinase autoinhibition

Spinocerebellar ataxia type 14 (SCA14) is a neurodegenerative disease caused by germline variants in the diacylglycerol (DAG)/Ca2+-regulated protein kinase Cγ (PKCγ), leading to Purkinje cell degeneration and progressive cerebellar dysfunction. Most of the identified mutations cluster in the DAG-sensing C1 domains. Here, we found with a FRET-based activity reporter that SCA14-associated PKCγ mutations, including a previously undescribed variant, D115Y, enhanced the basal activity of the kinase by compromising its autoinhibition. Unlike other mutations in PKC that impair its autoinhibition but lead to its degradation, the C1 domain mutations protected PKCγ from such down-regulation. This enhanced basal signaling rewired the brain phosphoproteome, as revealed by phosphoproteomic analysis of cerebella from mice expressing a human SCA14-associated H101Y mutant PKCγ transgene. Mutations that induced a high basal activity in vitro were associated with earlier average age of onset in patients. Furthermore, the extent of disrupted autoinhibition, but not agonist-stimulated activity, correlated with disease severity. Molecular modeling indicated that almost all SCA14 variants not within the C1 domain were located at interfaces with the C1B domain, suggesting that mutations in and proximal to the C1B domain are a susceptibility for SCA14 because they uniquely enhance PKCγ basal activity while protecting the enzyme from down-regulation. These results provide insight into how PKCγ activation is modulated and how deregulation of the cerebellar phosphoproteome by SCA14-associated mutations affects disease progression.

Essential Tremor Within the Broader Context of Other Forms of Cerebellar Degeneration

Essential tremor (ET) has recently been reconceptualized by many as a degenerative disease of the cerebellum. Until now, though, there has been no attempt to frame it within the context of these diseases. Here, we compare the clinical and postmortem features of ET with other cerebellar degenerations, thereby placing it within the broader context of these diseases. Action tremor is the hallmark feature of ET. Although often underreported in the spinocerebellar ataxias (SCAs), action tremors occur, and it is noteworthy that in SCA12 and 15, they are highly prevalent, often severe, and can be the earliest disease manifestation, resulting in an initial diagnosis of ET in many cases. Intention tremor, sometimes referred to as "cerebellar tremor," is a common feature of ET and many SCAs. Other features of cerebellar dysfunction, gait ataxia and eye motion abnormalities, are seen to a mild degree in ET and more markedly in SCAs. Several SCAs (e.g., SCA5, 6, 14, and 15), like ET, follow a milder and more protracted disease course. In ET, numerous postmortem changes have been localized to the cerebellum and are largely confined to the cerebellar cortex, preserving the cerebellar nuclei. Purkinje cell loss is modest. Similarly, in SCA3, 12, and 15, Purkinje cell loss is limited, and in SCA12 and 15, there is preservation of cerebellar nuclei and relative sparing of other central nervous system regions. Both clinically and pathologically, there are numerous similarities and intersection points between ET and other disorders of cerebellar degeneration.

NGS in Hereditary Ataxia: When Rare Becomes Frequent

The term hereditary ataxia (HA) refers to a heterogeneous group of neurological disorders with multiple genetic etiologies and a wide spectrum of ataxia-dominated phenotypes. Massive gene analysis in next-generation sequencing has entered the HA scenario, broadening our genetic and clinical knowledge of these conditions. In this study, we employed a targeted resequencing panel (TRP) in a large and highly heterogeneous cohort of 377 patients with a clinical diagnosis of HA, but no molecular diagnosis on routine genetic tests. We obtained a positive result (genetic diagnosis) in 33.2% of the patients, a rate significantly higher than those reported in similar studies employing TRP (average 19.4%), and in line with those performed using exome sequencing (ES, average 34.6%). Moreover, 15.6% of the patients had an uncertain molecular diagnosis. STUB1, PRKCG, and SPG7 were the most common causative genes. A comparison with published literature data showed that our panel would have identified 97% of the positive cases reported in previous TRP-based studies and 92% of those diagnosed by ES. Proper use of multigene panels, when combined with detailed phenotypic data, seems to be even more efficient than ES in clinical practice.

Episodic ataxia and severe infantile phenotype in spinocerebellar ataxia type 14: expansion of the phenotype and novel mutations

Introduction

Spinocerebellar ataxia type 14 (SCA14) is a dominantly inherited neurological disorder characterized by slowly progressive cerebellar ataxia. SCA14 is caused by mutations in PRKCG, a gene encoding protein kinase C gamma (PKCγ), a master regulator of Purkinje cells development.

Methods

We performed next-generation sequencing targeted resequencing panel encompassing 273 ataxia genes in 358 patients with genetically undiagnosed ataxia.

Results

We identified fourteen patients in ten families harboring nine pathogenic heterozygous variants in PRKCG, seven of which were novel. We encountered four patients with not previously described phenotypes: one with episodic ataxia, one with a spastic paraparesis dominating her clinical manifestations, and two children with an unusually severe phenotype.

Conclusions

Our study broadens the genetic and clinical spectrum of SCA14.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00415-021-10712-5.

Spinocerebellar ataxia type 14: refining clinicogenetic diagnosis in a rare adult-onset disorder

Objectives

Genetic variant classification is a challenge in rare adult‐onset disorders as in SCA‐PRKCG (prior spinocerebellar ataxia type 14) with mostly private conventional mutations and nonspecific phenotype. We here propose a refined approach for clinicogenetic diagnosis by including protein modeling and provide for confirmed SCA‐PRKCG a comprehensive phenotype description from a German multi‐center cohort, including standardized 3D MR imaging.

Methods

This cross‐sectional study prospectively obtained neurological, neuropsychological, and brain imaging data in 33 PRKCG variant carriers. Protein modeling was added as a classification criterion in variants of uncertain significance (VUS).

Results

Our sample included 25 cases confirmed as SCA‐PRKCG (14 variants, thereof seven novel variants) and eight carriers of variants assigned as VUS (four variants) or benign/likely benign (two variants). Phenotype in SCA‐PRKCG included slowly progressive ataxia (onset at 4–50 years), preceded in some by early‐onset nonprogressive symptoms. Ataxia was often combined with action myoclonus, dystonia, or mild cognitive‐affective disturbance. Inspection of brain MRI revealed nonprogressive cerebellar atrophy. As a novel finding, a previously not described T2 hyperintense dentate nucleus was seen in all SCA‐PRKCG cases but in none of the controls.

Interpretation

In this largest cohort to date, SCA‐PRKCG was characterized as a slowly progressive cerebellar syndrome with some clinical and imaging features suggestive of a developmental disorder. The observed non‐ataxia movement disorders and cognitive‐affective disturbance may well be attributed to cerebellar pathology. Protein modeling emerged as a valuable diagnostic tool for variant classification and the newly described T2 hyperintense dentate sign could serve as a supportive diagnostic marker of SCA‐PRKCG.

Zebrafish Models of Autosomal Dominant Ataxias

Hereditary dominant ataxias are a heterogeneous group of neurodegenerative conditions causing cerebellar dysfunction and characterized by progressive motor incoordination. Despite many efforts put into the study of these diseases, there are no effective treatments yet. Zebrafish models are widely used to characterize neuronal disorders due to its conserved vertebrate genetics that easily support genetic edition and their optic transparency that allows observing the intact CNS and its connections. In addition, its small size and external fertilization help to develop high throughput assays of candidate drugs. Here, we discuss the contributions of zebrafish models to the study of dominant ataxias defining phenotypes, genetic function, behavior and possible treatments. In addition, we review the zebrafish models created for X-linked repeat expansion diseases X-fragile/fragile-X tremor ataxia. Most of the models reviewed here presented neuronal damage and locomotor deficits. However, there is a generalized lack of zebrafish adult heterozygous models and there are no knock-in zebrafish models available for these diseases. The models created for dominant ataxias helped to elucidate gene function and mechanisms that cause neuronal damage. In the future, the application of new genetic edition techniques would help to develop more accurate zebrafish models of dominant ataxias.

Myoclonus-Ataxia Syndromes: A Diagnostic Approach

Background

A myriad of disorders combine myoclonus and ataxia. Most causes are genetic and an increasing number of genes are being associated with myoclonus-ataxia syndromes (MAS), due to recent advances in genetic techniques. A proper etiologic diagnosis of MAS is clinically relevant, given the consequences for genetic counseling, treatment, and prognosis.

Objectives

To review the causes of MAS and to propose a diagnostic algorithm.

Methods

A comprehensive and structured literature search following PRISMA criteria was conducted to identify those disorders that may combine myoclonus with ataxia.

Results

A total of 135 causes of combined myoclonus and ataxia were identified, of which 30 were charted as the main causes of MAS. These include four acquired entities: opsoclonus-myoclonus-ataxia syndrome, celiac disease, multiple system atrophy, and sporadic prion diseases. The distinction between progressive myoclonus epilepsy and progressive myoclonus ataxia poses one of the main diagnostic dilemmas.

Conclusions

Diagnostic algorithms for pediatric and adult patients, based on clinical manifestations including epilepsy, are proposed to guide the differential diagnosis and corresponding work-up of the most important and frequent causes of MAS. A list of genes associated with MAS to guide genetic testing strategies is provided. Priority should be given to diagnose or exclude acquired or treatable disorders.

Paraneoplastic cerebellar degeneration associated with anti-protein kinase Cgamma antibodies in a Chinese patient

Anti-protein kinase Cgamma (anti-PKCγ) antibodies are rare onconeural antibodies associated with paraneoplastic cerebellar degeneration (PCD). To date, only two patients with PCD and anti-PKCγ antibodies have been reported. Here, we report a Chinese patient with PCD and anti-PKCγ antibodies. Screening for tumor revealed lymphoepithelial carcinoma in tonsil. The patient's symptoms improved gradually after radiotherapy for the lymphoepithelial carcinoma and intravenous immunoglobulin immunotherapy.

Spinocerebellar ataxia type 14 caused by a nonsense mutation in the PRKCG gene

Spinocerebellar ataxia type 14 (SCA14) is an autosomal dominant neurodegenerative disorder characterized by cerebellar ataxia with myoclonus, dystonia, spasticity, and rigidity. Although missense mutations and a deletion mutation have been found in the protein kinase C gamma (PRKCG) gene encoding protein kinase C γ (PKCγ) in SCA14 families, a nonsense mutation has not been reported. The patho-mechanisms underlying SCA14 remain poorly understood. However, gain-of-function mechanisms and loss-of-function mechanisms, but not dominant negative mechanisms, were reported the patho-mechanism of SCA14. We identified the c.226C>T mutation of PRKCG, which caused the p.R76X in PKCγ by whole-exome sequencing in patients presenting cerebellar atrophy with cognitive and hearing impairment. To investigate the patho-mechanism of our case, we studied aggregation formation, cell death, and PKC inhibitory effect by confocal microscopy, western blotting with cleaved caspase 3, and pSer PKC motif antibodies, respectively. PKCγ(R76X)-GFP have aggregations the same as wild-type (WT) PKCγ-GFP. The PKCγ(R76X)-GFP inhibited PKC phosphorylation activity more than GFP alone. It also induced more apoptosis in COS7 and SH-SY5Y cells compared to WT-PKCγ-GFP and GFP. We first reported SCA14 patients with p.R76X in PKCγ who have cerebellar atrophy with cognitive and hearing impairment. Our results suggest that a dominant negative mechanism due to truncated peptides produced by p.R76X may be at least partially responsible for the cerebellar atrophy.

SCA13 causes dominantly inherited non-progressive myoclonus ataxia

INTRODUCTION

Spinocerebellar ataxia 13 (SCA13) is a rare autosomal dominant cerebellar ataxia. To our knowledge, its association to movement disorders has never been described. We aimed at reporting 8 new SCA13 cases with a focus on movement disorders especially myoclonus.

METHODS

We performed a detailed neurological examination and neurophysiological recording in 8 patients consecutively diagnosed with SCA13 between December 2013 and October 2015 and followed up in two French tertiary centers.

RESULTS

We identified mild subcortical myoclonus in all patients, with a homogenous clinical and electrophysiological pattern. Myoclonus ataxia was very slowly progressive, like the other symptoms of the disease, whatever the age of onset. Patients with R423H mutation had an earlier age of onset than patients with R420H mutation.

CONCLUSIONS

Myoclonus appears to be frequent in SCA13. SCA13 should be considered facing non-progressive autosomal dominant myoclonus ataxia, and polymyographic recording should be included in the diagnosis work.

Cerebellar neurochemical alterations in spinocerebellar ataxia type 14 appear to include glutathione deficiency

Autosomal dominant ataxia type 14 (SCA14) is a rare usually adult-onset progressive disorder with cerebellar neurodegeneration caused by mutations in protein kinase C gamma. We set out to examine cerebellar and extracerebellar neurochemical changes in SCA14 by MR spectroscopy. In 13 SCA14 patients and 13 healthy sex- and age-matched controls, 3-T single-voxel brain proton MR spectroscopy was performed in a cerebellar voxel of interest (VOI) at TE = 30 ms to obtain a neurochemical profile of metabolites with short relaxation times. In the cerebellum and in additional VOIs in the prefrontal cortex, motor cortex, and somatosensory cortex, a second measurement was performed at TE = 144 ms to mainly extract the total N-acetyl-aspartate (tNAA) signal besides the signals for total creatine (tCr) and total choline (tCho). The cerebellar neurochemical profile revealed a decrease in glutathione (6.12E-06 ± 2.50E-06 versus 8.91E-06 ± 3.03E-06; p = 0028) and tNAA (3.78E-05 ± 5.67E-06 versus 4.25E-05 ± 5.15E-06; p = 0023) and a trend for reduced glutamate (2.63E-05 ± 6.48E-06 versus 3.15E-05 ± 7.61E-06; p = 0062) in SCA14 compared to controls. In the tNAA-focused measurement, cerebellar tNAA (296.6 ± 42.6 versus 351.7 ± 16.5; p = 0004) and tCr (272.1 ± 25.2 versus 303.2 ± 31.4; p = 0004) were reduced, while the prefrontal, somatosensory and motor cortex remained unaffected compared to controls. Neuronal pathology in SCA14 detected by MR spectroscopy was restricted to the cerebellum and did not comprise cortical regions. In the cerebellum, we found in addition to signs of neurodegeneration a glutathione reduction, which has been associated with cellular damage by oxidative stress in other neurodegenerative diseases such as Parkinson's disease and Friedreich's ataxia.

EFNS/ENS Consensus on the diagnosis and management of chronic ataxias in adulthood

BACKGROUND AND OBJECTIVES

The ataxias are a challenging group of neurological diseases due the aetiological heterogeneity and the complexity of the genetic subtypes. This guideline focuses on the heredodegenerative ataxias. The aim is to provide a peer-reviewed evidence-based guideline for clinical neurologists and other specialist physicians responsible for the care of patients with ataxia.

METHODS

This guideline is based on systematic evaluations of the relevant literature and on three consensus meetings of the task force.

DIAGNOSIS

If acquired causes are ruled out, and if the disease course is rather slowly progressive, a (heredo)degenerative disease is likely. A positive family history gives much guidance. In the case of a dominant family history, first line genetic screening is recommended for spinocerebellar ataxia (SCA) 1, 2, 3, 6, 7 and 17 (level B), and in Asian patients also for dentatorubral-pallidoluysian atrophy (DRPLA). In the case of recessive disease, a stepwise diagnostic work-up is recommended, including both biochemical markers and targeted genetic testing, particularly aimed at Friedreich's ataxia, ataxia telangiectasia, ataxia due to vitamin E deficiency, polymerase gamma gene (POLG gene, various mutations), autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) and ataxia with oculomotor apraxia (AOA) types 1 and 2. If family history is negative, we still advise to screen for the more common dominant and recessive ataxias. In addition, if onset is below 45 years we recommend the full work-up for recessive ataxias; if onset is above 45 years we recommend to screen for fragile X mental retardation 1 FMR1 premutations (good practice points). In sporadic cases with an onset after 30 years, a diagnosis of multiple system atrophy should be considered (good practice point). In particular the genetic work-up will change over the upcoming years due to the diagnostic utility of new techniques such as gene panel diagnostics based on next generation sequencing for routine work-up, or even whole exome and genome sequencing for selected cases.

TREATMENT

Some of the rare recessive ataxias are treatable, but for most of the heredodegenerative ataxias treatment is purely symptomatic. Idebenone is not effective in Friedreich's ataxia (level A). Riluzole (level B) and amantadine (level C) might provide symptomatic relief, irrespective of exact etiology. Also, varenicline for SCA3 patients (level B) can be considered. There is level Class II evidence to recommend physiotherapy, and Class III data to support occupational therapy.

A conserved eEF2 coding variant in SCA26 leads to loss of translational fidelity and increased susceptibility to proteostatic insult

The autosomal dominant spinocerebellar ataxias (SCAs) are a genetically heterogeneous group of disorders exhibiting cerebellar atrophy and Purkinje cell degeneration whose subtypes arise from 31 distinct genetic loci. Our group previously published the locus for SCA26 on chromosome 19p13.3. In this study, we performed targeted deep sequencing of the critical interval in order to identify candidate causative variants in individuals from the SCA26 family. We identified a single variant that co-segregates with the disease phenotype that produces a single amino acid substitution in eukaryotic elongation factor 2. This substitution, P596H, sits in a domain critical for maintaining reading frame during translation. The yeast equivalent, P580H EF2, demonstrated impaired translocation, detected as an increased rate of -1 programmed ribosomal frameshift read-through in a dual-luciferase assay for observing translational recoding. This substitution also results in a greater susceptibility to proteostatic disruption, as evidenced by a more robust activation of a reporter gene driven by unfolded protein response activation upon challenge with dithiothreitol or heat shock in our yeast model system. Our results present a compelling candidate mutation and mechanism for the pathogenesis of SCA26 and further support the role of proteostatic disruption in neurodegenerative diseases.

Brain pathology of spinocerebellar ataxias

The autosomal dominant cerebellar ataxias (ADCAs) represent a heterogeneous group of neurodegenerative diseases with progressive ataxia and cerebellar degeneration. The current classification of this disease group is based on the underlying genetic defects and their typical disease courses. According to this categorization, ADCAs are divided into the spinocerebellar ataxias (SCAs) with a progressive disease course, and the episodic ataxias (EA) with episodic occurrences of ataxia. The prominent disease symptoms of the currently known and genetically defined 31 SCA types result from damage to the cerebellum and interconnected brain grays and are often accompanied by more specific extra-cerebellar symptoms. In the present review, we report the genetic and clinical background of the known SCAs and present the state of neuropathological investigations of brain tissue from SCA patients in the final disease stages. Recent findings show that the brain is commonly seriously affected in the polyglutamine SCAs (i.e. SCA1, SCA2, SCA3, SCA6, SCA7, and SCA17) and that the patterns of brain damage in these diseases overlap considerably in patients suffering from advanced disease stages. In the more rarely occurring non-polyglutamine SCAs, post-mortem neuropathological data currently are scanty and investigations have been primarily performed in vivo by means of MRI brain imaging. Only a minority of SCAs exhibit symptoms and degenerative patterns allowing for a clear and unambiguous diagnosis of the disease, e.g. retinal degeneration in SCA7, tau aggregation in SCA11, dentate calcification in SCA20, protein depositions in the Purkinje cell layer in SCA31, azoospermia in SCA32, and neurocutaneous phenotype in SCA34. The disease proteins of polyglutamine ataxias and some non-polyglutamine ataxias aggregate as cytoplasmic or intranuclear inclusions and serve as morphological markers. Although inclusions may impair axonal transport, bind transcription factors, and block protein quality control, detailed molecular and pathogenetic consequences remain to be determined.

SCA14 in Norway, two families with autosomal dominant cerebellar ataxia and a novel mutation in the PRKCG gene

OBJECTIVES

Despite a similar prevalence of autosomal dominant cerebellar ataxia (ADCA) in Norway compared to other European countries, less than 10% of the families are explained by the CAG trinucleotide expansions. We wanted to find the occurence of SCA14 in the dominant ataxia population and describe the phenotype.

METHODS

We screened a large dominant cerebellar ataxia cohort for mutations in the PRKCG gene. Patients were evaluated according to a standard clinical protocol for ataxia patients.

RESULTS

A novel mutation was found in two families, a C to A transversion altering Histidine to a Glutamine at codon 139, located in a highly concerved region in the gene. It completely co-segregated with the affected family members and was not seen in 576 control chromosomes. Genetic analysis revealed common alleles at three microsatellite markers between these two families suggesting a shared ancestral chromosome. Affected subjects displayed a mild, slowly progressive cerebellar syndrome that included gait and limb ataxia and saccadic pursuit and head tremor in one. Age at onset ranged from 10 to 45 years.

CONCLUSIONS

These are the first families with SCA14 reported from Scandinavia and a new mutation in the PRKCG gene. The occurrence in the Norwegian dominant ataxia cohort is 3.5%.

Spinocerebellar ataxia type 14

SCA14 is an autosomal dominant hereditary cerebellar ataxia that usually has an onset in early to mid adult life, with slow progression and normal lifespan. Although generally an uncomplicated cerebellar ataxia with gait imbalance, dysarthria, and nystagmus, there is occasionally sensory loss, hyperactive tendon reflexes, cognitive decline, or myoclonus. Brain MRI shows cerebellar atrophy. A single autopsy has shown loss of cerebellar Purkinje cells. The disease is caused by mutations in the protein kinase C gamma (PKCγ, PRKCG) gene with a hotspot for mutations in exon 4. Genetic testing for SCA14 is clinically available.

Autosomal dominant cerebellar ataxias

Cerebellar ataxias with autosomal dominant transmission (ADCA) are far rarer than sporadic cases of cerebellar ataxia. The identification of genes involved in dominant forms has confirmed the genetic heterogeneity of these conditions and of the underlying mechanisms and pathways. To date, at least 28 genetic loci and, among them, 20 genes have been identified. In many instances, the phenotype is not restricted to cerebellar dysfunction but includes more complex multisystemic neurological deficits. Seven ADCA (SCA1, 2, 3, 6, 7, 17, and dentatorubro-pallido-luysian atrophy) are caused by repeat expansions in the corresponding proteins; phenotype-genotype correlations have shown that repeat size influences the progression of the disease, its severity and clinical differences among patients, including the phenomenon of anticipation between generations. All other ADCA are caused either by non-coding repeat expansions, conventional mutations or large rearrangements in genes with different functions. This review will focus on the genetic features of ADCA and on the clinical differences among the different forms.

Congo red, an amyloid-inhibiting compound, alleviates various types of cellular dysfunction triggered by mutant protein kinase cγ that causes spinocerebellar ataxia type 14 (SCA14) by inhibiting oligomerization and aggregation

Several missense mutations in the protein kinase Cγ (γPKC) gene have been found to cause spinocerebellar ataxia type 14 (SCA14), an autosomal dominant neurodegenerative disease. We previously demonstrated that the mutant γPKC found in SCA14 is susceptible to aggregation that induces apoptotic cell death. Congo red is widely used as a histological dye for amyloid detection. Recent evidence has revealed that Congo red has the property to inhibit amyloid oligomers and fibril formation of misfolded proteins. In the present study, we examine whether Congo red inhibits aggregate formation and cytotoxicity of mutant γPKC. Congo red likely inhibits aggregate formation of mutant γPKC – green fluorescent protein (GFP) without affecting its expression level in SH-SY5Y cells. Congo red counteracts the insolubilization of recombinant mutant γPKC, suggesting that the dye inhibits aggregation of mutant γPKC by a direct mechanism. Congo red also inhibits aggregation and oligomerization of mutant γPKC-GFP in primary cultured cerebellar Purkinje cells. Moreover, the dye reverses the improper development of dendrites and inhibits apoptotic cell death in Purkinje cells that express mutant γPKC-GFP. These results indicate that amyloid-inhibiting compounds like Congo red may be novel therapeutics for SCA14.

Effect of trehalose on the properties of mutant {gamma}PKC, which causes spinocerebellar ataxia type 14, in neuronal cell lines and cultured Purkinje cells

Several missense mutations in the protein kinase Cγ (γPKC) gene have been found to cause spinocerebellar ataxia type 14 (SCA14), an autosomal dominant neurodegenerative disease. We previously demonstrated that the mutant γPKC found in SCA14 is susceptible to aggregation, which induces apoptotic cell death. The disaccharide trehalose has been reported to inhibit aggregate formation and to alleviate symptoms in cellular and animal models of Huntington disease, Alzheimer disease, and prion disease. Here, we show that trehalose can be incorporated into SH-SY5Y cells and reduces the aggregation of mutant γPKC-GFP, thereby inhibiting apoptotic cell death in SH-SY5Y cells and primary cultured Purkinje cells (PCs). Trehalose acts by directly stabilizing the conformation of mutant γPKC without affecting protein turnover. Trehalose was also found to alleviate the improper development of dendrites in PCs expressing mutant γPKC-GFP without aggregates but not in PCs with aggregates. In PCs without aggregates, trehalose improves the mobility and translocation of mutant γPKC-GFP, probably by inhibiting oligomerization and thereby alleviating the improper development of dendrites. These results suggest that trehalose counteracts various cellular dysfunctions that are triggered by mutant γPKC in both neuronal cell lines and primary cultured PCs by inhibiting oligomerization and aggregation of mutant γPKC.

Mutant protein kinase C gamma that causes spinocerebellar ataxia type 14 (SCA14) is selectively degraded by autophagy

Several causal missense mutations in the protein kinase Cgamma (gammaPKC) gene have been found in spinocerebellar ataxia type 14 (SCA14), an autosomal dominant neurodegenerative disease. We previously showed that mutant gammaPKC found in SCA14 is susceptible to aggregation and causes apoptosis. Aggregation of misfolded proteins is generally involved in the pathogenesis of many neurodegenerative diseases. Growing evidence indicates that macroautophagy (autophagy) is important for the degradation of misfolded proteins and the prevention of neurodegenerative diseases. In the present study, we examined whether autophagy is involved in the degradation of the mutant gammaPKC that causes SCA14. Mutant gammaPKC-GFP was transiently expressed in SH-SY5Y cells by using an adenoviral tetracycline-regulated system. Subsequently, temporal changes in clearance of aggregates and degradation of gammaPKC-GFP were evaluated. Rapamycin, an autophagic inducer, accelerated clearance of aggregates and promoted degradation of mutant gammaPKC-GFP, but it did not affect degradation of wild-type gammaPKC-GFP. These effects of rapamycin were not observed in embryonic fibroblast cells from Atg5-deficient mice, which are not able to perform autophagy. Furthermore, lithium, another type of autophagic inducer, also promoted the clearance of mutant gammaPKC aggregates. These results indicate that autophagy contributes to the degradation of mutant gammaPKC, suggesting that autophagic inducers could provide therapeutic potential for SCA14.

The spinocerebellar ataxias

Slowly progressive ataxia accompanied by cerebellar degeneration is often genetic in origin. The past 15 years have witnessed a revolution in our understanding of the causes of dominantly inherited ataxias, now known as the spinocerebellar ataxias (SCAs). Nearly 30 distinct genetic causes of SCA are known, numbered chronologically in order of discovery. All SCAs display classic cerebellar signs, and many display disabling noncerebellar features, most commonly brainstem dysfunction. Eye movement abnormalities are common, reflecting cerebellar and brainstem degeneration. Visual loss from retinal degeneration is rare in SCA, occurring most commonly and profoundly in SCA7. Although the SCAs are relentlessly progressive and currently untreatable, recent scientific advances have begun to shed light on various disease mechanisms that may lead to preventive therapies.

Loss of protein kinase Cgamma in knockout mice and increased retinal sensitivity to hyperbaric oxygen

OBJECTIVE

To determine if loss of protein kinase Cgamma (PKCgamma) results in increased structural damage to the retina by hyperbaric oxygen (HBO), a treatment used for several ocular disorders.

METHODS

Six-week-old mice were exposed in vivo to 100% HBO 3 times a week for 8 weeks. Eyes were dissected, fixed, embedded in Epon, sectioned, stained with toluidine blue O, and examined by light microscopy.

RESULTS

The thicknesses of the inner nuclear and ganglion cell layers were increased. Destruction of the outer plexiform layer was observed in the retinas of the PKCgamma-knockout mice relative to control mice. Exposure to HBO caused significant degradation of the retina in knockout mice compared with control mice. Damage to the outer segments of the photoreceptor layer and ganglion cell layer was apparent in central retinas of HBO-treated knockout mice.

CONCLUSIONS

Protein kinase Cgamma-knockout mice had increased retinal sensitivity to HBO. Results demonstrate that PKCgamma protects retinas from HBO damage.

CLINICAL RELEVANCE

Care should be taken in treating patients with HBO, particularly if they have a genetic disease, such as spinocerebellar ataxia type 14, a condition in which the PKCgamma is mutated and nonfunctional.

Loss of Purkinje cells in the PKCgamma H101Y transgenic mouse

Spinocerebellar ataxia type 14 (SCA14) is an autosomal, dominant neurodegenerative disorder caused by mutations in PKCgamma. The objective of this study was to determine effects of PKCgamma H101Y SCA14 mutation on Purkinje cells in the transgenic mouse. Results demonstrated that wild type PKCgamma-like Purkinje cell localization of HA-tagged PKCgamma H101Y mutant proteins, altered morphology and loss of Purkinje cells were observed in the PKCgamma H101Y SCA14 transgenic mouse at four weeks of age. Failure of stereotypical clasping responses in the hind limbs of transgenic mice was also observed. Further, PKCgamma H101Y SCA14 mutation caused lack of total cellular PKCgamma enzyme activity, loss of connexin 57 phosphorylation on serines, and activation of caspase-12 in the PKCgamma H101Y SCA14 transgenic mouse. Results clearly demonstrate a need for PKCgamma control of gap junctions for maintenance of Purkinje cells. This is the first transgenic mouse to our knowledge which models a human SCA14 mutation.

Expansion of the phenotypic spectrum of SCA14 caused by the Gly128Asp mutation in PRKCG

Two cases of spinocerebellar ataxia type 14 (SCA14) with a G128D mutation in the protein kinase C gamma gene (PRKCG) without a definite family history have been reported previously. Here, we describe the first familial cases of SCA14 with a G128D mutation in PRKCG. Among three family members, the chief complaints varied and included ataxic gait, cervical dystonia, and positional vertigo. Moreover, retinal degeneration and facial muscle weakness were observed, although these are not expected to be present in SCA14. Cerebral blood flow evaluation using single photon emission computed tomography (SPECT) also differed among family members. It is possible that patients with the G128D mutation suffering from SCA14 may sometimes be classified as unaffected due to the varying clinical signs among family members.

PKC gamma mutations in spinocerebellar ataxia type 14 affect C1 domain accessibility and kinase activity leading to aberrant MAPK signaling

Spinocerebellar ataxia type 14 (SCA14) is a neurodegenerative disorder caused by mutations in the neuronal-specific protein kinase C gamma (PKCgamma) gene. Since most mutations causing SCA14 are located in the PKCgamma C1B regulatory subdomain, we investigated the impact of three C1B mutations on the intracellular kinetics, protein conformation and kinase activity of PKCgamma in living cells. SCA14 mutant PKCgamma proteins showed enhanced phorbol-ester-induced kinetics when compared with wild-type PKCgamma. The mutations led to a decrease in intramolecular FRET of PKCgamma, suggesting that they ;open' PKCgamma protein conformation leading to unmasking of the phorbol ester binding site in the C1 domain. Surprisingly, SCA14 mutant PKCgamma showed reduced kinase activity as measured by phosphorylation of PKC reporter MyrPalm-CKAR, as well as downstream components of the MAPK signaling pathway. Together, these results show that SCA14 mutations located in the C1B subdomain ;open' PKCgamma protein conformation leading to increased C1 domain accessibility, but inefficient activation of downstream signaling pathways.

Enzymological analysis of mutant protein kinase Cgamma causing spinocerebellar ataxia type 14 and dysfunction in Ca2+ homeostasis

Spinocerebellar ataxia type 14 (SCA14) is an autosomal dominant neurodegenerative disease caused by mutations in protein kinase Cgamma (PKCgamma). Interestingly, 18 of 22 mutations are concentrated in the C1 domain, which binds diacylglycerol and is necessary for translocation and regulation of PKCgamma kinase activity. To determine the effect of these mutations on PKCgamma function and the pathology of SCA14, we investigated the enzymological properties of the mutant PKCgammas. We found that wild-type PKCgamma, but not C1 domain mutants, inhibits Ca2+ influx in response to muscarinic receptor stimulation. The sustained Ca2+ influx induced by muscarinic receptor ligation caused prolonged membrane localization of mutant PKCgamma. Pharmacological experiments showed that canonical transient receptor potential (TRPC) channels are responsible for the Ca2+ influx regulated by PKCgamma. Although in vitro kinase assays revealed that most C1 domain mutants are constitutively active, they could not phosphorylate TRPC3 channels in vivo. Single molecule observation by the total internal reflection fluorescence microscopy revealed that the membrane residence time of mutant PKCgammas was significantly shorter than that of the wild-type. This fact indicated that, although membrane association of the C1 domain mutants was apparently prolonged, these mutants have a reduced ability to bind diacylglycerol and be retained on the plasma membrane. As a result, they fail to phosphorylate TRPC channels, resulting in sustained Ca2+ entry. Such an alteration in Ca2+ homeostasis and Ca2+-mediated signaling in Purkinje cells may contribute to the neurodegeneration characteristic of SCA14.

Spectrum and prevalence of autosomal dominant spinocerebellar ataxia in Hokkaido, the northern island of Japan: a study of 113 Japanese families

Autosomal dominant cerebellar ataxia (ADCA) is a genetically heterogeneous group of neurodegenerative disorders. To shed further light on the clinical and genetic spectrum of ADCA in Japan, we conducted a study to determine the frequency of a new variety of different subtypes of SCAs among ADCA patients. This current study was carried out from April 1999 to December 2006 on the basis of patients with symptoms and signs of ADCA disorders. PCR and/or direct sequencing were evaluated in a total of 113 families. Among them, 35 families were found to have the mutation associated with SCA6, 30 with SCA3, 11 with SCA1, five with SCA2, five with DRPLA, and one with SCA14. We also detected the heterozygous -16C --> T single nucleotide substitution within the puratrophin-1 gene responsible for 16q22.1-linked ADCA in ten families. In this study, unusual varieties of SCA, including 27, 13, 5, 7, 8, 12, 17, and 16 were not found. Of the 113 patients, 14% had as yet unidentified ADCA mutations. The present study validates the prevalence of genetically distinct ADCA subtypes based on ethnic origin and geographical variation, and shows that 16q-linked ADCA has strong hereditary effects in patients with ADCAs in Japan.

Protection from ataxia-linked apoptosis by gap junction inhibitors

Mutations in the protein kinase C gamma (PKCgamma) gene cause spinocerebellar ataxia type 14 (SCA14), a heterogeneous neurodegenerative disorder. Synthetic peptides (C1B1) serve as gap junction inhibitors through activation of PKCgamma control of gap junctions. We investigated the neuroprotective potential of these peptides against SCA14 mutation-induced cell death using neuronal HT22 cells. The C1B1 synthetic peptides completely restored PKCgamma enzyme activity and subsequent control of gap junctions. PKCgamma SCA14 mutant proteins were shown to cause aggregation which initially resulted in endoplasmic reticulum (ER) stress and cell apoptosis as demonstrated by phosphorylation of PERK on Thr981, activation of caspase-12, increases in BiP/GRP78 protein levels, and consequent activation of caspase-3. Pre-incubation with C1B1 peptides completely abolished these SCA14 effects on ER stress and caspase-3 activation, suggesting that C1B1 peptides protect cells from apoptosis through inhibition of gap junctions by restoration of PKCgamma control of gap junctions, which may result in neuroprotection in SCA14.

PRKCG mutation (SCA-14) causing a Ramsay Hunt phenotype

Progressive myoclonic ataxia, also referred to as Ramsay Hunt syndrome, is characterized by a combination of myoclonus and cerebellar ataxia, infrequently accompanied by tonic-clonic seizures. Its differential diagnosis overlaps with progressive myoclonic epilepsy, a syndrome with myoclonus, tonic-clonic seizures, progressive ataxia and dementia. In patients with progressive myoclonic epilepsy, specific diseases can frequently be recognized, but the diagnostic yield in progressive myoclonic ataxia is much lower. We describe a patient who presented with multifocal myoclonus in his thirties and who later developed cerebellar ataxia and focal dystonia. His father was similarly affected. Genetic studies revealed a mutation in the protein kinase C gamma (PRKCG) gene, known to cause spinocerebellar ataxia type 14 (SCA-14). This case illustrates that both myoclonus and dystonia are part of the clinical spectrum in SCA-14 and that myoclonus can even be the presenting symptom. We suggest that SCA-14 should be considered in the differential diagnosis of progressive myoclonic ataxia.

Spinocerebellar ataxias: an update

PURPOSE OF REVIEW

Here we discuss recent advances regarding the molecular genetic basis of dominantly inherited ataxias.

RECENT FINDINGS

Important recent observations include insights into the mechanisms by which expanded polyglutamine causes cerebellar degeneration; new findings regarding how noncoding expansions may cause disease; the discovery that conventional (i.e. nonrepeat) mutations underlie recently identified ataxias; and growing recognition that multiple biological pathways, when perturbed, can cause cerebellar degeneration.

SUMMARY

The dominant ataxias, also known as spinocerebellar ataxias, continue to grow in number. Here we review the major categories of spinocerebellar ataxias: expanded polyglutamine ataxias; noncoding repeat ataxias; and ataxias caused by conventional mutations. After discussing features shared by these disorders, we present recent evidence supporting a toxic protein mechanism for the polyglutamine spinocerebellar ataxias and the recognition that both protein misfolding and perturbations in nuclear events represent key events in pathogenesis. Less is known about pathogenic mechanisms in spinocerebellar ataxias due to noncoding repeats, though a toxic RNA effect remains possible. Newly discovered, conventional mutations in spinocerebellar ataxias suggest a wide range of biological pathways can be disrupted to cause progressive ataxia. Finally, we discuss how new mechanistic insights can drive the push toward preventive treatment.

Protein kinase C gamma mutations in the C1B domain cause caspase-3-linked apoptosis in lens epithelial cells through gap junctions

Failure to control oxidative stress is closely related to aging and to a diverse range of human diseases. We have reported that protein kinase C gamma (PKCgamma) acts as a primary oxidative stress sensor in the lens. PKCgamma has a Zn-finger C1B stress switch domain, residues 101-150. Mutation, H101Y, in the C1B domain of PKCgamma proteins causes a failure of the PKCgamma oxidative stress response [Lin, D., Takemoto, D.J., 2005. Oxidative activation of protein kinase Cgamma through the C1 domain. Effects on gap junctions. J. Biol. Chem. 280, 13682-13693]. Some human neurodegenerative spinocerebellar ataxia type 14 are caused by mutations in the PKCgamma C1B domain. In the current study we have investigated the effects of these mutations on lens epithelial cell responses to oxidative stress. The results demonstrate that PKCgamma C1B mutants had lower basal enzyme activities and were not activated by H(2)O(2). Furthermore, the PKCgamma mutations caused a failure of endogenous wild type PKCgamma to be activated by H(2)O(2). These PKCgamma mutations abolished the effect of H(2)O(2) on phosphorylation of Cx43 and Cx50 by H(2)O(2) activation of PKCgamma. The cells with PKCgamma C1B mutations had more Cx43 and/or Cx50 gap junction plaques which were not decreased by H(2)O(2). Since open gap junctions could have a bystander effect this could cause apoptosis to occur. H(2)O(2) (100 microM, 3 h) activated a caspase-3 apoptotic pathway in the lens epithelial cells but was more severe in cells expressing PKCgamma mutations. The presence of 18alpha-glycyrrhetinic acid (AGA), an inhibitor of gap junctions, decreased Cx43 and Cx50 protein levels and gap junction plaque number. This reduction in gap junctions by AGA resulted in inhibition of H(2)O(2)-induced apoptosis. Our results demonstrate that there is a dominant negative effect of PKCgamma C1B mutations on endogenous PKCgamma which results in loss of control of gap junctions. Modeled structures suggest that the severity of C1B mutation effects may be related to the extent of loss of C1B structure. Mutations in the C1B domain of PKCgamma result in increased apoptosis in lens epithelial cells. This can be prevented by a gap junction inhibitor. Thus, propagation of apoptosis from cell-to-cell in lens epithelial cells may be through open gap junctions. The control of gap junctions requires PKCgamma.

Spinocerebellar ataxia 14: Novel mutation in exon 2 ofPRKCG in a German family

We describe a novel mutation in the gene coding for protein kinase C gamma (PRKCG) in patients of a German family affected with slowly progressive gait ataxia, dysarthria, and nystagmus. The G/T missense mutation occurred in exon 2 of PRKCG and results in a substitution of glycine by valine (G63V) in the evolutionarily highly conserved cysteine-rich region 1/C1 domain of PRKCG. Among the 20 mutations described to date, this is the first mutation located in exon 2 of PRKCG.