Ataxia with oculomotor apraxia type 2: clinical, biological and genotype/phenotype correlation study of a cohort of 90 patients

Progressive cerebellar atrophy: hereditary ataxias and disorders with spinocerebellar degeneration

The hereditary ataxias with onset in childhood are a group of heterogeneous disorders, usually with autosomal recessive inheritance. In many of them, magnetic resonance imaging (MRI) shows cerebellar atrophy. The most prominent exception to this is Friedreich's ataxia, where MRI shows normal cerebellar volume, but sometimes spinal cord atrophy. In several of the hereditary ataxias, the causative gene plays an important role in DNA repair: ataxia telangiectasia and ataxia telangiectasia-like disorder, and ataxia with oculomotor apraxia type I and II. Mitochondrial metabolism is impaired in another group of inherited ataxias including the emergent group of defects in coenzyme Q10 synthesis. Few of these disorders are amenable to effective treatment, the most important of these being vitamin E-responsive ataxia. The autosomal dominant spinocerebellar ataxias are rare in childhood. Some of them, especially SCA7 and SCA2, may begin in childhood or even infancy, family history being positive in these cases. Additional clinical clues such as presence or absence of neuropathy or oculomotor apraxia still help in making a definitive diagnosis albeit there are still many unsolved cases. In pontocerebellar hypoplasia, a neurodegenerative disease with prenatal onset, the genetic basis of the different subtypes has recently been elucidated and involves genes with different functions.

Peripheral nerve involvement in hereditary cerebellar and multisystem degenerative disorders

Hereditary ataxias (HA) encompass an increasing number of degenerative disorders characterized by progressive cerebellar ataxia usually accompanied by extracerebellar semeiology including peripheral nerve involvement. Classically, HA were classified according to their pathological hallmark comprising three main forms: (1) spinal form predominantly with degeneration of spinocerebellar tracts, posterior columns, and pyramidal tracts (Friedreich's ataxia, FA); (2) olivopontocerebellar atrophy (OPCA); and (3) cortical cerebellar atrophy (CCA). In the 1980s Harding proposed a clinico-genetic classification based upon age of onset, modality of transmission, and clinical semeiology. The main categories in this classification were as follows: (1) early onset cerebellar ataxia (EOCA) with age of onset below 25 years and usually with autosomal recessive (AR) transmission (this group encompasses FA and syndromes different from FA); (2) autosomal dominant cerebellar ataxia (ADCA) with adult onset and with either cerebellar-plus syndrome or pure cerebellar semeiology; and (3) idiopathic late onset onset cerebellar ataxia (ILOCA). With the advent of molecular genetics, the nosology of HA has been in a state of constant flux. At present EOCA comprises at least 17 genotypes (designated with the acronym of ARCA derived from AR cerebellar ataxia), whereas under the umbrella of ADCA 30 genotypes have been reported. In this chapter we will review peripheral nerve involvement in classical pathological entities (OPCA and CCA), ARCA, ADCA, and ILOCA paying special attention to the most prevalent syndromes in each category. As a general rule, nerve involvement is relatively common in any form of ataxia except ILOCA, the most common pattern being either sensory or sensorimotor neuronopathy with a dying-back process. An exception to this rule is AR spastic ataxia of Charlevoix-Saguenay where nerve conduction studies show the characteristic pattern of intermediate neuropathy implying that sacsin mutation causes both axonal and Schwann cell dysfunction.

The SETX missense variation spectrum as evaluated in patients with ALS4-like motor neuron diseases

Mutations in the senataxin (SETX) gene can cause amyotrophic lateral sclerosis 4 (ALS4), an autosomal dominant form of juvenile onset amyotrophic lateral sclerosis, or result in autosomal recessive ataxia with oculomotor apraxia type 2. Great caution regarding the possible disease causation, especially of missense variations, has to be taken. Here, we evaluated the significance of all previously reported SETX missense mutations as well as six newly identified variations in 54 patients suspected of having ALS4. Yet, epidemiologic and in silico evidence indicates that all newly identified variations and two previously published ALS4-related missense variations (C1554G and I2547T) are most likely non-pathogenic, demonstrating the problems of interpretation of SETX missense alleles in the absence of functional assays.

Recent advances in the genetics of cerebellar ataxias

The hereditary cerebellar ataxias are a clinically and genetically heterogeneous group of disorders that primarily affect the cerebellum; often there are additional features such as neuropathy, cognitive decline, or maculopathy that help define the clinical subtype of ataxia. They are commonly classified according to their mode of inheritance into autosomal dominant, autosomal recessive, X-linked, and mitochondrial forms. Great advances have been made in understanding the genetics of cerebellar ataxias in the last 15 years. At least 36 different forms of ADCA are known, 20 autosomal-recessive, two X-linked, and several forms of ataxia associated with mitochondrial defects are known to date. However, in about 40 % of suspected genetically determined ataxia cases, the underlying genetic defect remains undetermined. Although the majority of disease genes have been found in the last two decades, over the last 2 years the genetics has undergone a methodological revolution. New DNA sequencing technologies are enabling us to investigate the whole or large targeted proportions of the genome in a rapid, affordable, and comprehensive way. Exome and targeted sequencing has recently identified four new genes causing ataxia: TGM6, ANO10, SYT14, and rundataxin. This approach is likely to continue to discover new ataxia genes and make screening of existing genes more effective. Translating the genetic findings into isolated and overlapping disease pathways will help stratify patient groups and identify therapeutic targets for ataxia that have so far remained undiscovered.

Mutations in CIZ1 cause adult onset primary cervical dystonia

OBJECTIVE

Primary dystonia is usually of adult onset, can be familial, and frequently involves the cervical musculature. Our goal was to identify the causal mutation in a family with adult onset, primary cervical dystonia.

METHODS

Linkage and haplotype analyses were combined with solution-based whole-exome capture and massively parallel sequencing in a large Caucasian pedigree with adult onset, primary cervical dystonia to identify a cosegregating mutation. High-throughput screening and Sanger sequencing were completed in 308 Caucasians with familial or sporadic adult onset cervical dystonia and matching controls for sequence variants in this mutant gene.

RESULTS

Exome sequencing led to the identification of an exonic splicing enhancer mutation in exon 7 of CIZ1 (c.790A>G, p.S264G), which encodes CIZ1, Cip1-interacting zinc finger protein 1. CIZ1 is a p21(Cip1/Waf1) -interacting zinc finger protein expressed in brain and involved in DNA synthesis and cell-cycle control. Using a minigene assay, we showed that c.790A>G altered CIZ1 splicing patterns. The p.S264G mutation also altered the nuclear localization of CIZ1. Screening in subjects with adult-onset cervical dystonia identified 2 additional CIZ1 missense mutations (p.P47S and p.R672M).

INTERPRETATION

Mutations in CIZ1 may cause adult onset, primary cervical dystonia, possibly by precipitating neurodevelopmental abnormalities that manifest in adults and/or G1/S cell-cycle dysregulation in the mature central nervous system.

Mutations in rare ataxia genes are uncommon causes of sporadic cerebellar ataxia

BACKGROUND

Sporadic-onset ataxia is common in a tertiary care setting but a significant percentage remains unidentified despite extensive evaluation. Rare genetic ataxias, reported only in specific populations or families, may contribute to a percentage of sporadic ataxia.

METHODS

Patients with adult-onset sporadic ataxia, who tested negative for common genetic ataxias (SCA1, SCA2, SCA3, SCA6, SCA7, and/or Friedreich ataxia), were evaluated using a stratified screening approach for variants in 7 rare ataxia genes.

RESULTS

We screened patients for published mutations in SYNE1 (n = 80) and TGM6 (n = 118), copy number variations in LMNB1 (n = 40) and SETX (n = 11), sequence variants in SACS (n = 39) and PDYN (n = 119), and the pentanucleotide insertion of spinocerebellar ataxia type 31 (n = 101). Overall, we identified 1 patient with a LMNB1 duplication, 1 patient with a PDYN variant, and 1 compound SACS heterozygote, including a novel variant.

CONCLUSIONS

The rare genetic ataxias examined here do not significantly contribute to sporadic cerebellar ataxia in our tertiary care population.

SETX gene mutation in a family diagnosed autosomal dominant proximal spinal muscular atrophy

Autosomal dominant proximal spinal muscular atrophy (ADSMA) is a rare disorder with unknown gene defects in the majority of families. Here we describe a family where the diagnosis of juvenile and adult onset ADSMA was made in three individuals. Because of retained tendon reflexes an atypical course of juvenile amyotrophic lateral sclerosis (ALS4) was considered. SETX gene sequencing revealed the previously reported heterozygous missense mutation c.1166T<C, L389S in the patients. Moreover the index patient and his sister had an earlier age at onset (10 and 15 years) and a more pronounced weakness as compared to their father with an age at onset of 35 years. Both sibs additionally carried a second SETX missense mutation of unknown function V891A in trans. Altogether these results expand the phenotype associated with SETX mutations supporting the notion that patients with ADSMA should be investigated for SETX mutations.

Update on degenerative ataxias

PURPOSE OF REVIEW

Degenerative ataxias are a heterogeneous group of disorders that are clinically characterized by progressive ataxia. They can be subdivided into three major groups: the acquired ataxias, which are due to exogenous or endogenous nongenetic causes, the hereditary ataxias, and the nonhereditary degenerative ataxias. On the basis of a review of the literature published in 2009 and 2010, this review gives an update of the most recent developments in the field of ataxia.

RECENT FINDINGS

Using advanced methods of molecular genetic analysis, novel genes for recessive and dominant ataxias were identified. Recent imaging studies in dominantly inherited spinocerebellar ataxias (SCAs) focussed on the analysis of connectivity in the brain. Novel clinical assessment methods were developed and validated in large patient cohorts. Although a phase 3 trial of idebenone in Friedreich ataxia (FRDA) failed, a smaller phase 2 trial of riluzole in a mixed population of ataxia patients suggested a possible antiataxic action of this compound.

SUMMARY

Recent molecular advances underline the diversity of degenerative ataxias. With the progress in the development of clinical assessment methods for ataxia, the methodological requirements to run large interventional trials are now met.

Milestones in ataxia

The past 25 years have seen enormous progress in the deciphering of the genetic and molecular basis of ataxias, resulting in improved understanding of their pathogenesis. The most significant milestones during this period were the cloning of the genes associated with the common spinocerebellar ataxias, ataxia telangiectasia, and Friedreich ataxia. To date, the causative mutations of more than 30 spinocerebellar ataxias and 20 recessive ataxias have been identified. In addition, there are numerous acquired ataxias with defined molecular causes, so that the entire number of distinct ataxia disorders exceeds 50 and possibly approaches 100. Despite this enormous heterogeneity, a few recurrent pathophysiological themes stand out. These include protein aggregation, failure of protein homeostasis, perturbations in ion channel function, defects in DNA repair, and mitochondrial dysfunction. The clinical phenotypes of the most common ataxia disorders have been firmly established, and their natural history is being studied in ongoing large observational trials. Effective therapies for ataxias are still lacking. However, novel drug targets are under investigation, and it is expected that there will be an increasing number of therapeutic trials in ataxia.

A girl with an atypical form of ataxia telangiectasia and an additional de novo 3.14 Mb microduplication in region 19q12

A 9-year-old girl born to healthy parents showed manifestations suggestive of ataxia telangiectasia (AT), such as short stature, sudden short bouts of horizontal and rotary nystagmus, a weak and dysarthric voice, rolling gait, unstable posture, and atactic movements. She did not show several cardinal features typical of AT such as frequent, severe infections of the respiratory tract. In contrast, she showed symptoms not generally related to AT, including microcephaly, profound motor and mental retardation, small hands and feet, severely and progressively reduced muscle tone with slackly protruding abdomen and undue drooling, excess fat on her upper arms, and severe oligoarthritis. A cranial MRI showed no cerebellar hypoplasia and other abnormalities. In peripheral blood samples she carried a de novo duplication of 3.14 Mb in chromosomal region 19q12 containing six annotated genes, UQCRFS1, VSTM2B, POP4, PLEKHF1, CCNE1, and ZNF536, and a de novo mosaic inversion 14q11q32 (96% of metaphases). In a saliva-derived DNA sample only the duplication in 19q12 was detected, suggesting that the rearrangements in blood lymphocytes were acquired. These findings reinforced the suspicion that she had AT. AT was confirmed by strongly elevated serum AFP levels, cellular radiosensitivity and two inherited mutations in the ATM gene (c.510_511delGT; paternal origin and c.2922-50_2940del69; maternal origin). This case suggest that a defective ATM-dependent DNA damage response may entail additional stochastic genomic rearrangements. Screening for genomic rearrangements appears indicated in patients suspected of defective DNA damage responses.

Sen-sing RNA terminators

In this issue of Molecular Cell,Skourti-Stathaki et al. (2011) report that human Senataxin, like its yeast homolog Sen1, promotes termination by RNA polymerase II and resolves RNA/DNA duplexes formed during transcription. Their results may help uncover a cause of motor neuron degeneration.

Interpretation of genetic testing: variants of unknown significance

As the number of genes available for commercial sequencing increases and the promise of clinical whole-genome sequencing becomes a reality, the interpretation of the results of these tests becomes more challenging for the practicing neurologist as these studies have the potential to detect novel genetic variants. Such reports are becoming more frequent in general practice, and neurologists are often left to puzzle over the relevance of these "variants of unknown significance," as such genetic changes are often described, and how to communicate this information to the patients and their families. This article will briefly illustrate how clinicians can use such results in the care of their patients. Only genetic variants involving coding sequence will be considered, although similar methods may also be applied to changes such as noncoding alterations or copy number variations. It is also important to note that in some cases, particularly those involving tests that only sequence select exons, negative test results may also require special interpretation.

The functional neuroanatomy of dystonia

Dystonia is a neurological disorder characterized by involuntary twisting movements and postures. There are many different clinical manifestations, and many different causes. The neuroanatomical substrates for dystonia are only partly understood. Although the traditional view localizes dystonia to basal ganglia circuits, there is increasing recognition that this view is inadequate for accommodating a substantial portion of available clinical and experimental evidence. A model in which several brain regions play a role in a network better accommodates the evidence. This network model accommodates neuropathological and neuroimaging evidence that dystonia may be associated with abnormalities in multiple different brain regions. It also accommodates animal studies showing that dystonic movements arise with manipulations of different brain regions. It is consistent with neurophysiological evidence suggesting defects in neural inhibitory processes, sensorimotor integration, and maladaptive plasticity. Finally, it may explain neurosurgical experience showing that targeting the basal ganglia is effective only for certain subpopulations of dystonia. Most importantly, the network model provides many new and testable hypotheses with direct relevance for new treatment strategies that go beyond the basal ganglia. This article is part of a Special Issue entitled "Advances in dystonia".

[Peripheral neuropathies associated with hereditary cerebellar ataxias]

Inherited cerebellar ataxias constitute a complicated and heterogeneous group of neurodegenerative disorders affecting the cerebellum and/or spinocerebellar tract, spinal cord and peripheral nerves. A peripheral neuropathy is frequently seen in inherited cerebellar ataxias although it rarely reveals the disease. Moreover, the peripheral neuropathy is helpful for the diagnostic procedure and contributes to the functional prognosis of the disease. Thus, electroneuromyography is essential in the algorithm for the diagnosis of inherited cerebellar ataxias, as well as brain MRI (looking especially for cerebellar atrophy) and the assessment of several biomarkers (alpha-foetoprotein, vitamin E, albumin, LDL cholesterol, lactic acid, phytanic acid).

Senataxin mutations and amyotrophic lateral sclerosis

We studied three patients with mutations in the senataxin gene (SETX). One had juvenile onset of ALS. The second case resembled hereditary motor neuropathy. The third patient had an overlap syndrome of ataxia-tremor and motor neuron disease, phenotypes previously associated with SETX mutations. Our patients were all apparently sporadic, with no other affected relative. Two relatives of patient no. 2 carried the SETX c.4660T > G transversion but did not manifest motor neuron disease, abnormal eye movements, ataxia, or tremor suggesting that genetic or environmental modifiers may influence expression of this SETX polymorphism. Relatives of patients 1 and 3 were not available for examination or SETX mutation screening. Mutations causing ALS4 may be more frequent and heterogeneous than expected. Screening for SETX mutations should be considered in patients with apparently sporadic juvenile-onset ALS, hereditary motor neuropathy, and overlap syndromes with ataxia and motor neuron disease.

[Autosomal recessive cerebellar ataxias]

INTRODUCTION

Autosomal recessive cerebellar ataxias (ARCA) are heterogeneous and complex inherited neurodegenerative diseases that may affect the cerebellum and/or the spinocerebellar tract, the posterior column of the spinal cord and the peripheral nerves. Cerebellar ataxia is frequently proeminent and mostly associated with several neurological or extra-neurological signs, leading to a major disability before the age of 30.

STATE OF ART

Friedreich's ataxia (FRDA) is clearly the most frequent ARCA and several rarer entities have been described during the past fifteen years such as ataxia with oculomotor apraxia type 1 (AOA1) and type 2 (AOA2), ataxia with vitamin E deficiency (AVED) and autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS). The ACAR are characterized by both allelic and non-allelic genetic heterogeneity. They may be divided into three groups: spino-cerebellar ataxia with pure sensory neuropathy; cerebellar ataxia with sensori-motor axonal neuropathy; pure cerebellar ataxia (i.e. ataxia of purely cerebellar origin that may be associated with other symptoms). Common physiological pathways are involved in several ARCA, such as DNA repair deficiency (AOA1, ataxia telangiectasia [AT]…), RNA termination disorder (AOA2), mitochondrial defect (FRDA, sensory ataxic neuropathy with dysarthria and ophthalmoplegia [Sando]…), lipoprotein assembly defects (AVED, abetalipoproteinemia [ABL]), chaperon protein disorders (ARSACS, Marinesco-Sjögren syndrome [MSS]) or peroxysomal diseases (Refsum disease [RD]).

PERSPECTIVES

New nanotechnology methods and high throughput gene analysis as well as bioinformatics should lead to the identification of several new ARCAs in the next few years despite the rarity of these entities. However, the challenge of the next decades will be the discovery of efficient treatments for these disabling neurodegenerative disorders.

CONCLUSION

Clinicians should be aware of the more frequent ARCAs, especially FRDA, in addition to ARCAs for which treatment is available (FRDA, AVED, ABL and RD for instance).

(1)H MR spectroscopy in Friedreich's ataxia and ataxia with oculomotor apraxia type 2

BACKGROUND AND AIM

Friedreich's ataxia (FRDA) and ataxia with oculomotor apraxia type 2 (AOA2) are the two most frequent forms of autosomal recessive cerebellar ataxias. However, brain metabolism in these disorders is poorly characterized and biomarkers of the disease progression are lacking. We aimed at assessing the neurochemical profile of the pons, the cerebellar hemisphere and the vermis in patients with FRDA and AOA2 to identify potential biomarkers of these diseases.

METHODS

Short-echo, single-voxel proton ((1)H) magnetic resonance spectroscopy data were acquired from 8 volunteers with FRDA, 9 volunteers with AOA2, and 38 control volunteers at 4T. Disease severity was assessed by the Friedreich's Ataxia Rating Scale (FARS).

RESULTS

Neuronal loss/dysfunction was indicated in the cerebellar vermis and hemispheres in both diseases by lower total N-acetylaspartate levels than controls. The putative gliosis marker myo-inositol was higher than controls in the vermis and pons in AOA2 and in the vermis in FRDA. Total creatine, another potential gliosis marker, was higher in the cerebellar hemispheres in FRDA relative to controls. Higher glutamine in FRDA and lower glutamate in AOA2 than controls were observed in the vermis, indicating different mechanisms possibly leading to altered glutamatergic neurotransmission. In AOA2, total N-acetylaspartate levels in the cerebellum strongly correlated with the FARS score (p<0.01).

CONCLUSION

Distinct neurochemical patterns were observed in the two patient populations, warranting further studies with larger patient populations to determine if the alterations in metabolite levels observed here may be utilized to monitor disease progression and treatment.

RNA processing pathways in amyotrophic lateral sclerosis

RNA processing is a tightly regulated, highly complex pathway which includes RNA transcription, pre-mRNA splicing, editing, transportation, translation, and degradation of RNA. Over the past few years, several RNA processing genes have been shown to be mutated or genetically associated with amyotrophic lateral sclerosis (ALS), including the RNA-binding proteins TDP-43 and FUS/TLS. These findings suggest that RNA processing may represent a common pathogenic mechanism involved in development of ALS. In this review, we will discuss six ALS-related, RNA processing genes including their discovery, function, and commonalities.