Episodic Ataxias: Faux or Real?

Inherited episodic ataxia type 2 in pregnancy: A case report

An optimal materno‐neonatal outcome was obtained in a pregnancy with inherited episodic ataxia through a multidisciplinary team handling timing and mode of delivery.

Integration of multi-omics technologies for molecular diagnosis in ataxia patients

Background: Episodic ataxias are rare neurological disorders characterized by recurring episodes of imbalance and coordination difficulties. Obtaining definitive molecular diagnoses poses challenges, as clinical presentation is highly heterogeneous, and literature on the underlying genetics is limited. While the advent of high-throughput sequencing technologies has significantly contributed to Mendelian disorders genetics, interpretation of variants of uncertain significance and other limitations inherent to individual methods still leaves many patients undiagnosed. This study aimed to investigate the utility of multi-omics for the identification and validation of molecular candidates in a cohort of complex cases of ataxia with episodic presentation. Methods: Eight patients lacking molecular diagnosis despite extensive clinical examination were recruited following standard genetic testing. Whole genome and RNA sequencing were performed on samples isolated from peripheral blood mononuclear cells. Integration of expression and splicing data facilitated genomic variants prioritization. Subsequently, long-read sequencing played a crucial role in the validation of those candidate variants. Results: Whole genome sequencing uncovered pathogenic variants in four genes (SPG7, ATXN2, ELOVL4, PMPCB). A missense and a nonsense variant, both previously reported as likely pathogenic, configured in trans in individual #1 (SPG7: c.2228T>C/p.I743T, c.1861C>T/p.Q621*). An ATXN2 microsatellite expansion (CAG32) in another late-onset case. In two separate individuals, intronic variants near splice sites (ELOVL4: c.541 + 5G>A; PMPCB: c.1154 + 5G>C) were predicted to induce loss-of-function splicing, but had never been reported as disease-causing. Long-read sequencing confirmed the compound heterozygous variants configuration, repeat expansion length, as well as splicing landscape for those pathogenic variants. A potential genetic modifier of the ATXN2 expansion was discovered in ZFYVE26 (c.3022C>T/p.R1008*). Conclusion: Despite failure to identify pathogenic variants through clinical genetic testing, the multi-omics approach enabled the molecular diagnosis in 50% of patients, also giving valuable insights for variant prioritization in remaining cases. The findings demonstrate the value of long-read sequencing for the validation of candidate variants in various scenarios. Our study demonstrates the effectiveness of leveraging complementary omics technologies to unravel the underlying genetics in patients with unresolved rare diseases such as ataxia. Molecular diagnoses not only hold significant promise in improving patient care management, but also alleviates the burden of diagnostic odysseys, more broadly enhancing quality of life.

Case report: Episodic ataxia without ataxia?

Hereditary myopathies represent a clinically and genetically heterogeneous group of neuromuscular disorders, characterized by highly variable clinical presentations and frequently overlapping phenotypes with other neuromuscular disorders, likely influenced by genetic and environmental modifiers. Genetic testing is often challenging due to ambiguous clinical diagnosis. Here, we present the case of a family with clinical and Electromyography (EMG) features resembling a myotonia-like disorder in which Whole Exome Sequencing (WES) analysis revealed the co-segregation of two rare missense variants in UBR4 and HSPG2, genes previously associated with episodic ataxia 8 (EA8). A review of the literature highlighted a striking overlap between the clinical and the molecular features of our family and the previously described episodic ataxias (EAs), which raises concerns about the genotype-phenotype correlation, clinical variability, and the confounding overlap in these groups of disorders. This emphasizes the importance of thoroughly framing the patient's phenotype. The more clear-cut the diagnosis, the easier the identification of a genetic determinant, and the better the prognosis and the treatment of patients.

Genotype-phenotype relations for episodic ataxia genes: MDSGene systematic review

BACKGROUND

Most episodic ataxias (EA) are autosomal dominantly inherited and characterized by recurrent attacks of ataxia and other paroxysmal and non-paroxysmal features. EA is often caused by pathogenic variants in the CACNA1A, KCNA1, PDHA1, and SLC1A3 genes, listed as paroxysmal movement disorders (PxMD) by the MDS Task Force on the Nomenclature of Genetic Movement Disorders. Little is known about the genotype-phenotype correlation of the different genetic EA forms.

METHODS

We performed a systematic review of the literature to identify individuals affected by an episodic movement disorder harboring pathogenic variants in one of the four genes. We applied the standardized MDSGene literature search and data extraction protocol to summarize the clinical and genetic features. All data are available via the MDSGene protocol and platform on the MDSGene website (https://www.mdsgene.org/).

RESULTS

Information on 717 patients (CACNA1A: 491, KCNA1: 125, PDHA1: 90, and SLC1A3: 11) carrying 287 different pathogenic variants from 229 papers was identified and summarized. We show the profound phenotypic variability and overlap leading to the absence of frank genotype-phenotype correlation aside from a few key 'red flags'.

CONCLUSION

Given this overlap, a broad approach to genetic testing using a panel or whole exome or genome approach is most practical in most circumstances.

Paroxysmal movement disorders: Paroxysmal dyskinesia and episodic ataxia

Paroxysmal movement disorders have traditionally been classified into paroxysmal dyskinesia (PxD), which consists in attacks of involuntary movements (mainly dystonia and/or chorea) without loss of consciousness, and episodic ataxia (EA), which features spells of cerebellar dysfunction with or without interictal neurological manifestations. In this chapter, PxD will be discussed first according to the trigger-based classification, thus reviewing clinical, genetic, and molecular features of paroxysmal kinesigenic dyskinesia, paroxysmal nonkinesigenic dyskinesia, and paroxysmal exercise-induced dyskinesia. EA will be presented thereafter according to their designated gene or genetic locus. Clinicogenetic similarities among paroxysmal movement disorders have progressively emerged, which are herein highlighted along with growing evidence that their pathomechanisms overlap those of epilepsy and migraine. Advances in our comprehension of the biological pathways underlying paroxysmal movement disorders, which involve ion channels as well as proteins associated with the vesical synaptic cycle or implicated in neuronal energy metabolism, may represent the cornerstone for defining a shared pathophysiologic framework and developing target-specific therapies.

Episodic ataxia type 2 (EA2) with interictal myokymia and focal dystonia

Episodic ataxia type 1 and 2 (EA1 and EA2) are the most well-described of the episodic ataxias. They are autosomal dominantly inherited early-onset diseases characterized by attacks of cerebellar dysfunction. EA1 is clinically characterized by short episodes of ataxia with interictal myokymia, whereas EA2 is characterized by longer-lasting recurrent ataxia, slurred speech, and interictal nystagmus. We report on a patient with EA2 with interictal focal dystonia and also interictal myokymia, which is hitherto not reported as an interictal feature associated to EA2. The patient carries a previously described heterozygous pathogenic de novo frameshift variant in the CACNA1A gene, establishing the diagnosis of EA2. She had symptom onset at age 13 and from age 48 she developed interictal myokymia and focal dystonia as illustrated in Supplemental Movie S1. We conclude that interictal myokymia and focal dystonia may be interictal features associated to EA2 caused by the cerebellar pathophysiology of EA2. Episodes of ataxia were successfully treated with acetazolamide in low dose, whereas the interictal features were unresponsive to acetazolamide.

Neuroimaging in cerebellar ataxia in childhood: A review

Ataxia is one of the most common pediatric movement disorders and can be caused by a large number of congenital and acquired diseases affecting the cerebellum or the vestibular or sensory system. It is mainly characterized by gait abnormalities, dysmetria, intention tremor, dysdiadochokinesia, dysarthria, and nystagmus. In young children, ataxia may manifest as the inability or refusal to walk. The diagnostic approach begins with a careful clinical history including the temporal evolution of ataxia and the inquiry of additional symptoms, is followed by a meticulous physical examination, and, depending on the results, is complemented by laboratory assays, electroencephalography, nerve conduction velocity, lumbar puncture, toxicology screening, genetic testing, and neuroimaging. Neuroimaging plays a pivotal role in either providing the final diagnosis, narrowing the differential diagnosis, or planning targeted further workup. In this review, we will focus on the most common form of ataxia in childhood, cerebellar ataxia (CA). We will discuss and summarize the neuroimaging findings of either the most common or the most important causes of CA in childhood or present causes of pediatric CA with pathognomonic findings on MRI. The various pediatric CAs will be categorized and presented according to (a) the cause of ataxia (acquired/disruptive vs. inherited/genetic) and (b) the temporal evolution of symptoms (acute/subacute, chronic, progressive, nonprogressive, and recurrent). In addition, several illustrative cases with their key imaging findings will be presented.

Genes of the Glutamatergic System and Tardive Dyskinesia in Patients with Schizophrenia

Background: Tardive dyskinesia (TD) is an extrapyramidal side effect of the long-term use of antipsychotics. In the present study, the role of glutamatergic system genes in the pathogenesis of total TD, as well as two phenotypic forms, orofacial TD and limb-truncal TD, was studied. Methods: A set of 46 SNPs of the glutamatergic system genes (GRIN2A, GRIN2B, GRIK4, GRM3, GRM7, GRM8, SLC1A2, SLC1A3, SLC17A7) was studied in a population of 704 Caucasian patients with schizophrenia. Genotyping was performed using the MassARRAY Analyzer 4 (Agena Bioscience™). Logistic regression analysis was performed to test for the association of TD with the SNPs while adjusting for confounders. Results: No statistically significant associations between the SNPs and TD were found after adjusting for multiple testing. Since three SNPs of the SLC1A2 gene demonstrated nominally significant associations, we carried out a haplotype analysis for these SNPs. This analysis identified a risk haplotype for TD comprising CAT alleles of the SLC1A2 gene SNPs rs1042113, rs10768121, and rs12361171. Nominally significant associations were identified for SLC1A3 rs2229894 and orofacial TD, as well as for GRIN2A rs7192557 and limb-truncal TD. Conclusions: Genes encoding for mGlu3, EAAT2, and EAAT1 may be involved in the development of TD in schizophrenia patients.

Involvement of the Peripheral Nervous System in Episodic Ataxias

Episodic ataxias comprise a group of inherited disorders, which have a common hallmark—transient attacks of ataxia. The genetic background is heterogeneous and the causative genes are not always identified. Furthermore, the clinical presentation, including intraictal and interictal symptoms, as well as the retention and progression of neurological deficits, is heterogeneous. Spells of ataxia can be accompanied by other symptoms—mostly from the central nervous system. However, in some of episodic ataxias involvement of peripheral nervous system is a part of typical clinical picture. This review intends to provide an insight into involvement of peripheral nervous system in episodic ataxias.