Spinocerebellar ataxia type 4 and 16q22.1-linked Japanese ataxia are not allelic

A Review of Ocular Movement Abnormalities in Hereditary Cerebellar Ataxias

Cerebellar ataxias are a wide heterogeneous group of disorders that may present with fine motor deficits as well as gait and balance disturbances that have a significant influence on everyday activities. To review the ocular movements in cerebellar ataxias in order to improve the clinical knowledge of cerebellar ataxias and related subtypes. English papers published from January 1990 to May 2022 were selected by searching PubMed services. The main search keywords were ocular motor, oculomotor, eye movement, eye motility, and ocular motility, along with each ataxia subtype. The eligible papers were analyzed for clinical presentation, involved mutations, the underlying pathology, and ocular movement alterations. Forty-three subtypes of spinocerebellar ataxias and a number of autosomal dominant and autosomal recessive ataxias were discussed in terms of pathology, clinical manifestations, involved mutations, and with a focus on the ocular abnormalities. A flowchart has been made using ocular movement manifestations to differentiate different ataxia subtypes. And underlying pathology of each subtype is reviewed in form of illustrated models to reach a better understanding of each disorder.

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.

EFNS guidelines on the molecular diagnosis of ataxias and spastic paraplegias

BACKGROUND AND PURPOSE

These EFNS guidelines on the molecular diagnosis of neurogenetic disorders are designed to provide practical help for the general neurologist to make appropriate use of molecular genetics in diagnosing neurogenetic disorders.

METHODS

Literature searches were performed before expert members of the task force wrote proposals, which were discussed in detail until final consensus had been reached among all task force members.

RESULTS AND CONCLUSION

This paper provides updated guidelines for molecular diagnosis of two particularly complex groups of disorders, the ataxias and spastic paraplegias. Possibilities and limitations of molecular genetic diagnosis of these disorders are evaluated and recommendations are provided.

Spinocerebellar ataxia type 31 is associated with "inserted" penta-nucleotide repeats containing (TGGAA)n

Spinocerebellar ataxia type 31 (SCA31) is an adult-onset autosomal-dominant neurodegenerative disorder showing progressive cerebellar ataxia mainly affecting Purkinje cells. The SCA31 critical region was tracked down to a 900 kb interval in chromosome 16q22.1, where the disease shows a strong founder effect. By performing comprehensive Southern blot analysis and BAC- and fosmid-based sequencing, we isolated two genetic changes segregating with SCA31. One was a single-nucleotide change in an intron of the thymidine kinase 2 gene (TK2). However, this did not appear to affect splicing or expression patterns. The other was an insertion, from 2.5-3.8 kb long, consisting of complex penta-nucleotide repeats including a long (TGGAA)n stretch. In controls, shorter (1.5-2.0 kb) insertions lacking (TGGAA)n were found only rarely. The SCA31 repeat insertion's length inversely correlated with patient age of onset, and an expansion was documented in a single family showing anticipation. The repeat insertion was located in introns of TK2 and BEAN (brain expressed, associated with Nedd4) expressed in the brain and formed RNA foci in the nuclei of patients' Purkinje cells. An electrophoretic mobility-shift assay showed that essential splicing factors, serine/arginine-rich splicing factors SFRS1 and SFRS9, bind to (UGGAA)n in vitro. Because (TGGAA)n is a characteristic sequence of paracentromeric heterochromatin, we speculate that the insertion might have originated from heterochromatin. SCA31 is important because it exemplifies human diseases associated with "inserted" microsatellite repeats that can expand through transmission. Our finding suggests that the ectopic microsatellite repeat, when transcribed, might cause a disease involving the essential splicing factors.