Hereditary spastic paraplegia and hereditary ataxia, Part 2: A family demonstrating various phenotypic manifestations with the SCA3 genotype

Spinocerebellar ataxia type 3: subphenotypes in a cohort of Brazilian patients

Spinocerebellar ataxia type 3 (SCA3) involves cerebellar, pyramidal, extrapyramidal, motor neuron and oculomotor systems with strong phenotypic heterogeneity, that lead us to classify the disorder into different clinical subtypes according to the predominantly affected motor systems.

METHOD

The series comprises 167 SCA3 patients belonging to 68 pedigrees, studied from 1989-2013. These patients were categorized into seven different subphenotypes.

RESULTS

SCA3 cases were clustered according to the predominant clinical features. Three most common forms were subphenotype 2, characterized by ataxia and pyramidal symptom was observed in 67.5%, subphenotype 3 with ataxia and peripheral signs in 13.3%, and subphenotype 6 with pure cerebellar syndrome in 7.2%.

CONCLUSION

Our study was the first to systematically classify SCA3 into seven subphenotypes. This classification may be particularly useful for determination of a more specific and direct phenotype/genotype correlation in future studies.

Toward understanding Machado-Joseph disease

Machado-Joseph disease (MJD), also known as spinocerebellar ataxia type 3 (SCA3), is the most common inherited spinocerebellar ataxia and one of many polyglutamine neurodegenerative diseases. In MJD, a CAG repeat expansion encodes an abnormally long polyglutamine (polyQ) tract in the disease protein, ATXN3. Here we review MJD, focusing primarily on the function and dysfunction of ATXN3 and on advances toward potential therapies. ATXN3 is a deubiquitinating enzyme (DUB) whose highly specialized properties suggest that it participates in ubiquitin-dependent proteostasis. By virtue of its interactions with VCP, various ubiquitin ligases and other ubiquitin-linked proteins, ATXN3 may help regulate the stability or activity of many proteins in diverse cellular pathways implicated in proteotoxic stress response, aging, and cell differentiation. Expansion of the polyQ tract in ATXN3 is thought to promote an altered conformation in the protein, leading to changes in interactions with native partners and to the formation of insoluble aggregates. The development of a wide range of cellular and animal models of MJD has been crucial to the emerging understanding of ATXN3 dysfunction upon polyQ expansion. Despite many advances, however, the principal molecular mechanisms by which mutant ATXN3 elicits neurotoxicity remain elusive. In a chronic degenerative disease like MJD, it is conceivable that mutant ATXN3 triggers multiple, interconnected pathogenic cascades that precipitate cellular dysfunction and eventual cell death. A better understanding of these complex molecular mechanisms will be important as scientists and clinicians begin to focus on developing effective therapies for this incurable, fatal disorder.

The modulation of Amyotrophic Lateral Sclerosis risk by ataxin-2 intermediate polyglutamine expansions is a specific effect

Full expansions of the polyglutamine domain (polyQ≥34) within the polysome-associated protein ataxin-2 (ATXN2) are the cause of a multi-system neurodegenerative disorder, which usually presents as a Spino-Cerebellar Ataxia and is therefore known as SCA2, but may rarely manifest as Levodopa-responsive Parkinson syndrome or as motor neuron disease. Intermediate expansions (27≤polyQ≤33) were reported to modify the risk of Amyotrophic Lateral Sclerosis (ALS). We have now tested the reproducibility and the specificity of this observation. In 559 independent ALS patients from Central Europe, the association of ATXN2 expansions (30≤polyQ≤35) with ALS was highly significant. The study of 1490 patients with Parkinson's disease (PD) showed an enrichment of ATXN2 alleles 27/28 in a subgroup with familial cases, but the overall risk of sporadic PD was unchanged. No association was found between polyQ expansions in Ataxin-3 (ATXN3) and ALS risk. These data indicate a specific interaction between ATXN2 expansions and the causes of ALS, possibly through altered RNA-processing as a common pathogenic factor.

Other causes of ataxia in patients with SCA mutations

Autosomal dominant spinocerebellar ataxias (SCAs) are slowly progressive and have a variable clinical presentation. Overlapping clinical features among the SCAs make the clinical diagnosis of these ataxias difficult. Even when genetic testing identifies an SCA mutation, clinicians should be vigilant for other causes of neurological dysfunction in these patients. We report two patients who developed other causes of ataxia in the setting of SCA-3 and SCA-8 mutations, respectively.

Neuro-ophthalmologic findings in konzo, an upper motor neuron disorder in Africa

PURPOSE

To investigate the neuro-ophthalmological manifestations in konzo, a non-progressive symmetric spastic para/tetraparesis of acute onset associated with consumption of insufficiently processed bitter cassava roots combined with a low protein intake.

METHODS

Twenty-one Congolese konzo patients underwent neuro-ophthalmological investigations including visual acuity testing, assessment of light pupillary reflexes, evaluation of ocular motility and deviation, direct ophthalmoscopy, and visual field perimetry. Objective refraction including retinoscopy and keratometry, and slit-lamp biomicroscopy were also done.

RESULTS

Five patients had visual impairment, and 14 had temporal pallor of the optic disc. Fourteen presented visual field defects, the most frequent being concentric constriction and peripheral defects. Overall, 11 subjects had symptoms qualifying for the diagnosis of optic neuropathy. Two had spontaneous pendular nystagmus in primary position of gaze. Visual field defects and pallor of the optic discs were found in mild, moderate and severe forms of konzo. No correlation was found between the severity of the motor disability of konzo and the extent of visual field loss.

CONCLUSIONS

Konzo was associated with optic neuropathy and a few patients had nystagmus. Although the etiopathogenesis of this optic neuropathy remains to be elucidated, the symmetry of the involvement suggests a toxic origin. We suggest that cyanide causes the neuro-ophthalmological damage in konzo. However, the optic neuropathy in konzo patients does not resemble the features of the epidemic optic neuropathy in Tanzania, Cuba or Nigeria, Leber's hereditary optic neuropathy, tobacco amblyopia or vitamin B deficiency.