Germ-line CAG repeat instability causes extreme CAG repeat expansion with infantile-onset spinocerebellar ataxia type 2

CAG repeat mosaicism is gene specific in spinocerebellar ataxias

Expanded CAG repeats in coding regions of different genes are the most common cause of dominantly inherited spinocerebellar ataxias (SCAs). These repeats are unstable through the germline, and larger repeats lead to earlier onset. We measured somatic expansion in blood samples collected from 30 SCA1, 50 SCA2, 74 SCA3, and 30 SCA7 individuals over a mean interval of 8.5 years, along with postmortem tissues and fetal tissues from SCA1, SCA3, and SCA7 individuals to examine somatic expansion at different stages of life. We showed that somatic mosaicism in the blood increases over time. Expansion levels are significantly different among SCAs and correlate with CAG repeat lengths. The level of expansion is greater in individuals with SCA7 who manifest disease compared to that of those who do not yet display symptoms. Brain tissues from SCA individuals have larger expansions compared to the blood. The cerebellum has the lowest mosaicism among the studied brain regions, along with a high expression of ATXNs and DNA repair genes. This was the opposite in cortices, with the highest mosaicism and lower expression of ATXNs and DNA repair genes. Fetal cortices did not show repeat instability. This study shows that CAG repeats are increasingly unstable during life in the blood and the brain of SCA individuals, with gene- and tissue-specific patterns.

Juvenile Huntington's Disease and Other PolyQ Diseases, Update on Neurodevelopmental Character and Comparative Bioinformatic Review of Transcriptomic and Proteomic Data

Polyglutamine (PolyQ) diseases are neurodegenerative disorders caused by the CAG repeat expansion mutation in affected genes resulting in toxic proteins containing a long chain of glutamines. There are nine PolyQ diseases: Huntington’s disease (HD), spinocerebellar ataxias (types 1, 2, 3, 6, 7, and 17), dentatorubral-pallidoluysian atrophy (DRPLA), and spinal bulbar muscular atrophy (SBMA). In general, longer CAG expansions and longer glutamine tracts lead to earlier disease presentations in PolyQ patients. Rarely, cases of extremely long expansions are identified for PolyQ diseases, and they consistently lead to juvenile or sometimes very severe infantile-onset polyQ syndromes. In apparent contrast to the very long CAG tracts, shorter CAGs and PolyQs in proteins seems to be the evolutionary factor enhancing human cognition. Therefore, polyQ tracts in proteins can be modifiers of brain development and disease drivers, which contribute neurodevelopmental phenotypes in juvenile- and adult-onset PolyQ diseases. Therefore we performed a bioinformatics review of published RNAseq polyQ expression data resulting from the presence of polyQ genes in search of neurodevelopmental expression patterns and comparison between diseases. The expression data were collected from cell types reflecting stages of development such as iPSC, neuronal stem cell, neurons, but also the adult patients and models for PolyQ disease. In addition, we extended our bioinformatic transcriptomic analysis by proteomics data. We identified a group of 13 commonly downregulated genes and proteins in HD mouse models. Our comparative bioinformatic review highlighted several (neuro)developmental pathways and genes identified within PolyQ diseases and mouse models responsible for neural growth, synaptogenesis, and synaptic plasticity.

Genetic profile and clinical characteristics of Chinese patients with spinocerebellar ataxia type 2: A multicenter experience over 10 years

BACKGROUND AND PURPOSE

Spinocerebellar ataxia type 2 (SCA2) is the second most common type of spinocerebellar ataxia in China. However, data on the clinical and genetic features of Chinese SCA2 patients are scarce. This study aims to provide a comprehensive description of in the Chinese SCA2 cohort.

METHODS

A total of 135 patients with SCA2 from 92 families and 104 unrelated normal controls were recruited from three medical centers between 2008 and 2020. Sanger sequencing and TA cloning were used to determine the CAG repeat length and intrinsic structure. The clinical data of patients with SCA2, including electromyography, magnetic resonance imaging, positron-emission tomography, and clinical scale scores, were recorded.

RESULTS

The mean ± SD age at onset of SCA2 patients was 32.6 ± 11.9 years and the corresponding CAG repeat length was 42.1 ± 3.6. CAG repeat length accounted for 64% of the age-at-onset variance. We observed that patients had a significantly lower proportion of (CAG)₈ CAA(CAG)₄ CAA(CAG)₈ within normal alleles than normal controls (48.8% vs. 64.9%; p = 0.003), while the distribution of the proportion of (CAG)₁₃ CAA (CAG)₈ was the opposite. Peripheral neuropathy was frequent, occurring in 75.9% of the patients. Parkinsonism was relatively common, with a frequency of 11.8%. Two patients with parkinsonism had a significantly more severe reduction in dopamine transporter levels in the bilateral striatum than the one patient with pure ataxia. An infant-onset case of SCA2 with more than 180 CAG repeats was characterized by global development delay, hypotonia and hearing impairment.

CONCLUSIONS

This study describes the genetic profile and clinical characteristics of the largest SCA2 cohort to date in the Chinese population and analyzes inter-population differences. Many aspects of this study population were different from other populations with SCA2.

A clinical report of the massive CAG repeat expansion in spinocerebellar ataxia type 2: Severe onset in a Mexican child and review previous cases

The spinocerebellar ataxia type 2 is a neurodegenerative disease with autosomal dominant inheritance; clinically characterized by progressive cerebellar ataxia, slow ocular saccades, nystagmus, ophthalmoplegia, dysarthria, dysphagia, cognitive deterioration, mild dementia, peripheral neuropathy. Infantile onset is a rare presentation that only has been reported in four instances in the literature. In the present work a boy aged 5 years 7 months was studied due to horizontal gaze-evoked nystagmus, without saccades, ataxic gait, dysarthria, dysphagia, dysmetria, generalized spasticity mainly pelvic, bilateral Babinsky. The mother aged 27 years-old presented progressive cerebellar ataxia, dysarthria, dysmetria, dysdiadochokinesis, limb ataxia and olivopontocerebellar atrophy. The molecular analysis was made by identifying the expansion repeats in tandem by long PCR to analyze the repeats in the ATXN2 gene. We found an extreme CAG expansion repeats of ~884 repeats in the child. We describe a Mexican child affected by SCA2 with an infantile onset, associated with a high number of CAG repeats previously no reported and anticipation phenomenon.

Generation of an Atxn2-CAG100 knock-in mouse reveals N-acetylaspartate production deficit due to early Nat8l dysregulation

Spinocerebellar ataxia type 2 (SCA2) is an autosomal dominant neurodegenerative disorder caused by CAG-expansion mutations in the ATXN2 gene, mainly affecting motor neurons in the spinal cord and Purkinje neurons in the cerebellum. While the large expansions were shown to cause SCA2, the intermediate length expansions lead to increased risk for several atrophic processes including amyotrophic lateral sclerosis and Parkinson variants, e.g. progressive supranuclear palsy. Intense efforts to pioneer a neuroprotective therapy for SCA2 require longitudinal monitoring of patients and identification of crucial molecular pathways. The ataxin-2 (ATXN2) protein is mainly involved in RNA translation control and regulation of nutrient metabolism during stress periods. The preferential mRNA targets of ATXN2 are yet to be determined. In order to understand the molecular disease mechanism throughout different prognostic stages, we generated an Atxn2-CAG100-knock-in (KIN) mouse model of SCA2 with intact murine ATXN2 expression regulation. Its characterization revealed somatic mosaicism of the expansion, with shortened lifespan, a progressive spatio-temporal pattern of pathology with subsequent phenotypes, and anomalies of brain metabolites such as N-acetylaspartate (NAA), all of which mirror faithfully the findings in SCA2 patients. Novel molecular analyses from stages before the onset of motor deficits revealed a strong selective effect of ATXN2 on Nat8l mRNA which encodes the enzyme responsible for NAA synthesis. This metabolite is a prominent energy store of the brain and a well-established marker for neuronal health. Overall, we present a novel authentic rodent model of SCA2, where in vivo magnetic resonance imaging was feasible to monitor progression and where the definition of earliest transcriptional abnormalities was possible. We believe that this model will not only reveal crucial insights regarding the pathomechanism of SCA2 and other ATXN2-associated disorders, but will also aid in developing gene-targeted therapies and disease prevention.

Evidence of oxidative stress and mitochondrial dysfunction in spinocerebellar ataxia type 2 (SCA2) patient fibroblasts: Effect of coenzyme Q10 supplementation on these parameters

Spinocerebellar ataxia type 2 (SCA2) is a rare neurodegenerative disorder caused by a CAG repeat expansion in the ataxin-2 gene. We show increased oxidative stress, abnormalities in the antioxidant system, changes in complexes involved in oxidative phosphorylation and changes in mitochondrial morphology in SCA2 patient fibroblasts compared to controls, and we show that treatment with CoQ10 can partially reverse these changes. Together, our results suggest that oxidative stress and mitochondrial dysfunction may be contributory factors to the pathophysiology of SCA2 and that therapeutic strategies involving manipulation of the antioxidant system could prove to be of clinical benefit.

From normal gait to loss of ambulation in 6 months: a novel presentation of SCA17

Spinocerebellar ataxias are a group of rare and heterogeneous autosomal dominant disorders characterized by progressive ataxia and other features. Spinocerebellar ataxia 17 (SCA17) is one of the 32 subtypes described to date and is secondary to CAG/CAA repeat expansion in the gene coding for the TATA-box binding protein (TBP). SCA17 is clinically heterogeneous and typically presents with slowly evolving ataxia, dysarthria, dementia, depression, and other movement disorders such as chorea. More than 41 CAG/CAA repeats are considered diagnostic of SCA17, with more than 49 being associated with full penetrance. We report one patient presenting with isolated rapidly evolving ataxia who was found to have 44 CAG/CAA repeats in the TBP gene. This suggests that, while SCA17 typically slowly progresses over years, its repertoire of presentations should be expanded to include rapidly progressive isolated ataxia resembling paraneoplastic disorders or prion disease.

Infantile onset spinocerebellar ataxia 2 (SCA2): a clinical report with review of previous cases

Autosomal dominant cerebellar ataxia type I is a heterogeneous group of spinocerebellar ataxias with variable neurologic presentations, with age of onset varying from infancy to adulthood. Autosomal dominant cerebellar ataxia type I is composed mainly of 3 prevalent spinocerebellar ataxia types with different pathogenic loci, specifically spinocerebellar ataxia 1 (6p24-p23), spinocerebellar ataxia 2 (12q24.1), and spinocerebellar ataxia 3 (14q32.1). The shared pathogenic mutational event is the expansion of the CAG repeat that results in polyglutamine extended stretches in the encoded proteins. CAG repeat disorders generally show the phenomenon of anticipation, which is more often associated with paternal transmission. In this report, we describe a patient with infantile-onset spinocerebellar ataxia type 2 (~320 CAG repeat) who inherited the disease from his father (47 CAG repeats). We have summarized the clinical, neuroimaging, electroencephalographic (EEG), and molecular data of previous cases and attempt to highlight the most consistent findings. Our intent is to help treating clinicians to suspect this disorder and to offer timely genetic counseling for a currently potentially untreatable disorder.