Are interrupted SCA2 CAG repeat expansions responsible for parkinsonism?

ATXN2L primarily interacts with NUFIP2, the absence of ATXN2L results in NUFIP2 depletion, and the ATXN2-polyQ expansion triggers NUFIP2 accumulation

The cytoplasmic Ataxin-2 (ATXN2) protein associates with TDP-43 in stress granules (SG) where RNA quality control occurs. Mutations in this pathway underlie Spinocerebellar Ataxia type 2 (SCA2) and Amyotrophic Lateral Sclerosis. In contrast, Ataxin-2-like (ATXN2L) is predominantly perinuclear, more abundant, and essential for embryonic life. Its sequestration into ATXN2 aggregates may contribute to disease. In this study, we utilized two approaches to clarify the roles of ATXN2L. First, we identified interactors through co-immunoprecipitation in both wild-type and ATXN2L-null murine embryonic fibroblasts. Second, we assessed the proteome profile effects using mass spectrometry in these cells. Additionally, we examined the accumulation of ATXN2L interactors in the SCA2 mouse model, Atxn2-CAG100-KnockIn (KIN). We observed that RNA-binding proteins, including PABPN1, NUFIP2, MCRIP2, RBMS1, LARP1, PTBP1, FMR1, RPS20, FUBP3, MBNL2, ZMAT3, SFPQ, CSDE1, HNRNPK, and HNRNPDL, exhibit a stronger association with ATXN2L compared to established interactors like ATXN2, PABPC1, LSM12, and G3BP2. Additionally, ATXN2L interacted with components of the actin complex, such as SYNE2, LMOD1, ACTA2, FYB, and GOLGA3. We noted that oxidative stress increased HNRNPK but decreased SYNE2 association, which likely reflects the relocalization of SG. Proteome profiling revealed that NUFIP2 and SYNE2 are depleted in ATXN2L-null fibroblasts. Furthermore, NUFIP2 homodimers and SYNE1 accumulate during the ATXN2 aggregation process in KIN 14-month-old spinal cord tissues. The functions of ATXN2L and its interactors are therefore critical in RNA granule trafficking and surveillance, particularly for the maintenance of differentiated neurons.

Multisample motif discovery and visualization for tandem repeats

Tandem repeats (TRs) occupy a significant portion of the human genome and are a source of polymorphisms due to variations in sizes and motif compositions. Some of these variations have been associated with various neuropathological disorders, highlighting the clinical importance of assessing the motif structure of TRs. Moreover, assessing the TR motif variation can offer valuable insights into evolutionary dynamics and population structure. Previously, characterizations of TRs were limited by short-read sequencing technology, which lacks the ability to accurately capture the full TR sequences. As long-read sequencing becomes more accessible and can capture the full complexity of TRs, there is now also a need for tools to characterize and analyze TRs using long-read data across multiple samples. In this study, we present MotifScope, a novel algorithm for the characterization and visualization of TRs based on a de novo k-mer approach for motif discovery. Comparative analysis against established tools reveals that MotifScope can identify a greater number of motifs and more accurately represent the underlying repeat sequences. Moreover, MotifScope has been specifically designed to enable motif composition comparisons across assemblies of different individuals, as well as across long-read sequencing reads within an individual, through combined motif discovery and sequence alignment. We showcase potential applications of MotifScope in diverse fields, including population genetics, clinical settings, and forensic analyses.

An observational study of pleiotropy and penetrance of amyotrophic lateral sclerosis associated with CAG-repeat expansion of ATXN2

Spinocerebellar ataxia type 2 (SCA2) and amyotrophic lateral sclerosis (ALS) are both associated with a CAG-repeat expansion in ATXN2 and with TDP-43-positive neuronal cytoplasmic inclusions. The two disorders have been viewed as distinct entities, where an intermediate length expansion of 31-33 CAG-repeats is associated with sporadic ALS and a full length expansion of ≥34 CAG-repeats is associated with SCA2. We report the clinical phenotype of ATXN2-positive patients and their relatives, identified in three specialist ALS clinics, which force a reconsideration of this dichotomy. We also report the frequency of ATXN2 expansions in two large cohorts of ALS patients and in a population-matched cohort of controls. We report ten cases of familial ALS in which disease is associated with either an intermediate or a full-length ATXN2 CAG-repeat expansion. Pedigrees and patients feature additional phenotypes including parkinsonism, dementia and essential tremor (ET). We conclude that CAG-repeat expansions in ATXN2 exhibit pleiotropy and are associated with a disease spectrum that includes ALS, SCA2, and parkinsonism; to recognise this complexity we propose the new term 'ATXN2-related neurodegeneration'. We also observed sporadic ALS associated with full-length expansions. We conclude that ATXN2 CAG-repeat expansions, irrespective of length, should be considered a risk factor for ALS. Interrupted CAG-repeats were associated with an ALS phenotype in our data but we also identified ALS cases with uninterrupted expansions. Our findings have relevance for researchers, patients and families linked to CAG-repeat expansions in ATXN2.

Localization of Potential Energy in Hydrogen Bonds of the ATXN2 Gene

It is known that a number of neurodegenerative diseases, also called diseases of waiting, are associated with the expansion of the polyQ tract in the first exon of the ATXN2 gene. In the expanded polyQ tract, the probability of occurrence of non-canonical configurations (hairpins, G-quadruplexes, etc.) is significantly higher than in the normal one. Obviously, for their formation, the occurrence of open states (OSs) is necessary. Calculations were made for these processes using the angular mechanical model of DNA. It has been established that the probability of the large OS zones genesis in a DNA segment depends not only on the "strength" of the nucleotide sequence but also on the factors determining the dynamics of DNA; localization of the energy in the DNA molecule and the potential energy of interaction between pairs of nitrogenous bases also depend on environmental parameters. The potential energy of hydrogen bonds does not remain constant, and oscillatory movements lead to its redistribution and localization. In this case, OSs effectively dissipate the energy of oscillations. Thus, mathematical modeling makes it possible to calculate the localization of mechanical energy, which is necessary for the OSs formation, and to predict the places of their origin, taking into account the mechanical oscillations of the DNA molecule.

Substantia nigra degeneration in spinocerebellar ataxia 2 and 7 using neuromelanin-sensitive imaging

OBJECTIVE

Spinocerebellar ataxias (SCA) are neurodegenerative diseases with widespread lesions across the central nervous system. Ataxia and spasticity are usually predominant, but patients may also present with parkinsonism. We aimed to characterize substantia nigra pars compacta (SNc) degeneration in SCA2 and 7 using neuromelanin-sensitive imaging.

METHODS

Ataxic and preataxic expansion carriers with SCA2 (n=15) and SCA7 (n=15) and healthy controls (n=10) were prospectively recruited. Volume and signal-to-noise ratio (SNR) values of the SNc were extracted from neuromelanin-sensitive images. ROC curves were used to determine the metrics that best differentiated SCA participants. Correlations between imaging measurements, clinical variables, and plasma neurofilaments light chain (NfL) levels were investigated.

RESULTS

SCA2 participants had lower SNR values in the SNc than controls (110.2 ± 1.3 versus 113.2 ± 1.4; p < 0.001) and those with SCA7 (112.5 ± 2.1; p < 0.01). SNR in SCA7 participants and controls did not differ. In ataxic patients, SNc volumes were lower in SCA2 (0.13 ± 0.04; p = 0.06) and SCA7 (0.10 ± 0.03, p = 0.02) patients compared to controls (0.17 ± 0.04). Signal decrease was detected at the preataxic stage in SCA2, but not in SCA7. SCA2 participants showed prominent involvement of the associative and limbic nigral territories. SNR discriminated ataxic and preataxic SCA2 participants from controls (AUC ≥0.94). SNc volume differentiated ataxic SCA7 participants from controls (AUC = 1), but not preataxic ones. In SCA7, correlations were observed between SNc volume and time to onset, CAG repeats, clinical severity scores, and NfL.

CONCLUSIONS

Neuromelanin-sensitive imaging provides biomarkers of nigral degeneration in SCAs, detectable from the preataxic stage in SCA2, which could potentially serve as outcome measures in clinical trials.

Parkinson's families project: a UK-wide study of early onset and familial Parkinson's disease

The Parkinson's Families Project is a UK-wide study aimed at identifying genetic variation associated with familial and early-onset Parkinson's disease (PD). We recruited individuals with a clinical diagnosis of PD and age at motor symptom onset ≤45 years and/or a family history of PD in up to third-degree relatives. Where possible, we also recruited affected and unaffected relatives. We analysed DNA samples with a combination of single nucleotide polymorphism (SNP) array genotyping, multiplex ligation-dependent probe amplification (MLPA), and whole-genome sequencing (WGS). We investigated the association between identified pathogenic mutations and demographic and clinical factors such as age at motor symptom onset, family history, motor symptoms (MDS-UPDRS) and cognitive performance (MoCA). We performed baseline genetic analysis in 718 families, of which 205 had sporadic early-onset PD (sEOPD), 113 had familial early-onset PD (fEOPD), and 400 had late-onset familial PD (fLOPD). 69 (9.6%) of these families carried pathogenic variants in known monogenic PD-related genes. The rate of a molecular diagnosis increased to 28.1% in PD with motor onset ≤35 years. We identified pathogenic variants in LRRK2 in 4.2% of families, and biallelic pathogenic variants in PRKN in 3.6% of families. We also identified two families with SNCA duplications and three families with a pathogenic repeat expansion in ATXN2, as well as single families with pathogenic variants in VCP, PINK1, PNPLA6, PLA2G6, SPG7, GCH1, and RAB32. An additional 73 (10.2%) families were carriers of at least one pathogenic or risk GBA1 variant. Most early-onset and familial PD cases do not have a known genetic cause, indicating that there are likely to be further monogenic causes for PD.

Structural and Dynamical Properties of Nucleic Acid Hairpins Implicated in Trinucleotide Repeat Expansion Diseases

Dynamic mutations in some human genes containing trinucleotide repeats are associated with severe neurodegenerative and neuromuscular disorders-known as Trinucleotide (or Triplet) Repeat Expansion Diseases (TREDs)-which arise when the repeat number of triplets expands beyond a critical threshold. While the mechanisms causing the DNA triplet expansion are complex and remain largely unknown, it is now recognized that the expandable repeats lead to the formation of nucleotide configurations with atypical structural characteristics that play a crucial role in TREDs. These nonstandard nucleic acid forms include single-stranded hairpins, Z-DNA, triplex structures, G-quartets and slipped-stranded duplexes. Of these, hairpin structures are the most prolific and are associated with the largest number of TREDs and have therefore been the focus of recent single-molecule FRET experiments and molecular dynamics investigations. Here, we review the structural and dynamical properties of nucleic acid hairpins that have emerged from these studies and the implications for repeat expansion mechanisms. The focus will be on CAG, GAC, CTG and GTC hairpins and their stems, their atomistic structures, their stability, and the important role played by structural interrupts.

Repeat expansion disorders

An increasing number of repeat expansion disorders have been found to cause both rare and common neurological disease. This is exemplified in recent discoveries of novel repeat expansions underlying a significant proportion of several late-onset neurodegenerative disorders, such as CANVAS (cerebellar ataxia, neuropathy and vestibular areflexia syndrome) and spinocerebellar ataxia type 27B. Most of the 60 described repeat expansion disorders to date are associated with neurological disease, providing substantial challenges for diagnosis, but also opportunities for management in a clinical neurology setting. Commonalities in clinical presentation, overarching diagnostic features and similarities in the approach to genetic testing justify considering these disorders collectively based on their unifying causative mechanism. In this review, we discuss the characteristics and diagnostic challenges of repeat expansion disorders for the neurologist and provide examples to highlight their clinical heterogeneity. With the ready availability of clinical-grade whole-genome sequencing for molecular diagnosis, we discuss the current approaches to testing for repeat expansion disorders and application in clinical practice.

Stability of the CAG Tract in the ATXN2 Gene Depends on the Localization of CAA Interruptions

It is known that the presence of CAA codons in the CAG tract affects the nature and time of disease onset caused by the expansion of trinucleotide repeats. The mechanisms leading to the occurrence of these diseases should be sought not only at the level of the physiological role of the ATXN2 protein, but also at the DNA level. These mechanisms are associated with non-canonical configurations (hairpins) that can form in the CAG tract. The tendency of hairpins to slide along the corresponding threads is usually considered important to explain the expansion of the CAG tract. At the same time, hairpins occur in areas of open states. Previous studies on the role of CAA interruptions have suggested that, under certain conditions, they can stabilize the dynamics of the hairpin, preventing the expansion of the CAG tract. We calculated the probability of additional open state zones occurrence in the CAG tract using an angular mathematical model of DNA. The calculations made it possible to establish that CAA interruptions affect the stability of the CAG tract, and this influence, depending on the localization of the interruption, can both increase and decrease the stability of the CAG tract.

Clinical features, disease progression, and nuclear imaging in ATXN2-related parkinsonism in a longitudinal cohort

BACKGROUND

Spinocerebellar ataxia 2 (SCA2) with a low range of CAG repeat expansion of ATXN2 gene can present with predominant or isolated parkinsonism that closely resembles Parkinson's disease (PD). This study is aimed at comparing clinical features, disease progression, and nuclear imaging between ATXN2-related parkinsonism (ATXN2-P) and PD.

METHODS

Three hundred and seventy-seven clinically diagnosed PD with family history were screened by multiplex ligation-dependent probe amplification, whole-exome sequencing or target sequencing, and dynamic mutation testing of 10 SCA subtypes. The baseline and longitudinal clinical features as well as the dual-tracer positron emission tomography (PET) imaging were compared between ATXN2-P and genetically undefined familial PD (GU-fPD).

RESULTS

Fifteen ATXN2-P patients from 7 families and 50 randomly selected GU-fPD patients were evaluated. Significantly less resting tremor and more symmetric signs were observed in ATXN2-P than GU-fPD. No significant difference was found in motor progression and duration from onset to occurrence of fluctuation, dyskinesia, and recurrent falls between the two groups. Cognitive impairment and rapid-eye-movement sleep behavior disorder were more common in ATXN2-P. During follow-up, olfaction was relatively spared, and no obvious progression of cognition dysfunction evaluated by Mini-Mental State Examination scores was found in ATXN2-P. PET results of ATXN2-P demonstrated a symmetric, diffuse, and homogenous dopamine transporter loss of bilateral striatum and a glucose metabolism pattern inconsistent with that in PD.

CONCLUSIONS

Symmetric motor signs and unique nuclear imaging might be the clues to distinguish ATXN2-P from GU-fPD.

Sequence composition changes in short tandem repeats: heterogeneity, detection, mechanisms and clinical implications

Short tandem repeats (STRs) are a class of repetitive elements, composed of tandem arrays of 1-6 base pair sequence motifs, that comprise a substantial fraction of the human genome. STR expansions can cause a wide range of neurological and neuromuscular conditions, known as repeat expansion disorders, whose age of onset, severity, penetrance and/or clinical phenotype are influenced by the length of the repeats and their sequence composition. The presence of non-canonical motifs, depending on the type, frequency and position within the repeat tract, can alter clinical outcomes by modifying somatic and intergenerational repeat stability, gene expression and mutant transcript-mediated and/or protein-mediated toxicities. Here, we review the diverse structural conformations of repeat expansions, technological advances for the characterization of changes in sequence composition, their clinical correlations and the impact on disease mechanisms.

Genetic modifiers of repeat expansion disorders

Repeat expansion disorders (REDs) are monogenic diseases caused by a sequence of repetitive DNA expanding above a pathogenic threshold. A common feature of the REDs is a strong genotype-phenotype correlation in which a major determinant of age at onset (AAO) and disease progression is the length of the inherited repeat tract. Over a disease-gene carrier's life, the length of the repeat can expand in somatic cells, through the process of somatic expansion which is hypothesised to drive disease progression. Despite being monogenic, individual REDs are phenotypically variable, and exploring what genetic modifying factors drive this phenotypic variability has illuminated key pathogenic mechanisms that are common to this group of diseases. Disease phenotypes are affected by the cognate gene in which the expansion is found, the location of the repeat sequence in coding or non-coding regions and by the presence of repeat sequence interruptions. Human genetic data, mouse models and in vitro models have implicated the disease-modifying effect of DNA repair pathways via the mechanisms of somatic mutation of the repeat tract. As such, developing an understanding of these pathways in the context of expanded repeats could lead to future disease-modifying therapies for REDs.

Detection and discovery of repeat expansions in ataxia enabled by next-generation sequencing: present and future

Hereditary cerebellar ataxias are a heterogenous group of progressive neurological disorders that are disproportionately caused by repeat expansions (REs) of short tandem repeats (STRs). Genetic diagnosis for RE disorders such as ataxias are difficult as the current gold standard for diagnosis is repeat-primed PCR assays or Southern blots, neither of which are scalable nor readily available for all STR loci. In the last five years, significant advances have been made in our ability to detect STRs and REs in short-read sequencing data, especially whole-genome sequencing. Given the increasing reliance of genomics in diagnosis of rare diseases, the use of established RE detection pipelines for RE disorders is now a highly feasible and practical first-step alternative to molecular testing methods. In addition, many new pathogenic REs have been discovered in recent years by utilising WGS data. Collectively, genomes are an important resource/platform for further advancements in both the discovery and diagnosis of REs that cause ataxia and will lead to much needed improvement in diagnostic rates for patients with hereditary ataxia.

Genetic Movement Disorders Commonly Seen in Asians

The increasing availability of molecular genetic testing has changed the landscape of both genetic research and clinical practice. Not only is the pace of discovery of novel disease-causing genes accelerating but also the phenotypic spectra associated with previously known genes are expanding. These advancements lead to the awareness that some genetic movement disorders may cluster in certain ethnic populations and genetic pleiotropy may result in unique clinical presentations in specific ethnic groups. Thus, the characteristics, genetics and risk factors of movement disorders may differ between populations. Recognition of a particular clinical phenotype, combined with information about the ethnic origin of patients could lead to early and correct diagnosis and assist the development of future personalized medicine for patients with these disorders. Here, the Movement Disorders in Asia Task Force sought to review genetic movement disorders that are commonly seen in Asia, including Wilson's disease, spinocerebellar ataxias (SCA) types 12, 31, and 36, Gerstmann-Sträussler-Scheinker disease, PLA2G6-related parkinsonism, adult-onset neuronal intranuclear inclusion disease (NIID), and paroxysmal kinesigenic dyskinesia. We also review common disorders seen worldwide with specific mutations or presentations that occur frequently in Asians.

Genotype-Phenotype Correlations for ATX-TBP (SCA17): MDSGene Systematic Review

Spinocerebellar ataxia type 17 or ATX-TBP is a CAG/CAA repeat expansion disorder characterized by marked clinical heterogeneity. Reports of affected carriers with subthreshold repeat expansions and of patients with Parkinson's disease (PD) with expanded repeats have cast doubt on the established cutoff values of the expansions and the phenotypic spectrum of this disorder. The objective of this systematic review was to explore the genotype-phenotype relationships for repeat expansions in TBP to delineate the ATX-TBP phenotype and reevaluate the pathological range of repeat expansions. The International Parkinson and Movement Disorder Society Genetic Mutation Database (MDSGene) standardized data extraction protocol was followed. Clinically affected carriers of reported ATX-TBP expansions were included. Publications that contained repeat sizes in screened cohorts of patients with PD and/or healthy individuals were included for a separate evaluation of cutoff values. Phenotypic and genotypic data for 346 ATX-TBP patients were curated. Overall, 97.7% of the patients had ≥41 repeats, while 99.6% of patients with PD and 99.9% of healthy individuals had ≤42 repeats, with a gray zone of reduced penetrance between 41 and 45 repeats. Pure parkinsonism was more common in ATX-TBP patients with 41 to 45 repeats than in the group with ≥46 repeats, which conversely more often presented with a complex phenotype with mixed movement disorders. An updated genotype-phenotype assessment for ATX-TBP is provided, and new repeat expansion cutoff values of reduced penetrance (41-45 expanded repeats) and full penetrance (46-66 expanded repeats) are proposed. These adjusted cutoff values will have diagnostic and counseling implications and may guide future clinical trial protocol. © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Genetic Testing in Clinical Movement Disorders: A Case-Based Review

Genetics are fundamental to understanding the pathophysiology of neurological disease, including movement disorders. Genetic testing in clinical practice has changed dramatically over the last few decades. While the likelihood of establishing an etiological diagnosis is greater now with increased access to testing and more advanced technologies, clinicians face challenges when deciding whether to test, then selecting the appropriate test, and ultimately interpreting and sharing the results with patients and families. In this review, we use a case-based approach to cover core aspects of genetic testing for the neurologist, namely, genetic testing in Parkinson's disease, interpretation of inconclusive genetic test reports, and genetic testing for repeat expansion disorders using Huntington disease as a prototype.

The role of structural variations in Alzheimer's disease and other neurodegenerative diseases

Dozens of single nucleotide polymorphisms (SNPs) related to Alzheimer's disease (AD) have been discovered by large scale genome-wide association studies (GWASs). However, only a small portion of the genetic component of AD can be explained by SNPs observed from GWAS. Structural variation (SV) can be a major contributor to the missing heritability of AD; while SV in AD remains largely unexplored as the accurate detection of SVs from the widely used array-based and short-read technology are still far from perfect. Here, we briefly summarized the strengths and weaknesses of available SV detection methods. We reviewed the current landscape of SV analysis in AD and SVs that have been found associated with AD. Particularly, the importance of currently less explored SVs, including insertions, inversions, short tandem repeats, and transposable elements in neurodegenerative diseases were highlighted.

Neurogenetic motor disorders

Advances in the field of neurogenetics have practical applications in rapid diagnosis on blood and body fluids to extract DNA, obviating the need for invasive investigations. The ability to obtain a presymptomatic diagnosis through genetic screening and biomarkers can be a guide to life-saving disease-modifying therapy or enzyme replacement therapy to compensate for the deficient disease-causing enzyme. The benefits of a comprehensive neurogenetic evaluation extend to family members in whom identification of the causal gene defect ensures carrier detection and at-risk counseling for future generations. This chapter explores the many facets of the neurogenetic evaluation in adult and pediatric motor disorders as a primer for later chapters in this volume and a roadmap for the future applications of genetics in neurology.

Movement disorders and neuropathies: overlaps and mimics in clinical practice

Movement disorders as well as peripheral neuropathies are extremely frequent in the general population; therefore, it is not uncommon to encounter patients with both these conditions. Often, the coexistence is coincidental, due to the high incidence of common causes of peripheral neuropathy, such as diabetes and other age-related disorders, as well as of Parkinson disease (PD), which has a typical late onset. Nonetheless, there is broad evidence that PD patients may commonly develop a sensory and/or autonomic polyneuropathy, triggered by intrinsic and/or extrinsic mechanisms. Similarly, some peripheral neuropathies may develop some movement disorders in the long run, such as tremor, and rarely dystonia and myoclonus, suggesting that central mechanisms may ensue in the pathogenesis of these diseases. Although rare, several acquired or hereditary causes may be responsible for the combination of movement and peripheral nerve disorders as a unique entity, some of which are potentially treatable, including paraneoplastic, autoimmune and nutritional aetiologies. Finally, genetic causes should be pursued in case of positive family history, young onset or multisystemic involvement, and examined for neuroacanthocytosis, spinocerebellar ataxias, mitochondrial disorders and less common causes of adult-onset cerebellar ataxias and spastic paraparesis. Deep phenotyping in terms of neurological and general examination, as well as laboratory tests, neuroimaging, neurophysiology, and next-generation genetic analysis, may guide the clinician toward the correct diagnosis and management.

Spinocerebellar ataxias in Asia: Prevalence, phenotypes and management

This paper reviews and summarizes three main aspects of spinocerebellar ataxias (SCA) in the Asian population. First, epidemiological studies were comprehensively reviewed. Overall, the most common subtypes include SCA1, SCA2, SCA3, and SCA6, but there are large differences in the relative prevalence of these and other SCA subtypes between Asian countries. Some subtypes such as SCA12 and SCA31 are rather specific to certain Asian populations. Second, we summarized distinctive phenotypic manifestations of SCA patients of Asian origin, for example a frequent co-occurrence of parkinsonism in some SCA subtypes. Lastly, we have conducted an exploratory survey study to map SCA-specific expertise, resources, and management in various Asian countries. This showed large differences in accessibility, genetic testing facilities, and treatment options between lower and higher income Asian countries. Currently, many Asian SCA patients remain without a final genetic diagnosis. Lack of prevalence data on SCA, lack of patient registries, and insufficient access to genetic testing facilities hamper a wider understanding of these diseases in several (particularly lower income) Asian countries.

NOTCH2NLC-related disorders: the widening spectrum and genotype-phenotype correlation

GGC repeat expansion in the 5' untranslated region of NOTCH2NLC is the most common causative factor in neuronal intranuclear inclusion disease (NIID) in Asians. Such expanded GGC repeats have been identified in patients with leukoencephalopathy, essential tremor (ET), multiple system atrophy, Parkinson's disease (PD), amyotrophic lateral sclerosis and oculopharyngodistal myopathy (OPDM). Herein, we review the recently reported NOTCH2NLC-related disorders and potential disease-causing mechanisms. We found that visual abnormalities may be NOTCH2NLC-specific and should be investigated in other patients with NOTCH2NLC mutations. NOTCH2NLC GGC repeat expansion was rarely identified in patients of European ancestry, whereas the actual prevalence of the expansion in European patients may be potentially higher than reported, and the CGG repeats in LRP12/GIPC1 are suggested to be screened in European patients with NIID. The repeat size and interruptions in NOTCH2NLC GGC expansion confer pleiotropic effects on clinical phenotype, a pure and stable ET phenotype may be an early symptom of NIID, and GGC repeats in NOTCH2NLC possibly give rise to ET. An association may also exist between intermediate-length NOTCH2NLC GGC repeat expansion and patients affected by PD and ET. NOTCH2NLC-OPDM highly resembles NOTCH2NLC-NIID, the two disorders may be the variations of a single neurodegenerative disease, and there may be a disease-causing upper limit in size of GGC repeats in NOTCH2NLC, repeats over which may be non-pathogenic. The haploinsufficiency of NOTCH2NLC may not be primarily involved in NOTCH2NLC-related disorders and a toxic gain-of-function mechanism possibly drives the pathogenesis of neurodegeneration in patients with NOTCH2NLC-associated disorders.

Characterization of FMR1 Repeat Expansion and Intragenic Variants by Indirect Sequence Capture

Traditional methods for the analysis of repeat expansions, which underlie genetic disorders, such as fragile X syndrome (FXS), lack single-nucleotide resolution in repeat analysis and the ability to characterize causative variants outside the repeat array. These drawbacks can be overcome by long-read and short-read sequencing, respectively. However, the routine application of next-generation sequencing in the clinic requires target enrichment, and none of the available methods allows parallel analysis of long-DNA fragments using both sequencing technologies. In this study, we investigated the use of indirect sequence capture (Xdrop technology) coupled to Nanopore and Illumina sequencing to characterize FMR1, the gene responsible of FXS. We achieved the efficient enrichment (> 200×) of large target DNA fragments (~60-80 kbp) encompassing the entire FMR1 gene. The analysis of Xdrop-enriched samples by Nanopore long-read sequencing allowed the complete characterization of repeat lengths in samples with normal, pre-mutation, and full mutation status (> 1 kbp), and correctly identified repeat interruptions relevant for disease prognosis and transmission. Single-nucleotide variants (SNVs) and small insertions/deletions (indels) could be detected in the same samples by Illumina short-read sequencing, completing the mutational testing through the identification of pathogenic variants within the FMR1 gene, when no typical CGG repeat expansion is detected. The study successfully demonstrated the parallel analysis of repeat expansions and SNVs/indels in the FMR1 gene at single-nucleotide resolution by combining Xdrop enrichment with two next-generation sequencing approaches. With the appropriate optimization necessary for the clinical settings, the system could facilitate both the study of genotype-phenotype correlation in FXS and enable a more efficient diagnosis and genetic counseling for patients and their relatives.

Toxicity of pathogenic ataxin-2 in Drosophila shows dependence on a pure CAG repeat sequence

Spinocerebellar ataxia type 2 is a polyglutamine (polyQ) disease associated with an expanded polyQ domain within the protein product of the ATXN2 gene. Interestingly, polyQ repeat expansions in ATXN2 are also associated with amyotrophic lateral sclerosis (ALS) and parkinsonism depending upon the length of the polyQ repeat expansion. The sequence encoding the polyQ repeat also varies with disease presentation: a pure CAG repeat is associated with SCA2, whereas the CAG repeat in ALS and parkinsonism is typically interrupted with the glutamine encoding CAA codon. Here, we asked if the purity of the CAG sequence encoding the polyQ repeat in ATXN2 could impact the toxicity of the ataxin-2 protein in vivo in Drosophila. We found that ataxin-2 encoded by a pure CAG repeat conferred toxicity in the retina and nervous system, whereas ataxin-2 encoded by a CAA-interrupted repeat or CAA-only repeat failed to confer toxicity, despite expression of the protein at similar levels. Furthermore, the CAG-encoded ataxin-2 protein aggregated in the fly eye, while ataxin-2 encoded by either a CAA/G or CAA repeat remained diffuse. The toxicity of the CAG-encoded ataxin-2 protein was also sensitive to the translation factor eIF4H, a known modifier of the toxic GGGGCC repeat in flies. These data indicate that ataxin-2 encoded by a pure CAG versus interrupted CAA/G polyQ repeat domain is associated with differential toxicity, indicating that mechanisms associated with the purity of the sequence of the polyQ domain contribute to disease.

Atypical Phenotype in a Spinocerebellar Ataxia Type 2 Kindred

Background

Non-ataxic manifestations in autosomal dominant cerebellar ataxias are variable and influenced by CAG repeat length and age at onset. This report describes a genetically proven SCA2 kindred with an atypical phenotype resembling SCA3.

Case Report

The phenotype of five genetically proven patients with SCA2 in this report differed from the typical phenotype owing to persistence of reflexes late into the course of illness, absence of peripheral neuropathy, and very prominent facial twitches.

Discussion

Despite descriptions of typical phenotypes of SCA, significant variations occur, especially within kindreds. Caution should be exercised in clinical diagnoses of SCA, especially with atypical features.

Recalling the pathology of Parkinson's disease; lacking exact figure of prevalence and genetic evidence in Asia with an alarming outcome: A time to step-up

Parkinson's disease (PD) is the second most common and progressive neurodegenerative disease globally, with major symptoms like bradykinesia, impaired posture, and tremor. Several genetic and environmental factors have been identified but elucidating the main factors have been challenging due to the disease's complex nature. Diagnosis, prognosis, and management of such diseases are challenging and require effective targeted attention in developing countries. Recently, PD is growing rapidly in many crowded Asian countries as an alarming threat with inadequate knowledge of its prevalence, genetic architecture, and geographic distribution. This study gave an in-depth overview of the prevalence, incidence and genomic/genetics studies published so far in the Asian population. To the best of our knowledge, PD has increased significantly in several Asian countries, including China, South Korea, Japan, Thailand, and Israel over the past few years, requiring a greater level of care and attention. Genetic screening of families with PD at national levels and establishing an official database of PD cases are essential to get a comprehensive and conclusive view of the exact prevalence and genetic diversity of PD in the Asian population to properly manage and treat the disease.

30 years of repeat expansion disorders: What have we learned and what are the remaining challenges?

Tandem repeats represent one of the most abundant class of variations in human genomes, which are polymorphic by nature and become highly unstable in a length-dependent manner. The expansion of repeat length across generations is a well-established process that results in human disorders mainly affecting the central nervous system. At least 50 disorders associated with expansion loci have been described to date, with half recognized only in the last ten years, as prior methodological difficulties limited their identification. These limitations still apply to the current widely used molecular diagnostic methods (exome or gene panels) and thus result in missed diagnosis detrimental to affected individuals and their families, especially for disorders that are very rare and/or clinically not recognizable. Most of these disorders have been identified through family-driven approaches and many others likely remain to be identified. The recent development of long-read technologies provides a unique opportunity to systematically investigate the contribution of tandem repeats and repeat expansions to the genetic architecture of human disorders. In this review, we summarize the current and most recent knowledge about the genetics of repeat expansion disorders and the diversity of their pathophysiological mechanisms and outline the perspectives of developing personalized treatments in the future.

Conventional MRI findings in hereditary degenerative ataxias: a pictorial review

Purpose

Cerebellar ataxias are a large and heterogeneous group of disorders. The evaluation of brain parenchyma via MRI plays a central role in the diagnostic assessment of these conditions, being mandatory to exclude the presence of other underlying causes in determining the clinical phenotype. Once these possible causes are ruled out, the diagnosis is usually researched in the wide range of hereditary or sporadic ataxias.

Methods

We here propose a review of the main clinical and conventional imaging findings of the most common hereditary degenerative ataxias, to help neuroradiologists in the evaluation of these patients.

Results

Hereditary degenerative ataxias are all usually characterized from a neuroimaging standpoint by the presence, in almost all cases, of cerebellar atrophy. Nevertheless, a proper assessment of imaging data, extending beyond the mere evaluation of cerebellar atrophy, evaluating also the pattern of volume loss as well as concomitant MRI signs, is crucial to achieve a proper diagnosis.

Conclusion

The integration of typical neuroradiological characteristics, along with patient’s clinical history and laboratory data, could allow the neuroradiologist to identify some conditions and exclude others, addressing the neurologist to the more appropriate genetic testing.

What is the Pathogenic CAG Expansion Length in Huntington's Disease?

Huntington's disease (HD) (OMIM 143100) is caused by an expanded CAG repeat tract in the HTT gene. The inherited CAG length is known to expand further in somatic and germline cells in HD subjects. Age at onset of the disease is inversely correlated with the inherited CAG length, but is further modulated by a series of genetic modifiers which are most likely to act on the CAG repeat in HTT that permit it to further expand. Longer repeats are more prone to expansions, and this expansion is age dependent and tissue-specific. Given that the inherited tract expands through life and most subjects develop disease in mid-life, this implies that in cells that degenerate, the CAG length is likely to be longer than the inherited length. These findings suggest two thresholds- the inherited CAG length which permits further expansion, and the intracellular pathogenic threshold, above which cells become dysfunctional and die. This two-step mechanism has been previously proposed and modelled mathematically to give an intracellular pathogenic threshold at a tract length of 115 CAG (95% confidence intervals 70- 165 CAG). Empirically, the intracellular pathogenic threshold is difficult to determine. Clues from studies of people and models of HD, and from other diseases caused by expanded repeat tracts, place this threshold between 60- 100 CAG, most likely towards the upper part of that range. We assess this evidence and discuss how the intracellular pathogenic threshold in manifest disease might be better determined. Knowing the cellular pathogenic threshold would be informative for both understanding the mechanism in HD and deploying treatments.

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.

Essential phenotypes of NOTCH2NLC-related repeat expansion disorder

This scientific commentary refers to ‘Expansion of GGC repeat in human-specific NOTCH2NLC gene is associated with essential tremor’ by Sun et al. (doi: 10.1093/brain/awz372).

Dynamics of strand slippage in DNA hairpins formed by CAG repeats: roles of sequence parity and trinucleotide interrupts

DNA trinucleotide repeats (TRs) can exhibit dynamic expansions by integer numbers of trinucleotides that lead to neurodegenerative disorders. Strand slipped hairpins during DNA replication, repair and/or recombination may contribute to TR expansion. Here, we combine single-molecule FRET experiments and molecular dynamics studies to elucidate slipping dynamics and conformations of (CAG)_n TR hairpins. We directly resolve slipping by predominantly two CAG units. The slipping kinetics depends on the even/odd repeat parity. The populated states suggest greater stability for 5′-AGCA-3′ tetraloops, compared with alternative 5′-CAG-3′ triloops. To accommodate the tetraloop, even(odd)-numbered repeats have an even(odd) number of hanging bases in the hairpin stem. In particular, a paired-end tetraloop (no hanging TR) is stable in (CAG)_{n = even}, but such situation cannot occur in (CAG)_{n = odd}, where the hairpin is “frustrated’’ and slips back and forth between states with one TR hanging at the 5′ or 3′ end. Trinucleotide interrupts in the repeating CAG pattern associated with altered disease phenotypes select for specific conformers with favorable loop sequences. Molecular dynamics provide atomic-level insight into the loop configurations. Reducing strand slipping in TR hairpins by sequence interruptions at the loop suggests disease-associated variations impact expansion mechanisms at the level of slipped hairpins.

Parkinsonism in neurodegenerative diseases predominantly presenting with ataxia

The number of molecularly defined degenerative ataxia diseases is rapidly increasing, many of them involving complex multisystemic presentations including parkinsonism. The increasing number of novel ataxia genes -with most of them being ultra-rare - often makes it difficult for clinicians and scientists to identify the molecular diagnosis underlying these ataxia-parkinsonism syndromes. Here we aim to provide an overview on the most frequent diseases and molecular causes underlying ataxia-parkinsonism, focusing both on novel aspects of well-known causes of ataxia-parkinsonism (MSA-C, PSP-C, FXTAS, repeat-expansion spinocerebellar ataxias [SCAs], conventional mutation SCAs) as well as on more recently identified rare genetic causes of ataxia-parkinsonism (AT, POLG, SPG7). We demonstrate that frequency data and phenotype characteristics help to guide diagnostics in patients with unexplained ataxia-parkinsonism, while the newly identified rare genetic causes of ataxia-parkinsonism provide novel insights into molecular key pathways underlying the shared vulnerability of cerebellar and basal ganglia neurons.

Spinocerebellar ataxia 2 in a family with different phenotypes: Two case reports

RATIONALE

Spinocerebellar ataxia 2 (SCA2) is a genetic disease, mainly characterized by ataxia. A number of other neurological symptoms also have been described, such as Parkinsonism, cognitive dysfunction, autonomic dysfunction, even the signs of motor neuron disease and so on. Mostly, In the same family, clinical performance is the same in most cases. Here, we describe a father and his son who suffered from SCA2, but their first manifestations were different.

PATIENT CONCERNS

The father exhibited progressive bradykinesia and rigidity, which resulted in the dysfunction of walking and caring himself. He hoped to relieve his symptoms by taking medicine. But the son presented with ataxia which was mild that the discomfort did not affect his daily life with none treated.

DIAGNOSIS

Both of them were given SCA2 tests. Briefly, we designed primers around the CAG trinucleotide, repeated the spinal cerebellar ataxia subtype gene, performed PCR expansion, and then calculated the specific number of repetitions by capillary electrophoresis. Abnormal expansion was detected in them through SCA2 sequencing with different repeat numbers of CAG, and then they were diagnosed with SCA2 sequencing.

INTERVENTIONS

The father was treated with dopaminergic drugs, but the son was not administered treatment.

OUTCOMES

The father's symptoms are improved and he can take care of himself. The son has none difficulty in his daily life.

LESSONS

It is rare that different individuals in the same family with SCA2 have different manifestations. The genetic testing is a crucial method to diagnose the disease of SCA2.

Spinocerebellar ataxias: prospects and challenges for therapy development

The spinocerebellar ataxias (SCAs) comprise more than 40 autosomal dominant neurodegenerative disorders that present principally with progressive ataxia. Within the past few years, studies of pathogenic mechanisms in the SCAs have led to the development of promising therapeutic strategies, especially for SCAs caused by polyglutamine-coding CAG repeats. Nucleotide-based gene-silencing approaches that target the first steps in the pathogenic cascade are one promising approach not only for polyglutamine SCAs but also for the many other SCAs caused by toxic mutant proteins or RNA. For these and other emerging therapeutic strategies, well-coordinated preparation is needed for fruitful clinical trials. To accomplish this goal, investigators from the United States and Europe are now collaborating to share data from their respective SCA cohorts. Increased knowledge of the natural history of SCAs, including of the premanifest and early symptomatic stages of disease, will improve the prospects for success in clinical trials of disease-modifying drugs. In addition, investigators are seeking validated clinical outcome measures that demonstrate responsiveness to changes in SCA populations. Findings suggest that MRI and magnetic resonance spectroscopy biomarkers will provide objective biological readouts of disease activity and progression, but more work is needed to establish disease-specific biomarkers that track target engagement in therapeutic trials. Together, these efforts suggest that the development of successful therapies for one or more SCAs is not far away.

Length of Uninterrupted CAG, Independent of Polyglutamine Size, Results in Increased Somatic Instability, Hastening Onset of Huntington Disease

Huntington disease (HD) is caused by a CAG repeat expansion in the huntingtin (HTT) gene. Although the length of this repeat is inversely correlated with age of onset (AOO), it does not fully explain the variability in AOO. We assessed the sequence downstream of the CAG repeat in HTT [reference: (CAG)n-CAA-CAG], since variants within this region have been previously described, but no study of AOO has been performed. These analyses identified a variant that results in complete loss of interrupting (LOI) adenine nucleotides in this region [(CAG)n-CAG-CAG]. Analysis of multiple HD pedigrees showed that this LOI variant is associated with dramatically earlier AOO (average of 25 years) despite the same polyglutamine length as in individuals with the interrupting penultimate CAA codon. This LOI allele is particularly frequent in persons with reduced penetrance alleles who manifest with HD and increases the likelihood of presenting clinically with HD with a CAG of 36-39 repeats. Further, we show that the LOI variant is associated with increased somatic repeat instability, highlighting this as a significant driver of this effect. These findings indicate that the number of uninterrupted CAG repeats, which is lengthened by the LOI, is the most significant contributor to AOO of HD and is more significant than polyglutamine length, which is not altered in these individuals. In addition, we identified another variant in this region, where the CAA-CAG sequence is duplicated, which was associated with later AOO. Identification of these cis-acting modifiers have potentially important implications for genetic counselling in HD-affected families.

Spinocerebellar ataxia

The spinocerebellar ataxias (SCAs) are a genetically heterogeneous group of autosomal dominantly inherited progressive disorders, the clinical hallmark of which is loss of balance and coordination accompanied by slurred speech; onset is most often in adult life. Genetically, SCAs are grouped as repeat expansion SCAs, such as SCA3/Machado-Joseph disease (MJD), and rare SCAs that are caused by non-repeat mutations, such as SCA5. Most SCA mutations cause prominent damage to cerebellar Purkinje neurons with consecutive cerebellar atrophy, although Purkinje neurons are only mildly affected in some SCAs. Furthermore, other parts of the nervous system, such as the spinal cord, basal ganglia and pontine nuclei in the brainstem, can be involved. As there is currently no treatment to slow or halt SCAs (many SCAs lead to premature death), the clinical care of patients with SCA focuses on managing the symptoms through physiotherapy, occupational therapy and speech therapy. Intense research has greatly expanded our understanding of the pathobiology of many SCAs, revealing that they occur via interrelated mechanisms (including proteotoxicity, RNA toxicity and ion channel dysfunction), and has led to the identification of new targets for treatment development. However, the development of effective therapies is hampered by the heterogeneity of the SCAs; specific therapeutic approaches may be required for each disease.

The genetic landscape of Parkinson's disease

The cause of Parkinson's disease (PD) remains unknown in most patients. Since 1997, with the first genetic mutation known to cause PD described in SNCA gene, many other genes with Mendelian inheritance have been identified. We summarize genetic, clinical and neuropathological findings related to the 27 genes reported in the literature since 1997, associated either with autosomal dominant (AD): LRRK2, SNCA, VPS35, GCH1, ATXN2, DNAJC13, TMEM230, GIGYF2, HTRA2, RIC3, EIF4G1, UCHL1, CHCHD2, and GBA; or autosomal recessive (AR) inheritance: PRKN, PINK1, DJ1, ATP13A2, PLA2G6, FBXO7, DNAJC6, SYNJ1, SPG11, VPS13C, PODXL, and PTRHD1; or an X-linked transmission: RAB39B. Clinical and neuropathological variability among genes is great. LRRK2 mutation carriers present a phenotype similar to those with idiopathic PD whereas, depending on the SNCA mutations, the phenotype ranges from early onset typical PD to dementia with Lewy bodies, including many other atypical forms. DNAJC6 nonsense mutations lead to a very severe phenotype whereas DNAJC6 missense mutations cause a more typical form. PRKN, PINK1 and DJ1 cases present with typical early onset PD with slow progression, whereas other AR genes present severe atypical Parkinsonism. RAB39B is responsible for a typical phenotype in women and a variable phenotype in men. GBA is a major PD risk factor often associated with dementia. A growing number of reported genes described as causal genes (DNAJC13, TMEM230, GIGYF2, HTRA2, RIC3, EIF4G1, UCHL1, and CHCHD2) are still awaiting replication or indeed have not been replicated, thus raising questions as to their pathogenicity. Phenotypic data collection and next generation sequencing of large numbers of cases and controls are needed to differentiate pathogenic dominant mutations with incomplete penetrance from rare, non-pathogenic variants. Although known genes cause a minority of PD cases, their identification will lead to a better understanding their pathological mechanisms, and may contribute to patient care, genetic counselling, prognosis determination and finding new therapeutic targets.

Non-ataxic manifestations of Spinocerebellar ataxia-2, their determinants and predictors

INTRODUCTION

To evaluate the non-ataxic clinical manifestations in genetically proven Spinocerebellar ataxia 2 (SCA2) and identify their determinants and predictors.

METHODS

Seventy-three subjects with genetically proven SCA2 were evaluated clinically for the common non-ataxic manifestations. Based on the presence or absence of non-ataxic manifestations, patients were classified into groups and then compared for significant differences in the CAG repeat length, age at onset (AAO), duration of disease, and ataxia rating score. Predictors of non-ataxic symptoms were identified using multivariable binary logistic regression.

RESULTS

The most common non-ataxic clinical manifestations were peripheral neuropathy, extrapyramidal features, pyramidal signs, cognitive impairment and lower motor neuron signs. The CAG repeat length was inversely related to the AAO of symptoms (r = -0.46, p < .001). Patients with peripheral neuropathy and psychiatric symptoms had earlier AAO. Patients with cognitive impairment and extrapyramidal symptoms had higher CAG repeat length whereas presence of lower motor neuron signs was more common in patients with lower CAG repeat length.

CONCLUSION

The lower strength of association between CAG repeat length and AAO in our cohort suggests the presence of additional factors underlying the variability in AAO. Both CAG repeat length and AAO were identified as significant determinants and predictors of non-ataxic symptoms.

Parkinson's Disease: Biomarkers, Treatment, and Risk Factors

Parkinson's disease (PD) is a progressive neurodegenerative disorder caused mainly by lack of dopamine in the brain. Dopamine is a neurotransmitter involved in movement, motivation, memory, and other functions; its level is decreased in PD brain as a result of dopaminergic cell death. Dopamine loss in PD brain is a cause of motor deficiency and, possibly, a reason of the cognitive deficit observed in some PD patients. PD is mostly not recognized in its early stage because of a long latency between the first damage to dopaminergic cells and the onset of clinical symptoms. Therefore, it is very important to find reliable molecular biomarkers that can distinguish PD from other conditions, monitor its progression, or give an indication of a positive response to a therapeutic intervention. PD biomarkers can be subdivided into four main types: clinical, imaging, biochemical, and genetic. For a long time protein biomarkers, dopamine metabolites, amino acids, etc. in blood, serum, cerebrospinal liquid (CSF) were considered the most promising. Among the candidate biomarkers that have been tested, various forms of α-synuclein (α-syn), i.e., soluble, aggregated, post-translationally modified, etc. were considered potentially the most efficient. However, the encouraging recent results suggest that microRNA-based analysis may bring considerable progress, especially if it is combined with α-syn data. Another promising analysis is the advanced metabolite profiling of body fluids, called "metabolomics" which may uncover metabolic fingerprints specific for various stages of PD. Conventional pharmacological treatment of PD is based on the replacement of dopamine using dopamine precursors (levodopa, L-DOPA, L-3,4 dihydroxyphenylalanine), dopamine agonists (amantadine, apomorphine) and MAO-B inhibitors (selegiline, rasagiline), which can be used alone or in combination with each other. Potential risk factors include environmental toxins, drugs, pesticides, brain microtrauma, focal cerebrovascular damage, and genomic defects. This review covers molecules that might act as the biomarkers of PD. Then, PD risk factors (including genetics and non-genetic factors) and PD treatment options are discussed.

Spinocerebellar Ataxia Tethering PCR: A Rapid Genetic Test for the Diagnosis of Spinocerebellar Ataxia Types 1, 2, 3, 6, and 7 by PCR and Capillary Electrophoresis

Spinocerebellar ataxia (SCA) types 1, 2, 3, 6, and 7, associated with a (CAG)_n repeat expansion in coding sequences, are the most prevalent autosomal dominant ataxias worldwide (approximately 60% of the cases). In addition, the phenotype of SCA2 expansions has been now extended to Parkinson disease and amyotrophic lateral sclerosis. Their diagnosis is currently based on a PCR to identify small expanded alleles, followed by a second-level test whenever a false normal homozygous or a CAT interruption in SCA1 needs to be verified. Next-generation sequencing still does not allow efficient detection of these repeats. Here, we show the efficacy of a novel, rapid, and cost-effective method to identify and size pathogenic expansions in SCA1, 2, 3, 6, and 7 and recognize large alleles or interruptions without a second-level test. Twenty-five healthy controls and 33 expansion carriers were analyzed: alleles migrated consistently in different PCRs and capillary runs, and homozygous individuals were always distinguishable from heterozygous carriers of both common and large (>100 repeats) pathogenic CAG expansions. Repeat number could be calculated counting the number of peaks, except for the largest SCA2 and SCA7 alleles. Interruptions in SCA1 were always visible. Overall, our method allows a simpler, cost-effective, and sensibly faster SCA diagnostic protocol compared with the standard technique and to the still unadapted next-generation sequencing.

The central role of DNA damage and repair in CAG repeat diseases

Diseases such as Huntington's disease and certain spinocerebellar ataxias are caused by the expansion of genomic cytosine-adenine-guanine (CAG) trinucleotide repeats beyond a specific threshold. These diseases are all characterised by neurological symptoms and central neurodegeneration, but our understanding of how expanded repeats drive neuronal loss is incomplete. Recent human genetic evidence implicates DNA repair pathways, especially mismatch repair, in modifying the onset and progression of CAG repeat diseases. Repair pathways might operate directly on repeat sequences by licensing or inhibiting repeat expansion in neurons. Alternatively, or in addition, because many of the genes containing pathogenic CAG repeats encode proteins that themselves have roles in the DNA damage response, it is possible that repeat expansions impair specific DNA repair pathways. DNA damage could then accrue in neurons, leading to further expansion at repeat loci, thus setting up a vicious cycle of pathology. In this review, we consider DNA damage and repair pathways in postmitotic neurons in the context of disease-causing CAG repeats. Investigating and understanding these pathways, which are clearly relevant in promoting and ameliorating disease in humans, is a research priority, as they are known to modify disease and therefore constitute prevalidated drug targets.

NESSCA Validation and Responsiveness of Several Rating Scales in Spinocerebellar Ataxia Type 2

Spinocerebellar ataxia type 2 (SCA2), caused by a CAG expansion (CAGexp) at ATXN2, has a complex clinical picture. While validated ataxia scales are available, comprehensive instruments to measure all SCA2 neurological manifestations are required. This study aims to validate the Neurological Examination Score for the assessment of Spinocerebellar Ataxias (NESSCA) to be used in SCA2 and to compare its responsiveness to those obtained with other instruments. NESSCA, SARA, SCAFI, and CCFS scales were applied in symptomatic SCA2 patients. Correlations were done with age at onset, disease duration, CAGexp, and between scales. Responsiveness was estimated by comparing deltas of stable to worse patients after 12 months, according to Patient Global Impression of change, and the area under the curve (AUC) of the Receiver Operating Characteristics curve of scores range. Eighty-eight evaluations (49 patients) were obtained. NESSCA had an even distribution and correlated with disease duration (r = 0.55), SARA (r = 0.63), and CAGexp (rho = 0.32): both explained 44% of NESSCA variance. Deltas (95% CI) after 1 year in stable and worse patients were only significantly different for SARA. NESSCA, SARA, SCAFI, and CCFS AUC were 0.63, 0.81, 0.49, and 0.48, respectively. NESSCA is valid to be used in SCA2. However, the only instrument that presented good responsiveness to change in 1 year was SARA. We suggest that NESSCA can be used as a secondary outcome in future trials in SCA2 due to the burden of neurological disabilities related to disease progression.