The ins and outs of a polyglutamine neurodegenerative disease: spinocerebellar ataxia type 1 (SCA1)

Advances in Modeling Polyglutamine Diseases Using Genome Editing Tools

Polyglutamine (polyQ) diseases, including Huntington’s disease, are a group of late-onset progressive neurological disorders caused by CAG repeat expansions. Although recently, many studies have investigated the pathological features and development of polyQ diseases, many questions remain unanswered. The advancement of new gene-editing technologies, especially the CRISPR-Cas9 technique, has undeniable value for the generation of relevant polyQ models, which substantially support the research process. Here, we review how these tools have been used to correct disease-causing mutations or create isogenic cell lines with different numbers of CAG repeats. We characterize various cellular models such as HEK 293 cells, patient-derived fibroblasts, human embryonic stem cells (hESCs), induced pluripotent stem cells (iPSCs) and animal models generated with the use of genome-editing technology.

5'UTR-mediated regulation of Ataxin-1 expression

Expression of mutant Ataxin-1 with an abnormally expanded polyglutamine domain is necessary for the onset and progression of spinocerebellar ataxia type 1 (SCA1). Understanding how Ataxin-1 expression is regulated in the human brain could inspire novel molecular therapies for this fatal, dominantly inherited neurodegenerative disease. Previous studies have shown that the ATXN1 3'UTR plays a key role in regulating the Ataxin-1 cellular pool via diverse post-transcriptional mechanisms. Here we show that elements within the ATXN1 5'UTR also participate in the regulation of Ataxin-1 expression. PCR and PacBio sequencing analysis of cDNA obtained from control and SCA1 human brain samples revealed the presence of three major, alternatively spliced ATXN1 5'UTR variants. In cell-based assays, fusion of these variants upstream of an EGFP reporter construct revealed significant and differential impacts on total EGFP protein output, uncovering a type of genetic rheostat-like function of the ATXN1 5'UTR. We identified ribosomal scanning of upstream AUG codons and increased transcript instability as potential mechanisms of regulation. Importantly, transcript-based analyses revealed significant differences in the expression pattern of ATXN1 5'UTR variants between control and SCA1 cerebellum. Together, the data presented here shed light into a previously unknown role for the ATXN1 5'UTR in the regulation of Ataxin-1 and provide new opportunities for the development of SCA1 therapeutics.

Analysis of autosomal dominant spinocerebellar ataxia type 1 in an extended family of central India

BACKGROUND

Spinocerebeller ataxia type 1 (SCA1) is a specific type of ataxia among a group of inherited diseases of the central nervous system. In SCA1, genetic defects lead to impairment of specific nerve fibers carrying messages to and from the brain, resulting in the degeneration of the cerebellum, the coordination center of the brain. We investigated 24 members of an extended family in Gwalior city, India, some of which were earlier clinically diagnosed to be suffering from yet unconfirmed type of SCA neurodegenerative disorder.

MATERIALS AND METHODS

All the family members from each age group were screened clinically and the characteristics of those resembling with ataxia were recorded for diagnosis by MRI. The confirmed patients of the family were genetically tested by PCR based molecular testing to identify the type of SCA (i.e., SCA 1, 2, 3, 4, 6 or 7). Family tree of the disease inheritance was constructed by pedigree based method.

RESULT AND CONCLUSION

We found the clinical (symptoms and MRI) and genetic (Pedigree and PCR) results to be correlated. The PCR result revealed the disease to be of SCA 1 type being inherited in the family.

Impairment of spinal motor neurons in spinocerebellar ataxia type 1-knock-in mice

Spinocerebellar ataxia type 1 (SCA1) is an autosomal dominant neurodegenerative disorder caused by the expansion of polyglutamine repeats in the Ataxin-1 protein. An accumulating body of cerebellar, histological and behavioral analyses has proven that SCA1-knock-in mice (in which the endogenous Atxn1 gene is replaced with mutant Atxn1 that has abnormally expanded 154 CAG repeats) work as a good tool, which resembles the central nervous system pathology of SCA1 patients. However, the peripheral nervous system pathology of the model mice has not been studied despite the fact that the clinical manifestation is also characterized by peripheral involvement. We show here that spinal motor neurons are degenerated in SCA1-knock-in mice. Histologically, some spinal motor neurons of the SCA1-knock-in mice have polyglutamine aggregates in their nuclei and also thinner and demyelinated axons. Electrophysiological examinations of the mice showed slower nerve conduction velocities in spinal motor neurons and lower amplitudes of muscle action potential, compared to wild-type mice. Consistently, the mice displayed decrease in rearing number and total rearing time. These results suggest that the knock-in mice serve as a definite model that reproduces peripheral involvement and are therefore useful for research on the peripheral nervous system pathology in SCA1 patients.

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.

Autosomal dominant cerebellar ataxia type I: a review of the phenotypic and genotypic characteristics

Type I autosomal dominant cerebellar ataxia (ADCA) is a type of spinocerebellar ataxia (SCA) characterized by ataxia with other neurological signs, including oculomotor disturbances, cognitive deficits, pyramidal and extrapyramidal dysfunction, bulbar, spinal and peripheral nervous system involvement. The global prevalence of this disease is not known. The most common type I ADCA is SCA3 followed by SCA2, SCA1, and SCA8, in descending order. Founder effects no doubt contribute to the variable prevalence between populations. Onset is usually in adulthood but cases of presentation in childhood have been reported. Clinical features vary depending on the SCA subtype but by definition include ataxia associated with other neurological manifestations. The clinical spectrum ranges from pure cerebellar signs to constellations including spinal cord and peripheral nerve disease, cognitive impairment, cerebellar or supranuclear ophthalmologic signs, psychiatric problems, and seizures. Cerebellar ataxia can affect virtually any body part causing movement abnormalities. Gait, truncal, and limb ataxia are often the most obvious cerebellar findings though nystagmus, saccadic abnormalities, and dysarthria are usually associated. To date, 21 subtypes have been identified: SCA1-SCA4, SCA8, SCA10, SCA12-SCA14, SCA15/16, SCA17-SCA23, SCA25, SCA27, SCA28 and dentatorubral pallidoluysian atrophy (DRPLA). Type I ADCA can be further divided based on the proposed pathogenetic mechanism into 3 subclasses: subclass 1 includes type I ADCA caused by CAG repeat expansions such as SCA1-SCA3, SCA17, and DRPLA, subclass 2 includes trinucleotide repeat expansions that fall outside of the protein-coding regions of the disease gene including SCA8, SCA10 and SCA12. Subclass 3 contains disorders caused by specific gene deletions, missense mutation, and nonsense mutation and includes SCA13, SCA14, SCA15/16, SCA27 and SCA28. Diagnosis is based on clinical history, physical examination, genetic molecular testing, and exclusion of other diseases. Differential diagnosis is broad and includes secondary ataxias caused by drug or toxic effects, nutritional deficiencies, endocrinopathies, infections and post-infection states, structural abnormalities, paraneoplastic conditions and certain neurodegenerative disorders. Given the autosomal dominant pattern of inheritance, genetic counseling is essential and best performed in specialized genetic clinics. There are currently no known effective treatments to modify disease progression. Care is therefore supportive. Occupational and physical therapy for gait dysfunction and speech therapy for dysarthria is essential. Prognosis is variable depending on the type of ADCA and even among kindreds.

Molecular pathogenesis of spinocerebellar ataxia type 1 disease

Spinocerebellar ataxia type 1 (SCA1) is an autosomal-dominant neurodegenerative disorder characterized by ataxia and progressive motor deterioration. SCA1 is associated with an elongated polyglutamine tract in ataxin-1, the SCA1 gene product. As summarized in this review, recent studies have clarified the molecular mechanisms of SCA1 pathogenesis and provided direction for future therapeutic approaches. The nucleus is the subcellular site where misfolded mutant ataxin-1 acts to cause SCA1 disease in the cerebellum. The role of these nuclear aggregates is the subject of intensive study. Additional proteins have been identified, whose conformational alterations occurring through interactions with the polyglutamine tract itself or non-polyglutamine regions in ataxin-1 are the cause of SCA-1 cytotoxicity. Therapeutic hope comes from the observations concerning the reduction of nuclear aggregation and alleviation of the pathogenic phenotype by the application of potent inhibitors and RNA interference.

Antisense RNA sequences modulating the ataxin-1 message: molecular model of gene therapy for spinocerebellar ataxia type 1, a dominant-acting unstable trinucleotide repeat disease

Spinocerebellar ataxia type 1 (SCA1) is a dominant inherited disease caused by expanded trinucleotide repeats resulting in an increased polyglutamine tract in the gene product. As a potential therapeutic approach for SCA1, we tested antisense RNAs targeting two regions of the ataxin-1 message. Single-stranded regions around the translational initiation site and the intron 8 splice donor site of the ataxin-1 message were identified by computer-assisted RNA secondary structure prediction. Plasmids were generated to contain a 254-bp antisense sequence spanning the translation initiation site (pLasBDini) or a 317-bp sequence spanning the intron 8 splice donor site (pLasBDei) of the ataxin-1 message. These plasmids were transfected into Chinese hamster ovary cells engineered to express either expanded or unexpanded ataxin-1 message and protein. Reduced levels of mutant ataxin-1 message (82 CAG repeats), wild-type ataxin-1 message (30 CAG repeats), and ataxin-1 protein were observed by Northern and Western blot analyses in pLasBDini-transfected clones. pLasBDei-transfected 293 cells exhibited a shift in ataxin-1 message to a size several kilobases longer than that of the natural message. Reverse transcriptase/polymerase chain reaction assays demonstrated the retention of message spanning the intron 8 splice acceptor and the inability to amplify sequences between exons 8 and 9, implying that normal splicing of intron 8 had been interrupted. We conclude that antisense RNAs were effective in reducing or modifying ataxin-1 messages in transfected cells, and may be an effective genetic strategy for therapy of SCA1 and similar dominant-acting neurological disorders.

Activation of p38MAPK contributes to expanded polyglutamine-induced cytotoxicity

Background

The signaling pathways that may modulate the pathogenesis of diseases induced by expanded polyglutamine proteins are not well understood.

Methodologies/Principal Findings

Herein we demonstrate that expanded polyglutamine protein cytotoxicity is mediated primarily through activation of p38MAPK and that the atypical PKC iota (PKCι) enzyme antagonizes polyglutamine-induced cell death through induction of the ERK signaling pathway. We show that pharmacological blockade of p38MAPK rescues cells from polyglutamine-induced cell death whereas inhibition of ERK recapitulates the sensitivity observed in cells depleted of PKCι by RNA interference. We provide evidence that two unrelated proteins with expanded polyglutamine repeats induce p38MAPK in cultured cells, and demonstrate induction of p38MAPK in an in vivo model of neurodegeneration (spinocerebellar ataxia 1, or SCA-1).

Conclusions/Significance

Taken together, our data implicate activated p38MAPK in disease progression and suggest that its inhibition may represent a rational strategy for therapeutic intervention in the polyglutamine disorders.

Bidirectional expression of CUG and CAG expansion transcripts and intranuclear polyglutamine inclusions in spinocerebellar ataxia type 8

We previously reported that a (CTG)n expansion causes spinocerebellar ataxia type 8 (SCA8), a slowly progressive ataxia with reduced penetrance. We now report a transgenic mouse model in which the full-length human SCA8 mutation is transcribed using its endogenous promoter. (CTG)116 expansion, but not (CTG)11 control lines, develop a progressive neurological phenotype with in vivo imaging showing reduced cerebellar-cortical inhibition. 1C2-positive intranuclear inclusions in cerebellar Purkinje and brainstem neurons in SCA8 expansion mice and human SCA8 autopsy tissue result from translation of a polyglutamine protein, encoded on a previously unidentified antiparallel transcript (ataxin 8, ATXN8) spanning the repeat in the CAG direction. The neurological phenotype in SCA8 BAC expansion but not BAC control lines demonstrates the pathogenicity of the (CTG-CAG)n expansion. Moreover, the expression of noncoding (CUG)n expansion transcripts (ataxin 8 opposite strand, ATXN8OS) and the discovery of intranuclear polyglutamine inclusions suggests SCA8 pathogenesis involves toxic gain-of-function mechanisms at both the protein and RNA levels.

Coexistence of familial transthyretin amyloidosis ATTR Val30Met and spinocerebellar ataxia type 1 in a Japanese family--a follow-up autopsy report

Three brothers in a family with Val30Met transthyretin (TTR) amyloid polyneuropathy (FAP) in Iiyama, Japan were studied pathologically. In this family, affected members have been reported to show typical clinical features of FAP, and some have been documented to exhibit symptoms and signs of central nervous system (CNS) involvement consisting of cerebellar ataxia and pyramidal tract signs. After the original description, this family was regarded as a unique phenotype of this form of FAP; however, subsequent molecular genetic studies revealed that some patients and asymptomatic members in the family had Val30Met TTR and/or spinocerebellar ataxia type 1 (SCA1) gene mutations. In this study, pathological examination of two patients with both FAP and CNS symptoms showed (1) TTR-immunoreactive leptomeningeal and cerebrovascular amyloid deposition compatible with Val30Met TTR FAP, and (2) neuronal loss and gliosis mainly in the Purkinje cell layer, spinocerebellar system, olivo-ponto-cerebellar system, dentato-rubral system, gracile nuclei, cuneate nuclei, and various nuclei of cranial nerves, accompanied by anti-expanded polyglutamine tract antibody positive neuronal intranuclear inclusions, all of which were compatible with the pathological findings of SCA1. On the other hand, the remaining patient with FAP symptoms only showed the former pathological finding alone. The present study demonstrates, at the pathological level, that Val30Met TTR FAP and SCA1 coexist in the same family members, and that the CNS dysfunction seen in the patients in this family is ascribable to SCA1 pathology but not to CNS amyloidosis.

Imperfect CAG repeats form diverse structures in SCA1 transcripts

The expanded CAG repeat in the coding sequence of the spinocerebellar ataxia type 1 (SCA1) gene is responsible for SCA1, one of the hereditary human neurodegenerative diseases. In the normal SCA1 alleles usually 1-3 CAT triplets break the continuity of the CAG repeat tracts. Here we show what is the structural role of the CAU interruptions in the SCA1 transcripts. Depending on their number and localization within the repeat tract the interruptions either enlarge the terminal loop of the hairpin formed by the repeats, nucleate the internal loops in its stem structure, or force the repeats to fold into two smaller hairpins. Thus, the interruptions destabilize the CAG repeat hairpin, which is likely to decrease its ability to participate in the putative RNA pathogenesis mechanism driven by the long CAG repeat hairpins.

Diagnosis of five spinocerebellar ataxia disorders by multiplex amplification and capillary electrophoresis

The autosomal-dominant spinocerebellar ataxias (ADCA) are a heterogeneous group of neurodegenerative disorders with variable expression and phenotypic overlap. An accurate diagnosis relies on detection of a mutation in a specific causative gene, which is typically an abnormal number of CAG trinucleotide repeats. To streamline testing in a clinical setting, we converted our current panel of tests for the spinocerebellar ataxias (SCA) types SCA1, SCA2, SCA3, SCA6, and SCA7 from five independent amplification reactions analyzed by polyacrylamide gel electrophoresis (PAGE) to a single multiplex amplification reaction analyzed by capillary electrophoresis (CE). Multiplex amplification was facilitated by the use of chimeric primers; different lengths and fluorochromes distinguished the amplicons. During CE with commercially available molecular weight standards, the SCA amplicons migrated faster than predicted, thereby underestimating their length compared to that determined previously by PAGE. This was observed to varying degrees for each of the five loci, with the greatest size differential occurring in amplicons with greater (CAG)(n). To determine accurate amplicon length, and therefore an accurate number of CAG repeats, a size correction formula was calculated for each locus. This multiplex semi-automated assay has been reliable during 1 year of use in a clinical setting during which 57 samples were tested and five positive samples were detected.