Possible reduced penetrance of expansion of 44 to 47 CAG/CAA repeats in the TATA-binding protein gene in spinocerebellar ataxia type 17

Molecular Mechanisms of Spinocerebellar Ataxia Type 17

Spinocerebellar ataxia type 17 (SCA17) is a hereditary neurodegenerative disorder characterized by progressive motor and cognitive decline, leading to severe disability and death. SCA17 is caused by a CAG repeat expansion mutation in the TBP gene, resulting in the production of an abnormally long polyglutamine tract, which classifies it as a polyglutamine disorder. At present, there is no effective treatment for SCA17, and existing therapies provide only symptomatic relief. While the exact pathogenic mechanisms of SCA17 remain unclear, the TBP mutation affects a well-characterized transcription factor, making it an ideal model for studying polyglutamine-related neurodegeneration. Here, we review the clinical features of SCA17 and explore proposed mechanisms of its pathogenesis.

Dilemma in differentiation of spinocerebellar ataxia type 17 from Huntington's disease: comorbidity or independent disease?

Both Huntington's disease (HD) and Spinocerebellar ataxia 17 (SCA17) mutations showed expanded CAG repeats, with overlapping clinical manifestation: motor disorders, psychiatric symptoms and cognitive impairments. Therefore, SCA17 is also called Huntington like disease (HD-like, HDL) type 4. In this paper, we reported that one patient had 47 CAG repeats in HTT gene and 42 CAG repeats in TBP gene. There is a dilemma in differentiation of SCA 17 from HD in one patient, never been reported before. Is the diagnosis comorbidity of HD with SCA17 or HD only?

An Update on the Adult-Onset Hereditary Cerebellar Ataxias: Novel Genetic Causes and New Diagnostic Approaches

The hereditary cerebellar ataxias (HCAs) are rare, progressive neurologic disorders caused by variants in many different genes. Inheritance may follow autosomal dominant, autosomal recessive, X-linked or mitochondrial patterns. The list of genes associated with adult-onset cerebellar ataxia is continuously growing, with several new genes discovered in the last few years. This includes short-tandem repeat (STR) expansions in RFC1, causing cerebellar ataxia, neuropathy, vestibular areflexia syndrome (CANVAS), FGF14-GAA causing spinocerebellar ataxia type 27B (SCA27B), and THAP11. In addition, the genetic basis for SCA4, has recently been identified as a STR expansion in ZFHX3. Given the large and growing number of genes, and different gene variant types, the approach to diagnostic testing for adult-onset HCA can be complex. Testing methods include targeted evaluation of STR expansions (e.g. SCAs, Friedreich ataxia, fragile X-associated tremor/ataxia syndrome, dentatorubral-pallidoluysian atrophy), next generation sequencing for conventional variants, which may include targeted gene panels, whole exome, or whole genome sequencing, followed by various potential additional tests. This review proposes a diagnostic approach for clinical testing, highlights the challenges with current testing technologies, and discusses future advances which may overcome these limitations. Implementing long-read sequencing has the potential to transform the diagnostic approach in HCA, with the overall aim to improve the diagnostic yield.

Unraveling the genetic landscape of undiagnosed cerebellar ataxia in Brazilian patients

INTRODUCTION

Hereditary ataxias (HAs) encompass a diverse and genetically intricate group of rare neurodegenerative disorders, presenting diagnostic challenges. Whole-exome sequencing (WES) has significantly improved diagnostic success. This study aimed to elucidate genetic causes of cerebellar ataxia within a diverse Brazilian cohort.

METHODS

Biological samples were collected from individuals with sporadic or familial cerebellar ataxia, spanning various ages and phenotypes, excluding common SCAs and Friedreich ataxia. RFC1 biallelic AAGGG repeat expansion was screened in all patients. For AAGGG-negative cases, WES targeting 441 ataxia-related genes was performed, followed by ExpansionHunter analysis for repeat expansions, including the recently described GGC-ZFHX3. Variant classification adhered to ClinGen guidelines, yielding definitive or probable diagnoses.

RESULTS

The study involved 76 diverse Brazilian families. 16 % received definitive diagnoses, and another 16 % received probable ones. RFC1-related ataxia was predominant, with two definitive cases, followed by KIF1A (one definitive and one probable) and SYNE-1 (two probable). Early-onset cases exhibited higher diagnostic rates. ExpansionHunter improved diagnosis by 4 %.We did not detected GGC-ZFHX3 repeat expansion in this cohort.

CONCLUSION

This study highlights diagnostic complexities in cerebellar ataxia, even with advanced genetic methods. RFC1, KIF1A, and SYNE1 emerged as prevalent mutations. ZFHX3 repeat expansion seem to be rare in Brazilian population. Early-onset cases showed higher diagnostic success. WES coupled with ExpansionHunter holds promise as a primary diagnostic tool, emphasizing the need for broader NGS accessibility in Brazil.

The Diagnostic Landscape of Adult Neurogenetic Disorders

Neurogenetic diseases affect individuals across the lifespan, but accurate diagnosis remains elusive for many patients. Adults with neurogenetic disorders often undergo a long diagnostic odyssey, with multiple specialist evaluations and countless investigations without a satisfactory diagnostic outcome. Reasons for these diagnostic challenges include: (1) clinical features of neurogenetic syndromes are diverse and under-recognized, particularly those of adult-onset, (2) neurogenetic syndromes may manifest with symptoms that span multiple neurological and medical subspecialties, and (3) a positive family history may not be present or readily apparent. Furthermore, there is a large gap in the understanding of how to apply genetic diagnostic tools in adult patients, as most of the published literature focuses on the pediatric population. Despite these challenges, accurate genetic diagnosis is imperative to provide affected individuals and their families guidance on prognosis, recurrence risk, and, for an increasing number of disorders, offer targeted treatment. Here, we provide a framework for recognizing adult neurogenetic syndromes, describe the current diagnostic approach, and highlight studies using next-generation sequencing in different neurological disease cohorts. We also discuss diagnostic pitfalls, barriers to achieving a definitive diagnosis, and emerging technology that may increase the diagnostic yield of testing.

Genetic screening for Huntington disease phenocopies in Sweden: A tertiary center case series focused on short tandem repeat (STR) disorders

OBJECTIVE

To perform a screening for Huntington disease (HD) phenocopies in a Swedish cohort.

METHODS

Seventy-three DNA samples negative for HD were assessed at a tertiary center in Stockholm. The screening included analyses for C9orf72-frontotemporal dementia/amyotrophic lateral sclerosis (C9orf72-FTD/ALS), octapeptide repeat insertions (OPRIs) in PRNP associated with inherited prion diseases (IPD), Huntington's disease-like 2 (HDL2), spinocerebellar ataxia-2 (SCA2), spinocerebellar ataxia 3 (SCA3) and spinocerebellar ataxia-17 (SCA17). Targeted genetic analysis was carried out in two cases based on the salient phenotypic features.

RESULTS

The screening identified two patients with SCA17, one patient with IPD associated with 5-OPRI but none with nucleotide expansions in C9orf72 or for HDL2, SCA2 or SCA3. Furthermore, SGCE-myoclonic-dystonia 11 (SGCE-M-D) and benign hereditary chorea (BHC) was diagnosed in two sporadic cases. WES identified VUS in STUB1 in two patients with predominant cerebellar ataxia.

CONCLUSIONS

Our results are in keeping with previous screenings and suggest that other genes yet to be discovered are involved in the etiology of HD phenocopies.

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.

Complex Ataxia-Dementia Phenotype in Patients with Digenic TBP/STUB1 Spinocerebellar Ataxia

BACKGROUND AND OBJECTIVES

Spinocerebellar ataxias (SCAs) are autosomal dominant disorders with extensive clinical and genetic heterogeneity. We recently identified a form of SCA transmitted with a digenic pattern of inheritance caused by the concomitant presence of an intermediate-length expansion in TATA-box binding protein gene (TBP_40-46 ) and a heterozygous pathogenic variant in the Stip1-homologous and U-Box containing protein 1 gene (STUB1). This SCA^TBP/STUB1 represents the first example of a cerebellar disorder in which digenic inheritance has been identified.

OBJECTIVES

We studied a large cohort of patients with SCA^TBP/STUB1 with the aim of describing specific clinical and neuroimaging features of this distinctive genotype.

METHODS

In this observational study, we recruited 65 affected and unaffected family members from 21 SCA^TBP/STUB1 families and from eight families with monogenic SCA17. Their characteristics and phenotypes were compared with those of 33 age-matched controls.

RESULTS

SCA^TBP/STUB1 patients had multi-domain dementia with a more severe impairment in respect to patient carrying only fully expanded SCA17 alleles. Cerebellar volume and thickness of cerebellar cortex were reduced in SCA^TBP/STUB1 compared with SCA17 patients (P = 0.03; P = 0.008). Basal ganglia volumes were reduced in both patient groups, as compared with controls, whereas brainstem volumes were significantly reduced in SCA^TBP/STUB1 , but not in SCA17 patients.

CONCLUSIONS

The identification of the complex SCA^TBP/STUB1 phenotype may impact on diagnosis and genetic counseling in the families with both hereditary and sporadic ataxia. The independent segregation of TBP and STUB1 alleles needs to be considered for recurrence risk and predictive genetic tests. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Intermediate repeat expansions of TBP and STUB1: Genetic modifier or pure digenic inheritance in spinocerebellar ataxias?

PURPOSE

CAG/CAA repeat expansions in TBP_>49 are responsible for spinocerebellar ataxia (SCA) type 17 (SCA17). We previously detected cosegregation of STUB1 variants causing SCA48 with intermediate alleles of TBP in 2 families. This cosegregation questions the existence of SCA48 as a monogenic disease.

METHODS

We systematically sequenced TBP repeats in 34 probands of dominant ataxia families with STUB1 variants. In addition, we searched for pathogenic STUB1 variants in probands with expanded alleles of TBP_>49 (n = 2) or intermediate alleles of TBP_≥40 (n = 47).

RESULTS

STUB1 variants were found in half of the TBP_40-49 cohort. Mirroring this finding, TBP_40-49 alleles were detected in 40% of STUB1 probands. The longer the TBP repeat length, the more likely the occurrence of cognitive impairment (P = .0129) and the faster the disease progression until death (P = .0003). Importantly, 13 STUB1 probands presenting with the full SCA48 clinical phenotype had normal TBP_37-39 alleles, excluding digenic inheritance as the sole mode.

CONCLUSION

We show that intermediate TBP_40-49 alleles act as disease modifiers of SCA48 rather than a STUB1/TBP digenic model. This distinction from what has been proposed before has crucial consequences for genetic counseling in SCA48.

Digenic inheritance of STUB1 variants and TBP polyglutamine expansions explains the incomplete penetrance of SCA17 and SCA48

PURPOSE

This study aimed to unravel the genetic factors underlying missing heritability in spinocerebellar ataxia type 17 (SCA17) caused by polyglutamine-encoding CAG/CAA repeat expansions in the TBP gene. Alleles with >49 CAG/CAA repeats are fully penetrant. Most patients, however, carry intermediate TBP_41-49 alleles that show incomplete penetrance.

METHODS

Using next-generation sequencing approaches, we investigated 40 SCA17/TBP_41-54 index patients, their affected (n = 55) and unaffected (n = 51) relatives, and a cohort of patients with ataxia (n = 292).

RESULTS

All except 1 (30/31) of the index cases with TBP_41-46 alleles carried a heterozygous pathogenic variant in the STUB1 gene associated with spinocerebellar ataxias SCAR16 (autosomal recessive) and SCA48 (autosomal dominant). No STUB1 variant was found in patients carrying TBP_47-54 alleles. TBP_41-46 expansions and STUB1 variants cosegregate in all affected family members, whereas the presence of either TBP_41-46 expansions or STUB1 variants individually was never associated with the disease.

CONCLUSION

Our data reveal an unexpected genetic interaction between STUB1 and TBP in the pathogenesis of SCA17 and raise questions on the existence of SCA48 as a monogenic disease with crucial implications for diagnosis and counseling. They provide a convincing explanation for the incomplete penetrance of intermediate TBP alleles and demonstrate a dual inheritance pattern for SCA17, which is a monogenic dominant disorder for TBP_≥47 alleles and a digenic TBP/STUB1 disease (SCA17-DI) for intermediate expansions.

Autosomal Dominant Gene Negative Frontotemporal Dementia-Think of SCA17

SCA 17 is a rare, autosomal dominant disorder caused by TBP gene CAG/CAA repeat expansion. Ataxia and dementia are common. The presence of frontal dysfunction at outset of the disease may mimic frontotemporal dementia (FTD). Parkinsonism, chorea, dystonia, and pyramidal signs may occur. We report an Irish family with autosomal dominant partially penetrant frontal dementia with cerebellar atrophy due to SCA17 and present detailed neuropsychological assessment for the first time. A 44-year-old doctor presented with 18-month history of behavioral problems. She slowed down, became apathetic, and unable to multitask. She became more irritable and short tempered, and her work performance deteriorated. Brain MRI showed cerebellar atrophy and cerebellar hypometabolism was noted on FDG-PET. A sister developed personality changes at age 45 with apathy, and had problems with memory and social skills; another sister at age 39 became dysarthric and unsteady. A brother at age 52 demonstrated emotional lability, and became dysarthric, unsteady, and slowed down. Their mother aged 73 had an abnormal antalgic gait due to arthritis; their father was jocular and disinhibited. MAPT testing detected an exon 9 c.726C>T variant in the proband. Subsequent testing in nine siblings and both parents failed to show co-segregation with disease. SCA17 testing revealed a TBP gene 43 repeat expansion that co-segregated in all affected siblings and in the mother whose gait problems were initially attributed to arthritis. In over 80% of cases of FTD with clear autosomal dominant inheritance, causative gene defects involve MAPT, GRN, or C9orf72 mutations. A minority involves VCP, FUS, and CHMP2B. As evident from our case, SCA17 testing should also be considered, especially if cerebellar atrophy if found on imaging. Segregation analysis is crucial. MAPT variant (c.726C>T exon 9) detected in the family was deemed a polymorphism.

Complexity of the Genetics and Clinical Presentation of Spinocerebellar Ataxia 17

Spinocerebellar ataxia type 17 (SCA17) is a rare autosomal dominant neurodegenerative disease caused by a CAG repeat expansion in the TATA-box binding protein gene (TBP). The disease has a varied age at onset and clinical presentation. It is distinct from other SCAs for its association with dementia, psychiatric symptoms, and some patients presenting with chorea. For this reason, it is also called Huntington’s disease-like 4 (HDL-4). Here we examine the distribution of SCA17 allele repeat sizes in a United Kingdom-based cohort with ataxia and find that fully penetrant pathogenic alleles are very rare (5 in 1,316 chromosomes; 0.38%). Phenotype-genotype correlation was performed on 30 individuals and the repeat structure of their TBP genes was examined. We found a negative linear correlation between total CAG repeat length and age at disease onset and, unlike SCA1, there was no correlation between the longest contiguous CAG tract and age at disease onset. We were unable to identify any particular phenotypic trait that segregated with particular CAG/CAA repeat tract structures or repeat lengths. One individual within the cohort was homozygous for variable penetrance range SCA17 alleles. This patient had a similar age at onset to heterozygotes with the same repeat sizes, but also presented with a rapidly progressive dementia. A pair of monozygotic twins within the cohort presented 3 years apart with the sibling with the earlier onset having a more severe phenotype with dementia and chorea in addition to the ataxia observed in their twin. This appears to be a case of variable expressivity, possibly influenced by other environmental or epigenetic factors. Finally, there was an asymptomatic father with a severely affected child with an age at onset in their twenties. Despite this, they share the same expanded allele repeat sizes and sequences, which would suggest that there is marked difference in the penetrance of this 51-repeat allele. We therefore propose that the variable penetrance range extend from 48 repeats to incorporate this allele. This study shows that there is variability in the presentation and penetrance of the SCA17 phenotype and highlights the complexity of this disorder.

Spinocerebellar Ataxia Type 17 (SCA17)

In 1999, a polyglutamine expansion was identified in the transcription factor TATA-binding protein (TBP) in a patient with ataxia with negative family history. Subsequently, CAG/CAA repeat expansions in the TBP gene were identified in families with spinocerebellar ataxia (SCA), establishing this repeat expansion as the underlying mutation in SCA type 17 (SCA17). There are several characteristic differences between SCA17 and other polyglutamine diseases. First, SCA17 shows a complex and variable clinical phenotype, in some cases overlapping that of Huntington's disease. Second, compared to the other SCA subtypes caused by expanded trinucleotide repeats, anticipation in SCA17 kindreds is rare because of the characteristic structure of the TBP gene. And thirdly, SCA17 patients often have diagnostic problems that may arise from non-penetrance. Because the gap between normal and abnormal repeat numbers is very narrow, it is difficult to determine a cutoff value for pathologic CAG repeat number in SCA17. Herein, we review the clinical, genetic and pathologic features of SCA17.

The Pathogenic Role of Low Range Repeats in SCA17

INTRODUCTION

SCA17 is an autosomal dominant cerebellar ataxia with expansion of the CAG/CAA trinucleotide repeats in the TATA-binding protein (TBP) gene. SCA17 can have various clinical presentations including parkinsonism, ataxia, chorea and dystonia. SCA17 is diagnosed by detecting the expanded CAG repeats in the TBP gene; however, in the literature, pathologic repeat numbers as low as 41 overlap with normal repeat numbers.

METHODS

The subjects in this study included patients with involuntary movement disorders such as cerebellar ataxia, parkinsonism, chorea and dystonia who visited Seoul National University Hospital between Jan. 2006 and Apr. 2014 and were screened for SCA17. Those who were diagnosed with other genetic diseases or nondegenerative diseases were excluded. DNA from healthy subjects who did not have a family history of parkinsonism, ataxia, psychiatric symptoms, chorea or dystonia served as the control. In total, 5242 chromosomes from 2099 patients and 522 normal controls were analyzed.

RESULTS

The total number of patients included in the analysis was 2099 (parkinsonism, 1706; ataxia, 345; chorea, 37; and dystonia, 11). In the normal control, up to 44 repeats were found. In the 44 repeat group, there were 7 (0.3%) patients and 1 (0.2%) normal control. In 43 repeat group, there were 8 (0.4%) patients and 2 (0.4%) normal controls. In the 42 repeat group, there were 16 (0.8%) patients and 3 (0.6%) normal controls. In 41 repeat group, there were 48 (2.3%) patients and 8 (1.5%) normal controls. Considering the overlaps and non-significant differences in allelic frequencies between the patients and the normal controls with low-expansions, we could not determine a definitive cutoff value for the pathologic CAG repeat number of SCA17.

CONCLUSION

Because the statistical analysis between the normal controls and patients with low range expansions failed to show any differences so far, we must consider that clinical cases with low range expansions could be idiopathic movement disorders showing coincidental CAG/CAA expansions. Thus, we need to reconsider the pathologic role of low range expansions (41-42). Long term follow up and comprehensive investigations using autopsy and imaging studies in patients and controls with low range expansions are necessary to determine the cutoff value for the pathologic CAG repeat number of SCA17.

Parkinsonism in spinocerebellar ataxia

Spinocerebellar ataxia (SCA) presents heterogeneous clinical phenotypes, and parkinsonism is reported in diverse SCA subtypes. Both levodopa responsive Parkinson disease (PD) like phenotype and atypical parkinsonism have been described especially in SCA2, SCA3, and SCA17 with geographic differences in prevalence. SCA2 is the most frequently reported subtype of SCA related to parkinsonism worldwide. Parkinsonism in SCA2 has unique genetic characteristics, such as low number of expansions and interrupted structures, which may explain the sporadic cases with low penetrance. Parkinsonism in SCA17 is more remarkable in Asian populations especially in Korea. In addition, an unclear cutoff of the pathologic range is the key issue in SCA17 related parkinsonism. SCA3 is more common in western cohorts. SCA6 and SCA8 have also been reported with a PD-like phenotype. Herein, we reviewed the epidemiologic, clinical, genetic, and pathologic features of parkinsonism in SCAs.

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.

Molecular diagnosis of neurogenetic disorders involving trinucleotide repeat expansions

There are more than 15 known neurogenetic disorders involving trinucleotide repeat expansion. Expanded repeats range from small expansions of 20-100 copies to larger expansions of up to several thousand units. These dynamic expansions result in variability in age of onset, degree of severity and clinical presentation. Individuals carrying alleles in the intermediate range, known as premutation alleles, are often asymptomatic, but can potentially transmit a further expanded allele to his/her offspring. For autosomal dominant adult-onset disorders, carriers are asymptomatic prior to disease onset. With current molecular tools, it is now possible to determine the presence and number of expanded repeats for accurate diagnosis, presymptomatic testing and carrier status screening. This review examines some of the current approaches for molecular diagnosis and discusses the issues unique to triplet repeat diseases.

Where genotype is not predictive of phenotype: towards an understanding of the molecular basis of reduced penetrance in human inherited disease

Some individuals with a particular disease-causing mutation or genotype fail to express most if not all features of the disease in question, a phenomenon that is known as 'reduced (or incomplete) penetrance'. Reduced penetrance is not uncommon; indeed, there are many known examples of 'disease-causing mutations' that fail to cause disease in at least a proportion of the individuals who carry them. Reduced penetrance may therefore explain not only why genetic diseases are occasionally transmitted through unaffected parents, but also why healthy individuals can harbour quite large numbers of potentially disadvantageous variants in their genomes without suffering any obvious ill effects. Reduced penetrance can be a function of the specific mutation(s) involved or of allele dosage. It may also result from differential allelic expression, copy number variation or the modulating influence of additional genetic variants in cis or in trans. The penetrance of some pathogenic genotypes is known to be age- and/or sex-dependent. Variable penetrance may also reflect the action of unlinked modifier genes, epigenetic changes or environmental factors. At least in some cases, complete penetrance appears to require the presence of one or more genetic variants at other loci. In this review, we summarize the evidence for reduced penetrance being a widespread phenomenon in human genetics and explore some of the molecular mechanisms that may help to explain this enigmatic characteristic of human inherited disease.

Huntington's disease: how intermediate are intermediate repeat lengths?

BACKGROUND

Huntington's disease (HD) is a devastating heredoneurodegenerative disorder associated with a wide variety of neurological and psychiatric symptoms caused by an expanded CAG repeat in the HTT gene. The expansion mutation in HTT is dominantly transmitted and codes for a protein named huntingtin (htt).

HYPOTHESIS

One hypothesis, according to a multistep mechanism, is that the intergenerational transmission of the normal repeat size causes small, progressive CAG stretch elongations in the general population from one generation to another, until a critical pathological CAG repeat threshold is reached. Mutations may originate in the offspring from paternally transmitted CAG repeats, falling within an intermediate alleles (IA) range of 27 to 35 in repeat length.

CONCLUSIONS

There has been emerging evidence that some individuals with IAs might develop an HD phenotype. This presents a challenge for genetic counseling, because these individuals are often reassured that they are "disease free." However, there are many unanswered questions related to the role of IAs in the development of the HD phenotype and in the pathogenesis of HD.

Early-onset SCA17 with 43 TBP repeats: expanding the phenotype?

The SCA17 clinical phenotype includes characteristics associated with cerebellar and cortical atrophy such as ataxia, dementia, epilepsy, chorea and parkinsonian features. Here we describe the case of a 38-year-old male presenting with ataxia, cognitive impairment and seizures, who was found to carry 43 repeats on one allele of the TATA-binding protein (TBP) gene. Therefore, genetic analysis of TBP gene triplets was performed on the patient's entire family, identifying three asymptomatic carriers of the same allele. A neuroradiological phenotype appeared to segregate with this allele, suggesting that it may play at least a contributory role in the determination of SCA17.

Huntington's disease look-alikes

Huntington's disease (HD) is caused by a triplet repeat expansion in the IT15 gene on chromosome 4 encoding huntingtin. Gene mutations are found in about 99% of cases, with symptoms and signs suggestive of HD. This implies the existence of other causes of this syndrome, and, in recent years, several other distinct genetic disorders have been identified that can present with a clinical picture indistinguishable from HD, termed HD-like (HDL) syndromes. So far, four genes associated with HDL syndromes have been identified, including the prion protein gene (HDL1), the junctophilin 3 gene (HDL2) and, the gene encoding the TATA box-binding protein (HDL4). In addition, a single family with a recessively inherited HD phenocopy, the exact genetic basis of which is currently unknown (HDL3), has been described. These disorders, however, account for only a small proportion of HDL cases, and the list of HDL genes and conditions is set to grow. In this article, we review the currently identified HD phenocopy disorders and discuss clinical clues to facilitate further investigations. We will concentrate on the four so-called HDL syndromes mentioned above. Other genetic choreatic syndromes such as dentatorubral-pallidoluysian atrophy, neuroferritinopathy, pantothenate kinase-associated neurodegeneration, and chorea-acanthocytosis are also briefly discussed.

Spinocerebellar ataxia 17 (SCA17) and Huntington's disease-like 4 (HDL4)

Spinocerebellar ataxia 17 (SCA17) or Huntington's disease-like-4 is a neurodegenerative disease caused by the expansion above 44 units of a CAG/CAA repeat in the coding region of the TATA box binding protein (TBP) gene leading to an abnormal expansion of a polyglutamine stretch in the corresponding protein. Alleles with 43 and 44 repeats have been identified in sporadic cases and their pathogenicity remains uncertain. Furthermore, incomplete penetrance of pathological alleles with up to 49 repeats has been suggested. The imperfect nature of the repeat makes intergenerational instability extremely rare and de novo mutations are most likely the result of partial duplications. This is one of the rarer forms of autosomal dominant cerebellar ataxia but the associated phenotype is often severe, involving various systems (cerebral cortex, striatum, and cerebellum), with extremely variable age at onset (range: 3-75 years) and clinical presentation. This gene is thought to account for a small proportion of patients with a Huntington's disease-like phenotype and cerebellar signs. Parkinson's disease-like, Creutzfeldt-Jakob disease-like and Alzheimer disease-like phenotypes have also been described with small SCA17 expansions. The abnormal protein is expressed at the same level as its normal counterpart and forms neuronal intranuclear inclusions containing other proteins involved in protein folding or degradation. The increase in the size of the glutamine stretch enhances transcription in vitro, probably leading to transcription deregulation. Interestingly, the TBP protein mutated in SCA17 is recruited in the inclusions of other polyglutaminopathies, suggesting its involvement in the transcription down-regulation observed in these diseases.

Experience and outcome of 3 years of a European EQA scheme for genetic testing of the spinocerebellar ataxias

The European Molecular Genetics Quality Network (EMQN) has been organizing an external quality assessment (EQA) scheme for molecular genetic testing of trinucleotide repeat mutations in the spinocerebellar ataxias (SCAs) since 2004. DNA samples were validated by at least two independent labs and two different methods. Together with mock clinical case descriptions and requests for specific SCA gene analyses, these were sent to registered participants each year. Laboratories were asked to use their routine procedures and protocols. A panel of assessors reviewed the final returns, including genotype results and reports, to assess the quality of (1) genotyping and (2) interpretation and reporting. A description of methods and raw data were also requested and were very useful for the final analysis. Altogether, during 3 years, 239 reports were received from the laboratories. Overall genotype error rate ranged 1.1-5.2%, a significant cause of concern. Scores for interpretation and reporting also showed that there is still much room for progress, although performance has improved over this period of assessment. The consequences of suboptimal laboratory practices, genotyping errors and misdiagnosis and of incorrect or incomplete interpretation and reporting have wide implications for patient lives, as well as for health management and counselling of relatives. EQA schemes are an important part of quality assurance in molecular genetic laboratories, and their use should become a routine part of laboratory diagnostic practice. Current evidence shows also that it is important that laboratories participate on a yearly basis and that this becomes mandatory for reference laboratories.

Instability of expanded CAG/CAA repeats in spinocerebellar ataxia type 17

Trinucleotide repeat expansions are dynamic mutations causing many neurological disorders, and their instability is influenced by multiple factors. Repeat configuration seems particularly important, and pure repeats are thought to be more unstable than interrupted repeats. But direct evidence is still lacking. Here, we presented strong support for this hypothesis from our studies on spinocerebellar ataxia type 17 (SCA17). SCA17 is a typical polyglutamine disease caused by CAG repeat expansion in TBP (TATA binding protein), and is unique in that the pure expanded polyglutamine tract is coded by either a simple configuration with long stretches of pure CAGs or a complex configuration containing CAA interruptions. By small pool PCR (SP-PCR) analysis of blood DNA from SCA17 patients of distinct racial backgrounds, we quantitatively assessed the instability of these two types of expanded alleles coding similar length of polyglutamine expansion. Mutation frequency in patients harboring pure CAG repeats is 2-3 folds of those with CAA interruptions. Interestingly, the pure CAG repeats showed both expansion and deletion while the interrupted repeats exhibited mostly deletion at a significantly lower frequency. These data strongly suggest that repeat configuration is a critical determinant for instability, and CAA interruptions might serve as a limiting element for further expansion of CAG repeats in SCA17 locus, suggesting a molecular basis for lack of anticipation in SCA17 families with interrupted CAG expansion.

A small trinucleotide expansion in the TBP gene gives rise to a sporadic case of SCA17 with abnormal putaminal findings on MRI

A Japanese woman developed gait disturbances at 25 years of age, and subsequently underwent gradual changes in her personality. By the age of 42, she showed clear signs of dementia and cerebellar ataxia, and displayed behavioral abnormalities, choreic movements and hyperreflexia. The findings of MRI not only showed cerebellar and cerebral atrophy, but also revealed putaminal rim hyperintensity on T2-weighted images. We identified a heterozygously expanded CAG/CAA repeat (45/36) within the TATA-binding protein gene, leading to a diagnosis of SCA17. These results show that a 45 CAG/CAA repeat is pathological, giving rise to early-onset SCA17.

Case control analysis of repeat expansion size in ataxia

Spinocerebellar ataxias (SCAs) are a group of clinically and genetically heterogeneous neurological diseases. The expansion of unstable microsatellite repeats has been identified as the underlying pathogenic cause of 10 subtypes of autosomal dominant SCAs. The aetiology of sporadic SCA is unknown. The aim of this study was to investigate the effect of large normal repeats in patients presenting with sporadic or familial ataxia compared to a control population. The size of the expansion was determined using a fluorescent PCR approach in 10 common SCA genes: SCA-1 (ATXN1), SCA-2 (ATXN2), SCA-3 (ATXN3), SCA-6 (CACNA1A), SCA-7 (ATXN7), SCA-8 (ATXN8OS), SCA-10 (ATXN10), SCA-12 (PPP2R2B), SCA-17 (TBP) and DRPLA (ATN1), in 165 ataxia patients and 307 controls of Welsh origin. There was no difference between cases and controls in the distribution of the large normal alleles, or in the distribution of the combined CAG repeats. The normal allele distribution in the Welsh population was largely similar to that of other Caucasian populations. Our study failed to demonstrate an effect of large normal repeats on the susceptibility to develop ataxia.

Spinocerebellar ataxia type 17 is caused by mutations in the TATA-box binding protein

The spinocerebellar ataxia type 17 (SCA17) is characterized by cerebellar ataxia, dementia, and involuntary movements, including chorea and dystonia. In addition, psychiatric symptoms, pyramidal signs, and rigidity are common. MRI shows variable atrophy of the cerebrum, brainstem, and cerebellum. The autosomal dominantly inherited progressive neurodegenerative disorder is caused by an expanded CAA/CAG repeat coding for glutamine. Alleles of the normal range carry 25 to 42 glutamine residues, disease causing alleles 43 to 63. Alleles with 43 to 48 glutamine codons may be associated with incomplete penetrance. The mean age of onset is about 30 years for individuals with full-penetrance alleles, but ranges from three to 55 years.

Spinocerebellar ataxia type 17: extension of phenotype with putaminal rim hyperintensity on magnetic resonance imaging

We report on a 50-year-old woman who presented with an 8-year history of involuntary movements, unsteadiness, and cognitive decline. Examination revealed multidomain cognitive deficits, jerky ocular pursuit movements, hypometric saccades, gaze impersistence, dysarthria, upper limb dystonia, and widespread chorea. TATA-binding protein gene test revealed trinucleotide expansion allele sizes of 47 and 39 repeats, confirming the diagnosis of spinocerebellar ataxia type 17 (SCA-17). Magnetic resonance imaging (MRI) showed marked cerebellar atrophy and putaminal rim hyperintensity. This is the first case of SCA-17 reported to show MRI signal change in the basal ganglia, and extends the phenotypic manifestation of SCA-17.

Minimum prevalence of spinocerebellar ataxia 17 in the north east of England

OBJECTIVE

To determine the minimum prevalence of spinocerebellar ataxia type 17 (SCA17) in the north east of England.

PATIENTS AND METHODS

A defined region containing 2,516,500 individuals with 192 families with undiagnosed ataxia, 90 patients with a Huntington's disease-like phenotype and 292 controls. The number of (CAG/CAA)(n) repeats in the SCA17/TBP gene was determined by fluorescent PCR and sequenced in affected individuals.

RESULTS

The mean repeat size for 584 control alleles was 34 (S.D.=3.58), ranging from 25 to 40. Two index cases had larger alleles with repeat lengths greater than the control range. Affected family members presented in adult life with ataxia followed by extrapyramidal features and cognitive impairment. In one family 44 repeats were associated with a younger age of onset than has been previously described.

CONCLUSIONS

The minimum prevalence of SCA17 in the north east of England was 0.16/100,000 (upper 95% confidence interval 0.31/100,000).

Levels of mutant huntingtin influence the phenotypic severity of Huntington disease in YAC128 mouse models

Huntington disease (HD) is a devastating neuropsychiatric disease caused by expansion of a trinucleotide repeat (CAG) in the HD gene. Neuropathological changes include the appearance of N-terminal huntingtin fragments, decreased brain weight and apoptotic neuronal loss in a select subset of neurons located in the striatum. There is still controversy over whether homozygosity for the mutation in HD is associated with a more severe phenotype. In humans, resolution of this issue has been complicated by the small number of homozygous patients and difficulty in the definition of reliable phenotypic endpoints. In order to definitively determine whether there is a correlation between phenotypic severity and expression levels of mutant huntingtin, we undertook a behavioral and neuropathological assessment of YAC128 mice with varying levels of mutant huntingtin. The results reveal a clear relationship between levels of mutant huntingtin and phenotype defined by earlier age of onset, more rapid progression, enhanced striatal volume loss, acceleration of nuclear huntingtin fragment accumulation and increased sensitivity to NMDAR-mediated excitotoxicity. These results provide clear evidence in vivo supporting a more severe phenotype associated with increased levels of mutant huntingtin as seen in homozygotes for HD.

Expanded trinucleotide repeats in the TBP/SCA17 gene mapped to chromosome 6q27 are associated with schizophrenia

Schizophrenia has a complex and non-Mendelian mode of inheritance. Recently, trinucleotide repeat (TNR)-containing genes have been considered as the candidate genes predisposing to schizophrenia. The purpose of this study was to determine whether a genetic association could be observed between schizophrenia and the TNR polymorphisms within the KLHL1AS/SCA8, PPP2R2B/SCA12, and TBP/SCA17 genes. We studied 100 unrelated schizophrenia patients and 124 controls without evident neurodegenerative or psychiatric disorders. The overall allele frequency distributions of the KLHL1AS/SCA8 and PPP2R2B/SCA12 genes were not significantly different between the schizophrenic patients and the control subjects (P>0.05). The allele frequency distribution in the schizophrenic patients was significantly different from that in the controls at the TBP/SCA17 gene (P=0.0149), with an increased frequency of 36 repeats in the patients and two patients carrying 45 TNR expansions were identified. TBP/SCA17 is the TATA box binding protein gene mapped to chromosome 6q27. The study suggests that TNR expansions of the TBP/SCA17 gene may contribute to the genetic risk of schizophrenia in rare cases.

TATA-binding protein in neurodegenerative disease

TATA binding protein (TBP) is a general transcription factor that plays an important role in initiation of transcription. In recent years evidence has emerged implicating TPB in the molecular mechanism of a number of neurodegenerative diseases. Wild type TBP in humans contains a long polyglutamine stretch ranging in size from 29 to 42. It has been found associated with aggregated proteins in several of the polyglutamine disorders. Expansion in the CAA/CAG composite repeat beyond 42 has been shown to cause a cerebellar ataxia, SCA17. The involvement of such an important housekeeping protein in the disease mechanism suggests a major impact on the functioning of cells. The question remains, does TBP contribute to these diseases through a loss of normal function, likely to be catastrophic to a cell, or the gain of an aberrant function? This review deals with the function of TBP in transcription and cell function. The distribution of the polyglutamine coding allele lengths in TBP of the normal population and in SCA17 is reviewed and an outline is given on the reported cases of SCA17. The role of TBP in other polyglutamine disorders will be addressed as well as its possible role in other neurodegenerative diseases.

Small de novo duplication in the repeat region of the TATA-box-binding protein gene manifest with a phenotype similar to variant Creutzfeldt-Jakob disease

A 20-year-old North American patient developed rapidly progressive cognitive decline and pronounced ataxia, a phenotype compatible with prion disease. No structural changes were found in the PRNP gene, which excludes genetic prion disease, but the patient's PRNP codon 129 Met/Met genotype is known to predispose to variant Creutzfeldt-Jakob disease (vCJD). Further studies identified an expanded allele with 55 CAG/CAA repeats in the TBP gene. The increase of trinucleotide repeat number in the coding region of the TBP gene has previously been associated with spinocerebellar ataxia type 17 (SCA17). The patient's unaffected parents and siblings show normal-size TBP alleles with 37-38 repeats. Haplotype and nucleotide sequence analyses clearly indicate that the mutation has occurred de novo on a paternal chromosome by insertion/duplication of a (CAA)(CAG)(CAA)(CAG)(15) sequence. This report presents a second fully investigated sporadic case of SCA17 occurring as a result of a DNA rearrangement within the polymorphic TBP trinucleotide repeat region. Our findings suggest that patients suspected of vCJD should undergo testing for SCA17, Huntington's disease and other neurodegenerative disorders having phenotypic similarities with vCJD.