The "crab sign": an imaging feature of spinocerebellar ataxia type 48

Clinical and functional characterization of a novel STUB1 mutation in a Chinese spinocerebellar ataxia 48 pedigree

BACKGROUND

Spinocerebellar ataxias (SCAs) encompass a wide spectrum of inherited neurodegenerative diseases, primarily characterized by pathological changes in the cerebellum, spinal cord, and brainstem degeneration. Autosomal dominant spinocerebellar ataxia type 48 (SCA48) is a newly identified subtype of SCA, marked by early-onset ataxia and cognitive impairment, and is associated with mutations in the STIP1 homology and U-box-containing protein 1 (STUB1) gene. The STUB1 gene encodes the protein CHIP (C-terminus of HSC70-interacting protein) which functions as E3 ubiquitin ligase and is crucial to the development of neural systems.

RESULTS

Here, we reported a Chinese SCA48 family exhibited typical features and defined a novel missense mutation STUB1 c.755A>C (CHIP p. Y252S) through whole-exome sequencing. The variant was interpreted as a variant of uncertain significance, so we conducted a series of experiments using minigene plasmids to evaluate the pathogenicity of the variant. We found that the variant STUB1 c.755A>C caused a significant reduction of CHIP level and the loss function of ubiquitin ligase activity as the pathogenic STUB1 mutations reported before. Besides, we also found that the CHIP p. Y252S could cause tau aggregation, which is considered to contribute to the progression of neurodegenerative disorders.

CONCLUSIONS

We diagnose the SCA48 pedigree in China and highlight the role of decreased ubiquitination and increased tau aggregation in the pathogenesis of the novel STUB1 c.755C>A mutation.

Case Series of Cerebellar Ataxia with Tremor Due to Heterozygous STUB1 Variants (SCA48) without TBP Expansions: Further Evidence for SCA48 as a Monogenic Disease

Clinically-relevant variants in the STUB1 gene have been associated with an autosomal dominant spinocerebellar ataxia 48 (SCA48), a recently described inherited neurodegenerative condition that is characterised by cognitive and psychiatric changes. To describe the clinical phenotype and genetic findings of three new Australian probands with STUB1 to expand the current understanding of the spectrum of clinical presentation and natural history of SCA48. Clinical and genetic review of patients diagnosed with SCA48 ataxia drawn from our centres. The third case was derived from a collaborating centre (Royal Brisbane Hospital). We identified three unrelated SCA48 patients with heterozygous pathogenic STUB1 variants. All presented with slowly progressive cerebellar ataxia with tremor and additional findings of dysarthria, parkinsonism, hypertonia, cognitive and psychiatric symptoms. Age of onset varied from 34 to 65 years of age. Brain MRI showed significant diffuse cerebellar atrophy, affecting the vermis and cerebellar hemispheres. We identified two novel pathogenic variants of STUB1 gene, and one previously reported pathogenic variant. Genetic testing for intermediate expansions of TBP (SCA17) identified TBP repeats within the normal range of 25-40 in all 3 probands. Our case series expands the clinical spectrum of SCA48. We highlight the importance of tremor as part of the clinical phenotype including upper limb rest tremor and Parkinsonian signs. Our cases lacked pathological TBP expansions and provide additional evidence that STUB1 (SCA48) can manifest as a monogenic disease.

A New Case Series Suggests That SCA48 (ATX/STUB1) Is Primarily a Monogenic Disorder

BACKGROUND

Monoallelic, pathogenic STUB1 variants cause autosomal dominant cerebellar ataxia (ATX-STUB1/SCA48). Recently, a genetic interaction between STUB1 variants and intermediate or high-normal CAG/CAA repeats in TBP was suggested, indicating digenic inheritance or a disease-modifying role for TBP expansions.

OBJECTIVE

To determine the presence and impact of intermediate or high-normal TBP expansions in ataxic patients with heterozygous STUB1 variants.

METHODS

We describe 21 patients with ataxia carrying a heterozygous STUB1 variant and determined TBP repeat length.

RESULTS

A total of 15 of 21 patients (71%) carried a normal TBP <40 allele, 4 (19%) carried an intermediate TBP 41-42 allele, and two carried a high-normal TBP 40 allele (9.5%). Five of six carriers (83%) of both STUB1 variants and TBP 40-42 alleles showed marked cognitive impairment.

CONCLUSIONS

SCA48 is predominantly a monogenic disorder, because most patients carried an isolated, heterozygous STUB1 variant and presented with the typical combined phenotype of ataxia and cognitive dysfunction. Still, co-occurrence of TBP 41-42 or high-normal TBP40 alleles was relatively frequent and associated with marked cognitive defects (28.5%), suggesting a modifying effect on clinical expression in some cases.

A Review of Brain and Pituitary Gland MRI Findings in Patients with Ataxia and Hypogonadism

The association of cerebellar ataxia and hypogonadism occurs in a heterogeneous group of disorders, caused by different genetic mutations often associated with a recessive inheritance. In these patients, magnetic resonance imaging (MRI) plays a pivotal role in the diagnostic workflow, with a variable involvement of the cerebellar cortex, alone or in combination with other brain structures. Neuroimaging involvement of the pituitary gland is also variable. Here, we provide an overview of the main clinical and conventional brain and pituitary gland MRI imaging findings of the most common genetic mutations associated with the clinical phenotype of ataxia and hypogonadism, with the aim of helping neuroradiologists in the identification of these disorders.

A Review of Ocular Movement Abnormalities in Hereditary Cerebellar Ataxias

Cerebellar ataxias are a wide heterogeneous group of disorders that may present with fine motor deficits as well as gait and balance disturbances that have a significant influence on everyday activities. To review the ocular movements in cerebellar ataxias in order to improve the clinical knowledge of cerebellar ataxias and related subtypes. English papers published from January 1990 to May 2022 were selected by searching PubMed services. The main search keywords were ocular motor, oculomotor, eye movement, eye motility, and ocular motility, along with each ataxia subtype. The eligible papers were analyzed for clinical presentation, involved mutations, the underlying pathology, and ocular movement alterations. Forty-three subtypes of spinocerebellar ataxias and a number of autosomal dominant and autosomal recessive ataxias were discussed in terms of pathology, clinical manifestations, involved mutations, and with a focus on the ocular abnormalities. A flowchart has been made using ocular movement manifestations to differentiate different ataxia subtypes. And underlying pathology of each subtype is reviewed in form of illustrated models to reach a better understanding of each disorder.

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.

The molecular basis of spinocerebellar ataxia type 48 caused by a de novo mutation in the ubiquitin ligase CHIP

The spinocerebellar ataxias (SCAs) are a class of incurable diseases characterized by degeneration of the cerebellum that results in movement disorder. Recently, a new heritable form of SCA, spinocerebellar ataxia type 48 (SCA48), was attributed to dominant mutations in STIP1 homology and U box-containing 1 (STUB1); however, little is known about how these mutations cause SCA48. STUB1 encodes for the protein C terminus of Hsc70 interacting protein (CHIP), an E3 ubiquitin ligase. CHIP is known to regulate proteostasis by recruiting chaperones via a N-terminal tetratricopeptide repeat domain and recruiting E2 ubiquitin-conjugating enzymes via a C-terminal U-box domain. These interactions allow CHIP to mediate the ubiquitination of chaperone-bound, misfolded proteins to promote their degradation via the proteasome. Here we have identified a novel, de novo mutation in STUB1 in a patient with SCA48 encoding for an A52G point mutation in the tetratricopeptide repeat domain of CHIP. Utilizing an array of biophysical, biochemical, and cellular assays, we demonstrate that the CHIPA52G point mutant retains E3-ligase activity but has decreased affinity for chaperones. We further show that this mutant decreases cellular fitness in response to certain cellular stressors and induces neurodegeneration in a transgenic Caenorhabditis elegans model of SCA48. Together, our data identify the A52G mutant as a cause of SCA48 and provide molecular insight into how mutations in STUB1 cause SCA48.

MRI CNS Atrophy Pattern and the Etiologies of Progressive Ataxias

MRI shows the three archetypal patterns of CNS volume loss underlying progressive ataxias in vivo, namely spinal atrophy (SA), cortical cerebellar atrophy (CCA) and olivopontocerebellar atrophy (OPCA). The MRI-based CNS atrophy pattern was reviewed in 128 progressive ataxias. A CNS atrophy pattern was identified in 91 conditions: SA in Friedreich’s ataxia, CCA in 5 acquired and 72 (24 dominant, 47 recessive,1 X-linked) inherited ataxias, OPCA in Multi-System Atrophy and 12 (9 dominant, 2 recessive,1 X-linked) inherited ataxias. The MRI-based CNS atrophy pattern may be useful for genetic assessment, identification of shared cellular targets, repurposing therapies or the enlargement of drug indications in progressive ataxias.

NGS in Hereditary Ataxia: When Rare Becomes Frequent

The term hereditary ataxia (HA) refers to a heterogeneous group of neurological disorders with multiple genetic etiologies and a wide spectrum of ataxia-dominated phenotypes. Massive gene analysis in next-generation sequencing has entered the HA scenario, broadening our genetic and clinical knowledge of these conditions. In this study, we employed a targeted resequencing panel (TRP) in a large and highly heterogeneous cohort of 377 patients with a clinical diagnosis of HA, but no molecular diagnosis on routine genetic tests. We obtained a positive result (genetic diagnosis) in 33.2% of the patients, a rate significantly higher than those reported in similar studies employing TRP (average 19.4%), and in line with those performed using exome sequencing (ES, average 34.6%). Moreover, 15.6% of the patients had an uncertain molecular diagnosis. STUB1, PRKCG, and SPG7 were the most common causative genes. A comparison with published literature data showed that our panel would have identified 97% of the positive cases reported in previous TRP-based studies and 92% of those diagnosed by ES. Proper use of multigene panels, when combined with detailed phenotypic data, seems to be even more efficient than ES in clinical practice.

Episodic ataxia and severe infantile phenotype in spinocerebellar ataxia type 14: expansion of the phenotype and novel mutations

Introduction

Spinocerebellar ataxia type 14 (SCA14) is a dominantly inherited neurological disorder characterized by slowly progressive cerebellar ataxia. SCA14 is caused by mutations in PRKCG, a gene encoding protein kinase C gamma (PKCγ), a master regulator of Purkinje cells development.

Methods

We performed next-generation sequencing targeted resequencing panel encompassing 273 ataxia genes in 358 patients with genetically undiagnosed ataxia.

Results

We identified fourteen patients in ten families harboring nine pathogenic heterozygous variants in PRKCG, seven of which were novel. We encountered four patients with not previously described phenotypes: one with episodic ataxia, one with a spastic paraparesis dominating her clinical manifestations, and two children with an unusually severe phenotype.

Conclusions

Our study broadens the genetic and clinical spectrum of SCA14.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00415-021-10712-5.

A de novo STUB1 variant associated with an early adult-onset multisystemic ataxia phenotype

Background

Biallelic STUB1 variants are a well-established cause of autosomal-recessive early-onset multisystemic ataxia (SCAR16). Evidence for STUB1 variants causing autosomal-dominant ataxia (SCA48) so far largely relies on segregation data in larger families. Presenting the first de novo occurrence of a heterozygous STUB1 variant, we here present additional qualitative evidence for STUB1-disease as an autosomal-dominant disorder.

Methods

Whole exome sequencing on an index patient with sporadic early-onset ataxia, followed by Sanger sequencing in all family members, was used to identify causative variants as well as to rule out alternative genetic hits and intronic STUB1 variants. STUB1 mRNA and protein levels in PBMCs in all family members were analysed using qRT-PCR and Western Blot.

Results

A previously unreported start-lost loss-of-function variant c.3G>A in the start codon of STUB1 was identified in the index case, occurring de novo and without evidence for a second (potentially missed) variant (e.g., intronic or copy number) in STUB1. The patient showed an early adult-onset multisystemic ataxia complicated by spastic gait disorder, distal myoclonus and cognitive dysfunction, thus closely mirroring the systems affected in autosomal-recessive STUB1-associated disease. In line with the predicted start-lost effect of the variant, functional investigations demonstrated markedly reduced STUB1 protein expression in PBMCs, whereas mRNA levels were intact.

Conclusion

De novo occurrence of the loss-of-function STUB1 variant in our case with multisystemic ataxia provides a qualitatively additional line of evidence for STUB1-disease as an autosomal-dominant disorder, in which the same neurological systems are affected as in its autosomal-recessive counterpart. Moreover, this finding adds support for loss-of-function as a mechanism underlying autosomal-dominant STUB1-disease, thus mirroring its autosomal-recessive counterpart also in terms of the underlying mutational mechanism.

Spinocerebellar ataxia type 14: refining clinicogenetic diagnosis in a rare adult-onset disorder

Objectives

Genetic variant classification is a challenge in rare adult‐onset disorders as in SCA‐PRKCG (prior spinocerebellar ataxia type 14) with mostly private conventional mutations and nonspecific phenotype. We here propose a refined approach for clinicogenetic diagnosis by including protein modeling and provide for confirmed SCA‐PRKCG a comprehensive phenotype description from a German multi‐center cohort, including standardized 3D MR imaging.

Methods

This cross‐sectional study prospectively obtained neurological, neuropsychological, and brain imaging data in 33 PRKCG variant carriers. Protein modeling was added as a classification criterion in variants of uncertain significance (VUS).

Results

Our sample included 25 cases confirmed as SCA‐PRKCG (14 variants, thereof seven novel variants) and eight carriers of variants assigned as VUS (four variants) or benign/likely benign (two variants). Phenotype in SCA‐PRKCG included slowly progressive ataxia (onset at 4–50 years), preceded in some by early‐onset nonprogressive symptoms. Ataxia was often combined with action myoclonus, dystonia, or mild cognitive‐affective disturbance. Inspection of brain MRI revealed nonprogressive cerebellar atrophy. As a novel finding, a previously not described T2 hyperintense dentate nucleus was seen in all SCA‐PRKCG cases but in none of the controls.

Interpretation

In this largest cohort to date, SCA‐PRKCG was characterized as a slowly progressive cerebellar syndrome with some clinical and imaging features suggestive of a developmental disorder. The observed non‐ataxia movement disorders and cognitive‐affective disturbance may well be attributed to cerebellar pathology. Protein modeling emerged as a valuable diagnostic tool for variant classification and the newly described T2 hyperintense dentate sign could serve as a supportive diagnostic marker of SCA‐PRKCG.