Spinocerebellar ataxia type 17-digenic TBP/STUB1 disease: neuropathologic features of an autopsied patient

Movement Disorders in Hereditary Cerebellar Ataxia

BACKGROUND

Hereditary cerebellar ataxia (HCA) represents a complex group of disorders, with a wide spectrum of neurological symptoms. Among these, non-ataxia movement disorders (MD) have been increasingly acknowledged, with variable frequency across different forms.

OBJECTIVES

To characterize the type and frequency of MD in patients with HCA. To identify factors associated with MD and analyze their impact on disability.

METHODS

We conducted a prospective study starting in 2017, with annual visits according to a structured protocol. Patients were selected from the study database and their clinical and genetic features analyzed.

RESULTS

The cohort comprised 193 symptomatic patients. Machado-Joseph disease (MJD, also SCA3 or ATX-ATXN3) and cerebellar ataxia, neuropathy and vestibular areflexia syndrome (ATX-RFC1) were the most common autosomal dominant (AD) or recessive forms, with a frequency of 14.0% and 15.0%, respectively. MD were present in 95 (54.4%), with dystonia being the most common (49.2%). Tremor was identified in 10.9%, Parkinsonism in 4.1% and chorea in 3.6% patients. Myoclonus and tics were rare (2.6% and 0.5%). The presence of MD was associated with AD inheritance and ATXN3. MD, regardless of type, correlated with higher SARA score at baseline, increased fall frequency, confinement to wheelchair, and earlier occurrence of falls and of permanent use of walking aid.

CONCLUSIONS

Movement disorders, particularly dystonia, were common in our cohort. This highlights the possible role of the cerebellum in MD, but also extra-cerebellar involvement in some HCA. Presence of MD significantly worsened motor disability, highlighting the need for strategies of early identification and tailored management.

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.

Disease-associated mutations in C-terminus of HSP70 interacting protein (CHIP) impair its ability to negatively regulate mitophagy

C-terminus of HSP70 interacting protein (CHIP) is an E3 ubiquitin ligase and HSP70 cochaperone. Mutations in the CHIP encoding gene are the cause of two neurodegenerative conditions: spinocerebellar ataxia autosomal dominant type 48 (SCA48) and autosomal recessive type 16 (SCAR16). The mechanisms underlying CHIP-associated diseases are currently unknown. Mitochondrial dysfunction, specifically dysfunction in mitochondrial autophagy (mitophagy), is increasingly implicated in neurodegenerative diseases and loss of CHIP has been demonstrated to result in mitochondrial dysfunction in multiple animal models, although how CHIP is involved in mitophagy regulation has been previously unknown. Here, we demonstrate that CHIP acts as a negative regulator of the PTEN-induced kinase 1 (PINK1)/Parkin-mediated mitophagy pathway, promoting the degradation of PINK1, impairing Parkin translocation to the mitochondria, and suppressing mitophagy in response to mitochondrial stress. We also show that loss of CHIP enhances neuronal mitophagy in a PINK1 and Parkin dependent manner in Caenorhabditis elegans. Furthermore, we find that multiple disease-associated mutations in CHIP dysregulate mitophagy both in vitro and in vivo in C. elegans neurons, a finding which could implicate mitophagy dysregulation in CHIP-associated diseases.

RNA Editing by ADAR Adenosine Deaminases in the Cell Models of CAG Repeat Expansion Diseases: Significant Effect of Differentiation from Stem Cells into Brain Organoids in the Absence of Substantial Influence of CAG Repeats on the Level of Editing

Expansion of CAG repeats in certain genes is a known cause of several neurodegenerative diseases, but exact mechanism behind this is not yet fully understood. It is believed that the double-stranded RNA regions formed by CAG repeats could be harmful to the cell. This study aimed to test the hypothesis that these RNA regions might potentially interfere with ADAR RNA editing enzymes, leading to the reduced A-to-I editing of RNA and activation of the interferon response. We studied induced pluripotent stem cells (iPSCs) derived from the patients with Huntington's disease or ataxia type 17, as well as midbrain organoids developed from these cells. A targeted panel for next-generation sequencing was used to assess editing in the specific RNA regions. Differentiation of iPSCs into brain organoids led to increase in the ADAR2 gene expression and decrease in the expression of protein inhibitors of RNA editing. As a result, there was increase in the editing of specific ADAR2 substrates, which allowed identification of differential substrates of ADAR isoforms. However, comparison of the pathology and control groups did not show differences in the editing levels among the iPSCs. Additionally, brain organoids with 42-46 CAG repeats did not exhibit global changes. On the other hand, brain organoids with the highest number of CAG repeats in the huntingtin gene (76) showed significant decrease in the level of RNA editing of specific transcripts, potentially involving ADAR1. Notably, editing of the long non-coding RNA PWAR5 was nearly absent in this sample. It could be stated in conclusion that in most cultures with repeat expansion, the hypothesized effect on RNA editing was not confirmed.

Two more families supporting the existence of monogenic spinocerebellar ataxia 48

The reduced penetrance of TBP intermediate alleles and the recently proposed possible digenic TBP/STUB1 inheritance raised questions on the possible mechanism involved opening a debate on the existence of SCA48 as a monogenic disorder. We here report clinical and genetic results of two apparently unrelated patients carrying the same STUB1 variant(c.244G > T;p.Asp82Tyr) with normal TBP alleles and a clinical picture fully resembling SCA48, including cerebellar ataxia, dysarthria and mild cognitive impairment. This report provides supportive evidence that this specific ataxia can also occur as a monogenic disease, considering classical TBP allelic ranges.

GDF9His209GlnfsTer6/S428T and GDF9Q321X/S428T bi-allelic variants caused female subfertility with defective follicle enlargement

BACKGROUND

Antral follicles consist of an oocyte cumulus complex surrounding by somatic cells, including mural granulosa cells as the inner layer and theca cells as the outsider layer. The communications between oocytes and granulosa cells have been extensively explored in in vitro studies, however, the role of oocyte-derived factor GDF9 on in vivo antral follicle development remains elusive due to lack of an appropriate animal model. Clinically, the phenotype of GDF9 variants needs to be determined.

METHODS

Whole-exome sequencing (WES) was performed on two unrelated infertile women characterized by an early rise of estradiol level and defect in follicle enlargement. Besides, WES data on 1,039 women undergoing ART treatment were collected. A Gdf9Q308X/S415T mouse model was generated based on the variant found in one of the patients.

RESULTS

Two probands with bi-allelic GDF9 variants (GDF9His209GlnfsTer6/S428T, GDF9Q321X/S428T) and eight GDF9S428T heterozygotes with normal ovarian response were identified. In vitro experiments confirmed that these variants caused reduction of GDF9 secretion, and/or alleviation in BMP15 binding. Gdf9Q308X/S415T mouse model was constructed, which recapitulated the phenotypes in probands with abnormal estrogen secretion and defected follicle enlargement. Further experiments in mouse model showed an earlier expression of STAR in small antral follicles and decreased proliferative capacity in large antral follicles. In addition, RNA sequencing of granulosa cells revealed the transcriptomic profiles related to defective follicle enlargement in the Gdf9Q308X/S415T group. One of the downregulated genes, P4HA2 (a collagen related gene), was found to be stimulated by GDF9 protein, which partly explained the phenotype of defective follicle enlargement.

CONCLUSIONS

GDF9 bi-allelic variants contributed to the defect in antral follicle development. Oocyte itself participated in the regulation of follicle development through GDF9 paracrine effect, highlighting the essential role of oocyte-derived factors on ovarian response.

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.