Deciphering the natural history of SCA7 in children

Syndromic retinitis pigmentosa

Retinitis pigmentosa (RP) is a progressive inherited retinal dystrophy, characterized by the degeneration of photoreceptors, presenting as a rod-cone dystrophy. Approximately 20-30% of patients with RP also exhibit extra-ocular manifestations in the context of a syndrome. This manuscript discusses the broad spectrum of syndromes associated with RP, pathogenic mechanisms, clinical manifestations, differential diagnoses, clinical management approaches, and future perspectives. Given the diverse clinical and genetic landscape of syndromic RP, the diagnosis may be challenging. However, an accurate and timely diagnosis is essential for optimal clinical management, prognostication, and potential treatment. Broadly, the syndromes associated with RP can be categorized into ciliopathies, inherited metabolic disorders, mitochondrial disorders, and miscellaneous syndromes. Among the ciliopathies associated with RP, Usher syndrome and Bardet-Biedl syndrome are the most well-known. Less common ciliopathies include Cohen syndrome, Joubert syndrome, cranioectodermal dysplasia, asphyxiating thoracic dystrophy, Mainzer-Saldino syndrome, and RHYNS syndrome. Several inherited metabolic disorders can present with RP, including Zellweger spectrum disorders, adult Refsum disease, α-methylacyl-CoA racemase deficiency, certain mucopolysaccharidoses, ataxia with vitamin E deficiency, abetalipoproteinemia, several neuronal ceroid lipofuscinoses, mevalonic aciduria, PKAN/HARP syndrome, PHARC syndrome, and methylmalonic acidaemia with homocystinuria type cobalamin (cbl) C disease. Due to the mitochondria's essential role in supplying continuous energy to the retina, disruption of mitochondrial function can lead to RP, as seen in Kearns-Sayre syndrome, NARP syndrome, primary coenzyme Q10 deficiency, SSBP1-associated disease, and long chain 3-hydroxyacyl-CoA dehydrogenase deficiency. Lastly, Cockayne syndrome and PERCHING syndrome can present with RP, but they do not fit the abovementioned hierarchy and are thus categorized as miscellaneous. Several first-in-human clinical trials are underway or in preparation for some of these syndromic forms of RP.

Long-Term Follow-Up before and during Riluzole Treatment in Six Patients from Two Families with Spinocerebellar Ataxia Type 7

BACKGROUND

Currently no curative treatment exists for spinocerebellar ataxias (SCAs). Riluzole repurposing was proposed as a symptomatic treatment in different types of cerebellar ataxia. We report a long-term-follow up under riluzole treatment in SCA type 7.

METHODS

Six patients received Riluzole 50 mg twice daily on a compassionate use program for a mean of 4.8 years (range 3.5-9). We measured ataxia onset and progression through the Scale for the Assessment and Rating of Ataxia (SARA), and collected extensive ophthalmological data before and after Riluzole treatment. Electrocardiogram and laboratory profile for drug safety were performed every six months.

RESULTS

Riluzole treatment showed no effect on visual function in two patients with an advanced retinal damage. Improvements of visual function occurred in four patients followed by ophthalmologic stability up to 5 years after starting treatment. Two patients had a less steep deterioration of ataxia after treatment compared to pre-treatment, during the first 2,5 years of therapy. One showed soon after therapy an improvement of the SARA score, and then overall stability lasting 3,5 years, followed by ataxia worsening. One visually impaired patient without neurological impairment did not worse until the last visit after 3,5 years of follow-up. The remaining 2 patients showed an improvement of SARA scores soon after therapy, and an overall stability lasting respectively 5 and 3 years. No adverse event was registered during the observation period.

DISCUSSION

This study suggests a possible beneficial action of Riluzole in SCA7 and provides a detailed description of the ophthalmologic profile of these patients.

Diagnostic uplift through the implementation of short tandem repeat analysis using exome sequencing

To date, approximately 50 short tandem repeat (STR) disorders have been identified; yet, clinical laboratories rarely conduct STR analysis on exomes. To assess its diagnostic value, we analyzed STRs in 6099 exomes from 2510 families with mostly suspected neurogenetic disorders. We employed ExpansionHunter and REViewer to detect pathogenic repeat expansions, confirming them using orthogonal methods. Genotype-phenotype correlations led to the diagnosis of thirteen individuals in seven previously undiagnosed families, identifying three autosomal dominant disorders: dentatorubral-pallidoluysian atrophy (n = 3), spinocerebellar ataxia type 7 (n = 2), and myotonic dystrophy type 1 (n = 2), resulting in a diagnostic gain of 0.28% (7/2510). Additionally, we found expanded ATXN1 alleles (≥39 repeats) with varying patterns of CAT interruptions in twelve individuals, accounting for approximately 0.19% in the Korean population. Our study underscores the importance of integrating STR analysis into exome sequencing pipeline, broadening the application of exome sequencing for STR assessments.

Genetic Epidemiology and Clinical Characteristics of Patients with Spinocerebellar Ataxias in an Unexplored Brazilian State, Using Strategies for Resource-Limited Settings

Spinocerebellar ataxias (SCAs) have a worldwide average prevalence of 2.7 cases per 100,000 individuals, with significant geographic variability. This study aimed to develop resource-limited strategies to detect and characterize the frequency and genetic-clinical profile of SCAs in an unexplored population from Alagoas State, a low Human Development Index state in northeastern Brazil. Active search strategies were employed to identify individuals with a diagnosis or clinical suspicion of SCAs, and a protocol for clinical and molecular evaluation was applied in collaboration with a reference center in Neurogenetics. A total of 73 individuals with SCAs were identified, with a minimum estimated prevalence of 2.17 cases per 100,000 inhabitants. SCA3 was the most common type (75.3%), followed by SCA7 (15.1%), SCA1 (6.8%), and SCA2 (2.7%). Patients with SCA3 subphenotype 2 were the most predominant. Detailed analysis of patients with SCA3 and SCA7 revealed age at onset and clinical features congruent with other studies, with gait disturbance and reduced visual capacity in SCA7 as the main initial manifestations. The study also identified many asymptomatic individuals at risk of developing SCAs. These findings demonstrate that simple and collaborative strategies can enhance the detection capacity of rare diseases such as SCAs in resource-limited settings and that Alagoas State has a minimum estimated prevalence of SCAs similar to the world average.

Autosomal dominant cerebellar ataxias: new genes and progress towards treatments

Dominantly inherited spinocerebellar ataxias (SCAs) are associated with phenotypes that range from pure cerebellar to multisystemic. The list of implicated genes has lengthened in the past 5 years with the inclusion of SCA37/DAB1, SCA45/FAT2, SCA46/PLD3, SCA47/PUM1, SCA48/STUB1, SCA50/NPTX1, SCA25/PNPT1, SCA49/SAM9DL, and SCA27B/FGF14. In some patients, co-occurrence of multiple potentially pathogenic variants can explain variable penetrance or more severe phenotypes. Given this extreme clinical and genetic heterogeneity, genome sequencing should become the diagnostic tool of choice but is still not available in many clinical settings. Treatments tested in phase 2 and phase 3 studies, such as riluzole and transcranial direct current stimulation of the cerebellum and spinal cord, have given conflicting results. To enable early intervention, preataxic carriers of pathogenic variants should be assessed with biomarkers, such as neurofilament light chain and brain MRI; these biomarkers could also be used as outcome measures, given that clinical outcomes are not useful in the preataxic phase. The development of bioassays measuring the concentration of the mutant protein (eg, ataxin-3) might facilitate monitoring of target engagement by gene therapies.

Extreme phenotypic heterogeneity in non-expansion spinocerebellar ataxias

Although the best-known spinocerebellar ataxias (SCAs) are triplet repeat diseases, many SCAs are not caused by repeat expansions. The rarity of individual non-expansion SCAs, however, has made it difficult to discern genotype-phenotype correlations. We therefore screened individuals who had been found to bear variants in a non-expansion SCA-associated gene through genetic testing, and after we eliminated genetic groups that had fewer than 30 subjects, there were 756 subjects bearing single-nucleotide variants or deletions in one of seven genes: CACNA1A (239 subjects), PRKCG (175), AFG3L2 (101), ITPR1 (91), STUB1 (77), SPTBN2 (39), or KCNC3 (34). We compared age at onset, disease features, and progression by gene and variant. There were no features that reliably distinguished one of these SCAs from another, and several genes-CACNA1A, ITPR1, SPTBN2, and KCNC3-were associated with both adult-onset and infantile-onset forms of disease, which also differed in presentation. Nevertheless, progression was overall very slow, and STUB1-associated disease was the fastest. Several variants in CACNA1A showed particularly wide ranges in age at onset: one variant produced anything from infantile developmental delay to ataxia onset at 64 years of age within the same family. For CACNA1A, ITPR1, and SPTBN2, the type of variant and charge change on the protein greatly affected the phenotype, defying pathogenicity prediction algorithms. Even with next-generation sequencing, accurate diagnosis requires dialogue between the clinician and the geneticist.

CAG Repeat Expansion in THAP11 Is Associated with a Novel Spinocerebellar Ataxia

BACKGROUND

More than 50 loci are associated with spinocerebellar ataxia (SCA), and the most frequent subtypes share nucleotide repeats expansion, especially CAG expansion.

OBJECTIVE

The objective of this study was to confirm a novel SCA subtype caused by CAG expansion.

METHODS

We performed long-read whole-genome sequencing combined with linkage analysis in a five-generation Chinese family, and the finding was validated in another pedigree. The three-dimensional structure and function of THAP11 mutant protein were predicted. Polyglutamine (polyQ) toxicity of THAP11 gene with CAG expansion was assessed in skin fibroblasts of patients, human embryonic kidney 293 and Neuro-2a cells.

RESULTS

We identified THAP11 as the novel causative SCA gene with CAG repeats ranging from 45 to 100 in patients with ataxia and from 20 to 38 in healthy control subjects. Among the patients, the number of CAA interruptions within CAG repeats was decreased to 3 (up to 5-6 in controls), whereas the number of 3' pure CAG repeats was up to 32 to 87 (4-16 in controls), suggesting that the toxicity of polyQ protein was length dependent on the pure CAG repeats. Intracellular aggregates were observed in cultured skin fibroblasts from patients. THAP11 polyQ protein was more intensely distributed in the cytoplasm of cultured skin fibroblasts from patients, which was replicated with in vitro cultured neuro-2a transfected with 54 or 100 CAG repeats.

CONCLUSIONS

This study identified a novel SCA subtype caused by intragenic CAG repeat expansion in THAP11 with intracellular aggregation of THAP11 polyQ protein. Our findings extended the spectrum of polyQ diseases and offered a new perspective in understanding polyQ-mediated toxic aggregation. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Accumulation of senescence observed in spinocerebellar ataxia type 7 mouse model

Spinocerebellar ataxia type 7 (SCA7) is a neurodegenerative disease caused by a trinucleotide CAG repeat. SCA7 predominantly causes a loss of photoreceptors in the retina and Purkinje cells of the cerebellum. Severe infantile-onset SCA7 also causes renal and cardiac irregularities. Previous reports have shown that SCA7 results in increased susceptibility to DNA damage. Since DNA damage can lead to accumulation of senescent cells, we hypothesized that SCA7 causes an accumulation of senescent cells over the course of disease. A 140-CAG repeat SCA7 mouse model was evaluated for signs of disease-specific involvement in the kidney, heart, and cerebellum, tissues that are commonly affected in the infantile form. We found evidence of significant renal abnormality that coincided with an accumulation of senescent cells in the kidneys of SCA7^140Q/5Q mice, based on histology findings in addition to RT-qPCR for the cell cycle inhibitors p16^Ink4a and p21^Cip1 and senescence-associated ß-galactosidase (SA-ßgal) staining, respectively. The Purkinje layer in the cerebellum of SCA7^140Q/5Q mice also displayed SA-ßgal⁺ cells. These novel findings offer evidence that senescent cells accumulate in affected tissues and may possibly contribute to SCA7’s specific phenotype.

Clinical and genetic spectrums of 413 North African families with inherited retinal dystrophies and optic neuropathies

Background

Inherited retinal dystrophies (IRD) and optic neuropathies (ION) are the two major causes world-wide of early visual impairment, frequently leading to legal blindness. These two groups of pathologies are highly heterogeneous and require combined clinical and molecular diagnoses to be securely identified. Exact epidemiological studies are lacking in North Africa, and genetic studies of IRD and ION individuals are often limited to case reports or to some families that migrated to the rest of the world. In order to improve the knowledge of their clinical and genetic spectrums in North Africa, we reviewed published data, to illustrate the most prevalent pathologies, genes and mutations encountered in this geographical region, extending from Morocco to Egypt, comprising 200 million inhabitants.

Main body

We compiled data from 413 families with IRD or ION together with their available molecular diagnosis. The proportion of IRD represents 82.8% of index cases, while ION accounted for 17.8%. Non-syndromic IRD were more frequent than syndromic ones, with photoreceptor alterations being the main cause of non-syndromic IRD, represented by retinitis pigmentosa, Leber congenital amaurosis, and cone-rod dystrophies, while ciliopathies constitute the major part of syndromic-IRD, in which the Usher and Bardet Biedl syndromes occupy 41.2% and 31.1%, respectively. We identified 71 ION families, 84.5% with a syndromic presentation, while surprisingly, non-syndromic ION are scarcely reported, with only 11 families with autosomal recessive optic atrophies related to OPA7 and OPA10 variants, or with the mitochondrial related Leber ION. Overall, consanguinity is a major cause of these diseases within North African countries, as 76.1% of IRD and 78.8% of ION investigated families were consanguineous, explaining the high rate of autosomal recessive inheritance pattern compared to the dominant one. In addition, we identified many founder mutations in small endogamous communities.

Short conclusion

As both IRD and ION diseases constitute a real public health burden, their under-diagnosis in North Africa due to the absence of physicians trained to the identification of inherited ophthalmologic presentations, together with the scarcity of tools for the molecular diagnosis represent major political, economic and health challenges for the future, to first establish accurate clinical diagnoses and then treat patients with the emergent therapies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13023-022-02340-7.

Spinocerebellar ataxia type 2 from an evolutionary perspective: Systematic review and meta-analysis

Dominant diseases due to expanded CAG repeat tracts, such as spinocerebellar ataxia type 2 (SCA2), are prone to anticipation and worsening of clinical picture in subsequent generations. There is insufficient data about selective forces acting on the maintenance of these diseases in populations. We made a systematic review and meta-analysis on the effect of the CAG length over age at onset, instability of transmissions, anticipation, de novo or sporadic cases, fitness, segregation of alleles, and ancestral haplotypes. The correlation between CAG expanded and age at onset was r²  = 0.577, and transmission of the mutant allele was associated with an increase of 2.42 CAG repeats in the next generation and an anticipation of 14.62 years per generation, on average. One de novo and 18 sporadic cases were detected. Affected SCA2 individuals seem to have more children than controls. The expanded allele was less segregated than the 22-repeat allele in children of SCA2 subjects. Several ancestral SCA2 haplotypes were published. Data suggest that SCA2 lineages may tend to disappear eventually, due to strong anticipation phenomena. Whether or not the novel cases come from common haplotypes associated with a predisposition to further expansions is a question that needs to be addressed by future studies.