The genetic basis of early-onset hereditary ataxia in Iran: results of a national registry of a heterogeneous population

BACKGROUND

To investigate the genetics of early-onset progressive cerebellar ataxia in Iran, we conducted a study at the Children's Medical Center (CMC), the primary referral center for pediatric disorders in the country, over a three-year period from 2019 to 2022. In this report, we provide the initial findings from the national registry.

METHODS

We selected all early-onset patients with an autosomal recessive mode of inheritance to assess their phenotype, paraclinical tests, and genotypes. The clinical data encompassed clinical features, the Scale for the Assessment and Rating of Ataxia (SARA) scores, Magnetic Resonance Imaging (MRI) results, Electrodiagnostic exams (EDX), and biomarker features. Our genetic investigations included single-gene testing, Whole Exome Sequencing (WES), and Whole Genome Sequencing (WGS).

RESULTS

Our study enrolled 162 patients from various geographic regions of our country. Among our subpopulations, we identified known and novel pathogenic variants in 42 genes in 97 families. The overall genetic diagnostic rate was 59.9%. Notably, we observed PLA2G6, ATM, SACS, and SCA variants in 19, 14, 12, and 10 families, respectively. Remarkably, more than 59% of the cases were attributed to pathogenic variants in these genes.

CONCLUSIONS

Iran, being at the crossroad of the Middle East, exhibits a highly diverse genetic etiology for autosomal recessive hereditary ataxia. In light of this heterogeneity, the development of preventive strategies and targeted molecular therapeutics becomes crucial. A national guideline for the diagnosis and management of patients with these conditions could significantly aid in advancing healthcare approaches and improving patient outcomes.

TPP1 Variants in Iranian patients: A Novel Pathogenic Homozygous Variant Causing Neuronal Ceroid Lipofuscinosis 2

Introduction

TPP1 variants have been identified as a causative agent of neuronal ceroid lipofuscinosis 2 disease, that ataxia is one of its clinical features. Therefore, here, molecular study of TPP1 variants is presented in an Iranian cohort and a novel pathogenic variant is described.

Methods

This investigation was conducted as a cross-sectional study in a tertiary referral hospital, Children's Medical Center, Pediatrics Center of Excellence. Clinical presentations and pedigrees were documented. Patients with cerebellar ataxia were enrolled in this study. Next-generation sequencing was applied to confirm the diagnosis. Segregation and bioinformatics analyses were also done for the variants using Sanger sequencing.

Results

Forty-five patients were included in our study. The mean age of onset was 104 (+55.60) months (minimum = 31 months, maximum = 216 months). The majority of cases (73.3%) were born to consanguineous parents and only 1 patient (2.2%) had an affected sibling. Of the 45 patients, only 1 patient with a novel pathogenic variant (c.1425_1425+1delinsAT, p.A476Cfs*15) in the TPP1 gene was identified.

Discussion

The main strength of current study is the relatively large sample size. Besides, a novel pathogenic variant could be important toward the diagnosis and management of this condition. With significant advances in various therapies, early diagnosis could improve the treatments using personalized-based medicine.

History of Ataxias and Paraplegias with an Annotation on the First Description of Striatonigral Degeneration

The aim of this paper is to carry out a historical overview of the evolution of the knowledge on degenerative cerebellar disorders and hereditary spastic paraplegias, over the last century and a half. Original descriptions of the main pathological subtypes, including Friedreich's ataxia, hereditary spastic paraplegia, olivopontocerebellar atrophy and cortical cerebellar atrophy, are revised. Special attention is given to the first accurate description of striatonigral degeneration by Hans Joachim Scherer, his personal and scientific trajectory being clarified. Pathological classifications of ataxia are critically analysed. The current clinical-genetic classification of ataxia is updated by taking into account recent molecular discoveries. We conclude that there has been an enormous progress in the knowledge of the nosology of hereditary ataxias and paraplegias, currently encompassing around 200 genetic subtypes.

Expanding the phenotype of AFG3L2 mutations: Late-onset autosomal recessive spinocerebellar ataxia

The AFG3L2 gene encodes AFG3-like protein 2, which is a subunit of human mitochondrial ATPases associated with various cellular protease activities (m-AAA). The clinical spectrum of AFG3L2 mutations is broad. Dominant AFG3L2 mutations can cause autosomal dominant spinocerebellar ataxia type 28 (SCA28), whereas biallelic AFG3L2 mutations may lead to spastic ataxia 5 (SPAX5). However, the role of AFG3L2 mutations in autosomal recessive spinocerebellar ataxia (SCAR) remains elusive. The aim of this study is to delineate the clinical features and spectrum of AFG3L2 mutations in a Taiwanese cohort with cerebellar ataxia. Mutational analyses of AFG3L2 were carried out by targeted resequencing in a cohort of 133 unrelated patients with molecularly undetermined cerebellar ataxia. We identified one single patient carrying compound heterozygous mutations in AFG3L2, p.[R632*];[V723M] (c.[1894C > T];[2167G > A]). The patient has suffered from apparently sporadic and slowly progressive cerebellar ataxia, ptosis, and ophthalmoparesis since age 55 years. These findings expand the clinical spectrum of AFG3L2 mutations and suggest a new subtype of late-onset SCAR caused by biallelic AFG3L2 mutations.

Autosomal Recessive Cerebellar Ataxia 1: First Case Report Depicting a Variant in SYNE1 Gene in a Chilean Patient

Autosomal recessive cerebellar ataxia type 1 (ARCA-1) or spinocerebellar ataxia autosomal recessive type 8 (SCAR8) is a slowly progressive neurodegenerative disorder that occurs due to mutations in the spectrin repeat containing nuclear envelope protein 1 (SYNE1) gene. Previously considered a rare cause of ARCA, related to French-Canadian patients from Beauce, Quebec, Canada, SYNE1 ataxia is now known to be of worldwide distribution. We present the case report of a 54-year-old male patient with the genetic diagnosis of SYNE1 ataxia, presenting with a SYNE1 gene mutation never described in Chilean population before.

Late-onset autosomal recessive cerebellar ataxia and neuropathy with a novel splicing mutation in the ATM gene

Autosomal recessive cerebellar ataxias comprise many types of diseases. The most frequent autosomal recessive cerebellar ataxias are Friedreich ataxia, but other types are relatively rare. We encountered a consanguineous family with two cases of late-onset cerebellar ataxia with neuropathy. We performed whole-exome sequencing in one patient and confirmed by Sanger sequencing in other family members. Neurological examination revealed cerebellar ataxia, hand tremor, and neck dystonia, distal muscle wasting, and diminished tendon reflexes. The patients had no conjunctival telangiectasia or immunodeficiency. Blood examination revealed slightly elevated α-fetoprotein. Brain MRI demonstrated marked cerebellar atrophy and mild brainstem atrophy. The electrophysiologic study and nerve biopsy showed axonal neuropathy. Whole-exome sequencing revealed a novel homozygous missense variant (NM_000051.3: c.496G > C) in the ataxia-telangiectasia mutated gene. This homozygous variant was found in another patient, co-segregated within the family members-this variant results in aberrant splicing (skipping exon 5) on RT-PCR analysis. We identified the ataxia-telangiectasia mutated variant in an adult, late-onset autosomal recessive cerebellar ataxias family. We should consider ataxia-telangiectasia even in late-onset autosomal recessive cerebellar ataxias without telangiectasia or immunodeficiency.

A new MRI marker of ataxia with oculomotor apraxia

PURPOSE

Evaluate the specificity and sensitivity of disappearance of susceptibility weighted imaging (SWI) dentate nuclei (DN) hypointensity in oculomotor apraxia patients (AOA).

METHOD

In this prospective study, 27 patients with autosomal genetic ataxia (AOA (n = 11), Friedreich ataxia and ataxia with vitamin E deficit (n = 4), and dominant genetic ataxia (n = 12)) were included along with fifteen healthy controls. MRIs were qualitatively classified for the presence or absence of DN hypointensity on FLAIR and SWI sequences. The MRIs were then quantitatively studied, with measurement of a ratio of DN over brainstem white matter signal intensity through manual delineation. The institutional review board approved this study, and written informed consent was obtained. In the cross-sectional analysis, the Mann-Whitney test was applied.

RESULTS

Qualitatively, the eleven AOA patients presented absence of both DN SWI and FLAIR hyposignals; three dominant genetic ataxia patients had moderate SWI DN hyposignal and absent FLAIR hyposignal; the thirteen remaining subjects presented normal SWI and FLAIR DN hyposignal. Absence of DN SWI hypointensity was 100% sensitive and specific to AOA. Quantitative signal intensity ratio (mean ± standard deviation) of the AOA group (98·96 ± 5·37%) was significantly higher than in control subjects group (76.40 ± 8.34%; p < 0.001), dominant genetic ataxia group (81·15 ± 9·94%; p < 0·001), and Friedreich ataxia and ataxia with vitamin E deficit group (87·56 ± 2·78%; p < 0·02).

CONCLUSION

This small study shows that loss of the normal hypointensity in the dentate nucleus on both SWI and FLAIR imaging at 3 T is a highly sensitive and specific biomarker for AOA.

Compound Heterozygous Inheritance of Mutations in Coenzyme Q8A Results in Autosomal Recessive Cerebellar Ataxia and Coenzyme Q10 Deficiency in a Female Sib-Pair

Autosomal recessive ataxias are characterised by a fundamental loss in coordination of gait with associated atrophy of the cerebellum. There is significant clinical and genetic heterogeneity amongst inherited ataxias; however, an early molecular diagnosis is essential with low-risk treatments available for some of these conditions. We describe two female siblings who presented early in life with unsteady gait and cerebellar atrophy. Whole exome sequencing revealed compound heterozygous inheritance of two pathogenic mutations (p.Leu277Pro, c.1506+1G>A) in the coenzyme Q8A gene (COQ8A), a gene central to biosynthesis of coenzyme Q (CoQ). The paternally derived p.Leu277Pro mutation is predicted to disrupt a conserved motif in the substrate-binding pocket of the protein, resulting in inhibition of CoQ₁₀ production. The maternal c.1506+1G>A mutation destroys a canonical splice donor site in exon 12 affecting transcript processing and subsequent protein translation. Mutations in this gene can result in primary coenzyme Q₁₀ deficiency type 4, which is characterized by childhood onset of cerebellar ataxia and exercise intolerance, both of which were observed in this sib-pair. Muscle biopsies revealed unequivocally low levels of CoQ_10, and the siblings were subsequently established on a therapeutic dose of CoQ₁₀ with distinct clinical evidence of improvement after 1 year of treatment. This case emphasises the importance of an early and accurate molecular diagnosis for suspected inherited ataxias, particularly given the availability of approved treatments for some subtypes.

SYNE1 related cerebellar ataxia presents with variable phenotypes in a consanguineous family from Turkey

SYNE1 related autosomal recessive cerebellar ataxia type 1 (ARCA1) is a late-onset cerebellar ataxia with slow progression originally demonstrated in French-Canadian populations of Quebec, Canada. Nevertheless, recent studies on SYNE1 ataxia have conveyed the condition from a geographically limited pure cerebellar recessive ataxia to a complex multisystem phenotype that is relatively common on the global scale. To determine the underlying genetic cause of the ataxia phenotype in a consanguineous family from Turkey presenting with very slow progressive cerebellar symptoms including dysarthria, dysmetria, and gait ataxia, we performed SNP-based linkage analysis in the family along with whole exome sequencing (WES) in two affected siblings. We identified a homozygous variant in SYNE1 (NM_033071.3: c.13086delC; p.His4362GlnfsX2) in all four affected siblings. This variant presented herein has originally been associated with only pure ataxia in a single case. We thus present segregation and phenotypic manifestations of this variant in four affected family members and further extend the pure ataxia phenotype with upper motor neuron involvement and peripheral neuropathy. Our findings in turn established a precise molecular diagnosis in this family, demonstrating the use of WES combined with linkage analysis in families as a powerful tool for establishing a quick and precise genetic diagnosis of complex neurological phenotypes.

Ataxia with oculomotor apraxia type 2 fibroblasts exhibit increased susceptibility to oxidative DNA damage

Ataxia with oculomotor apraxia type 2 (AOA2) is an autosomal recessive cerebellar ataxia associated with mutations in SETX, which encodes the senataxin protein, a DNA/RNA helicase. We describe the clinical phenotype and molecular characterization of a Colombian AOA2 patient who is compound heterozygous for a c.994 C>T (p.R332W) missense mutation in exon 7 and a c.6848_6851delCAGA (p.T2283KfsX32) frameshift deletion in SETX exon 21. Immunocytochemistry of patient-derived fibroblasts revealed a normal cellular distribution of the senataxin protein, suggesting that these mutations do not lead to loss or mis-localization of the protein, but rather that aberrant function of senataxin underlies the disease pathogenesis. Furthermore, we used the alkaline comet assay to demonstrate that patient-derived fibroblast cells exhibit an increased susceptibility to oxidative DNA damage. This assay provides a novel and additional means to establish pathogenicity of SETX mutations.

Molecular diagnosis of autosomal recessive cerebellar ataxia in the whole exome/genome sequencing era

Autosomal recessive cerebellar ataxias (ARCA) are a clinically and genetically heterogeneous group of rare neurodegenerative disorders characterized by autosomal recessive inheritance and an early age of onset. Progressive ataxia is usually the prominent symptom and is often associated with other neurological or additional features. ARCA classification still remains controversial even though different approaches have been proposed over the years. Furthermore, ARCA molecular diagnosis has been a challenge due to phenotypic overlap and increased genetic heterogeneity observed within this group of disorders. Friedreich’s ataxia and ataxia telangiectasia have been reported as the most frequent and well-studied forms of ARCA. Significant progress in understanding the genetic etiologies of the ARCA has been achieved during the last 15 years. The methodological revolution that has been observed in genetics over the last few years has contributed significantly to the molecular diagnosis of rare diseases including the ARCAs. Development of high throughput technologies has resulted in the identification of new ARCA genes and novel mutations in known ARCA genes. Therefore, an improvement in the molecular diagnosis of ARCA is expected. Moreover, based on the fact that many patients still remain undiagnosed, additional forms of ataxia are expected to be identified. We hereby review the current knowledge on the ARCAs, focused on the genetic findings of the most common forms that were molecularly characterized before the whole exome/genome era, as well as the most recently described forms that have been elucidated with the use of these novel technologies. The significant contribution of whole-exome sequencing or whole-genome sequencing in the molecular diagnosis of ARCAs is discussed.