Genetic Analysis of Hereditary Ataxias in Peru Identifies SCA10 Families with Incomplete Penetrance

Spinocerebellar Ataxia Type 10 (SCA 10) in Brazil

Spinocerebellar ataxia type 10 (SCA10) is an autosomal dominant inherited ataxia caused by the expansion of ATTCT pentanucleotide repeats in intron 9 of the ATXN10 gene. This rare form of SCA has predominantly been observed in individuals of Indigenous American and East Asian descent. Notably, in Mexico and the southern Brazilian states of Paraná and Santa Catarina, SCA10 is identified as the second most prevalent type of spinocerebellar ataxia. Initially, the phenotype described in Mexico featured a combination of cerebellar ataxia and epilepsy-a presentation also observed in other Latin American and Asian countries, as well as some Brazilian states. However, in Paraná and Santa Catarina, the predominant manifestation of SCA10 is pure cerebellar ataxia, which is distinguished from the presentations seen in other regions.

Hereditary Ataxias in Argentina

Hereditary or genetic ataxias are hundreds of disorders characterized by large phenotypic, genetic, and epidemiological heterogeneity. In Argentina, 35 genetic ataxias have been identified, with SCA1 (ATX-ATXN1), SCA2 (ATX-ATXN2), SCA3 (ATX-ATXN3), and Friedreich ataxia (ATX-FXN) as the most prevalent causes, reflecting the epidemiology of most Western European countries, the main origin of immigration to the country. Genetic diagnostic studies of ataxia cohorts in Argentina have found high rates of undiagnosed patients, ranging from 65 to 82%. Deep phenotyping, comprehensive genetic testing, and knowledge of the prevalence of different genetic ataxias are essential for an accurate diagnostic and treatment approach in clinical practice. This narrative review proposes a targeted, tiered genetic diagnostic approach for undiagnosed patients based on the Argentinian epidemiological and healthcare system data. Future national efforts should support comprehensive screening studies on ataxia cohorts, including testing for repeat expansions in RFC1 and FGF14 genes. In addition, establishing a trial-ready patient registry for genetic ataxias, enhancing networking with international clinical and research initiatives, and developing specialized centers for interdisciplinary care of genetic ataxia patients are recommended.

The impact of interrupted ATXN10 expansions on clinical findings of spinocerebellar ataxia type 10

BACKGROUND

Spinocerebellar ataxia type 10 (SCA10), due to an ATTCT repeat expansion in ATXN10, has variable expressivity and the role of presence (ATTCTint +) and absence (ATTCTint-) of interruptions in the repeat is not clear. We aimed to describe the relations between ATTCTint + and age at onset, seizures, and neurologic severity in ataxic and non-ataxic carriers from Brazil.

METHODS

Family, age at onset (AO), and seizures data plus DNA were obtained from symptomatic carriers already diagnosed in Porto Alegre, Curitiba, and São Paulo, Brazil. Patients and their relatives were invited to be evaluated through Scale of Assessment and Rating of Ataxia (SARA) and other clinical scales; a SARA > 2.5 classified subjects as ataxic carriers. Repeat-primed PCR (RP-PCR) defined the expansions with (ATTCTint +) or without (ATTCTint-) interruptions. Comparisons were performed for a p level of 0.05.

RESULTS

Among 78 ataxic carriers, earlier AO (p = 0.039) and higher occurrences of epilepsy (p < 0.0001) were seen in subjects with ATTCTint + than in those with ATTCTint-. Clinical scales were worse in 34 ataxics than in 7 non-ataxics and 10 related controls (p = 0.006) and did not discriminate non-ataxics from controls. The 11 ataxic ATTCTint + carriers had higher SARA scores per year of disease duration than the 23 ATTCTint- carriers (r = 0.879, beta = 0.45, p = 0.0001).

DISCUSSION

ATTCTint + carriers had worse clinical findings than ATTCTint- carriers: earlier AO, more seizures, and worse ataxia scores. Interruptions in the expanded repeat have a real impact in SCA10 phenotype.

Novel Intermediate ATXN10 Alleles in the Healthy Peruvian Population: A Matter of Indigenous American Ethnic Origin

Spinocerebellar ataxia type 10 (SCA10) is a neurodegenerative disease predominant in Latin American individuals with Indigenous American ancestry. SCA10 is caused by an expansion of ATTCT repeat within the ATXN10 gene. Healthy individuals carry 9-32 ATTCT repeats, whereas SCA10 patients carry an expansion of 280 repeats and higher. Recently, intermediate alleles (over than 32 repeats) have been identified in healthy Peruvian Indigenous American individuals, with unclear significance. This study aims to characterize the variability of the ATTCT repeats within the ATXN10 gene across self-declared Indigenous American and Mestizo subpopulations from Peru. A total of 871 samples (754 Mestizo and 117 Indigenous American) were analyzed using PCR, and RP-PCR when suspecting apparent homozygosity due to larger alleles. 8.7% of the total of healthy individuals (76/871) carry at least one intermediate allele. The 14-repeat allele being the most common for both subpopulations (41.5%). Intermediate alleles were detected in the Peruvian population (4.5%) with a significantly higher frequency among self-declared Indigenous American compared to Mestizo, suggesting a possible association with the ethnic origin. The G allele at the SNP rs41524547 had a frequency of 51.39% in individuals with intermediate alleles, with not significantly difference between subpopulations. Further analysis should be performed to confirm the size and composition of ATTCT repeat tract, as well as the contribution of rs41524547 in SCA10.

ATXN10 Gene Expansions in Mexican Patients with Ataxia Without Epilepsy

Spinocerebellar ataxia type 10 (SCA10) is an autosomal dominant (AD) neurodegenerative disorder prevalent in the Americas, particularly in Mexico. Clinical manifestations include progressive ataxia and epilepsy. However, it can exhibit wide phenotypic variability and even reduced penetrance. Because the diagnostic overlaps with other ataxias, molecular diagnosis is essential. This cross-sectional study conducted a retrospective review and analysis of 183 DNA samples from a laboratory registry of patients with ataxia who were suspected of having AD ataxia (n = 86; negative for ATXN1, ATXN2, ATXN3, ATXN7, TBP, and ATN1 genes) or sporadic ataxia (n = 97). Triplet repeat-primed PCR (TP-PCR) was performed to identify ATXN10 gene expansions. 19.6% (n = 36) of the samples showed ATXN10 expansions, with a higher proportion of hereditary AD cases (30.2%; n = 26) compared to sporadic cases (10.3%; n = 10). Clinical information was available in only 23 registries, with manifestations predominantly including cerebellar signs, but notably not epilepsy. The frequency of SCA10 in our country underlines the need to change the diagnostic suspicion, as the absence of epilepsy challenges previous diagnostic assumptions. As this is a study from a laboratory registry, we are aware of certain limitations.

Extended haplotype with rs41524547-G defines the ancestral origin of SCA10

Spinocerebellar ataxia type 10 (SCA10) is a rare autosomal dominant ataxia caused by a large expansion of the (ATTCT)n repeat in ATXN10. SCA10 was described in Native American and Asian individuals which prompted a search for an expanded haplotype to confirm a common ancestral origin for the expansion event. All patients with SCA10 expansions in our cohort share a single haplotype defined at the 5'-end by the minor allele of rs41524547, located ~35 kb upstream of the SCA10 expansion. Intriguingly, rs41524547 is located within the miRNA gene, MIR4762, within its DROSHA cleavage site and just outside the seed sequence for mir4792-5p. The world-wide frequency of rs41524547-G is less than 5% and found almost exclusively in the Americas and East Asia-a geographic distribution that mirrors reported SCA10 cases. We identified rs41524547-G(+) DNA from the 1000 Genomes/International Genome Sample Resource and our own general population samples and identified SCA10 repeat expansions in up to 25% of these samples. The reduced penetrance of these SCA10 expansions may be explained by a young (pre-onset) age at sample collection, a small repeat size, purity of repeat units, or the disruption of miR4762-5p function. We conclude that rs41524547-G is the most robust at-risk SNP allele for SCA10, is useful for screening of SCA10 expansions in population genetics studies and provides the most compelling evidence to date for a single, prehistoric origin of SCA10 expansions sometime prior to or during the migration of individuals across the Bering Land Bridge into the Americas.

Machado Joseph-Disease Is Rare in the Peruvian Population

Spinocerebellar ataxia type 3 or Machado-Joseph disease (MJD/SCA3) is the most prevalent autosomal dominant cerebellar ataxia worldwide, but its frequency varies by geographic region. We describe MJD/SCA3 patients diagnosed in a tertiary healthcare institution in Peru. In a cohort of 341 individuals (253 probands) with clinical ataxia diagnosis, seven MJD/SCA3 probands were identified and their pedigrees extended, detecting a total of 18 MJD/SCA3 cases. Out of 506 alleles from all probands from this cohort, the 23-CAG repeat was the most common ATXN3 allele (31.8%), followed by the 14-CAG repeat allele (26.1%). Normal alleles ranged from 12 to 38 repeats while pathogenic alleles ranged from 64 to 75 repeats. We identified 80 large normal (LN) alleles (15.8%). Five out of seven families declared an affected family member traced back to foreign countries (England, Japan, China, and Trinidad and Tobago). MJD/SCA3 patients showed ataxia, accompanied by pyramidal signs, dysarthria, and dysphagia as well as abnormal oculomotor movements. In conclusion, ATXN3 allelic distribution in non-MJD/SCA3 patients with ataxia is similar to the distribution in normal individuals around the world, whereas LN allele frequency reinforces no correlation with the frequency of MJD/SCA3. Evidence of any atypical MJD/SCA3 phenotype was not found. Furthermore, haplotypes are required to confirm the foreign origin of MJD/SCA3 in the Peruvian population.

Molecular Mechanisms in Pentanucleotide Repeat Diseases

The number of neurodegenerative diseases resulting from repeat expansion has increased extraordinarily in recent years. In several of these pathologies, the repeat can be transcribed in RNA from both DNA strands producing, at least, one toxic RNA repeat that causes neurodegeneration by a complex mechanism. Recently, seven diseases have been found caused by a novel intronic pentanucleotide repeat in distinct genes encoding proteins highly expressed in the cerebellum. These disorders are clinically heterogeneous being characterized by impaired motor function, resulting from ataxia or epilepsy. The role that apparently normal proteins from these mutant genes play in these pathologies is not known. However, recent advances in previously known spinocerebellar ataxias originated by abnormal non-coding pentanucleotide repeats point to a gain of a toxic function by the pathogenic repeat-containing RNA that abnormally forms nuclear foci with RNA-binding proteins. In cells, RNA foci have been shown to be formed by phase separation. Moreover, the field of repeat expansions has lately achieved an extraordinary progress with the discovery that RNA repeats, polyglutamine, and polyalanine proteins are crucial for the formation of nuclear membraneless organelles by phase separation, which is perturbed when they are expanded. This review will cover the amazing advances on repeat diseases.