Analysis of ATXN2 trinucleotide repeats in Korean patients with amyotrophic lateral sclerosis

Beyond C9orf72: repeat expansions and copy number variations as risk factors of amyotrophic lateral sclerosis across various populations

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder which is characterized by the loss of both upper and lower motor neurons in the central nervous system. In a significant fraction of ALS cases - irrespective of family history- a genetic background may be identified. The genetic background of ALS shows a high variability from one ethnicity to another. The most frequent genetic cause of ALS is the repeat expansion of the C9orf72 gene. With the emergence of next-generation sequencing techniques and copy number alteration calling tools the focus in ALS genetics has shifted from disease causing genes and mutations towards genetic susceptibility and risk factors.In this review we aimed to summarize the most widely recognized and studied ALS linked repeat expansions and copy number variations other than the hexanucleotide repeat expansion in the C9orf72 gene. We compare and contrast their involvement and phenotype modifying roles in ALS among different populations.

The Clinical and Ploynucleotide Repeat Expansion Analysis of ATXN2, NOP56, AR and C9orf72 in Patients With ALS From Mainland China

Background

Repeat expansions, including those in C9orf72 and ATXN2, have been implicated in amyotrophic lateral sclerosis (ALS). However, there have been few studies on the association of AR and NOP56 repeat expansion with ALS, especially in China. Accordingly, we aimed to evaluate the frequency of C9orf72 and ATXN2 repeat mutations and investigate whether NOP56 and AR repeat expansion are risk factors for ALS.

Methods

In this study, 736 ALS patients and several hundred healthy controls were recruited. Polymerase chain reaction (PCR) and repeat-primed PCR (RP-PCR) were performed to determine the repeat lengths in C9orf72, ATXN2, AR, and NOP56.

Results

GGGGCC repeats in C9orf72 were observed in six ALS patients (0.8%, 6/736) but not in any of the controls (0/365). The patients with pathogenic GGGGCC repeats showed shorter median survival times than those with a normal genotype (p = 0.006). Regarding ATXN2 CAG repeats, we identified that intermediate repeat lengths (29–34 copies) were associated with ALS (p = 0.033), and there was no difference in clinical characteristics between the groups with and without intermediate repeats (p > 0.05). Meanwhile, we observed that there was no association between the repeat size in AR and NOP56 and ALS (p > 0.05).

Conclusions

Our results demonstrated that pathogenetic repeats in C9orf72 are rare in China, while intermediate CAG repeats in ATXN2 are more frequent but have no effect on disease phenotypes; the repeat size in AR and NOP56 may not be a risk factor for ALS.

Mutation spectrum of amyotrophic lateral sclerosis in Central South China

To analyze the mutational spectrum of known ALS causative genes in China ALS patients. We comprehensively analyzed 51 ALS causative genes by combining different sequencing technologies in 753 unrelated ALS patients from Central South China. The mean age at onset (AAO) was 53.7±11.4 years. The AAO was earlier in the autosomal dominant (AD) ALS patients than in the sporadic ALS (sALS) patients. Bulbar onset was more frequent in females than in males. SOD1 was the most frequently mutated gene in the AD-ALS and the sALS patients, followed by the ATXN2 and FUS genes in the AD-ALS patients and the NEK1 and CACNA1H genes in the sALS patients. Patients with RDVs in the SOD1 or FUS genes had an earlier AAO than the mean AAO of all the patients, while the patients with RDVs in the NEK1 gene showed later onset. SOD1 gene was the most commonly mutated gene in ALS patients in China, followed by ATXN2 and NEK1. The phenotype might be determined synergistically by sex and genetic variants.

Molecular Mechanisms and Therapeutics for Spinocerebellar Ataxia Type 2

The effective therapeutic treatment and the disease-modifying therapy for spinocerebellar ataxia type 2 (SCA2) (a progressive hereditary disease caused by an expansion of polyglutamine in the ataxin-2 protein) is not available yet. At present, only symptomatic treatment and methods of palliative care are prescribed to the patients. Many attempts were made to study the physiological, molecular, and biochemical changes in SCA2 patients and in a variety of the model systems to find new therapeutic targets for SCA2 treatment. A better understanding of the uncovered molecular mechanisms of the disease allowed the scientific community to develop strategies of potential therapy and helped to create some promising therapeutic approaches for SCA2 treatment. Recent progress in this field will be discussed in this review article.

Association of ATXN2 intermediate-length CAG repeats with amyotrophic lateral sclerosis correlates with the distributions of normal CAG repeat alleles among individual ethnic populations

Intermediate-length CAG repeats in ATXN2 have been widely shown to be a risk factor for sporadic amyotrophic lateral sclerosis (SALS). To evaluate the association of ATXN2 intermediate-length CAG repeat alleles with an increased risk of SALS, we investigated distributions of CAG repeat alleles in 394 patients with SALS and 490 control individuals in the Japanese population. In the intermediate-length repeat units of 29 or more, we identified one SALS patient with 31 repeat units and two control individuals with 30 repeat units. Thus, no significant differences in the carrier frequency of intermediate-length CAG repeat alleles were detected between patients with SALS and control individuals. When we investigated the distribution of "large normal alleles" defined as ATXN2 CAG repeats ranging from 24 up to 33 in the Japanese population compared with those in other populations in previous studies, the frequency of large normal alleles was significantly higher in the European and North American series than in the Japanese series. Moreover, these frequencies in the Turkish, Chinese, Korean, and Brazilian (Latin American) series were also higher than that in the Japanese series. These results raise the possibility that the frequencies of large normal alleles in individual populations underlie the frequencies of ALS risk alleles in the corresponding populations.