Mutational mechanism for DAB1 (ATTTC)n insertion in SCA37: ATTTT repeat lengthening and nucleotide substitution.

Dynamic mutations by microsatellite instability are the molecular basis of a growing number of neuromuscular and neurodegenerative diseases. Repetitive stretches in the human genome may drive pathogenicity, either by expansion above a given threshold, or by insertion of abnormal tracts in nonpathogenic polymorphic repetitive regions, as is the case in spinocerebellar ataxia type 37 (SCA37). We have recently established that this neurodegenerative disease is caused by an (ATTTC)n insertion within an (ATTTT)n in a noncoding region of DAB1. We now investigated the mutational mechanism that originated the (ATTTC)n insertion within an ancestral (ATTTT)n . Approximately 3% of nonpathogenic (ATTTT)n alleles are interspersed by AT-rich motifs, contrarily to mutant alleles that are composed of pure (ATTTT)n and (ATTTC)n stretches. Haplotype studies in unaffected chromosomes suggested that the primary mutational mechanism, leading to the (ATTTC)n insertion, was likely one or more T>C substitutions in an (ATTTT)n pure allele of approximately 200 repeats. Then, the (ATTTC)n expanded in size, originating a deleterious allele in DAB1 that leads to SCA37. This is likely the mutational mechanism in three similar (TTTCA)n insertions responsible for familial myoclonic epilepsy. Because (ATTTT)n tracts are frequent in the human genome, many loci could be at risk for this mutational process.

Verification of Inter-laboratorial Genotyping Consistency in the Molecular Diagnosis of Polyglutamine Spinocerebellar Ataxias.

The polyglutamine spinocerebellar ataxias (SCAs) constitute a clinically and genetically heterogeneous group of rare late-onset neurodegenerative disorders, caused by CAG expansions in the coding region of the respective genes. Given their considerable clinical overlapping, differential diagnosis relies on molecular testing. Laboratory best practice guidelines for molecular genetic testing of the SCAs were released in 2010 by the European Molecular Genetics Quality Network, following the recognition of gross genotyping errors by some diagnostic laboratories. The main goal of this study was to verify the existence of inter-laboratorial consistency comparing genotypes for SCA1, SCA2, SCA3, SCA6 and SCA7 obtained by independent diagnostic laboratories. The individual impact of different methodological issues on the genotype for the several SCAs was also analysed. Four international collaborative diagnostic laboratories provided 79 samples and the respective SCA genotypes. Samples were genotyped in-house for all SCAs using an independent methodology; comparison of the allele size obtained with the one provided by the collaborative laboratories was performed. Globally, no significant differences were identified, a result which could be reflecting the fulfilment of recommendations for the molecular testing of SCAs and demonstrating an improvement in genotyping accuracy.

Large-scale assessment of polyglutamine repeat expansions in Parkinson disease.

OBJECTIVES: We aim to clarify the pathogenic role of intermediate size repeat expansions of SCA2, SCA3, SCA6, and SCA17 as risk factors for idiopathic Parkinson disease (PD). METHODS: We invited researchers from the Genetic Epidemiology of Parkinson's Disease Consortium to participate in the study. There were 12,346 cases and 8,164 controls genotyped, for a total of 4 repeats within the SCA2, SCA3, SCA6, and SCA17 genes. Fixed- and random-effects models were used to estimate the summary risk estimates for the genes. We investigated between-study heterogeneity and heterogeneity between different ethnic populations. RESULTS: We did not observe any definite pathogenic repeat expansions for SCA2, SCA3, SCA6, and SCA17 genes in patients with idiopathic PD from Caucasian and Asian populations. Furthermore, overall analysis did not reveal any significant association between intermediate repeats and PD. The effect estimates (odds ratio) ranged from 0.93 to 1.01 in the overall cohort for the SCA2, SCA3, SCA6, and SCA17 loci. CONCLUSIONS: Our study did not support a major role for definite pathogenic repeat expansions in SCA2, SCA3, SCA6, and SCA17 genes for idiopathic PD. Thus, results of this large study do not support diagnostic screening of SCA2, SCA3, SCA6, and SCA17 gene repeats in the common idiopathic form of PD. Likewise, this largest multicentered study performed to date excludes the role of intermediate repeats of these genes as a risk factor for PD.