Gene Deregulation and Underlying Mechanisms in Spinocerebellar Ataxias With Polyglutamine Expansion

Aging is an inevitable outcome of life, characterized by progressive decline in tissue and organ function and increased risk of mortality. Accumulating evidence links aging to genetic and epigenetic alterations. Given the reversible nature of epigenetic mechanisms, these pathways provide promising avenues for therapeutics against age-related decline and disease. In this review, we provide a comprehensive overview of epigenetic studies from invertebrate organisms, vertebrate models, tissues, and in vitro systems. We establish links between common operative aging pathways and hallmark chromatin signatures that can be used to identify "druggable" targets to counter human aging and age-related disease.

Spinocerebellar ataxia type 1 (SCA1) is an autosomal dominant disorder characterized by neurodegeneration of the cerebellum, spinal cord and brainstem. A 1.2-Megabase stretch of DNA from the short arm of chromosome 6 containing the SCA1 locus was isolated in a yeast artificial chromosome contig and subcloned into cosmids. A highly polymorphic CAG repeat was identified in this region and was found to be unstable and expanded in individuals with SCA1. There is a direct correlation between the size of the (CAG)n repeat expansion and the age-of-onset of SCA1, with larger alleles occurring in juvenile cases. We also show that the repeat is present in a 10 kilobase mRNA transcript. SCA1 is therefore the fifth genetic disorder to display a mutational mechanism involving an unstable trinucleotide repeat.

Background: Hereditary cerebellar ataxias (HCA) and hereditary spastic paraplegias (HSP) are two groups of neurodegenerative disorders that usually present with progressive gait impairment, often leading to permanent disability. Advances in genetic research in the last decades have improved their diagnosis and brought new possibilities for prevention and future treatments. Still, there is great uncertainty regarding their global epidemiology. Summary: Our objective was to assess the global distribution and prevalence of HCA and HSP by a systematic review and meta-analysis of prevalence studies. The MEDLINE, ISI Web of Science and Scopus databases were searched (1983-2013) for studies performed in well-defined populations and geographical regions. Two independent reviewers assessed the studies and extracted data and predefined methodological parameters. Overall, 22 studies were included, reporting on 14,539 patients from 16 countries. Multisource population-based studies yielded higher prevalence values than studies based primarily on hospitals or genetic centres. The prevalence range of dominant HCA was 0.0-5.6/10 5 , with an average of 2.7/10 5 (1.5-4.0/10 5 ). Spinocerebellar ataxia type 3 (SCA3)/Machado-Joseph disease was the most common dominant ataxia, followed by SCA2 and SCA6. The autosomal recessive (AR) HCA (AR-HCA) prevalence range was 0.0-7.2/10 5 , the average being 3.3/10 5 (1.8-4.9/10 5 ). Friedreich ataxia was the most frequent AR-HCA, followed by ataxia with oculomotor apraxia or ataxia-telangiectasia. The prevalence of autosomal dominant (AD) HSP (AD-HSP) ranged from 0.5 to 5.5/10 5 and that of AR-HSP from 0.0 to 5.3/10 5 , with pooled averages of 1.8/10 5 (95% CI: 1.0-2.7/10 5 ) and 1.8/10 5 (95% CI: 1.0-2.6/10 5 ), respectively. The most common AD-HSP form in every population was spastic paraplegia, autosomal dominant, type 4 (SPG4), followed by SPG3A, while SPG11 was the most frequent AR-HSP, followed by SPG15. In population-based studies, the number of families without genetic diagnosis after systematic testing ranged from 33 to 92% in the AD-HCA group, and was 40-46% in the AR-HCA, 45-67% in the AD-HSP and 71-82% in the AR-HSP groups. Key Messages: Highly variable prevalence values for HCA and HSP are reported across the world. This variation reflects the different genetic make-up of the populations, but also methodological heterogeneity. Large areas of the world remain without prevalence studies. From the available data, we estimated that around 1:10,000 people are affected by HCA or HSP. In spite of advances in genetic research, most families in population-based series remain without identified genetic mutation after extensive testing.