Polyglutamine disease: from pathogenesis to therapy

Polyglutamine diseases are inherited neurodegenerative conditions arising from expanded trinucleotide CAG repeats in the disease-causing gene, which are translated into polyglutamine tracts in the resultant protein. Although these diseases share a common type of mutation, emerging evidence suggests that pathogenesis is complex, involving disruption of key cellular pathways, and varying with the disease context. An understanding of polyglutamine disease mechanisms is critical for development of novel therapeutics. Here we summarise theories of molecular pathogenesis, and examine ways in which this knowledge is being harnessed for therapy, with reference to work under way at the University of Cape Town. Despite a plethora of preclinical data, clinical trials of therapies for polyglutamine diseases have had only limited success. However, recently initiated trials, including those using gene silencing approaches, should provide valuable insights into the safety and efficacy of therapies directly targeting polyglutamine pathogenesis. This is particularly relevant in the South African context, where the frequencies of two polyglutamine diseases, spinocerebellar ataxia types 1 and 7, are among the highest globally

Design of RNAi Hairpins for Mutation-Specific Silencing of Ataxin-7 and Correction of a SCA7 Phenotype

Spinocerebellar ataxia type 7 is a polyglutamine disorder caused by an expanded CAG repeat mutation that results in neurodegeneration. Since no treatment exists for this chronic disease, novel therapies such post-transcriptional RNA interference-based gene silencing are under investigation, in particular those that might enable constitutive and tissue-specific silencing, such as expressed hairpins. Given that this method of silencing can be abolished by the presence of nucleotide mismatches against the target RNA, we sought to identify expressed RNA hairpins selective for silencing the mutant ataxin-7 transcript using a linked SNP. By targeting both short and full-length tagged ataxin-7 sequences, we show that mutation-specific selectivity can be obtained with single nucleotide mismatches to the wild-type RNA target incorporated 3′ to the centre of the active strand of short hairpin RNAs. The activity of the most effective short hairpin RNA incorporating the nucleotide mismatch at position 16 was further studied in a heterozygous ataxin-7 disease model, demonstrating significantly reduced levels of toxic mutant ataxin-7 protein with decreased mutant protein aggregation and retention of normal wild-type protein in a non-aggregated diffuse cellular distribution. Allele-specific mutant ataxin7 silencing was also obtained with the use of primary microRNA mimics, the most highly effective construct also harbouring the single nucleotide mismatch at position 16, corroborating our earlier findings. Our data provide understanding of RNA interference guide strand anatomy optimised for the allele-specific silencing of a polyglutamine mutation linked SNP and give a basis for the use of allele-specific RNA interference as a viable therapeutic approach for spinocerebellar ataxia 7

A New Class of Stem Cells in South Africa: Introducing Induced Pluripotent Stem cells (iPS cells)

Created from adult rather than embryonic cells, induced pluripotent stem (iPS) cells represent a breakthrough in stem cell science, and their pioneers have been recognised with the 2012 Nobel Prize in Medicine. These cells offer new hope in the treatment of pathogenetic diseases, but there is still a way to go on the road to effective therapeutic applications

Clinical and genetic analysis of spinocerebellar ataxia type 7 (SCA7) in Zambian families

Background: To date, 43 types of Spinocerebellar Ataxias (SCAs) have been identified. A subset of the SCAs are caused by the pathogenic expansion of a CAG repeat tract within the corresponding gene. Ethnic and geographic differences are evident in the prevalence of the autosomal dominant SCAs. Few descriptions of the clinical phenotype and molecular genetics of the SCAs are available from the African continent. Established studies mostly concern the South African populations, where there is a high frequency of SCA1, SCA2 and SCA7. The SCA7 mutation in South Africa (SA) has been found almost exclusively in families of indigenous Black African ethnic origin. Objective: To present the results of the first clinical description of seven Zambian families presenting with autosomal dominant SCA, as well as the downstream molecular genetic analysis of a subset of these families. Methods: The study was undertaken at the University Teaching Hospital in Lusaka, Zambia. Ataxia was quantified with the Brief Ataxia Rating Scale derived from the modified international ataxia rating scale. Molecular genetic testing for 5 types of SCA (SCA1, SCA2, SCA3, SCA6 and SCA7) was performed at the National Health Laboratory Service at Groote Schuur Hospital and the Division of Human Genetics, University of Cape Town, SA. The clinical and radiological features were evaluated in seven families with autosomal dominant cerebellar ataxia. Molecular genetic analysis was completed on individuals representing three of the seven families. Results: All affected families were ethnic Zambians from various tribes, originating from three different regions of the country (Eastern, Western and Central province). Thirty-four individuals from four families had phenotypic features of SCA7. SCA7 was confirmed by molecular testing in 10 individuals from 3 of these families. The age of onset of the disease varied from 12 to 59 years. The most prominent phenotypic features in these families were gait and limb ataxia, dysarthria, visual loss, ptosis, ophthalmoparesis/ophthalmoplegia, pyramidal tract signs, and dementia. Affected members of the SCA7 families had progressive macular degeneration and cerebellar atrophy. All families displayed marked anticipation of age at onset and rate of symptom progression. The pathogenic SCA7 CAG repeat ranges varied from 47 to 56 repeats. Three additional families were found to have clinical phenotypes associated with autosomal dominant SCA, however, DNA was not available for molecular confirmation. The age of onset of the disease in these families varied from 19 to 53 years. The most common clinical picture in these families included a combination of cerebellar symptoms with slow saccadic eye movements, peripheral neuropathy, dementia and tremor. Conclusion: SCA is prevalent in ethnic Zambian families. The SCA7 families in this report had similar clinical presentations to families described in other African countries. In all families, the disease had an autosomal dominant pattern of inheritance across multiple generations. All families displayed anticipation of both age of onset and the rate of disease progression. Further clinical and molecular investigations of the inherited ataxias in a larger cohort of patients is important to understand the natural history and origin of SCAs in the Zambian population

Gene Mapping of Usher Syndrome Type IIa: Localization of the Gene to a 2.1-cM Segment on Chromosome 1q41

Usher syndrome type II is associated with hearing loss and retinitis pigmentosa but not with any vestibular problems. It is known to be genetically heterogeneous, and one locus (termed USH2A) has been linked to chromosome 1q41. In an effort to refine the localization of USH2A, the genetic map of the region between and adjacent to the marker loci previously recognized as flanking USH2A (D1S70 and PPOL) is updated. Analysis of marker data on 68 Usher II families places the USH2A gene into a 2.1-cM region between the markers D1S237 and D1S229. The gene for transforming growth factor β2 (TGFB2) and the gene for the homeodomain box (HLX1) are both eliminated as candidates for USH2A, by virtue of their localization outside these flanking markers. The earlier finding of genetic heterogeneity was confirmed in six new families, and the proportion of unlinked Usher II families is estimated at 12.5%. The placement of the USH2A gene into this region will aid in the physical mapping and isolation of the gene itself