Research Overview & Genetic testing
The inherited Spinocerebellar ataxias (SCAs) are a broad group of inherited neurodegenerative diseases characterized by selective cerebellar degeneration and other neurological symptoms. In South Africa, the main research focus has been on SCA types 1 and 7. Both of these disorders are dominantly inherited and caused by the expansion of a CAG repeat in the associated gene (ataxin-1 and ataxin-7). The translation of this repeat leads to an extended polyglutamine tract within the downstream protein, resulting in both SCA1 and SCA7 being classified as polyglutamine repeat disorders.
In South Africa, molecular genetic testing is available for seven of the SCAs (types 1, 2, 3, 6, 7, 12 and 17) through the National Health Laboratory Services (NHLS) in Cape Town. Patients may be referred for genetic testing through their clinician, or patients may attend the special monthly Neurogenetics Clinic at Groote Schuur Hospital.
History of the Research Group
In 1994 Dr Alan Bryer completed a PhD at UCT on the Inherited Ataxias in SA. After a diagnostic molecular genetic testing service was established at UCT in 1995, a research project was undertaken looking at SCA7 in SA (G Solomon : Hons 2002) on which the current research approach focussing on the inherited ataxias is based (Greenberg et al. 2006).
In 2006 a PhD research project using gene silencing was initiated aimed at developing a novel therapeutic strategy for the treatment of macular degeneration, which is typically associated with SCA7. SCA7 belongs to a broad class of genetic neurodegenerative diseases caused by abnormal expansions of an unstable trinucleotide repeat, in this case, in the ataxin-7 gene (atxn7). SCA7 normally accounts for 3-5% of all forms of ataxia worldwide, but is more common in SA. In particular, SCA7 is exclusively found in SA families of Black African origin & a potential founder effect has been suggested (Greenberg et al. 2006, Smith et al. 2012). Based on the observation that a characteristic SNP distinguishes between the normal & the mutant allele in most SA SCA7 patients, this research project was aimed at developing an RNAi-based strategy targeting the disease-associated allele. Given the poor specificity of the expanded repeated tract as a target for RNAi, this disease-associated SNP represents an intriguing alternative strategy for selective disease-allele silencing. State-of-the-art methodology has been used to date and this will eventually also include developing an appropriate AAV-mediated delivery of shRNA to the eye. This SCA7 gene-silencing research project resulted in a successfully completed PhD by Dr Janine Scholefield in 2008. Dr Scholefield completed a post doctoral fellowship with Professor Matthew Wood at the University of Oxford, and is now at the CSIR.
In 2008, a similar MSc project was undertaken by Fiona Baine, looking at SCA1 with the intention to identify a suitable SNP for allele-specific silencing of the disease-causing gene in South African SCA1 patients. Danielle Smith (nee Berkowitz) joined the group in 2009 and completed an MSc project investigating the expression of the heat shock proteins in SCA7 patient-derived cells, in addition to developing an allele-specific qPCR assay to measure the expression of each ataxin-7 allele. In 2010 Ms Alice Foxon completed an Honour’s project entitled “A Haplotype study of South African SCA7 patients”. Reinette Weideman registered in 2011 to do her Honours degree at UCT and elected to work with the neurogenetics research group. Her research project was entitled: “Epidemiology of SCA1 in South African Populations” and the aim of the project was to investigate the occurrence of haplotypes in SCA1 patients, and the presence of CAT interruptions known to stabilise the disease-causing mutation. Mr Gift Pule completed an Honour’s project in 2012, focussing on the study of somatic instability in SCA7 patient-derived cells.
The main focus of the SCA research group is currently on the use of induced pluripotent stem (iPS) cells in order to model the SCA7 phenotype in vitro. This “disease-in-a-dish” approach is being used to better understand the pathogenic mechanisms involved in SCA7, and to create suitable cell-based models for testing of future therapies.
In 2012 Lauren Watson was awarded a PhD for her thesis which was based on a research project that focussed on the generation of an in vitro disease model for SCA7, supervised by Professors Jacquie Greenberg (UCT), Matthew Wood (Oxford/UCT) and Marco Weinberg (Wits). Key elements of her work included the evaluation of transcriptional dysregulation in SCA7 patient-derived cells, as well as the creation of iPSC-derived neuronal cells from patients, with a view to developing an accurate cell-based model for the evaluation of gene-based therapies. Lauren was awarded a prestigious Commonwealth Split-Site Doctoral Scholarship in 2010. This allowed her to spend time in Professor Matthew Wood’s laboratory at the University of Oxford from October 2010 to February 2011, where she, together with collaborator Dr Janine Scholefield, successfully generated (to our knowledge) the first iPSC lines from South African SCA7 patients. Lauren returned to Oxford in September of 2011, to complete the process of neuronal differentiation on the iPSC lines generated during her first visit, to conclude the investigation into transcriptional dysregulation in SCA7 patient cells, and to determine the accuracy of the neuronal cell model in predicting efficacy of RNAi-based therapy.
From 2013-2014 Lauren was a post doctoral fellow within the Divisions of Human Genetics and Cell Biology at UCT, continuing her work on neuronal differentiation.
In 2014 Danielle Smith was awarded her PhD degree, after focussing her research on the retinal phenotype associated with SCA7. The main aim of her project was to elucidate some of the key players involved in the selective degeneration of photoreceptors in SCA7 patients, through the derivation of retinal cells from iPSCs. Danielle was also involved in the continuous improvement of diagnostic tests for the SCAs offered by the NHLS, and completed a study to determine the frequency and distribution of the SCAs in South Africa. Future work will focus on broadening this study to include other African countries.
In summary, the overall SCA7 project aims to investigate the molecular pathogenesis of SCA7 through the generation of SCA7 patient-derived disease-relevant cell types (neuronal cells, RPE and photoreceptors differentiated from iPSCs), with a view to developing therapies. The ultimate objective being to refine the locally developed gene-silencing form of therapy for the treatment of this specific dominant form of inherited ataxia. The goal of the overall project is to determine efficacy and safety using iPSCs as pre-clinical cellular models, in preparation for human clinical trials, as proof-of-principle, so that the therapy can ultimately be developed for SA SCA7 patients.
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