Polyglutamine ataxias: From Clinical and Molecular Features to Current Therapeutic Strategies

Spinocerebellar ataxias are a large group of heterogeneous diseases that all involve selective neuronal degeneration and accompanied cerebellar ataxia. These diseases can be further broken down into discrete groups according to their underlying molecular genetic cause. The most common are the polyglutamine ataxias, of which there are six; Spinocerebellar ataxia type 1, 2, 3, 6, 7 and 17. These diseases are characterised by a pathological expanded cytosine–adenine–guanine (CAG) repeat sequence, in the protein coding region of a given gene. Common clinical features include lack of coordination and gait ataxia, speech and swallowing difficulties, as well as impaired hand and motor functions. The polyglutamine spinocerebellar ataxias are typically late onset diseases that are progressive in nature and often lead to premature death, for which there is currently no known cure or effective treatment strategy. Although caused by the same molecular mechanism, the causative gene and associated protein differ for each disease. The exact mechanism by which disease pathogenesis is caused remains elusive. However, the variable (CAG)n repeats are codons that may be translated to an expanded glutamine tract, leading to conformational changes in the protein, giving it a toxic gain of function. Several pathogenic pathways have been implicated in polyglutamine spinocerebellar ataxia diseases, such as the hallmark feature of neuronal nuclear inclusions, protein misfolding and aggregation, as well as transcriptional dysregulation. These pathways are attractive avenues for potential therapeutic interventions, as the potential to treat more than one disease exists. Research is ongoing, and several promising therapies are currently underway in an attempt to provide relief for this devastating class of diseases.