Proteinopathies associated to repeat expansion disorders

The most common neurodegenerative disorders, such as Alzheimer's or Parkinson's diseases, are characterized by synaptic dysfunction, neuronal loss and proteinaceous aggregates in central nervous system. The deposition of misfolded proteins constitutes neuropathological hallmarks of these diseases, grouped in the generic term of proteinopathies. Apart from these, other neurodegenerative diseases are characterized by genetic abnormalities like unstable repetitive simple sequence tracts (microsatellites) dispersed throughout the human genome. They are called repeat expansion disorders and include, for example, Huntington's disease or frontotemporal dementia/amyotrophic lateral sclerosis phenotypes associated to an expansion in C9ORF72. The presence of the expanded DNA tract leads to molecular alterations at the DNA, RNA and protein levels associated to distinct mechanisms, such as loss-of-function (LOF), gain-of-function (GOF) including misfolding of physiological or mutant proteins, favoring their polymerization and aggregation. Therefore, specific proteinopathies also arise from these repeat expansion disorders. The molecular description of the nature and location of expanded tracts, highlighting the consequences onto clinical phenotypes will be first described. Specific focuses on the three pathomechanisms of the repeat expansions associated to proteinopathies will then be addressed. Lastly, we will show how progress in the understanding of these different mechanisms has led to recent advances in new/innovative therapeutic approaches and emergence of associated biomarkers.